Radiocarbon Dating of the El Fin Del Mundo Clovis

Late Pleistocene Palehydrologic Reconstructions and Radiocarbon Dating in the Southeastern Basin and Range, USA

Item Type text; Electronic Dissertation

Authors Kowler, Andrew

Publisher The University of Arizona.

Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.

Download date 02/10/2021 10:48:34

Link to Item http://hdl.handle.net/10150/347134 LATE PLEISTOCENE PALEOHYDROLOGIC RECONSTRUCTIONS AND RADIOCARBON DATING IN THE SOUTHEASTERN BASIN AND RANGE, USA

Andrew L. Kowler

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

In the Graduate College

THE UNIVERSITY OF ARIZONA

2015

THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE

As members of the Dissertation Committee, we certify that we have read the dissertation prepared by Andrew L. Kowler, titled “Late Pleistocene Paleohydrologic Reconstructions and Radiocarbon Dating in the Southeastern Basin and Range, USA” and recommend that it be accepted as fulfilling the dissertation requirement for the Degree of Doctor of Philosophy.

______Date: (10/28/14) Vance T. Holliday

______Date: (10/28/14) Jay Quade

______Date: (10/28/14) Andrew S. Cohen

______Date: (10/28/14) Owen K. Davis

Final approval and acceptance of this dissertation is contingent upon the candidate’s submission of the final copies of the dissertation to the Graduate College.

I hereby certify that I have read this dissertation prepared under my direction and recommend that it be accepted as fulfilling the dissertation requirement.

______Date: (10/28/14) Dissertation Director: Vance T. Holliday

STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the copyright holder.

SIGNED: Andrew L. Kowler

ACKNOWLEDGMENTS

Thanks to my amazing wife Astrid and wonderful mother and father for their unconditional love and support. Eileen Kowler and Roberta and Ava Shaffer—thanks for all the encouragement. I am grateful to Martin Rabenhorst for introducing me to soils; to Bruce James for teaching me why they have been important to society for millennia; to my advisor, Vance Holliday, for enabling me to pursue exciting research questions at the intersection of soils, geology, and archaeology by including me in his projects and writing dozens of letters on my behalf; to Jay Quade for recruiting me into the department and teaching me that I am capable of answering most of my own questions. He and Wally Broecker heavily influenced the direction of my doctoral research and helped me to get the paleolakes project off the ground. I express sincere gratitude to Vance Haynes, John Hawley, and Joseph Schreiber, who not only serve as mentors and role models but, in retirement, helped me as committee members would. Mark Aldenderfer and John Olsen brought me to places that I never dreamt of seeing. Andy Cohen brought me into his paleolimnology family. I have benefitted tremendously from the tutelage of scientists who often go unnoticed: Chris Eastoe, David Dettman, and George Burr. At the Arizona AMS facility, Tim Jull made it possible for me to afford radiocarbon dates, and Todd Lange, Rich Cruz, and Lori Hewitt carried out my every odd request—essential for the success of my projects. Darrell Kaufman and Aradhna Tripati provided me with opportunities to work beyond the scope of what was locally available. Rolfe Mandel and Kathleen Nicoll made sure I knew that I was doing a good job. David Killick, David Miller, Greg Hodgins, Jeff Pigati, Julio Betancourt, Owen Davis, Paul Fish, Richard Hereford, Tim Jull, and Warren Beck maintained an active interest in my work and left their doors open to me. Phil Pearthree, Joe Cook, Anne Youberg, and Larry Fellows helped me in my quest to integrate soils into geologic research by including me in fieldwork. Manuel Palacios-Fest took me under his wing and is a mentor and collaborator par excellence; he and Jordon Bright have gone over the top to help me unlock the paleohydrologic secrets of Willcox basin. Bill Reitze, Christine Gans, Jesse Ballenger, Josh Reuther, Michael McGlue, Patricio DeSouza, Shane Miller, and Randy Haas exemplify teamwork. Years of stimulating research discussions with Alex Pullen, Chris Roos, Fumie Iizuka, James Girardi, James Mayer, Majie Fan, Martin Pepper, Preston Smith, Sarah Ivory, Steph McAfee, and Dana Rosenstein kept me stimulated. Art Vokes and Jeff Nekola confirmed my inkling that even the best-trained geologist needs to rely on specialists to identify snails correctly. Ben McElhaney was there to repair antique equipment that should not have been permitted on a 21st century campus, and because of him and Bill Reitze I learned to shop for used equipment and grew a lab of my own. Ben Beshaw, Catherine Lehman, Cathy Snider, Sharon Bouck, Heather Alvarez, Denise Galindo, Gail Zizzo, and Sylvia Quintero helped me to realize my funding vision by greasing the accounting wheels. Owen Davis, thank you for sitting on my defense committee without hesitation. Eneida Guerra de Lima and Dirk Jensen took care of my IT needs. Without fail, Kiriyaki Xiluri came through for me each and every time I needed to plot a poster—and she made sure that I could afford it. Finally, Anne Chase champions the student’s cause, and I cannot overstate my gratitude for her assistance from day one of my existence in the department.

DEDICATION

To Astrid and Sam, the lights of my life. And to special people who are not here to witness my accomplishment: my grandparents Sam and Miriam, Mike and Rose; my aunts Muriel, Lillian, and Florence; and Isabel Chicquor.

TABLE OF CONTENTS

ABSTRACT...... 10

INTRODUCTION...... 12

REFERENCES...... 15

PRESENT STUDY...... 17

APPENDIX A: PALEOHYDROLOGY OF PLAYAS VALLEY, SOUTHWESTERN NEW MEXICO (32ON) AND CLIMATIC IMPLICATIONS FOR WESTERN NORTH AMERICA DURING THE FINAL PLEISTOCENE GLACIATION...... 18 Abstract...... 18 Introduction...... 19 Site Description...... 21 Methods...... 23 Mapping and Stratigraphy...... 23 Stable Isotope Analysis...... 23 Radiocarbon Dating...... 24 Results...... 24 Gastropod Fauna...... 24 Radiocarbon-Dating Considerations...... 25 Sedimentology/Stratigraphy, Fauna, and Chronology...... 26 Depositional Environments...... 27 Stable Isotope Data ...... 29 Results...... 29 Paleoenvironmental and Geochronologic Implications...... 29 Paleohydrologic Reconstruction...... 30 Synthesis...... 30 Summary of Nearby Paleo-Lake Records ...... 31 Comparison with Paleohydrologic Records from the Western U.S...... 32 Temporal Correlation with H1 and Changes in the North Atlantic...... 34 Response of Atmospheric Circulation in Southwestern New Mexico...... 35 Conclusions...... 37 Acknowledgments...... 38 References...... 39 Figure Captions...... 49

Tables...... 51 Figures...... 53

APPENDIX B: PALEOHYDROLOGY OF WILLCOX BASIN, SOUTHEASTERN ARIZONA AND IMPLICATIONS FOR ATMOSPHERIC CIRCULATION OVER INTERIOR WESTERN NORTH AMERICA DURING THE FINAL PLEISTOCENE DEGLACIATION...... 62 Abstract...... 62 Introduction...... 63 Physical Setting...... 65 Geology and Hydrology...... 65 Climate and Ecology...... 66 Previous Investigations...... 67 Methods...... 68 Mapping, Stratigraphy, and Sampling...... 68 Sediment Sampling and Processing for Faunal Analysis...... 69 Radiocarbon Dating...... 69 Stable Isotope Analysis...... 70 Dating Considerations...... 70 Fauna...... 71 Ostracoda...... 71 Mollusca...... 72 Sedimentology, Stratigraphy, and Geochronology...... 73 Group 1...... 73 Sedimentology and Geochronology...... 73 Lower Group 1...... 73 Upper Group 1...... 74 Fauna...... 75 Paleoenvironmental Reconstruction...... 76 Group 2...... 77 Sedimentology and Geochronology...... 77 Fauna...... 78 Paleoenvironmental Reconstruction...... 79 Group 3...... 79 Sedimentology and Geochronology...... 79 Fauna...... 80 Paleoenvironmental Reconstruction...... 80 Stable Isotopes...... 81 Carbonate 18OValues...... 82 7

Interpretation of 18O values...... 82 Cement...... 83 Biogenic Carbonate...... 84 Paleohydrologic Reconstruction...... 84 Summary of Regional Paleo-Lake Records...... 86 Regional Correlation...... 88 Early Deglacial (19-15.0 ka)………...... 88 Late Deglacial (15.0-10.5 ka)………...... 89 Regional Hydrologic Responses to North Atlantic Climate Forcings...... 92 Early Deglaciation and H1...... 94 Termination 1 and Late Deglaciation...... 95 Conclusions...... 96 Acknowledgments...... 97 References...... 98 Table Captions...... 107 Figure captions...... 107 Tables...... 109 Figures...... 115

APPENDIX C: RADIOCARBON DATING OF CHARCOAL FROM THE EL FIN DEL MUNDO 14 CLOVIS SITE IN NORTHERN SONORA, MEXICO AND IMPLICATIONS FOR AMS C MEASUREMENT IN SMALL SAMPLES...... 123 Abstract...... 123 Introduction...... 124 Site Description...... 125 Radiocarbon Systematics...... 126 Background Levels and Process Blanks...... 126 Small-Sample Effect...... 127 Background levels...... 127 Machine-Induced Fractionation (MIF)...... 128 Methods...... 128 Charcoal Identification and Subsampling...... 128 Radiocarbon Processing...... 129 Age Formulation...... 130 Results...... 130 Discussion...... 131 Correction Schemes and Associated Error Propagation……...... 131

Inter-comparison of Results from Different Charcoal Samples and Fractions….133 Conclusions...... 134 Acknowledgments...... 135 References...... 136 Figure Captions...... 138 Tables...... 139 Figures...... 141

ABSTRACT

A dearth of reliably-dated paleolake records from the southern Basin and Range has limited knowledge of past water balance changes there, precluding a more complete understanding of late Pleistocene atmospheric circulation across western North America.

Paleoshorelines in closed basins throughout the region can provide accurately dated records of local effective moisture variations, representing a largely untapped source of paleohydrologic information. This dissertation presents paleohydrologic reconstructions from depositional successions in two basins at 32oN, approximately 100 km apart: Willcox basin, in southeastern

Arizona, and Playas Valley, in southwestern New Mexico. Also presented are the results of 14C dating of charcoal samples from the El Fin del Mundo Clovis archaeological site, in northwestern Sonora, Mexico. In depth analysis of these results allowed constraint of the “small sample effect” on the charcoal ages, found to be smaller than 1 of analytical uncertainty. The magnitude of the problem in ages from miniscule shell samples in the Willcox and Playas chronologies was found to be similar.

The successions record moist pluvial conditions from ~20-13 ka in Playas, and >37-11 ka in Willcox, with most dates younger than 19 ka—before which there is no solid evidence for lake transgressions. There is clear evidence for overlapping highstands between ~18.3 and 17.9 ka and a brief highstand of Cochise at ~12.9 ka, coinciding with Heinrich events H1b and H0, respectively. Temporal concordance between wet periods and perturbations in the North

Atlantic ocean and/or southern Laurentide ice sheet supports the idea that abrupt paleoclimatic changes in the southwestern U.S. occurred in response to large-scale atmospheric linkages to the northern high latitudes. The H1b highstands fill a hiatus in 14C dates compiled from paleoshorelines throughout the western U.S., and correspond to the first part of a lowstand in

paleo-Lake Estancia (35oN), in north-central New Mexico. Anti-phasing within New Mexico suggests that the newly documented highstands resulted from an increase in southerly-sourced precipitation. This is consistent with paleoenvironmental evidence from southern Arizona and

New Mexico that points toward periodic intensification of the summer monsoon during the late

Pleistocene.

INTRODUCTION

Long-range climate forecasts for western North America call for an increase in mean annual temperatures by the end of this century, along with concomitant decreases in precipitation. Forecasts also suggest that the dry spell currently gripping the southwestern U.S. is only the first in a series of decadal-scale droughts, each of increasing severity and extent.

Individual models differ by a factor of three in their regional predictions of temperature and precipitation changes (IPCC, 2014), fueling current efforts to improve them. Although advances in computing will continue to improve output agreement, assessing model accuracy hinges on independent validation using pre-Industrial paleoclimatic archives. Accordingly, climate proxy records (e.g., from lake sediments, speleothems, and tree rings) have become increasingly important for the long-term context that they provide with respect to the relationship between temperature and drought under different climatic regimes (e.g., Woodhouse et al., 2009). As such, paleohydrologic records constitute the only available tool with which to test models that predict the responses of regional hydrologic cycles to rising temperatures and greenhouse gas concentrations.

Additionally, paleohydrologic records provide important information to land managers and policy makers about past changes in natural water supplies that have occurred on decadal to millennial time scales. This is especially important for desert regions of the western U.S., where large urban populations are approaching or exceeding their carrying capacities, and where the sustainability of water use-active (e.g., large-scale agriculture and mining) and -passive (e.g., livestock grazing) operations is in doubt. Ecologists are likewise interested in paleohydrologic records because they are needed for understanding ecosystem resilience on different time scales.

Further, they inform decisions on whether to manage and conserve exotic elements of modern

ecosystems, or to restore extinct elements. Large-scale reorganizations in the structure of ecological communities caused by changes in effective moisture (e.g., Breshears et al., 2005) will impact land use and conservation strategies, doubly affecting water use-passive stakeholders. Finally, the cultural and economic well-being of indigenous communities has long depended upon the sustainability of extremely limited hydrologic resources.

Today, changes in the El Nino Southern Oscillation (ENSO) and the North American summer monsoon are among the key causes of precipitation variability in the semi-arid southwestern U.S., yet little is truly understood about their past impacts on water balance due to a dearth of available proxies. Dendroclimatic archives offer the longest continuous (<2 ka) records of temperature and precipitation within this area (e.g., Gray et al., 2004; Salzer, 2005;

Knight et al., 2010); however, the limited number of high-resolution paleotemperature records in comparison to the paleohydrologic reconstructions developed from them precludes a more complete assessment of the precipitation-temperature relationship. Also, precipitation variability reconstructions should not necessarily be looked to as analogues for current/forecasted change beyond the current century. This is because boundary conditions will be altered significantly in light of what is forecasted to be the largest change in the global climate system since the

Pleistocene-Holocene transition. It is therefore instructive to consider variability occurring under differing configurations of boundary conditions, requiring the availability of reconstructions over much longer time scales. Extending as far back as the conventional limit of AMS radiocarbon dating (~50 ka), paleobotanical remains in packrat middens provide rich accounts of the vegetation present at discrete points in time and space—enabling reconstruction of seasonal climatic conditions across large areas of the southwestern U.S. (e.g., Van Devender, 1990;

Holmgren et al., 2007).

In contrast, cave speleothems from sites in southern Arizona and New Mexico provide continuous, decadally-resolved time series of paleoclimatic and paleohydrologic changes, spanning centuries to millennia during the late Pleistocene and Holocene. Spectral analysis of

18O records is the primary means used for extracting information about past modes of climate variability from them, through comparison to records from ice cores, marine and lake sediments, and other speleothems throughout the world. In speleothems records, unfortunately, it is generally not possible to parse apart changes owing to variability, precipitation (amount, seasonality, and source), seasonal temperature, and 18O systematics within/above the cave.

Thus, statistical exercises are of limited value in the absence of confirmation about the true paleoclimatic significance of the 18O “climate” proxy.

Paleolake records offer the possibility of recognizing signals of major water balance change in speleothem (e.g., Hudson and Quade, 2013) and other climate proxy records. Fossil shorelines are the geomorphic footprints of ancient lakes in arid closed basins, and thus document past hydroclimatic variability in large, dry continental interior areas, such as the western U.S., where other paleoclimate archives are generally sparse. Another major advantage of shoreline records is that they commonly contain a variety of materials that can be reliably dated using the radiocarbon method—well understood and economical in comparison to most other Quaternary dating methods. Equally importantly, they enable constraint of the effective moisture changes required to create them, thus revealing the relative magnitude and direction of spatial and temporal variability within a region. Finally, shoreline chronologies are unique in that they only record periods of positive hydrologic balance; from no other kind of paleoenvironmental record can this hydroclimatic state be unambiguously detected or hydrologic balance so accurately quantified.

Geologic studies of ancient shorelines in the western U.S. began over a century ago with the pioneering study of G.K. Gilbert on mega- Lake Bonneville (Gilbert, 1890). Since the development of radiometric dating techniques, knowledge of lake-level histories from there has expanded tremendously (e.g., Benson, 2004). However, paleoshorelines in the southern Basin and Range have received surprisingly little attention compared to those in the Great Basin, leaving much to be learned about the paleohydrology of a region that is comparatively water- starved. My dissertation research helps to fill this knowledge gap through geologic investigations within an area straddled by speleothem records—one to the west from Cave-of- the-Bells, in southeastern Arizona (Wagner et al., 2010), and one to the east from Ft. Stanton

Cave, in southern New Mexico (Asmerom et al., 2010).

REFERENCES

Asmerom, Y., Polyak, V.J., Burns, S.J., 2010. Variable Winter Moisture in the Southwestern United States Linked to Rapid Glacial Climate Shifts. Nature Geosciences 3:114-117.

Barros, V.R., Field, C.B., Dokken, D.J., Mastrandrea, M.D., Mach, K.J., Bilir, T.E., Chatterjee, M., Ebi, K.L., Estrada, Y.O., Genova, R.C., Girma, B., Kissel, E.S., Levy, A.N., Knight, T.A., Meko, D.M., Baisan, C.H., 2010. A Bimillennial-Length Tree-Ring Reconstruction of Precipitation for the Tavaputs Plateau, Northeastern Utah. Quaternary Research 73:107-117.

Benson, L., 2004. Western lakes. In: Developments in Quaternary Science Vol. 1, pp. 185-204.

Breshears, D.D., Cobb, N.S., Rich, P.M., Price, K.P., Allen, C.D., Balice, R.G., Romme, W.H., Kastens, J.H., Floyd, M.L., Belnap, J., Anderson, J.J., Myers, O.B., Meyer, C.W., 2005. Regional Vegetational Die-Off in Response to Global-Change-Type Drought. Proceedings of the National Academy of Science 102 (42):15144-15148.

Gilbert G.K., 1890, Lake Bonneville. U.S. Geological Survey Monograph 1:1-275.

Gray, S.T., Jackson, S.T., Betancourt, J.L., 2004. Tree-Ring Based Reconstructions of Interannual to Decadal Precipitation Variability for Northeastern Utah since 1226 A.D. Journal of the American Water Resources Association 40:940-960.

Holmgren, C.A., Norris, J., Betancourt, J.L., 2007. Inferences about Winter Temperatures and Summer Rains from the Late Quaternary Record of C4 Perennial Grasses and C3 Desert Shrubs in the Northern Chihuahuan Desert. Journal of Quaternary Science 22(2):141-161.

Hudson, A.M. and Quade, J., 2013. Long-Term East-West Asymmetry in Monsoon Rainfall on the Tibetan Plateau. Geology41(3): 351-354.

IPCC, 2014. Climate Change 2014 Report: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects.

MacCracken, P.R., Mastrandrea, P.R., and White, L.L. (Eds.): Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 688pp. Cambridge University Press.

Salzer, M. W., and Kipfmueller, K. F., 2005. Reconstructed Temperature and Precipitation on a Millennial Timescale from Tree-Rings in the Southern Colorado Plateau, USA. Climatic Change 70:465-487.

Van Devender, T.R., 1990. Late Quaternary Vegetation and Climate of the Chihuahuan Desert, United States and Mexico. In Betancourt J.L., Van Devender, T.R., Martin, P.S. (Eds): Packrat Middens—the Last 40,000 Years of Biotic Change, pp104-133. University of Arizona Press (Tucson).

Wagner, J.D.M., Cole, J.E., Beck, J.W., Patchett, P.J., Henderson, G.M., Barnett, H.R., 2010. Moisture Variability in the Southwestern United States Linked to Abrupt Glacial Climate Change. Nature Geoscience 3(2):110-113.

Woodhouse, C.A., Mekob, D.M., MacDonald, G.M., Stahled, D.W., Cooke, E.R., 2009. A 1,200-Year Perspective of 21st Centu Drought in Southwestern North America. PNAS 107(50):21283-21288.

PRESENT STUDY

Willcox basin hosted paleo-Lake Cochise (Appendix A) and is located along the boundary of the Sonoran and Chihuahuan deserts, with an average basin floor elevation of 1,260 m. Northern Playas Valley, ~100 km to the east and ~100 m higher in elevation, hosted paleo-

Lake Playas (Appendix B). Differences between the two paleolake basins include that Willcox basin is topographically closed, allowing paleo-Lake Cochise to respond fully to changes in water balance, whereas a low-lying sill in Playas Valley limited the size of paleo-Lake Playas.

Given the spatial proximity of these basins, it is likely that the paleohydrologic changes affecting them were similar in direction, if not magnitude. In combination, their paleoshorelines provide a robust record of regional paleohydrologic change at ~32oN during the last Deglacial period, when most of them formed.

Deposits that contained little or no carbon-bearing material were either left undated, or dated using miniscule shells, or shell fragments, approaching the lower sample-size limit for

AMS 14C dating. These conditions necessitated a correction for the “small sample effect”— which, if not accounted for, causes systematic age overestimation and error underestimation that can compromise the reliability of radiocarbon chronologies. Thus, my work on dating small charcoal samples from the El Fin del Mundo Clovis Paleoindian site in northwestern Sonora,

Mexico (Appendix C), provides a basis for assessing the magnitude of this problem in ages within the Willcox and Playas records.

APPENDIX A: PALEOHYDROLOGY OF PLAYAS VALLEY, SOUTHWESTERN NEW MEXICO (32ON) AND CLIMATIC IMPLICATIONS FOR WESTERN NORTH AMERICA DURING THE FINAL PLEISTOCENE GLACIATION

ABSTRACT

A succession of paleo-shoreline and -wetland deposits exposed around Playas Dry Lake

(32oN) records pluvial conditions in southwestern New Mexico during the last glacial- interglacial transition. Deposit ages are constrained by 14C dates from fossil gastropods, spanning ~20-13 ka. We dated semi-aquatic Succinids, known to provide reliable ages, and aquatic Lymnaeids, subject to 14C reservoir effects, here less than ±1σ of true deposit age.

Reworked Succinids in Unit 5, 7, and 8 reveal the existence of wet meadows along the valley floor, and hence wetter-than-modern conditions, at ~19.5, ~18.6, ~18.0, ~16.2, and ~13.6 ka;

Lymnaeids in Unit 5 constrain a highstand during the wettest period on record, from ~18.4-17.9 ka. This, together with evidence for contemporaneous paleohydrologic changes in the western

U.S. from 31-40oN—the catastrophic Bonneville (40±2oN) flood, highstands of paleo-Lakes

Franklin (40oN) and Columbus (38oN), and abrupt regression of paleo-Lakes Lahontan (40±2oN) and Estancia (35oN)—shows that a brief but intense wet episode at ~18.3 ka was followed by drying above 34oN and continued inundation to the south.

The contemporaneity of these events with Heinrich event H1b and sudden expansion of the Laurentide ice sheet not only suggests a causal linkage, but raises new questions about the nature of large- and regional-scale atmospheric responses to Heinrich events. The Playas highstand is best explained by an increase in southerly-sourced precipitation, consistent with late

Pleistocene paleobotanical evidence from the Sonoran and Chihuahuan deserts for periodic enhancement of the summer monsoon below 35oN. Development of additional shoreline

chronologies from the southern Basin and Range would constrain the spatial and temporal variability of the paleo-monsoon, and shed light on the nature and drivers of centennial-scale climatic variability observed in speleothem 18O records. Finally, it would serve to test the recently advanced hypothesis that Deglacial-age highstands in the Great Basin resulted from intensification of the summer monsoon.

INTRODUCTION

For over a century, geologists have reconstructed ancient lake levels from Pleistocene- age lake deposits throughout the interior western U.S. Collectively, these records reveal spatial and temporal patterns of past hydroclimatic variability, allowing paleoclimatologists to address hypotheses about changes in the global climate system during the latest Pleistocene. Preserved shoreline features appear as parallel arcs or concentric circles in hydrographically closed basins, and are the geomorphic footprints of ancient lakes—thus constraining their surface areas and documenting the relative magnitude of required effective moisture increases. Because many of the well-developed paleoshorelines in the western U.S. date to late MIS 2 (e.g., Waters, 1986;

Benson et al., 1990; Oviatt et al., 1992; Licciardi, 2001; Bacon et al, 2006; Garcia and Stokes,

2006; Orme and Orme, 2008; Munroe and Laabs, 2013), they are an excellent source of paleohydrologic information for the last glacial-interglacial transition (~19-11 ka). Moreover, they can be used to test the idea that hydrologic variability in the Great Basin correlated temporally with coupled changes in the ocean, cryosphere, and atmosphere in the northern

Atlantic region (e.g. Phillips et al., 1994; Oviatt, 1997).

Shoreline chronologies from pluvial lakes in the western U.S. indicate that most late

Pleistocene highstands occurred during the first part of the Deglacial interval (see compilation in

Munroe and Laabs, 2012), coinciding with Heinrich event 1 (H1). Oviatt (1997) and Benson et al. (1998) were among the first to recognize the close correspondence between Heinrich events and late Pleistocene lake cycles in western North America, based on shoreline chronologies for paleo-Lakes Bonneville and Lahontan, respectively. The breakup of the continental ice sheets during Heinrich events caused the launch of “iceberg armadas” into the open ocean, resulting in the near-halt/shutdown of the Atlantic Meridional Overturning Circulation (AMOC). The sedimentologic footprint of Heinrich events is most clearly identifiable as a single layer of detrital calcite-bearing ‘ice-rafted’ debris (IRD) in cores from ~40-55oN in the North Atlantic

(Ruddiman, 1977).

Core SU81-18, from offshore Iberia (37oN), is arguably the best-dated core in the subtropical North Atlantic (Bard et al., 2000), where the current 14C reservoir age of 400 14C yr

BP is thought to have remained constant through time (Bard et al., 1994). This stands in contrast to more northerly sites, which appear to have experienced abrupt changes in 14C content (Bard et al., 1994; Stocker and Wright, 1998). In the IRD belt, most evidence for H1 comes from a single

IRD layer, whereas SU81-18 contains two layers, corresponding to separate phases of H1 (Bard et al., 2000). These phases are constrained by basal 14C ages of ~18.3 ka (H1b) and ~17 ka

(H1a), with peak sedimentation occurring at ~17.5 and ~16.5 ka, respectively. On the basis of mineralogy, Bard et al. (2000) correlated the upper/younger layer (H1a) to the H1 layer in the

IRD belt, although Broecker et al. (2009) reached the opposite conclusion.

Denton et al. (2006) dubbed the period roughly corresponding to H1 the “Mystery

14 Interval”, corresponding to the coupled pCO2 increase and C decrease in the atmosphere from

~17.5-14.6 ka (Broecker, 2009). Broecker et al. (2009) proposed that H1b induced the abrupt

AMOC shutdown at ~17.5 ka, and that H1a induced the AMOC’s resumption at ~16 ka. Further,

they posited that shutdown of the AMOC caused the simultaneous onset of cold conditions in the

North Atlantic and dry conditions in the Great Basin, and that the AMOC resumption triggered large-lake episodes in the Great Basin. However, the proposed shift actually occurred a millennium later than most highstands, causing further speculation about the influence of H1 on water balance in the western U.S. (e.g., Broecker and Putnam, 2012).

There small number of well-dated shoreline records from basins south of 35oN, in the southwestern U.S., has hampered efforts to reconstruct late Pleistocene atmospheric circulation across western North America. In this part of the southwsestern U.S., the only well-dated

Deglacial-age beach deposit is lower Unit B of Waters (1989), formed between 17 and 16 cal kyr

BP by paleo-Lake Cochise (32oN) in southeastern Arizona (Fig. 1); however, the beach sediments above and below remain undated. Toward improving current understanding of the chronology of hydrologic changes in the southwestern U.S., we report results from our recent study of the late Quaternary stratigraphic record in Playas Valley (Fig. 1), a closed basin in southwestern New Mexico. Our hydrologic reconstruction is based on aspects of the sedimentology, mineralogy, geochemistry, and microfauna of wetland and lake deposits around

Playas Dry Lake.

SITE DESCRIPTION

Playas Dry Lake (N32o W109o) (Fig. 1) lies in the lower/northern hydrologic subbasin of

Playas Valley, in the panhandle of southwestern New Mexico in the northern Chihuahuan Desert

(Hawley, 1986). The northern Playas subbasin is 960 km2 in area, and the average elevation of

Playas Dry Lake is 1,307 m (Fig. 2).

The subbasin is bounded on most sides by mountain ranges (Fig. 2). South Pyramid Peak and the Brockman Hills bound the basin in the north. The eastern divide is a segment of the

Continental Divide, comprised of the Coyote Hills to the northeast and the Little Hatchet Range directly to the east. The western divide is comprised of the Gillespie Mountains to the southwest and the northern Animas Range to the northwest. Finally, a broad alluvial plain separates the northern and southern subbasins. When paleo-Lake Playas reached Hatchet Gap (~1,318 m today)—separating the Little Hatchet and Big Hatchet Ranges—it would have overflowed into the Hachita-Moscos basin to the southeast (Hawley, 1993) (Fig. 3).

Modern imagery reveals several shoreline features between ~1,314 and ~1,316 m around the northern perimeter of the playa, providing evidence of overflow in the past. Playas Dry Lake now covers ~45 km2, or ~5% of total basin area, whereas paleo-Lake Playas at the spillway, would have reached a depth of ~11 m and covered ~85 km2, or ~9% of total basin area—nearly double the area of the modern playa-lake (Fig. 2a). Thus, these paleoshorelines can be used to constrain the minimum amount of effective moisture required to create them. Measurement of the potentiometric gradient between the lower Playas subbasin and adjacent basins, made prior to intensive ground-water pumping (Reeder, 1957; Doty, 1960), suggests that ground-water outflowed in the recent historic past. However, the rate and direction of inter-basin flow may have been different during wetter periods in the past. Within the subbasin, runoff directly recharges the water table through channel floors, and ground-water discharges from seeps and springs at high elevations and along the southwestern playa-margin. In years with unusually high seasonal rainfall, the playa itself is inundated by runoff transported by North Playas Draw

(Fig. 3) and smaller draws that drain the bordering bajadas (Fig. 3). A minor fraction of total runoff comes from the Big Hatchet Range. Most of the bounding mountain ranges are composed

of Tertiary volcanic and noncarbonate sedimentary rocks, with the exception of the Big Hatchet

Range, the crest of the Little Hatchet Range, and the northern tip of the Northern Animas Range, all containing Paleozoic-age limestone.

Based on long-term meteorological records furnished by the Western Regional Climate

Center (www.wrcc.dri.edu/climatedata.html), mean precipitation and temperature values are nearly equivalent at the nearby townships of Hachita (1931-2003) and Animas (1923-2013). At both of these locales, annual precipitation (summer-dominated) and temperature averages are 27 cm/yr and 16oC, respectively, and seasonal temperatures range from 25oC (JJA) to 6oC (DJF).

METHODS

Mapping and Stratigraphy

Elevations shown in Fig. 2 are based on the 1/3 arc-second elevation data obtained from the National Elevation Dataset (2014). As depicted in Fig. 3, we divided the sedimentary succession into eight allostratigraphic units (North American Commission on Stratigraphic

Nomenclature, 1983). Unit contacts were defined by mapping of continuous disconformities or by inference from lithostratigraphy and 14C dating. Field descriptions (Table 1) draw from

Schoeneberger et al. (2012). Where present, fossil gastropods were sampled and identified on the basis of descriptions in Bequaert and Miller (1973) and Burch (1982).

Stable Isotope Analysis

For each shell analyzed, a fragment of the aperture was oven dried (40oC), pulverized, and then processed on an automated sample preparation device (Kiel III) fitted to a Finnigan

MAT 252 mass spectrometer in the Laboratory of Environmental Isotopes at the University of

Arizona. Carbonate isotope results are reported in standard  permil notation, relative to the

Vienna Pee Dee Belemnite (VPDB) standard. 18O and 13C values were normalized to NBS-19 based on internal lab standards. Precision of repeated standards are ± 0.11‰ for 18O and ±

0.07‰ for 13C.

Radiocarbon Dating

Gastropod shells were rinsed in distilled water and their condition inspected under binocular microscope. Depending on resilience of the material, specimens were briefly sonicated

(2-5 seconds) up to three times prior to a final rinse with distilled water, and then crushed to allow inspection of the interior surface for remaining detritus. Clean fragments were selected and, if more material was required, impure fragments were immersed in 0.01N HCl until only the pure shell separate remained. All shells were processed at the University of Arizona AMS

Radiocarbon facility. Calibrated radiocarbon dates are given as the 2σ range and median probability of possible calendar year ages before 1950 A.D. (“present”), or cal yr BP. All calibrations were generated by Calib 7.0 (Stuiver et al., 2012), based on the IntCal13

Radiocarbon Calibration Data Set (2012) reported and described in Reimer et al. (2013).

RESULTS

Gastropod Fauna and their Paleoenvironmental Implications

When encountered in the field, snails were identified to the family level, including semi- aquatic and aquatic snails. Bequaert and Miller (1973) provide a key for mollusks in Arizona, and Burch (1982) provides a key for freshwater aquatic mollusks across North America. The most abundant taxa found in the studied sections belong to lung-breathing (pulmonate) families,

including aquatic Lymnaeidae and semi-aquatic Succineidae. Succinids require permanent water to survive in areas with hot, summer-dominated climates. In areas with cooler summers— perhaps like Playas Valley during the late Pleistocene—they can survive in moist micro- environments (e.g. under leaf litter or on rotting vegetation) in the absence of standing water.

Where present along the margins of lakes, marshes, and streams, Succinids spend most of their time feeding on water-side vegetation, and less of it submerged in the water. On the other hand,

Lymnaeids spend most of their time submerged, and thus require standing water, although some species can tolerate seasonal desiccation (Clark et al., 2003; Harrold and Guralnick, 2010).

Radiocarbon-Dating Considerations

Eleven 14C dates were obtained from four sections at Playas, spanning 19,480±230 to

13,550±180 cal yr BP. All of these dates are from well-preserved aragonitic gastropod shells: five from Succinids, and six from Lymnaeids. No other macrofossils or organic matter were found for 14C dating. Both of these taxa are aquatic to some extent, raising questions about their utility for reconstructing lake history. This is because aquatic snails derive some of their shell carbon from dissolved inorganic carbon (DIC), which can be significantly out-of-equilibrium

14 with atmospheric CO2. In turn, this can lead to well-documented C reservoir effects and apparent ages in excess of the true ages (e.g. Riggs, 1984). Since Succinids are partially terrestrial and might derive some shell carbon from carbonate minerals (Goodfriend and Hood,

1983; Goodfriend and Stipp, 1983; Goodfriend, 1987), they could, in theory, also yield dates that are too-old. Finally, in order for gastropod shells to provide reliable ages, their inorganic carbon fraction must resist exchange with exogenous carbon following shell synthesis. Fortunately,

such contamination has been detected only in specimens older than ~25 cal kyr BP (Pigati et al,

2009; Pigati et al., 2010; Rech et al., 2011), thus minimizing concerns about our samples.

Recent detailed studies of terrestrial and semi-aquatic snails demonstrate that, with the exception of very few genera, Succinids secrete shell aragonite with 14C activity matching that of concurrent atmospheric CO2 (Pigati et al. 2004, 2010). This is due to the facts that Succinids spend much of their time feeding on terrestrial vegetation near the water’s edge, and can extract calcium from carbonate minerals without incorporating inorganic carbon into their shells. Thus,

Succinid shells yield reliable 14C dates that firmly establish the ages of the units in which they are found, barring reworking or alteration. On the other hand, Lymnaeids are fully aquatic, subjecting their 14C dates to reservoir effects. For this reason, the conservative view would regard dates from Lymnaeids as maxima. One way to test for reservoir effects is to date co- occurring shells of Succinids and Lymnaeids, but unfortunately they were not found together in the sections examined, likely due to strongly differing habitat preferences.

Sedimentology/Stratigraphy, Fauna, and Chronology

Four 1-3 m-thick stratigraphic sections were described (Table 1) from parallel highway and railroad cuts 2-5 m above the vegetated valley floor, laterally spanning ~100 m (Fig. 2b).

These exposures reveal a succession of intercalated marls, sands, and clays comprising eight allostratigraphic units (Fig. 4). Following is a summary of the sedimentologic characteristics of each unit, in order from youngest to oldest.

Based on observation in a hand-dug pit at the base of Pl-Loc1 and P09-Sec2, Unit 1 is an olive blocky clay with abundant redox-stained root channels, and carbonate root casts and nodules, containing sparse or no snails, and having a sharp upper contact with Unit 2. Unit 2 is a

30-40 cm-thick massive white sandy marl containing sparse mollusks, and redox-stained root channels, and having a gradual upper contact. The upper contact is sharp at Pl-Loc3W(30m), where the unit is platy and indurated. Unit 3 is a truncated, mottled blocky clay containing sparse mollusks and abundant carbonate nodules, and having a wavy sharp upper contact.

At Pl-Loc1, Unit 4 is a ~50 cm-thick, mottled, light grey, bedded medium sand, with intervening lenses of silt and coarse sand, grading down-dip to a white fine-sandy marl having a sharp wavy upper contact. Carbonate root casts extend into the unit from the lower contact of upper Unit 5, and iron oxide concentrations occur at the cast tips (Fig. 8). Lower Unit 5, only found at Pl-Loc1, is a ~40 cm-thick white calcareous sand containing abundant aquatic

Lymnaeids. It is unconformably overlain by a 40 cm-thick, weakly cemented massive carbonate with a 20 cm-thick platy calcrete cap. Upper Unit 5 is a 10-20 cm-thick, strongly cemented massive carbonate, locally platy, containing sparse Succinids in the upper few centimeters and having a wavy sharp upper contact. Unit 6 is a light yellowish brown prismatic calcareous sandy clay containing few or no snails and having a wavy sharp upper contact. Unit 7 is a pale brown massive calcareous sand thickening in the down-dip direction, containing sparse Succinids and having a wavy gradual upper contact. Unit 8, at the top of the sequence, is a white sandy marl, coarsening and thickening down-dip to a moderately cemented calcareous sand, containing sparse Succinids in the upper five centimeters.

Depositional Environments

Bellow 1,309 m, Unit 5 is a 10-30 cm-thick platy calcrete buried by inset deposits, Units

6-8 (Figs. 2b, 2c). Establishing chronostratigraphic relations between individual deposits and paleoshorelines will require direct dating of the shorelines. Units 1-4 could not be dated, as they

lack co-depositional snails (Table 1, Fig. 4). Five of eleven shell dates were obtained from

Succinids and are therefore considered reliable, whereas all other dates were obtained from

Lymnaeids and thus provide maximum ages. Lymnaeids in Units 5, and one in Unit 4, yielded statistically indistinguishable 14C dates (at 1σ) between ~17.9 and ~18.4 cal kyr BP. Finally,

Succinids in Units 5, 7, and 8 yielded dates out of stratigraphic order: ~19.5, ~18.6, ~18.0, ~16.2, and ~13.6 cal kyr BP.

This stratigraphic disorder likely arose from reworking of shell-bearing deposits of multiple ages. Supporting evidence for this includes (1) absence of Lymnaeids, (2) generally low abundance of snails within Units 5, 7, and 8, and (3) occurrence of Succinids slightly below the upper contact of Unit 5. Finally, the match between a Lymnaeid date (AA-86061) from Unit

5 and a Succinid date (AA-83627) from Unit 7 suggests that reservoir effects are negligible.

The following evidence suggests that the Succinids were deposited from fluvial reworking of nearby paleo-wetland sediments. First, the gastropod assemblages within Units 5,

7, and 8 contain semi-aquatic taxa but lack aquatic taxa, commonly the case in wet meadows.

Next, the lack of stratigraphic order suggests that the Succinids and host sediments were reworked. Finally, the lack of equivalent-age snails within each unit suggests that all were reworked from sporadic flow fed by runoff, as ground-water-fed streams would have supported snail populations.

Excluding reworked Succinids near the upper contact of Unit 5, the gastropod assemblage in Unit 5 features abundant Lymnaeids in the absence of non-aquatic taxa. This suggests that Unit 5 was deposited near the edge of a lake or through-flowing wetland. A lack of abrasion indicates that the shells were transported very short distances. The only Lymnaeid found in Unit 4 returned a date of ~17.9 cal kyr BP, overlapping at 1 with dates from

Lymnaeids in lower Unit 5. This, along with the near-absence of snails in Unit 4, provides evidence of translocation—consistent with evidence for bioturbation in Unit 4.

Stable Isotope Data

Results

Eleven of twelve dated shells were analyzed for their stable carbon and oxygen isotopic

18 18 composition (Table 2; Fig. 6). Lymnaeid shells yielded  O values ( Oshell) ranging from -9.3 to -7.4‰ (mean: -8.4‰; n=5) with a single value of -0.2‰, and those of Succinids yielded

13 13 values of -9.6, -7.7, and -1.2‰. Shell  C values ( Cshell) from Lymnaeids range from -8.4 to

-6.3‰ (mean: -7.0‰; n=5), with one relatively positive value of -2.1‰. Values from Succinids are -9.2, -4.2, and +0.5‰.

Paleoenvironmental and Geochronologic Implications

18 Not accounting for temperature-dependent fractionation,  Oshell values of aquatic and

18 18 semi-aquatic snails reflect the O content of the host water. Excluding two outliers,  Oshell values are < -7‰, reflecting minimal influence from evaporation. This is also suggested by the relatively narrow range of variability (≤2.5‰). Typically, biogenic carbonates formed in closed basin lakes yield more positive values (e.g. ostracode valves from neighboring paleo-Lake San

18 Agustin; Phillips et al., 1992), reflecting extensive evaporation (Talbot, 1990).  Oshell values from snails at Playas reflect an open geohydrologic system, such as an overflowing lake or

18 through-flowing wetland. Finally, outlying  Oshell values exceed others by at least 6‰, possibly reflecting evaporation in a desiccating or lake.

13  Cshell values from aquatic mollusks are significantly influenced by dissolved inorganic

- carbon (DIC) (Fritz and Poplowski, 1974)—predominately bicarbonate (HCO3 ) from pH 6.3-

10.4 (Hem, 1985)—and dietary (organic) carbon, often from 13C-depleted sources such as algae.

13 13 If the latter were not true, then in theory one could reconstruct the C content of DIC ( CDIC)

13 from  Cshell values using the carbon enrichment factor (Ԑarag — HCO3) of 2.8‰, observed from 10-

o 13 40 C by Romanek et al. (1992). However, the significance of  CDIC values would still be difficult to determine given that DIC in surface water can originate from multiple sources. These include CO2 in the soil or ground-water (either during recharge or during/after discharge) and/or limestone along the flow path. Further, selective uptake of 12C by aquatic vegetation can result in significant 13C-enrichment of DIC (McKenzie, 1985). These factors generally preclude the

13 use of  Cshell values from aquatic snails for paleoenvironmental reconstruction or detecting the presence/magnitude of ancient hard-water effects.

Paleohydrologic Reconstruction

The combined evidence from stratigraphy, sedimentology, fauna, and stable isotopes reflects the alternation of three primary hydrologic states in Playas Valley during the latest

Pleistocene. These include (1) extremely wet conditions between ~18.4 and ~17.9 cal kyr BP

(represented by aquatic snails in Unit 5) likely reflecting a highstand, (2) moderately wet conditions at ~19.5, ~18.6, ~18.0, ~16.2, and ~13.6 cal kyr BP (represented by reworked

Succinids in Units 5-8) likely reflecting extensive valley bottom wetlands, and (3) much drier, but wetter-than-modern, conditions at times not represented in the depositional record.

SYNTHESIS 30

Summary of Nearby Paleo-Lake Records

Lake-level records from the southern Basin and Range come from the Estancia basin

(Anderson et al., 2000; Anderson et al., 2002) and the Plains of San Agustin (Markgraf et al.,

1983, 1984; Phillips et al., 1992; Holliday et al., 2006), both in central New Mexico; paleo-Lake

Cochise (Waters, 1989), in southeastern Arizona; and paleo-Lake Cloverdale (Krider, 1998), in southwestern New Mexico, between paleo-Lakes Cochise and Playas. The only dated paleoshorelines include those of paleo-Lake Cloverdale (Krider, 1998) and Cochise (Waters,

1989), yielding charcoal dates of ~24 and 17-16 cal kyr BP, respectively, whereas the Estancia and San Agustin records are based on proxies in deep-lake sediments.

The San Agustin record is sparsely dated and completely untested for 14C reservoir effects, whereas a date of 130±65 14C yr BP from Artemia cysts from modern playa muds in the

Estancia basin suggests that 14C reservoir effects were negligible there. However, if one assumes complete exchange between modern DIC and atmospheric CO2—and accounts for the “bomb effect” (in sampling year 1990), using a fMC value of ~1.16 (Levin and Kromer, 2004)—a reservoir correction of ~1,300 14C yr would correspond to a date of 13,000 14C yr BP. Although this remains a possibility, the temporal concordance of dates from the deep-lake muds and nearshore deposits of Estancia suggests otherwise.

The San Agustin basin has yielded limited paleoenvironmental information for the period of interest. Numerous attempts at reconstructing its lake-level history have been hampered by a lack of dated shorelines. This ~90 km-long basin contains three interconnected subbasins: Horse

Spring (2,106 m), C–N (2,102 m), and White Lake (2,120 m). Interpretation of proxy records in combination with mapped shorelines (Weber, 1994) suggests that all three subbasins were submerged by a freshwater lake during the LGM (Markgraf et al., 1983, 1984; Phillips et al.,

1992). After ca. 19 ka, the lake remained fresh but regressed below the spillway into the Horse

Spring subbasin as marsh-like conditions persisted in the C-N basin. After ~15 ka, lake level declined irregularly. Radiocarbon dates from Bat Cave (Holliday et al., 2006; Hill and Holliday,

2011) show that the shoreline had receded to ≤2,105 m by 13.2 ka. Following that, the lake underwent a transition from fresh to alkaline to saline prior to its disappearance in the middle

Holocene. A more detailed reconstruction of lake level change in the Plains of San Agustin will require direct dating of shoreline landforms and establishment of robustly dated sediment core records in all three subbasins.

The Paleolake Estancia record is the synthesis of several data sets, including (1) sedimentological, (2) geochemical (Sr/Ca ratios), (3) faunal (ostracodes), and (4) geomorphic

(Allen and Anderson, 2000; Anderson et al., 2002). Collectively, these records indicate that paleo-Lake Estancia experienced maximum freshening on several occasions between ~23 and

~18.2 cal kyr BP, reaching no higher than ~1,890 m. Abrupt regression of the lake at ~18.2 cal kyr BP kicked off a lowstand, ending with Estancia’s transgression to ~1880 m at ~16.8 cal kyr

BP. Subsequent transgressions to similar or higher elevations occurred at ~16.2 and ~15.7 cal kyr BP. A date of ~14.6 cal kyr BP from the truncated surface of the continuous lake bed sequence constrains the timing of terminal Pleistocene desiccation. Subsequently, Estancia may have oscillated below ~1875 m; 14C dates reported in Anderson et al. (2002) constrain the timing of a possible lowstand to between ~13.4 and ~11.1 ka.

Comparison with Paleohydrologic Records from the Western U.S.

The wet interval in Playas Valley at ~19.5 cal kyr BP coincided with highstands in the

Estancia (Allen and Anderson, 2000) and San Agustin (Phillips et al., 1992) records, and a brief

highstand of paleo-Lake Clover (Munroe and Laabs, 2012) in the northern Great Basin. The subsequent wet episode, at ~18.6 cal kyr BP, coincided with a highstand of Estancia from ~18.8-

18.2 cal kyr BP. Contemporaneous changes are recorded in the Ft. Stanton speleothem record by

18O troughs centered on ~19.5 and ~18.6 ka. This temporal correspondence between the geologic and speleothem records suggests that the Succinids from Playas formed during episodic wetland expansions. Further, paleoenvironmental archives from southern Nevada (~35oN) suggest that conditions there were cold and dry during this time, as follows. A hiatus between

19.5 and 17.7 ka in the paleowetland record of Quade et al. (2003) suggests dry conditions, and

Lachniet et al. (2011) inferred a centuries-long cold/dry episode centered on ~18.6 ka from the

Pinnacle Cave speleothem record.

The most intense wet interval in our record produced a highstand from ~18.4-17.9 cal kyr

BP, overlapping with the rise and fall of 18O values in the Ft. Stanton record, lowstands of paleo-Lakes Estancia and Lahontan, and the Bonneville flood (McGee et al., 2012). The contemporaneity of these events with a highstand of paleo-Lake Franklin (Munroe and Laabs,

2013) in the northern Great Basin and paleo-Lake Columbus (Kurth et al., 2011) in the southern

Great Basin by ~18.2 ka reflects an abrupt reorganization of large-scale atmospheric circulation at that time. The subsequent wet episode in Playas Valley at ~16.2 cal kyr BP coincided with highstands of Estancia, Cochise, and numerous lakes in the Great Basin, including Franklin

(Munroe and Laabs, 2013) and Railroad (Lillquist, 1994).

The ~13.6 cal kyr BP wet episode in Playas Valley does not correspond to any known paleolake transgressions in the southern Basin and Range. However, paleobotanical evidence for a drying trend from ~15.2-13.7 cal kyr BP, from packrat middens in Playas Valley (Holmgren et al., 2003) and south-central New Mexico (Betancourt et al., 2001), corresponds to a sharp

decline in 18O values in the Ft. Stanton record between ~14.1 and ~13.6 ka. Records from other parts of the western U.S. reflect a contemporaneous late-glacial resurgence in effective moisture.

Evidence for this includes transgressions of Lake Chewaucan (Licciardi, 2001) and Franklin

(Munroe and Laabs, 2013) in the northern Great Basin at ~13.8 cal kyr BP, and the resumption of spring discharge in southern Nevada as early as ~14 cal kyr BP (Quade et al., 1998).

Temporal Correlation with H1 and Changes in the North Atlantic

It is now widely accepted that the melting of iceberg armadas in the northern Atlantic during Heinrich events stabilized the water column and suppressed formation of North Atlantic

Deep Water, resulting in slowing or stoppage of the Atlantic Meridional Overturning Circulation

(AMOC) (McManus et al., 2004). At these times, abrupt and often dramatic paleohydrologic shifts occurred throughout both hemispheres. Paleoshorelines in the western U.S. yield insights into the nature of regional atmospheric circulation during H1 by revealing spatial and temporal patterns of hydrologic change. However, because Heinrich events result from complex interactions of the atmosphere-ocean-cryosphere system on millennial timescales, it can be difficult to pinpoint the trigger(s) of any single event.

On the basis of AMOC intensity, Stanford et al. (2011) defined H1 sensu lato from ~19-

14.6 ka, dividing it into four phases: (1) initiation of sustained meltwater release at ~19 ka, causing AMOC slowing, (2) IRD deposition within the “IRD belt” (H1 sensu stricto) from ~17.5 ka, causing near-halt of the AMOC, (3) complete AMOC shutdown and resultant freshwater pooling in the Nordic Seas from ~16.5 ka, and (4) purging of freshwater into the North Atlantic from ~15.5-14.6 cal kyr BP, after which the AMOC resumed abruptly. H1 sensu stricto is difficult to date because of uncertainties related to 14C reservoir age, the duration of individual

IRD events, and when IRD events reached peak intensity. Individual IRD events appear to have lasted 500±250 kyr on average, generally peaking mid-way through (Hemming, 2004).

As reviewed above, H1 is best constrained by 14C dates from two IRD layers in core SU-

118. Based on this, H1b began at ~18.3 cal kyr BP and peaked at ~17.5 cal kyr BP, and H1a began at ~17 cal kyr BP and peaked at ~16.5 cal kyr BP (Bard et al., 2000). However, recent work demonstrates that North Atlantic Deep Water formation and concomitant changes in global climate are disproportionately affected by freshening of the Nordic Seas (Stanford et al., 2011a).

Additionally, Lowell et al. (1999) found to grow in response to northern Atlantic sea-surface cooling. The occurrence of wetlands in Playas Valley at ~19.5, ~18.6, and ~16.2 cal kyr BP is consistent with this as well as the general notion that paleoclimatic changes in the southwestern

U.S. corresponded to those in the northern Atlantic region. At ~19.5 ka, the southern Laurentide ice sheet retreated from its LGM maximum. By ~18.6 cal kyr BP, it had begun its ‘binge’ phase advance (Dyke et al., 2004), concurrent with the cold/dry episode in the southern Great Basin inferred from the Pinnacle Cave record (Lachniet et al., 2011). At ~16.2 cal kyr BP, a slight decrease in the intensity of polar vortex circulation coincided with, or slightly followed, the

AMOC shutdown (Stanford et al., 2011a and references therein). The most recent wet period in our record, at ~13.6 cal kyr BP, coincides with peak flux during meltwater pulse MWP1a

(Stanford et al., 2011b). Finally, the Playas highstand (~18.4-17.9 cal kyr BP) coincided with the start of H1b (Bard et al., 2000) and southern Laurentide advance.

Response of Atmospheric Circulation in Southwestern New Mexico

Modeling results and paleoceanographic records show that AMOC suppression correspond to depressed North Pacific sea-surface temperatures and a strengthened wintertime

Aleutian Low (Okumura et al., 2009). This would have caused westerly storm tracks to divert southward, ushering moist Pacific air masses into the western U.S. in winter, thus providing a plausible explanation for the widespread occurrence of highstands there during H1. The ‘shifting

Westerlies’ hypothesis can also explain how each of the Succinid dates from Playas coincides with one or more highstands in distant parts of western North America. Additionally, it offers a plausible explanation for the synchronicity of geographically disparate paleo-Lakes Estancia

(35oN) and Lahontan (~40oN) at the start of H1b: both records show an abrupt drop in lake level at ~18.3 cal kyr BP, with lowstands persisting until simultaneous transgressions at ~17 cal kyr

BP.

However, given the geographic proximity of paleo-Lakes Estancia and Playas, anti- phasing between them from ~18.3-17.9 cal kyr BP requires another explanation. While it is plausible that the Westerlies provided most of the moisture required to maintain Estancia’s lowstand, the Playas highstand would have required precipitation with a southerly source.

Throughout the Holocene, southern-sourced precipitation has appeared in summertime with the

North American monsoon, strongest in northwestern Mexico, southern New Mexico, and

Arizona. In addition, C4 vegetation in packrat middens—from Playas Valley (Holmgren et al.,

2003), the Tularosa basin (32-33o N) (Betancourt et al., 2001) in southern New Mexico, and the

San Simon Valley (Holmgren et al., 2006) in southeastern Arizona—indicates that this region was part of an expansive C4 grassland. This grassland extended as far south as the Bolson de

Mapimi (26-27oN) in Mexico and Big Bend (29oN) in western Texas (e.g., Van Devender, 1990;

Van Devender, 1995), providing strong evidence of summer monsoonal activity. Based on this, we suggest that the Playas highstand resulted from intensified monsoonal circulation. This may have also helped to sustain the lowstand of Estancia, near the northern fringe of the monsoon.

Lyle et al. (2012) proposed that intensified monsoonal circulation caused tropical Pacific moisture to penetrate episodically into the Great Basin during the Deglacial period, accounting for the highstands there. To explain the linkage between the monsoon and the North Atlantic,

Lyle et al. (2012) invoked a weakening or displacement of the northeast Pacific High, along with attendant suppression of the winds that normally block the flow of air masses from the Gulfs of

California and Mexico into the Great Basin. Whether or not the monsoon penetrated as far north as the Great Basin, our record suggests that Estancia lay near its northern limit from ~18.3-17.9 cal kyr BP.

CONCLUSIONS

The chronostratigraphy, sedimentology, fauna, and stable isotope geochemistry of shoreline and wetland deposits around Playas Dry Lake document moisture balance changes in southwestern New Mexico during the last glacial-interglacial transition. Deposit ages are constrained by 14C dates from fossil gastropods, spanning ~20-13 cal kyr BP. Wet meadows present at ~19.5, ~18.6, ~18.0, ~16.2, and ~13.6 cal kyr BP resulted from wetter-than-modern conditions, and a highstand(s) from ~18.4-17.9 occurred during the wettest interval on record.

Each of these events corresponds to one or more highstands elsewhere in the western

U.S., as well as with changes in the northern Atlantic region. These include the initial retreat of the Laurentide from its LGM maximum (~19.5 ka) and re-advance during its “binge” phase

(~19-17.5 ka), H1b (~18.3-17.5 ka), H1a (~17-16 ka), AMOC shutdown (~16.5-15.5 ka), and peak freshwater flux during meltwater pulse MWP1a (~13.8-13.4 ka). Further, the initial transgression at ~18.3 cal kyr BP coincided with other major paleohydrologic events in the northern Great Basin, including the Bonneville flood and brief highstands of paleo-Lakes

Franklin and Columbus. This shows that a brief but intense increase in effective moisture caused inundation of the western U.S. at ~18.3 cal kyr BP. Immediately thereafter, paleo-Lakes

Estancia (35oN) and Lahontan (40±2oN) fell to lowstands, and were thus anti-phased with Playas from ~18.3-17.9 ka. The contemporaneity of these events with H1b and advance of the southern

Laurentide suggests that H1b had a profound effect on regional atmospheric circulation throughout the western U.S.

In light of its proximity to Estancia, the Playas highstand is best explained by increased southerly-sourced precipitation below 35oN in New Mexico. This explanation is consistent with paleobotanical evidence of C4 grassland stretching from west Texas to central New Mexico over the past 40 kyr, and with the rise and fall of 18O values between ~18.4 and ~18.0 ka in the Ft.

Stanton speleothem record. Simultaneous drying to the north of Playas could be explained by a reduction in westerly-sourced winter precipitation and/or reduction in southerly-sourced summer precipitation. Testing of these hypotheses will entail (1) further dating of Unit 5 in Playas

Valley, to determine if/when paleo-Lake playas experienced other highstands, and (2) development of shoreline chronologies across the southern Basin and Range, for comparison to existing lake-level records from the western U.S. In conclusion, new paleohydrologic evidence from Playas Valley shows that the atmospheric mechanisms regulating centennial-scale climatic variability and water balance in southwestern New Mexico during the latest Pleistocene are more complex than previously understood.

ACKNOWLEDGMENTS

Support for this work came in part through funding from the Comer Science and

Education Foundation and NSF-EAR 1103379 (Global Change) to JQ. An equal portion came

from graduate research scholarships awarded to AK from the H. Wesley Peirce, Bert S. Butler, and Maxwell Short Foundations, GSA, Sigma Xi, the UA College of Science, and

ChevronTexaco. Special thanks to staff at the Arizona AMS facility: Tim Jull assisted with sample costs, and Todd Lange, Rich Cruz, and Lori Hewitt provided technical support. Michael

McGlue drafted the base map in Fig. 2a. Jay Quade made comments and structural revisions through the Results section which, along with additional comments from Vance Holliday and

Andy Cohen, greatly improved this manuscript. Finally, Freeport McMoran made this study possible by granting access to the exposures.

REFERENCES

Adams, K.D. and Wesnousky, S.G., 1998. Shoreline Processes and the Age of the Lake Lahontan Highstand in the Jessup Embayment, Nevada. Geological Society of America Bulletin 110:1318-1332.

Allen, B.D. and Anderson, R.Y., 2000. A Continuous, High-Resolution Record of Late Pleistocene Climate Variability from the Estancia Basin, New Mexico. Geological Society of America Bulletin 112(9):1444-1458.

Alley, R.B. and MacAyeal, D.R., 1994. Ice-Rafted Debris Associated with Bing/Purge Oscillations of the Laurentide ice sheet. Paleoceanography 9(4):503-511.

Anderson, R.Y., Allen, B.D., Menking, K.M., 2002. Geomorphic Expression of Abrupt Climate Change in Southwestern North America at the Glacial Termination. Quaternary Research 57:371-381.

Aravena, R., Warner, B.G., MacDonald, G.M., Hanf, K.I., 1992. Carbon Isotope Composition of Lake Sediments in Relation to Lake Productivity and Radiocarbon Dating. Quaternary Research 37:333-345.

Asmerom, Y., Polyak, V.J., Burns, S.J., 2010. Variable Winter Moisture in the Southwestern United States Linked to Rapid Glacial Climate Shifts. Nature Geosciences 3:114-117.

Bacon, S.N., Burkea, R.M., Pezzopanea, S.K., Jayko, A.S., 2006. Last Glacial Maximum and Holocene Lake Levels of Owens Lake, Eastern California, USA. Quaternary Science Reviews 25(11-12):1264-1282.

Bard, E., Arnold, M., Mangerud, J., Paterne, M., Labeyrie, L., Duprat, J., Me´ lie` res, M.-A., Sønstegaard, E., Duplessy, J.-C., 1994. The North Atlantic Atmosphere-Sea Surface 14C Gradient during the Younger Dryas Climatic Event. Earth and Planetary Science Letters 126: 275-287.

Bard, E., Rostek, F., Turon, J., Gendreau, S., 2000. Hydrological Impact of Heinrich Events in the Subtropical Northeast Atlantic. Science 289(5483):1321-1324.

Barker, S., Diz P., Vautravers, M.J., Pike, J., Knorr, G., Hall, I.R., Broecker, W.S., 2009. Interhemispheric Atlantic Seesaw Response during the Last Deglaciation. Nature 457(26):1097- 1102.

Benson, L., 1993. Factors Affecting 14C Ages of Lacustrine Carbonates: Timing and Duration of the Last Highstand Lake in the Lahontan Basin. Quaternary Research 39:163-174.

Benson, L., 2004. Western Lakes. In Gillespie, A.R., Porter, S.C., Atwater, B.F. (Eds.): The Quaternary Period in the United States, pp. 185-04. Elsevier.

Benson, L.V. and Thompson, R.S., 1987. Lake-Level Variation in the Lahontan Basin for the Past 50,000 Years. Quaternary Research 28:69-85.

Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, C.G., Robinson, S.W., Smith, G.I., Stine, S., 1990. Chronology of Expansion and Contraction of Four Great Basin Lake Systems during the Past 35,000 Years. Palaeogeography, Palaeoclimatology, Palaeoecology 78:241-286.

Benson, L., Kashgarian, M., Rubin, M., 1995. Carbonate deposition, Pyramid Lake Subbasin, Nevada: 2. Lake Levels and Polar Jet Stream Positions Reconstructed from Radiocarbon Ages and Elevations of Carbonates (Tufas) Deposited in the Lahontan Basin. Palaeogeography, Palaeoclimatology, Palaeoecology 117:1-30.

Benson, L.V., May, H.M., Antweiler, R.C., Brinton, T.I., Kashgarian, M., Smoot, J.P., Lund, S.P., 1998. Continuous Lake-Sediment Records of Glaciation in the Sierra Nevada between 52,600 and 12,500 14C yr B.P. Quaternary Research 50(2):113-127.

Betancourt, J.L., Rylander, K.A., Peñalba, C., McVickar, J.L., 2001. Late Quaternary Vegetation History of Rough Canyon, South-Central New Mexico, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 165:71-95.

Bequaert, J.C. and Miller, W.B., 1973. The Mollusks of the Arid Southwest, with an Arizona Check List: 271pp. University of Arizona Press, Tucson, AZ.

Holmgren, C.A., Betancourt, J.L., Rylander, K.A., 2006. A 36,000-yr Vegetation History from the Peloncillo Mountains, Southeastern Arizona, USA.Palaeogeography, Palaeoclimatology, Palaeoecology 240:405-422.

Bottinga, Y., 1969. Calculated Fractionation Factors for Carbon and Hydrogen Isotope Exchange in the System Calcite-Carbon Dioxide-Graphite-Methane-Hydrogen-Water Vapor. Geochimica et Cosmochimica Acta 33:49-64.

Brennan, R. and Quade, J., 1997. Reliable Late-Pleistocene Stratigraphic Ages and Shorter Ground-Water Travel Times from 14C in Fossil Snails from the Southern Great Basin. Quaternary Research 47(3):329-336.

Broecker, W.S., 2009. The Mysterious 14C Decline. Radiocarbon 51(1):109-119.

Broecker, W.S. and Barker, S., 2007. A 190 Per mil Drop in Atmopshere’s 14C during the “Mystery Interval” (17.5 to 14.5 ka). Earth and Planetary Science Letters 256:90-99.

Broecker, W.S., Denton, G.H., Edwards, R.L., Cheng, H., Alley, R.B., Putnam, A.E., 2010. Putting the Younger Dryas Cold Event into Context. Quaternary Science Reviews 29(9):1078- 1081.

Broecker, W.S., McGee, D., Adams, K.D., Cheng, H., Edwards, L, Oviatt, C.G., Quade, J., 2009. A Great Basin-Wide Dry Episode during the First Half of the Mystery Interval? Quaternary Science Reviews 28(25):2557-2563.

Broecker, W.S. and Putnam, A.E., 2012. How did the Hydrologic Cycle Respond to the Two- Phase Mystery Interval? Quaternary Science Reviews 57:17-25.

Burch, J.B., 1982. Freshwater Snails (Mollusca: Gastropoda) of North America. U.S. Environmental Protection Agency (Monitoring and Support Laboratory). EPA contract No. 600/3-82-026.

Cerling, T. E., 1984. The Stable Isotopic Composition of Modern Soil Carbonate and its Relationship to Climate. Earth and Planetary Science Letters 71:229-240.

Clark, P.U., Shakun, J.D., Baker, P.A., Bartlein, P.J., Brewer, S., Brook, E., Carlson, A.E., Cheng, H., Kaufman, D.S., Liu, Z., Marchitto, T.M., Mix, A.C., Morrill, C., Otto-Bliesner, B.L., Pahnkeo, K., Russell, J.M., Whitlock, C., Adkins, J.F., Blois, J.L., Clarka, J., Colman. S.M., Curry, W.B., Flower, B.P., He, F., Johnson, T.C., Lynch-Stieglitz, J., Markgraf, V., McManus, J., Mitrovica, J.X., Moreno, P.I., Williams, J.W., 2012. Global Climate Evolution during the Last Deglaciation. Proceedings of the National Academy of Sciences (online): www.pnas.org/cgi/doi/ 10.1073/pnas.1116619109.

Clarke, A.H., Hovingh, P., Clarke, J., 2003. Survey of the Freshwater Mollusks and Crayfishes of Eastern Colorado, at Altitudes of Less than 6,000 Feet. Ecosearch, Inc.: Final Report for the Colorado Division of Wildlife, State of Colorado: Contract No. PSC-701-2001.

Connell, S.D., Hawley, J.W., Love, D.W., 2005. Late Cenozoic Drainage Development in the Southeastern Basin and Range of New Mexico, Southeasternmost Arizona, and Western Texas.

In Lucas, S.G., Morgan, G.S., Zeigler, K.E. (Eds.): New Mexico’s Ice Ages. New Mexico Museum of Natural History and Science Bulletin No. 28:125-150.

Connin S.L., Betancourt, J.L., Quade, J., 1998. Late Pleistocene C4 Plant Dominance and Summer Rainfall in the Southwestern United States from Isotopic Study of Herbivore Teeth. Quaternary Research 50:197-193.

Denton, G.H., Alley, R.B., Comer, G., Broecker, W.S., 2005. The Role of Seasonality in Abrupt Climate Change. Quaternary Science Reviews 24:1159-1182.

Denton, G.H., Anderson, R.F., Toggweiler, J.R., Edwards, R.L., Schaefer, J.M., Putnam, A.E., 2010. The Last Glacial Termination. Science 328:1652-1656.

Denton, G.H., Broecker, W.S., Alley, R.B., 2006. The Mystery Interval 17.5 to 14.5 Kyrs Ago. PAGES News 14:14-16.

Doty, G.C. 1960. Reconnaissance of Ground Water In Playas Valley, Hidalgo County, New Mexico. New Mexico State Engineer Office Technical Report No. 15, 40 pp. plus maps.

Dyke, A.S., 2004. An Outline of North American Deglaciation with Emphasis on Central and Northern Canada. In Ehlers, J. and Gibbard, P.L. (Eds.): Quaternary Glaciations - Extent and Chronology, Part II: North America, pp. 373-424. Elsevier (Amsterdam).

Dyke, A.S., Andrews, J.T., Clark, P.U., England, J.H., Miller, G.H., Shaw, J., Veillette, J.J., 2002. The Laurentide and Innuitian Ice Sheets during the Last Glacial Maximum. Quaternary Science Reviews 21:9-31.

Ehleringer, J.R., 1988. Carbon Isotope Ratios and Physiological Processes in Aridland Plants. In Rundel, P.W., Ehleringer, J.R., and Nagy, K.A. (Eds.): Symposium Proceedings: Applications of Stable Isotope Ratios to Ecological Research, pp. 41-54. Springer-Verlag, New York.

Friedli, H., Lotscher, H., Oeshger, H., Siegenthaler, U., Stauffer, B., 1986. Ice Core Record of 13 12 the C/ C Ratio of Atmospheric CO2 in the Past Two Centuries. Nature 324:237-238.

Fritz, P., 1984. Radiocarbon Dating of Marl Deposits—Can We Assess the Hardwater Effect? In Mahaney, W.C. (Ed.): Correlation of Quaternary Chronologies:15-21. Geobooks: Norwich, CT.

Fritz, P. and Poplawski, S., 1974. 18O and 13C in the Shells of Freshwater Molluscs and their Environments. Earth and Planetary Science Letters 24:91-98.

García, A.F. and Stokes, M., 2006. Late Pleistocene Highstand and Recession of a Small, High- Altitude Pluvial Lake, Jakes Valley, Central Great Basin, USA. Quaternary Research 65(1):179- 186.

Goodfriend, G.A., 1987. Radiocarbon Age Anomalies in Shell Carbonate of Land Snails from Semi-Arid Areas. Radiocarbon 29:159-167.

Goodfriend, G.A. and Hood, D., 1983. Carbon Isotope Analysis of Land Snail Shells: Implications for Carbon Sources and Radiocarbon Dating. Radiocarbon 25:810-830.

Goodfriend, G.A. and Stipp, J.J., 1983. Limestone and the Problem of Radiocarbon Dating of Land-Snail Shell Carbonate. Geology 11:575-577.

Hanebuth, T., Stattegger, K., Bojanowski, A., 2009. Termination of the Last Glacial Maximum Sea-Level Lowstand: the Sunda Shelf Data Revisited. Global and Planetary Change 66:76-84.

Harrold, M.N. and Guralnick, R.P., 2010. A Field Guide to the Freshwater Mollusks of Colorado. Colorado Division of Wildlife.

Hawley, J.W., 1986. Physiographic Provinces and Landforms of New Mexico. In Williams, J.L. (Ed.): New Mexico Maps. Albuquerque, NM: University of New Mexico Press. 28-31.

Hawley, J.W., 1993. Geomorphic Setting and Late Quaternary History of Pluvial Lake Basins in the Southern New Mexico Region. New Mexico Bureau of Mines and Minerals Resources Open-File Report No. 391, 28pp.

Haynes Jr., C.V., Long, A., Jull, A.J.T., 1987. Radiocarbon Dates at Willcox Playa, Arizona, Bracket the Clovis Occupation Surface. Current Research in the Pleistocene 4:124-126.

Hem, J.D., 1985. Study and Interpretation of the Chemical Characteristics of Natural Water: U.S. Geological Survey Water Supply Paper 2254 (3rd edition), 263pp.

Hemming, S.R., 2004. Heinrich Events: Massive Late Pleistocene Detritus Layers of the North Atlantic and their Global Climate Imprint. Review of Geophysics 42(RG1005):1-43.

Hill, M.E. and Holliday, V.T.H., 2011. Paleoindian and Later Occupations along Ancient Shorelines of the San Agustin Plains, New Mexico. Journal of Field Archaeology 36(1):3-20.

Holmgren, C.A., Penalba, M.C., Rylander, K.A., Betancourt, J.L., 2003. A 16,000 14C yr B.P. Packrat Midden Series from the USA-Mexico Borderlands. Quaternary Research 60:319-329.

Holliday, V.T., Weber, R.H., Mayer, J.H., 2006. Geoarchaeological Investigations at the Mockingbird Gap Folsom Site, New Mexico. Current Research in the Pleistocene 23:112-114.

IntCal13 Radiocarbon Calibration Data Set, 2012: http://www.radiocarbon.org/IntCal13.htm

Krider, P.R., 1998. Paleoclimatic Significance of Late Quaternary Lacustrine and Alluvial Stratigraphy, Animas Valley, New Mexico. Quaternary Research 50:283-289.

Kurth, G., Phillips, F.M., Reheis, M.C., Redwine, J.L., Paces, J.B., 2011. Cosmogenic Nuclide and Uranium-Series Dating of Old, High Shorelines in the Western Great Basin, USA. Geological Society of America Bulletin 123:744-768.

14 Levin, I. and Kromer, B., 2004. The Tropospheric CO2 Level in Mid-Latitudes of the Northern Hemisphere (1959-2003). Radiocarbon 46(3):1261-1272.

Licciardi, Joseph M. Chronology of Latest Pleistocene Lake-Level Fluctuations in the Pluvial Lake Chewaucan Basin, Oregon, USA. Journal of Quaternary Science 16(6): 545-553.

Lillquist, K.D., 1994. Late Quaternary Lake Franklin: Lacustrine Chronology, Coastal Geomorphology, and Hydro-Isostatic Deflection in Ruby Valley and Northern Butte Valley, Nevada. PhD Thesis, University of Utah.

Lohmann, K.C. and Walker, J.C.G., 1989. The 18O Record of Phanerozoic Abiotic Marine Calcite Cements. Geophysical Research Letters 16:319-322.

Lowell, T.V., Hayward, R.K., Denton, G.H., 1999. Role of Climate Oscillations in Determining Ice-Margin Position: Hypothesis, Examples, and Implications. Geological Society of America Special Paper 337:193-203.

Lyle, M., Heusser, L., Ravelo, C., Yamamoto, M., Barron, J., Diffenbaugh, N.S., Herbert, T., Andreasen, D., 2012. Out of the Tropics: The Pacific, Great Basin Lakes, and Late Pleistocene Water Cycle in the Western United States. Science 337:1629-1633.

Markgraf, V., Bradbury, J.P., Forester, R.M., McCoy, W., Singh, G., Sternberg. R.S., 1983. Paleoenvironmental Reassessment of the 1.6-Million-Year-Old Record from San Agustin basin, New Mexico. In Chapin, C.E. and Callender, J.F. (Eds.): Socorro Region II. 34th New Mexico Geological Society Field Conference Guidebook, pp. 291-297. New Mexico Geological Society (Socorro).

Markgraf, V., Bradbury, J.P., Forester, R.M., Singh, G., Sternberg, R.S., 1984. San Agustin Plains, New Mexico: Age and Paleoenvironmental Potential Reassessed. Quaternary Research 22:336-343.

McGee, D., Quade, J., Edwards, R.L., Broecker, W.S., Cheng, H., Reiners, P.W., Evenson, N. Lacustrine Cave Carbonates: Novel Archives of Paleohydrologic Change in the Bonneville Basin (Utah,USA). Earth and Planetary Science Letters:352:182-194.

McKenzie, J.A., 1985. Carbon isotopes and Productivity in the Lacustrine and Marine Environment. In Stumm, W. (Ed.): Chemical Processes in Lakes, pp. 99-118. Wiley Interscience (New York).

McManus, J.F., Francois, R., Gherardi, J.M., Keigwin, L.D., Brown-Leger, S., 2004. Collapse and Rapid Resumption of Atlantic Meridional Circulation Linked to Deglacial Climate Changes. Nature 428:834-837. 44

Mifflin, M.D. and Wheat, M.M., 1979. Pluvial Lakes and Estimated Pluvial Climates of Nevada. Nevada Bureau of Mines and Geology Bulletin No. 94, 57pp.

Munroe, J.S. and Laabs, B.J.C., 2013. Latest Pleistocene History of Pluvial Lake Franklin, Northeastern Nevada, USA. Geological Society of America Bulletin 125:322-342.

Munroe, J.S., and Laabs, B.J., 2012. Temporal Correspondence between Pluvial Lake Highstands in the Southwestern US and Heinrich Event 1. Journal of Quaternary Science 28(1):49-58.

National Elevation Data Set, 2014: http://ned.usgs.gov

Okumura, Y.M., Deser., C., Hu, A., Timmermann, A., Xie, S.P., 2009. North Pacific Climate Response to Freshwater Forcing in the Subarctic North Atlantic: Oceanic and Atmospheric Pathways. Journal of Climate 22:1424-1445.

Oviatt, C.G., 1997. Lake Bonneville Fluctuations and Global Climate Change. Geology 25:155- 158.

Oviatt, C.G., Currey, D.R., Sack, D., 1992. Radiocarbon Chronology of Lake Bonneville, Eastern Great Basin, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 99(3-4):225- 241.

Orme, A.R. and Orme, A.J., 2008. Late Pleistocene Shorelines of Owens Lake, California, and their Hydroclimatic and Tectonic Implications. In Reheis, M.C., Hershler, R., Miller, D.M. (Eds.): Late Cenozoic Drainage History of the Southwestern Great Basin and Lower Colorado River Region: Geologic and Biotic Perspectives. Geological Society of America Special Paper 439:207-225.

Phillips, F.M., Campbell, A.R., Kruger, C., Johnson, P., Roberts, R., and Keyes, E., 1992. A Reconstruction of the Response of the Water Balance in Western United States Lake Basins to Climatic Change: Water Resources Research Institute Report No. 269, 167 pp. Las Cruces, New Mexico.

Phillips, F.M., Campbell, A.R., Smith, G.I. Bischoff, J.L., 1994. Interstadial Climatic Cycles: a Link between Western North America and Greenland? Geology 22:1115-1118.

Pigati, J.S., Bright, J.E., Shanahan, T.M., Mahan, S.A., 2009. Late Pleistocene Paleohydrology Near the Boundary of the Sonoran and Chihuahuan Deserts, Southeastern Arizona, USA. Quaternary Science Reviews 28:286-300.

Pigati, J.S., Quade, J., Shanahan, T.M., Haynes Jr., C.V., 2004. Radiocarbon Dating of Minute Gastropods and New Constraints on the Timing of Spring-Discharge Deposits in Southern Arizona, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 204:33–45.

Pigati, J.S., Rech, J.A., Nekola, J.C., 2010. Radiocarbon Dating of Small Terrestrial Gastropod Shells in North America. Quaternary Geochronology 5:519-532.

Pigati, J.S., Rech, J.A., Quade, J., Bright, J., 2014. Desert Wetlands in the Geologic Record. Earth Science Reviews 132:67-81.

Quade, J., 1986. Late Quaternary Environmental Changes in the Upper Las Vegas Valley, Nevada. Quaternary Research, 26: 340-357.

Quade, J., Forester, R.M., Pratt, W.L., Carter, C., 1998. Black Mats, Springfed Streams, and Late-Glacial-Age Recharge in the Southern Great Basin. Quaternary Research 49:129-148.

Quade, J., Forester, R.M., Whelan, J.F., 2003. Late Quaternary Paleohydrologic and Paleotemperature Change in Southern Nevada. Geological Society of America Special Paper 368:165-188.

Rech, J.A., Pigati, J.S., Lehmann, S.B., McGimpsey, C.N., Grimley, D.A., Nekola, J.C., 2011. Assessing Open-System Behavior of Carbon-14 in Terrestrial Gastropod Shells. Radiocarbon 53(2):325-335.

Reeder, H.O., 1957. Ground Water in Animas Valley, Hidalgo County, New Mexico. New Mexico State Engineer Office Technical Report No. 11. 101 p.

Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Ramsey, C.B., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, Weyhenmeyer, J., 2009. IntCal09 and MARINE09 Radiocarbon Age Calibration Curves, 0- 50,000 Years Cal BP. Radiocarbon 51:1111-1150.

Riggs, A.C., 1984. Major Carbon-14 Deficiency in Modern Snail Shells from Southern Nevada Springs. Science 224:58-61.

Romanek, C.S., Grossman, E.L., Morse, J.W., 1992. Carbon Isotopic Fractionation in Synthetic Aragonite and Calcite: Effects of Temperature and Precipitation Rate. Geochimica et Cosmohimica Acta 56:419-430.

Ruddiman, 1977. Late Quaternary Deposition of Ice-Rafted Sand in the Subpolar North Atlantic (40-65°N). Geological Society of America Bulletin 88:1813-1827.

Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., and Soil Survey Staff, 2012. Field Book for Describing and Sampling Soils, Version 3.0. Natural Resources Conservation Service (U.S. Department of Agriculture). National Soil Survey Center (Lincoln, NE).

Schwennesen, A.T., 1918. Ground Water in the Animas, Playas, Hachita, and San Luis basins, New Mexico. U.S. Geological Survey Water-Supply Paper 422, 35 pp. 46

Shanahan, T.S., Pigati, J.S., Dettman, D.L., Quade, J., 2005. Isotopic Variability in the Aragonite Shells of Freshwater Gastropods Living in Springs with Nearly Constant Temperature and Isotopic Composition. Geochimica et Cosmochimica Acta 69(16):3949-3966.

Stanford, J.D., Rohling, E.J., Bacon, S., Roberts, A.P., Grousset, F.E., Bolshaw, M., 2011a. A New Concept for the Paleoceanographic Evolution of Heinrich Event 1 in the North Atlantic.

Stanford, J.D., Hemingway, R., Rohling, E.J., Challenor, P.G., Medina-Elizalde, M., Lester, A.J., 2011b. Sea-Level Probability for the Last Deglaciation: a Statistical Analysis of Far-Field Records. Global and Planetary Change 79:193-203.

Stocker, T.F. and Wright, D.G., 1998. Radiocarbon 40:359.

Stuiver, M., Reimer, P.J., Reimer, R.W., 2012. Calib 7.0 Radiocarbon Calibration program: http://calib.qub.ac.uk/calib/

Talbot, M.R., 1990. A Review of the Palaeohydrological Interpretation of Carbon and Oxygen Isotopic Ratios in Primary Lacustrine Carbonates. Chemical Geology 80:261-279.

Van Devender, T.R., 1995. Desert Grassland History. In McClaran, M.P. and Van Devender, T.R. (Eds.): The Desert Grassland: Changing Climates, Evolution, Biogeography, and Community Dynamics, pp. 65-99. University of Arizona Press (Tucson).

Van Devender, T.R., 1990. Late Quaternary Vegetation and Climate of the Chihuahuan Desert, United States and Mexico. In Betancourt, J.L., Van Devender, T.R., Martin, P.S. (Eds.): Packrat Middens—the Last 40,000 Years of Biotic Change, pp. 104-133. University of Arizona Press (Tucson).

Van Devender, T.R, Betancourt, J.L., Wimberley, M., 1984. Biogeographic Implications of a Packrat Midden Sequence from the Sacramento Mountains, South-Central New Mexico. Quaternary Research 22:344-360.

Waelbroeck, C., Labeyrie, L., Duplessy, J., Guiot, J., Labracherie, M., Leclaire, H., Duprat, J., 1998. Improving Past Sea Surface Temperature Estimates Based on Planktonic Fossil Faunas. Paleoceanography 13:272-283.

Waters, M.R., 1989. Late Quaternary Lacustrine History and Paleoclimatic Significance of Pluvial Lake Cochise, Southeastern Arizona. Quaternary Research 32(1):1-11.

Weber, R.H., 1994. Pluvial Lakes of the Plains of San Agustin. In Chamberlin, R.M., Kues, B.S., Cather, S.M., Barker, J.M., McIntosh, W.C. (Eds.): Mogollon Slope, West-Central New Mexico and East-Central Arizona. New Mexico Geological Society 45th Annual Field Conference Guidebook, pp. 9-11.

Wells, S.G., Brown, W.J., Enzel, Y., 2003. Late Quaternary Geology and Paleohydrology of Pluvial Lake Mojave, Southern California. Geological Society of America Special Paper 368:79-114.

Western Regional Climate Center: www.wrcc.dri.edu/climatedata.html

Zic, M., Negrini, R.M., Wigand, P.E., 2002. Evidence of Synchronous Climate Change across the Northern Hemisphere between the North Atlantic and the Northwestern Great Basin, United States. Geology 30:635-638.

FIGURE CAPTIONS

Figure 1. Regional map showing locations of pluvial lakes in the western U.S. along with paleoenvironmental records discussed herein.

Figure 2. (a.) Digital Elevation Model of the Lower Playas subbasin and parts of adjacent hydrologic basins, with inset map (lower left) showing its location within New Mexico. Three lake area scenarios are shaded, labeled with elevation and area: Playas Dry Lake (white) at 1,307 m/45 km2, midstand paleo-Lake Playas (gray) at the elevation of the lowest paleo-shorelines at 1,314 m/45 km2, and highstand paleo-Lake Playas (black) at the elevation of Hatchet Gap at 1,318 m/85 km2. Discontinuous paleo-shoreline features are found along the northern periphery of Playas Dry Lake, demarcated by white long-dashed line. Dotted arrows represent possible flowpath of historic ground- water outflow, and the solid arrow over Hatchet Gap shows the pathway of overflow/spillage of paleo-Lake Playas at times during the late Pleistocene. The solid box delimits the study area, in the southern portion of the subbasin. (b.) Enlarged aerial image of the study area shows locations of stratigraphic sections depicted in the cross-sectional diagram (Figure 4), and transects represented in Figure 2c. Man-made linear features include defunct railroad beds, dirt road, range fences, and the paved Smelter Road. (c.) Simplified fence diagrams illustrating surface topography, and stratigraphic relations between lacustrine Unit 5 and inset alluvial Units 6-8. The white dashed line delineates the edge of the paleo-channel, with southeastward-pointing white arrows indicating the direction of dip.

Figure 3. Oblique aerial image of Playas Valley. The dashed white line delineates the Lower Playas hydrologic subbasin, the heavy line with arrow shows overflow of paleo-Lake Playas across Hatchet Gap during latest Pleistocene highstand(s), the dotted line delineates Playas Dry Lake, and the gray-shaded zone shows the fan-delta at the terminus of North Playas Draw.

Figure 4. Cross-section of stratigraphic profiles corresponding to the sections marked in Figure 2b, depicted in the following order (from left to right): A→A’→B’→B. Snails are represented by thick black spirals for Succinids and thin black spirals for Lymnaeids.

Figure 5. Panoramic photograph taken from the railroad bed, showing the easternmost stratigraphic exposure, including P09-Sec2 (B’). The tread produced by post-exposure weathering of the profile face is labeled to distinguish it from geologic strata.

13 18 14 Figure 6.  Cshell and  Oshell values from C-dated fossil Succinids and Lymnaeids.

Figure 7. Oblique south-facing view across the top of the PL-Loc3 sediment bloc exposed by the old railroad bed (out of view to the left/west) and Smelter Road (out of view to the right/east), showing proveniences of dated Succinids from upper Unit 5. The dashed line separates the white carbonate surface of Unit 5 from the darker sandy surface of the inset Units 7 and 8.

Figure 8. Central PL-Loc1 profile (west-facing view) showing the stratigraphic provenience of dated Lymnaeids, all from lower Unit 5, as well as redox mottles and carbonate root casts in Unit 4, the casts originating in upper Unit 5. Relatively dark, amorphous spots in Unit 5 are not natural features, but rather are areas that were wetted in the course of profile description.

Figure 9. Selection of paleohydrologic records from the western U.S., including published hydrographs of paleo-Lakes (a.) Lahontan, (b.) Estancia, and (c.) Playas, in addition to (d.) the speleothem 18O records from speleothems FS-2 from Ft. Stanton Cave, in southeastern New Mexico, and COB 01-02 from Cave-of-the-Bells, in southeastern Arizona. The Playas hydrograph (c.) is based on dates from Succinids (solid circles) and Lymnaeids (open circles). Elevation of the snail-bearing deposits corresponds to the relative intensity of the wet periods; aquatic Lymnaeids require extremely wet conditions, such as could be found along the shoreline of paleo-Lake Playas, and Succinids require the permanently moist habitat provided by wet meadows.

Table 1

Table 2

Figure 1

B PL-Loc3W(8m) N P09-Sec2 PYRAMID MTN ? N Playas Draw

1,318 m 85 km2 1,314 ? 65 km2 1,307 m 45 km2 PL-Loc3W(30m)

PL-Loc1 100 m

1,312 A Unit 5 C HATCHET GAP 175 m 1,307 A’ 1,309 100 m B Unit 7 B’ New Mexico Unit 5 paleo- channel 1,307

Figure 2 2

Figure 3

A PL-Loc1

A’ PL-Loc3W (8m) B’ B P09-Sec2 PL-Loc3W (30m)

175 m 25 m 100 m

Figure 4

Little Hatchet Range B’

A’ 8 AA83627: 18.0 ka 7 5 4 6

2 m 1 5 4 5 4 3 4 3

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

APPENDIX B: PALEOHYDROLOGY OF WILLCOX BASIN, SOUTHEASTERN ARIZONA AND IMPLICATIONS FOR ATMOSPHERIC CIRCULATION OVER INTERIOR WESTERN NORTH AMERICA DURING THE FINAL PLEISTOCENE DEGLACIATION

ABSTRACT

A succession of fossil wetland and shoreline deposits in Willcox basin (32oN) provides a record of late Quaternary paleohydrologic change in southeastern Arizona. In this study, we examined stratigraphy from trenches dug across the prominent shoreline berm parallelling the western perimeter of Willcox Playa. Sedimentological, geochemical, faunal, and radiocarbon evidence from twenty-eight formally named units indicates that (1) MIS 2 and late MIS 3 were relatively dry, (2) intervening wet periods were characterized by ephemeral wet meadows and perhaps lowstands, and (3) paleo-Lake Cochise experienced a series of highstands from ~19.2-

12.8 cal kyr BP. We divided the succession into three stratigraphic packages, described as follows, from bottom to top. Lower Group 1, consisting of white marls and intercalated tan, olive, and grey calcareous clays, was deposited between >37 and no later than 33 ka. Deposits within it contain ostracodes commonly associated with shallow water tables and ephemeral seeps and, in some cases, semi-aquatic and terrestrial gastropods commonly found together in wet meadows. Similar in appearance, upper Group 1was deposited from ~33-18 ka. Units 1-4, spanning ~33-21 ka, are in most ways indistinguishable from deposits in lower Group 1; however, Units 5-10, spanning ~19.5-17.5 ka, also contain lacustrine ostracodes and wet meadow snails—not typically found together. Group 2 constitutes a ~2 m-thick package of sands and gravels emplaced along the shoreline of paleo-Lake Cochise between ~17 and ~14 ka.

Finally, Group 3 (Units 17-22), mantling the berm, was emplaced between 14 and 11 ka and consists of organic-rich brown and grey silty deposits. 18O values from >400 ostracode valves

and >80 semi-aquatic gastropod shells from deposits throughout the succession vary by ≤10‰, and there is no consistent trend within individual taxa through time, likely reflecting ephemeral and variably evaporative ground-water discharge environments.

The paleohydrologic record from Willcox basin matches that of other closed basins in the interior western U.S. in showing that Deglacial-age highstands were larger than earlier lake expansions. Those in Willcox basin coincided with (1) the start of deglaciation and meltwater release and attendant AMOC slowdown at ~19 ka, (2) the start of Heinrich event H1b and simultaneous re-advance of the southern Laurentide ice sheet at ~18.3 ka, and (3) AMOC shutdown from ~17.5-14.5 ka. Later wet periods also coincided with major changes in the North

Atlantic, including Heinrich H0 (~12.9 ka) and peak discharge from meltwater pulses MWP1a

(~13.8 ka) and MWP1b (~10.9 ka). Future paleohydrologic studies should focus on defining the magnitude and direction of regional hydroclimatic change to shed light on the strength and nature of North Atlantic teleconnections during the final Pleistocene deglaciation.

INTRODUCTION

For over a century, geologists have reconstructed lake-level fluctuations from late

Pleistocene shoreline deposits in the numerous internally drained basins in the arid interior of the western U.S. (e.g., Russell, 1889; Gilbert, 1890). Collectively, these records reveal spatial and temporal patterns of hydrologic change across the region, enabling us to test hypotheses about the nature of coupled oceanic-atmospheric circulation during the late Pleistocene. Preserved shoreline features can provide accurate, well-dated records of lake surface area change, thereby constraining changes in effective moisture from one lake cycle to the next (Mifflin and Wheat,

1979; Benson and Mifflin, 1986; Benson and Paillet, 1989; Munroe and Laabs, 2012). Further,

because most well-preserved paleoshorelines in the western U.S. date to MIS 2 and 3 (Munroe and Laabs, 2012), they are an ideal source of paleohydrologic information for the last glacial- interglacial transition.

In the northern Great Basin, Mega-lakes Lahontan and Bonneville exceeded the dimensions of their LGM counterparts (Benson and Thompson, 1987; Oviatt et al., 1992) during the Deglacial period. Until very recently, it was widely believed that this could be explained by the retreat of the Laurentide ice sheet (Antevs, 1938), thus permitting the Polar Jet Stream to pass through the northern Great Basin (Kutzbach and Wright, 1985; Benson and Thompson, 1987).

An implicit assumption of the ‘Jet Stream hypothesis’ is that paleolakes in the southern Basin and Range experienced larger expansions during the LGM than subsequently. While much is known about the history of late Pleistocene paleolakes across the Great Basin and Mojave deserts, there is very little corresponding information from the southern Basin and Range—a key requirement for reconstructing late Pleistocene atmospheric circulation across western North

America. Currently, interpretations of most records from basins south of 35oN are equivocal.

More recent paleolake studies suggest an alternative hypothesis that Heinrich (H) events, and H1 in particular, had significant impacts on Great Basin lake levels (Oviatt, 1997; Benson et al., 1998; Broecker 1994; Quade and Broecker, 2009; Munroe and Laabs, 2012). The largest expansions in the Great Basin occurred at ca. 17 cal kyr BP (Munroe and Laabs, 2012), coinciding with major releases of ice-rafted debris (IRD) into the North Atlantic IRD belt between 17.5 and 16.7 ka. This induced the slowing/halt of the Atlantic Meridional Overturning

Circulation (AMOC) and ushered in the H1 stadial (17.5-14.6 ka). Denton et al. (2006) dubbed this period the “Mystery Interval” on the basis of a then-unexplained ~30 ppm decline in

14 atmospheric CO2, further discussed in Broecker and Barker (2007) and Broecker (2009).

Broecker et al. (2009) subdivided the Mystery Interval into a dry first half and wet second half based on records from paleo-Lakes Lahontan and Estancia in Nevada and New Mexico, respectively. Broecker et al. (2009) postulated that the first major IRD release triggered lake contractions and lowstands in western North America (henceforth referred to as the “Lahontan-

Estancia lowstand interval”, or LEL) and that a second release triggered lake expansions of unprecedented proportion since >30 ka. However, Broecker and Putnam (2012) note that the

LEL began at ~18 cal kyr BP and ended with the onset of highstands at ~17 cal kyr BP, whereas the timing they proposed for the IRD events is ~17.5 and ~16 cal kyr BP.

Ultimately, resolving the chronological relationship among changes in the North Atlantic ocean and atmospheric circulation across western North America during the Deglacial period will require additional well-dated stratigraphic records from paleolake basins in the southwestern

U.S. Accordingly, we present the results of our recent investigation of late Pleistocene-age shoreline and wetland deposits in Willcox basin (32oN), in southeastern Arizona (Fig. 1).

PHYSICAL SETTING

Geology and Hydrology

Willcox basin is a hydrologic subbasin in the northern Sulphur Springs Valley in southeastern Arizona (Fig. 2). Covering 130 km2, Willcox Playa occupies the lowest position in the basin at ~3,900 km2, whereas Mt. Graham, at 3,265 m, is the highest point in the adjacent

Pinaleño Mountains. Lacustrine processes began to influence basin sedimentation during the

Quaternary period.

Willcox basin is one of the few closed geohydrologic systems within the region (Langer et al., 1984), rendering change in lake surface area a sensitive indicator of past effective moisture

shifts. No perennial streams flow into the basin, but several perennial springs did, prior to intensive ground-water pumping in the basin. Based on piezometric data, Gardner and Heilweil

(2009) concluded that most ground-water originated as recharge at the mountain-front, and along the losing reaches of ephemeral streams, documenting that subsurface flow has historically followed surface topography.

Recharge also reaches the basin along several large washes during years of intense seasonal rainfall, accounting for a large portion of paleo-Lake Cochise’s water budget. As a consequence, Willcox Playas is periodically inundated by ephemeral surface runoff. Fine- grained lacustrine deposits near the basin center create a shallow (<30 m) perched aquifer (Oram,

1993), allowing Willcox Playa to hold an ephemeral lake during exceptionally wet years.

Climate and Ecology

Topographic relief in the southeastern Basin and Range is commonly several kilometers, causing significant spatial variability in mean annual temperature and precipitation. Two wet seasons punctuate otherwise arid conditions, rendering the climate semi-arid. The North

American summer monsoon, a regional-scale atmospheric circulation pattern resulting from westward expansion of the Bermuda High (Adams and Comrie 1997), produces convective storms accounting for over half of mean annual precipitation. The remainder is delivered during the cooler months by Pacific frontal storms embedded in the Westerlies. Based on long-term meteorological records (1913-2013) furnished by the Western Regional Climate Center

(www.wrcc.dri.edu/climatedata.html) for Pearce/Sunsites, Arizona—nearly equivalent in elevation to the study site—mean annual temperature is 16.4oC, mean winter (DJF) temperature

is 11.9oC, and mean summer (JJA) temperature is 24.5oC; mean annual precipitation is 31.2 cm, mean summer precipitation is 18.3 cm, and mean winter precipitation is 5.2 cm.

Ecologically, Willcox basin is located within a tropical-subtropical desert biome, including Arizona upland (650-1,100 m), semi-desert grassland (1,000-2,000 m), interior chaparral (1,000-2,000 m), oak-pine woodland (1,600-1,900 m), and ponderosa pine forest

(>1,900 m) (Brown, 1982). The Arizona upland contains C3 leguminous trees and shrubs, various cacti and succulents (CAM), and C4 grasses. Willcox Playa sits at the bottom of the basin, where the Arizona upland and semi-desert grassland ecozones merge. Thus, the area around the barren playa flat is predominantly C4 grassland with interspersed shrubs and succulents.

Previous Investigations

Willcox Playa is ringed by several conspicuous paleoshorelines which attracted the attention of Meinzer and Kelton (1913), who proposed the existence of a paleolake with area of

310 km2, or 8% of the total basin area. They based this on observations from artificial exposures of the beach ridge rimming the western perimeter of Willcox Playa at ~1,274 m. Corroborating and expanding upon the findings of Meinzer and Kelton (1913), Robinson (1965) and Schreiber et al. (1972) reported the results of detailed sedimentological investigations of the ridge, but provided no age control for the beach gravels.

Based primarily on 14C dating of the “Willcox White Marl” and the carbonate nodules collected from its denuded surface adjacent to the playa, Long (1966) recognized two distinct late Pleistocene lake cycles. During the first, from >33-16 cal kyr BP, the lake would have reached >1,290 m, whereas the aforementioned beach ridge (~1,274 m) was constructed by

smaller lakes between 13.5 and 12 cal kyr BP. Haynes et al. (1987) also described the stratigraphy of beach and nearshore deposits from an exposure along the ridge, obtaining a date of 15.7 cal kyr BP from charcoal in Unit B3. This unit is comprised of intercalated/cross-bedded gravels and silts, which Haynes et al. (1987) observed near the base of a 4 m-deep section at

Locality A (Fig. 2). Later, Waters (1989) constrained a constructional phase of the beach ridge from ~17-16 cal kyr BP based on five charcoal dates, from the lower member of a cross-bedded gravelly beach deposit at Locality B (Fig. 2), which he named Unit B1/B2 (Fig. 3).

METHODS

Mapping, Stratigraphy, and Sampling

Exposures created in eight trenches (Fig. 4) allowed us to identify twenty-two allostratigraphic units—disconformity-bounded strata, per definition of the North American

Commission on Stratigraphic Nomenclature (2005)—which we traced in profiles or otherwise inferred from lithology, sedimentology, and 14C dating. Field descriptions (Table 1) utilize terms and abbreviations defined in Schoeneberger et al. (2012). Where present, fossil gastropods were sampled, and identified based on illustrations, photographs, and descriptions provided by

Bequaert and Miller (1973) and Burch (1982). Stratigraphic profiling (Fig. 4) and14C dating

(Table 2) were conducted between spring 2009 and autumn 2012. Absolute elevation of the berm was obtained from 30 m Digital Elevation Model data (National Elevation Data Set: http://www.NED.gov), and along-transect elevation change (±1 m) was measured using an abney level with stadia rod. Our study site, labelled Locality C in Fig. 2, overlaps with the Grim locality of Waters (1989).

Sediment Sampling and Processing for Faunal Analysis

Fifteen to thirty mg of bulk sediment samples (Fig. 4) were processed for ostracodes according to technical procedure HP-78, R1 described in Forester (1991). Most units were sampled from several trenches to ensure that lateral variability in the faunal and isotopic composition of ostracode assemblages was captured.

Radiocarbon Dating

Carbonates were rinsed in distilled water and the condition of mollusk shells and ostracode valves inspected under binocular microscope, for secondary cementation and taphonomic evidence of reworking. Depending upon its resilience, each shell was briefly sonicated (2-5 seconds) up to three times prior to a final rinse with distilled water. Next, shells were crushed and both interior and exterior surfaces inspected again for adhering detritus. A similar procedure was used to inspect and purify ostracode calcite prior to 14C dating, except that

50% (by mass) of the ostracode concentrate was leached, rendering sonication unnecessary. All carbonate samples were soaked in H2O2 at room temperature until reaction with non-refractory organic matter was complete, followed by an overnight soak in distilled water. Specimens with dense cement were sonicated to facilitate removal of adhering detritus.

All shells were processed at the University of Arizona on the cryogenic extraction lines

14 of the Arizona AMS Facility. For C dating of carbonates, CO2 was cryogenically purified following in vacuo hydrolysis in 100% H3PO4. Impurities (H2O, SOx, NOx, and halides) were removed with Cu and Ag traps at ~600 °C, and the purified CO2 was divided into two portions.

One aliquot was converted to graphite by catalytic reduction of CO, and submitted for 14C analysis, whereas the other aliquot provided δ13C, which was used to adjust the 14C measurement

for biologic fractionation. The calibrated radiocarbon dates (Table 2, Fig. 4) are expressed as the

2σ range and median probability of possible calendar year ages before 1950 (“present”), abbreviated as cal yr BP. Calibrations were performed using Calib 7.0 (Stuiver et al., 2012), based on the IntCal13 Radiocarbon Calibration Data Set (2012) described in Reimer et al.

(2013).

Stable Isotope Analysis

Shell and sedimentary carbonate samples were oven dried (50oC) and pulverized, whereas ostracode valves were left intact. For larger species such as Heterocypris glaucus, 1-2 valves sufficed for a single analysis, whereas for 4-6 valves were usually required for other taxa.

Samples were processed on an automated sample preparation device (Kiel III) fitted to a

Finnigan MAT 252 mass spectrometer at the University of Arizona Laboratory of Environmental

Isotopes. 18O and 13C values were normalized to NBS-19 based on internal lab standards.

Precision of repeated standards are ± 0.11‰ for 18O and ± 0.07‰ for 13C. Carbonate isotopic results are reported in standard  per mil notation relative to the Vienna Pee Dee Belemnite

(VPDB).

DATING CONSIDERATIONS

The chronology is mostly based on shell dates from Succinids, which secrete shell

14 aragonite with C activity matching that of concurrent atmospheric CO2 (Pigati et al. 2004,

2010). Gastropod aragonite resists exchange with exogenous carbon following death and burial, such that contamination has not been detected in specimens younger than ca. 25 cal kyr BP

(Pigati et al, 2009; Pigati et al., 2010; Rech et al., 2011).

This leaves only a handful of dates from aquatic carbonates in question, given that their carbon derives from dissolved inorganic carbon (DIC). Nonetheless, DIC in the lake and in ground-water were probably in equilibrium with atmospheric CO2. This is likely true because

(1) there is no major carbonate bedrock in the basin, otherwise a major contributor of DIC, and

(2) paleo-Lake Cochise was shallow (<12 m) with a high surface area-to-volume ratio, which

0 2 should have promoted rapid (10 -10 yr) exchange between DIC and atmospheric CO2.

FAUNA

Ostracoda

Ostracoda is a class of minute crustaceans with bivalves composed of calcite and chitin, representing the most abundant calcitic microfossils found in continental aquatic sediments.

Many species have autoecological preferences that are so particular that they only populate specific hydrologic environments (e.g., lake, wetland, etc.) or sub-environments (e.g., wetland: aquifer, spring, stream, wet meadow, marsh, etc.). In contrast, other ostracodes have broad tolerance and are generally poor paleoenvironmental indicators. We examined fossil assemblages in forty-eight sediment samples distributed throughout the succession (Fig. 4) for their taxonomic composition and abundance (Table 3; Fig. 6), organizing the taxa into three environmental settings: permanent lake (lake), ephemeral surface water (temporary pond), and ground-water (either phreatic or discharge). Whole-assemblage abundances range from 0-43 valves/g sediment; assemblages with >7 valves/g are dominated by lacustrine taxa, whereas smaller fossil populations are dominated by taxa from the other categories.

Several taxa occur in significant abundance, making them useful for paleohydrologic reconstruction efforts. Lacustrine taxa include Limnocythere ceriotuberosa, L. sappaensis, and

H. glaucus. Candona patzcuaro occurs in playa-lakes and ephemeral wetlands (Furtos, 1933;

Horne, 1993), and C. truncata was first identified in a temporary pond in Ohio (Furtos, 1933).

Although both are drought-resistant, they are also found in perennial freshwater settings.

Cypridopsis vidua and Potamocypris spp. are generally associated with springs (Forester, 1991), although they are also found at the interface between lake- and ground-water in the littoral zone.

“Interstitial” taxa are morphologically adapted for existence within the saturated pore space of the matrix in the phreatic zone. The taxonomic affiliation of these ostracodes is not important to our research objectives since their ground-water origin can be inferred on the basis of a few basic morphological characteristics.

Mollusca

Semi-aquatic, terrestrial, and aquatic snail taxa were present among the sections we examined; when encountered in the field, snails were identified to the family level, excepting terrestrial taxa. References used for mollusk identification are Bequaert and Miller (1973) for mollusks of the southwestern U.S. and Arizona, and Burch (1982) for freshwater aquatic snails of North America. All snails encountered belong to lung-breathing (pulmonate) families, including the semi-aquatic Succineidae and aquatic Lymnaeidae, Planorbidae, and Physidae.

Succinids require permanent water to survive in areas with hot, summer-dominated climates, whereas in areas with cooler summers they can survive in permanently moist micro- environments (e.g. under leaf litter or on rotting vegetation). Otherwise, they spend most of their time in and around the shallows of wetlands and lakes, feeding on water-side vegetation. In contrast, Lymnaeids, Planorbids, and Physids spend most of their time in the water, although some species can tolerate seasonal desiccation (Clark et al., 2003; Harrold and Guralnick, 2010).

More specifically, Planorbids (“pond snails”) prefer open areas of standing/slow-moving water, such as can be found in ponds (Clark et al., 2003). Similarly, Lymnaeids and Physids live in shallow water, but are most often found among aquatic plants near the water’s edge.

SEDIMENTOLOGY, STRATIGRAPHY, AND GEOCHRONOLOGY

We divided the succession into three groups, and further into twenty-five units (1-oldest,

25-youngest), on the basis of the deposits’ physical characteristics and age (Table 4). Group 1 includes Units 1-10 (upper Group 1)—clays with intercalated marls—and the underlying marl deposits (lower Group 1) that remain uncorrelated (i.e. unassigned, or UA) with respect to the main numbered sequence. Upper Group 1 is inset into lower Group 1 (Fig. 4). Group 2 includes

Units 11-18 and consists of intercalated sands and gravels. Group 3 includes Units 19-22 and consists of silty deposits, variably enriched in organic matter. Following is a summary of the stratigraphy and sedimentology (Table 1; Fig. 4), geochronology (Table 2), and fauna (Table 3) of the Locality C succession.

Group 1

Sedimentology and Geochronology

Lower Group 1

Unit UA-1 is >60 cm-thick, composed of weakly cemented sandy marl with patches of fine gravel throughout, with carbonate root casts occurring locally, having a is diffuse or abrupt upper contact. Lower UA-2 is a chalky white marl with locally abundant rhizoliths, one of which yielded a date of ~23.3 cal kyr BP. Upper UA-2 contains carbonate nodules, rhizoliths, and lenses of olive clay containing dispersed shell fragments; some parts of the marl are chalky

and others are composed of indurated nodules. Unit UA-3 is 15-30 cm-thick, composed of sandy marl with intact gastropods throughout, including Succinids, one of which yielded a date of

~36.7 cal kyr BP.

Upper Group 1

Unit 1 is a >30 cm-thick olive clay having a sharp upper erosional contact with Unit 2.

Unit 2 is a light yellowish brown sand, 10-20 cm-thick. Unit 3 is a 30-40 cm-thick olive grey to pale olive sand with zones of abundant gastropod shells, crushed and intact, having a diffuse upper contact with Unit 3/4. Unit 3 also contains iron oxide-stained fine root casts originating in overlying units and lenses of olive clay containing dispersed shell fragments. Succinids from

Unit 3 yielded dates (n=3) between 32 and 30 cal kyr BP and a single date of ~19 cal kyr BP.

Unit 3/4 is a ~30 cm-thick crumbly marl containing abundant carbonate root casts and iron oxide-lined root channels. The diffuse upper contact and transition zone between Units 3/4 and 4 contains white calcareous clay with soft, very coarse manganese oxide concentrations. Upper

Unit 4 has similar characteristics but is light brownish grey and contains fewer redox concentrations. In most places, Unit 4/5 is a ~20 cm-thick marl; where the marl is absent, the uppermost ~20 cm of Unit 4 is heavily manganese oxide-stained.

Units 5-7 are similar in composition and equivalent in age. Unit 5 is composed of light brownish grey calcareous silty clay with abundant carbonate nodules and rhizoliths. Thin (5-15 cm), discontinuous manganese oxide-stained horizons resembling lower Unit 4 occur at various depths within these units; these horizons are bounded by faint carbonate bands (~1 cm-thick), sharp along the upper boundary and diffuse along the lower boundary. Succinids in Units 5-7 dated to 20.5 (n=1), 19.5 (n=1), 19.4 (n=1), and 19.0 (n=4) cal kyr BP, most not in stratigraphic

order. Unit 6, ~10 cm-thick, comprises a platy erosional remnant of Unit 5. Lower Unit 7 is composed of stratified beds of greyish brown sand, silt, and clay; the upper part is greyish brown very gravelly clay with abundant rip-up clasts, reworked carbonate nodules, rhizoliths, and gravels. Succinids throughout the unit yielded dates (n=4) from 19.2-19.0 cal kyr BP.

Unit 8 is a 10-20 cm-thick white calcareous clay resting atop Unit UA-3. A Succinid in

Unit 8, ~5cm above the upper contact of Unit UA-3, yielded a date of ~18.3 cal kyr BP, and a

Succinid from ~5 cm below the upper contact of Unit 8 dated to ~17.7 cal kyr BP. Unit 9 is a pale brown blocky clay containing gravel stringers, sand lenses, and dispersed shell fragments.

Unit 10A is a 5-10 cm-thick pale olive clay containing abundant carbonate nodules and sand fraction composed almost entirely of ostracode valves. Nodules yielded dates of ~9.6 and ~6.6 cal kyr BP, and bulk ostracodes yielded a date of ~17.4 cal kyr BP. Unit 10B is similar to 10A but is thinner, sandier, and devoid of nodules.

Fauna

Ostracode assemblages in Units 1, 2, 3, and 3/4 are dominated by C. patzcuaro juveniles; interstitial and lacustrine taxa are sparse. Assemblages in Units 4 and 4/5 are dominated by interstitial taxa and contain traces of others, although Unit 4 is largely devoid of ostracodes.

Ostracode assemblages in Units 5, 6, 8, and 10 are dominated by lacustrine taxa in moderate to high abundances, followed by C. patzcuaro, which is nearly absent in Unit 10. Units 7 and 9 contain mostly lacustrine taxa in low to moderate abundances, followed by C. patzcuaro. The exception to this is one sample from Unit 7, in which C. patzcuaro is followed in abundance by lacustrine taxa.

Units UA-1, UA-2, 1, 2, 3/4, 4, 4/5, and 10 are devoid of intact shells, whereas Units UA-

3, 3, 5, and 8 contain abundant shells, mostly from Succinids but some from terrestrial taxa. The sand lenses in lower Unit 7 contain shells, from Succinids and terrestrial snails, some of which are crushed, pointing toward a reworked origin for the entire faunal assemblage.

Paleoenvironmental Reconstruction

Faunal and sedimentological evidence from units older than Unit 5 suggests that they are composed of calcium carbonate that formed near the ground surface, in the capillary fringe or in freshly discharged ground-water, between >37 and ~20 cal kyr BP. Evidence from Units 5, 8, and 10 suggests that they were emplaced in the nearshore zone from ~19.2-18.9, ~18.3-17.7, and

~17.4 cal kyr BP, respectively. However, this interpretation of Unit 8 is tentative, as it hinges on two Succinid dates. With the exception of Unit 4, the prevalence and variable abundance of C. patzcuaro juveniles indicate that ground-water discharge was sufficient to sustain ephemeral ponds just long enough for juveniles to hatch. This could reflect rapid oscillations of the water table induced by seasonal fluxes of high elevation snow-melt. Succinids in Units UA-3 and 3 reflect permanent surface moisture, typical of wet meadows, seeps, and stream banks. In Unit 3, the high concentrations of fractured shell and sand, in addition to the occurrence of intact

Succinids as well as ostracodes associated with ground-water and ephemeral ponds, suggests that deposition occurred in a spring-fed channel draining a seep or wet meadow.

Multiple lines of evidence suggest that Units 4 and 4/5 also formed in association with a shallow water table and/or ephemeral ground-water discharge. First, the lack of Succinids and near-absence of C. patzcuaro probably reflects the brevity of individual discharge events. Next, the clayey texture and dominance of interstitial ostracodes reflects low energy deposition

associated with ponding of ground-water discharge. Finally, the hydromorphic character of the unit (i.e., gleying and iron/manganese oxide concentrations) owes to a fluctuating water table.

This would have promoted the oxidation/removal of organic matter and redistribution of carbonates, and iron and manganese oxides, within the capillary fringe.

The co-occurrence of nearshore lacustrine taxa, interstitial taxa, C. patzcuaro juveniles, and Succinids in Unit 5 suggests that it was formed both in a wet meadow and a lake, perhaps alternating seasonally due to large changes in lake area. The lack of stratigraphic order among

Succinid dates owes to extensive bioturbation, as evidenced by the abundance of root and burrow casts, many of which contain Succinid shells. Unit 7 was reworked from Unit 5. Root- bioturbation can explain the presence of a ~19 cal kyr BP Succinid in Unit 3. Dates from

Succinids in Units 5 and 7 constrain the timing of uncharacteristically wet conditions, resulting in a highstand during the last of three periods: ~20.5, ~19.5, and ~19.2-18.9 cal kyr BP.

Group 2

Sedimentology and Geochronology

Group 2 is inset into upper Group 1 and dips toward the basin, although Units 14, 16, and

17 dip slightly in the opposite direction. Unit 11 is a 5-20 cm-thick indurated white pebble gravel conglomerate; basal carbonate cement yielded a date of ~15.5 cal kyr BP. Directly overlying Units 6, 7, and 11, Unit 12 is a 1-4 cm-thick light yellowish brown sand; a reworked

Planorbid fragment dated to ~14.2 cal kyr BP. Unit 13 is a ≤70 cm-thick unconsolidated cross- bedded gravel, dipping slightly upslope; basal gravels are commonly coated with a thin, discontinuous carbonate crust. A fragment of crust yielded a date of ~14.2 cal kyr BP, matching the age of the Planorbid fragment in Unit 12.

Unit 14 is a pale olive calcareous sandy loam, with a high concentration of gravels reworked from Unit 13 in the middle part. Unit 14 directly overlies Unit 7 and the distal portions of Units 13 and 12; the lower contact with Unit 7 is diffuse or abrupt. Within Unit 14, adjacent

Succinid and Lymnaeid shells yielded identical dates (n=2) of ~15.2 cal kyr BP, indicating that the sediment was reworked from a deposit older than the overlying unit (Unit 12). Unit 15 is a massive sand, unconsolidated in the lower part (15A) and cemented in the upper part (15B).

Unit 15B correlates to Unit B3 of Waters (1989). Shells are extremely rare in 15A, but a date of

~14.4 cal kyr BP on a Succinid from 4 cm below the upper contact shows that it was reworked from a coeval deposit, perhaps Unit 12. Unit 18 represents small patches of conglomerate resting in micro-depressions in the dissolution-pitted surface of Unit 15B; carbonate cement from the matrix of Unit 15B yielded a date of ~13.1 cal kyr BP. Unit 16 is a light yellowish brown sand with Succinids, yielding dates (n=2) of ~14.3 and ~13.86 cal kyr BP; Comparison to dates from other units suggests that the older Succinid was reworked, rendering the date from the younger Succinid a maximum age. Unit 17 is an unconsolidated light brownish grey blocky calcareous sandy loam. A Succinid resting on the lower contact, reworked from Unit 16, yielded a date of ~14.2 cal kyr BP. Bulk carbonate from the upper part of the unit yielded a date of

~13.84 cal kyr BP, providing a maximum age.

Fauna

Excepting Units 15 and 16, lacustrine taxa dominate the ostracode assemblages in Group

2, for which whole-assemblage abundance ranges from 2-12 valves/g. The assemblages in Units

15 and 16 are dominated by C. patzcuaro juveniles, with sparse L. ceriotuberosa. Unit 15 also contains C. truncata in significant abundance, with very few Potamocypris. Gastropods are

abundant in Units 14 and 16; Unit 14 is dominated by Succinids, followed by terrestrial taxa, whereas Unit 16 contains only Succinids.

Paleoenvironmental Reconstruction

The coarse-grained textures and high abundances of lacustrine ostracodes in Group 2 deposits suggest that they were deposited in the nearshore zone between ~17.4 and ~13.1 cal kyr

BP. However, the co-occurrence and high abundance of C. patzcuaro juveniles and reworked

Succinids in Units 15 and 16 suggests that the Succinids were transported in a stream channel to the nearshore zone during a brief transgression at ~13.9 cal kyr BP. Reworked Succinids in

Units 16 and 15B constrain the timing of a wet interval between ~14.4 and 14.1 cal kyr BP.

Carbonate cement in Units 11, 13, 15, and 18 likely formed during or shortly after deposition of each in the nearshore zone during brief transgressions at ~15.5, ~14.2, and ~13.1 cal kyr BP.

Group 3

Sedimentology and Geochronology

Unit 19 is a 10-20 cm-thick light brownish grey calcareous sandy loam containing

Succinids, one of which yielded a date of ~12.9 cal kyr BP. Unit 20 is 30-120 cm-thick, composed of light brownish grey sandy loam, with lenses of calcareous silt and 20-40 cm-thick beds of massive fine sandy loam in the lower half of the unit. Two Succinids and one Physid in

Unit 20 yielded dates (n=3) between 12.8 and 12.7 cal kyr BP. Unit 21 is a gravelly calcareous sand containing rare Succinids and having a clear, undulating erosional upper contact. The

Succinids yielded dates (n=2) of ~10.9 and ~10.8 cal kyr BP. Unit 22 is a dark brown gravelly fine sandy loam containing reworked carbonate-coated gravels similar in appearance to those in

underlying units. Unit 21 contains rodent burrows infilled with darker-colored material from

Unit 22, whereas those in Unit 22 are infilled with lighter-colored material from Unit 21. Unit

22 is the lithostratigraphic equivalent of Unit E of Waters (1989), who reported a charcoal date of 260±80 14C yr BP from a buried archaeological site ~10 km northwest of Locality C.

Fauna

The ostracode assemblage of Unit 19 is dominated by L. ceriotuberosa with some

Potamocypris, and sparse concentrations of other Limnocythere spp., C. patzcuaro juveniles, and

C. vidua. At its base, Unit 20 contains C. patzcuaro juveniles in unprecedented abundance, with few C. truncata in equal abundance to those in Unit 15B, and sparse lacustrine and spring taxa— typical of most deposits in Group 2, and several in Group 1. Units 21 and 22 are devoid of ostracodes, and Unit 20 contains C. patzcuaro in its upper part.

Gastropods in Unit 19 are limited to a few Succinids above the lower contact. Unit 20 contains very few gastropods in the lower 20 cm. However, a 50 cm-thick bed of slightly cemented sediment contains clusters of extremely large Succinids with dispersed Physids and terrestrial taxa. Aside from the two dated Succinids from Unit 21, Units 21 and 22 are devoid of gastropods.

Paleoenvironmental Reconstruction

During a very brief highstand at ~12.9 cal kyr BP, Unit 19 was deposited in a lagoon with contributions from ground-water discharge. This is indicated by the mixture of semi-aquatic snails (Succinids) with ostracodes dominated by lacustrine taxa (>90% L. ceriotuberosa), with sparse taxa indicative of springs and ephemeral ponds. The clear contact between Units 19 and

20 suggests that the transition from lagoon to marsh was abrupt. The lower half of Unit 20 was deposited near the edge of a marsh, as indicated by the extremely high abundance of root channel voids, brownish color (i.e., relative high organic matter content), unique faunal assemblage, and geomorphic position (Fig. 4). Its gastropod assemblage consists of Succinids, followed by

Physids (mostly juveniles) and other aquatic snail taxa. Near the base of Unit 20, the ostracode assemblage contains mostly pond taxa in extremely high abundances, mixed with spring and lake taxa in low abundances. Throughout the rest of Unit 20, the gastropod and ostracode assemblage lacks spring and lacustrine taxa, and the uniquely high abundance of C. truncata indicates that the transition from lagoon to marsh was complete by the time Unit 20 deposition began.

Unit 21 was deposited during or after the last wet episode recorded in our succession.

Near the base, coarse sands and Succinids constrain the timing of this episode to ~10.9 cal kyr

BP. The relatively high silt content of overlying Unit 22 suggests an eolian origin. We postulate that dense stands of phreatophytes tapping a relatively shallow water table acted as a windbreak, intercepting dust from the surface of nearby desiccated wetlands. Seasonal alternation of wet, biologically active conditions and relatively dry, windy conditions during the Holocene may explain the thickness and high levels of silt and organic matter in Unit 22. Corroborating evidence comes from the occurrence of middle Holocene-age carbonate nodules and rhizoliths within a discrete, 10 cm-thick horizon overlying Group 2, probably marking the shallow capillary fringe upon rebound of the water table.

STABLE ISOTOPES

Stable isotope measurements from 17 units were made on ostracode valves (n=328), gastropod shells (n=89), and/or nonbiogenic carbonate (n=11) (Appendix 2, Fig. 10) from

samples shown in Fig. 4. Ostracodes and snails are each represented within eleven units, nonbiogenic carbonate is represented within five units, and all three carbonate types were found in Unit 20. Data were obtained from ostracodes in Unit 3, 5-10, 17, and 19-20; from snails in

Units 3, 5, 7-8, 12-13, 16, 18, and 20-21; and from nonbiogenic carbonate in Units 11, 13, 18, and 20.

Carbonate 18OValues

18O (VPDB) values range from -6.7 to +3.3‰. Values range from -6.7 to +3.3‰ for ostracode valves; from -7.5 to +1.5‰ for snail shells; and from -9.1 to -4.5‰ for cements. For ostracodes, values for L. ceriotuberosa range from -3.3 to +2.7‰ with an average of -1.5‰

(n=97); for H. glaucus, from -3.3 to +0.5‰ with an average of -1.5‰ (n=63); for C. patzcuaro juveniles, from -6.7 to +3.3‰ with an average of -2.2‰ (n=112); for C. truncata, from -5.7 to

-5.2‰ with an average of -5.5‰ (n=5); for Potamocypris, from -0.7 to +1.0‰ (n=2); and for L. sappaensis, from -4.2 to -0.9 with an average of -2.2‰ (n=23). For snails, values for Succinids range from -7.5 to +0.5‰ with an average of -3.9‰ (n=86), and values for Physids range from -

5.4 to +1.5‰ with an average of -2.4‰ (n=3). For cement, values range from -9.1 to -4.5‰ with an average of -6.4‰ (n=11).

Interpretation of 18O values

18O values of biogenic and non-biogenic carbonates reflect 18O content of the host water, along with a temperature-dependent fractionation (Kim and O’neil, 1997). Ostracode valves and snail shells are composed of calcite and aragonite, respectively, with aragonite enriched by ~1‰ compared to calcite. In addition, some biogenic carbonates are enriched

relative to isotopic equilibrium values due to physiological processes (“vital effects”). Xia et al.

(1997) examined lab-cultured Candona rawsoni, finding a vital effect of ~1‰. However, this correction may not accurately estimate the true vital effect, as the lab-cultured valves weighed less than natural specimens. Von Grafenstein (1999) reported a vital effect of ~2‰ from naturally occurring freshwater Candona spp. in Europe, and Colman et al. (1994) provided indirect evidence for a similar vital effect for Candona subtriangulata in cores from Lake

Michigan.

Finally, non-biogenic carbonates form passively due to changes in hydrochemistry and temperature, whereas biogenic carbonates are secreted under the specific conditions dictated by the environmental preferences of individual taxa. All of these factors must be taken into account in evaluating and comparing 18O records from different taxa and carbonate types within and among units, allowing us to examine the spatial and temporal variability in the 18O content of ancient host water. To do this, we apply the appropriate corrections in the discussion that follows, including a 2‰ correction for Candona spp.

Cement

Cements yielded 18O (VPDB) values between -9.1 and -4.5‰. Unit 20 yielded values

(n=2) from -7.1 to -6.5‰; Unit 18 (n=3), from -6.8 to -6.6‰; Unit 13 (n=2), from -9.1 to -6.2; and Unit 11 (n=4), from -5.8 to -4.4‰. These ranges and minima suggest that the cements formed from ground-water, and not lake-water, perhaps as ground-water discharge continued as the lake regressed. Values from Unit 11 are more positive than in other units. This either reflects increased evaporation or higher ground-water 18O values. An increase in ground-water

18O could signify a change in the source/composition of meteoric water and/or the ratio of snowmelt to rainfall.

Biogenic Carbonate

18O (VPDB) values for Succinids and C. patzcuaro overlap with one another, and their minima fall within the combined range for the cements, suggesting that they both formed in ground-water discharge settings. Among all snail and ostracode taxa, the overall intra-unit variability is greatest for Succinids and, although slightly less so, C. patzcuaro. Physids from

Unit 20 yielded values overlapping with co-occurring Succinids. Minimum values from

Succinids, Physids, and Potamocypris are similar, suggesting a common origin in ground-water- fed ponds. Succinids throughout the sequence yielded similar ranges of values; this suggests that the 18O content of the ground-water discharge remained stable over long time scales.

Succinids in Units 5, 6, and 7 co-occur with ostracode taxa from all habitats, enabling their values to be compared. Within these units and throughout the succession, most 18O values from lacustrine ostracode taxa range from -4-0‰, whereas most C. patzcuaro from these units yielded values between ~ -2 and ~ -5‰; finally, Succinids in Units 5 and 7 yielded values between ~ -3 and ~ -7‰. All ranges overlap, but values for lacustrine taxa are the most positive, and overlap least with other taxa, reflecting extensive evaporation as is common in lakes. Lastly,

H. glaucus in Units 5 and 6 yielded values similar to those from other lacustrine taxa.

PALEOHYDROLOGIC RECONSTRUCTION

Sedimentologic, isotopic, and chronologic evidence from our depositional sequence indicates alternation of different hydrologic conditions at Locality C, corresponding to three 84

primary hydrologic states in Willcox basin. These include (1) shallow, fluctuating water tables with localized spring-discharge and ephemeral pools under a slightly wetter-than-modern climate, (2) high water tables supporting extensive wetlands, including wet meadows with spring-fed channels and ephemeral ponds and possibly a small, semi-permanent lake under a much wetter-than-modern climate, and (3) elevated water tables and high runoff levels sustaining highstands during the wettest periods.

From >37 to ~20 cal kyr BP, shallow fluctuating water tables caused periodic increase of localized ground-water discharge along the distal bajada. This resulted in alternation between seasonally saturated soils during drier periods, and wet meadows during wetter periods. During exceptionally wet intervals, these meadows may have fed streams draining into a small, semi- permanent lake or large valley floor wetland. Such conditions may have occurred at >37, ~20.5, and ~19.5 cal kyr BP. The first unequivocal lake transgression did not occur until ~19.2 cal kyr

BP, reaching 1,274 m on one or more occasions until its permanent regression at ~18.9 cal kyr

BP. Dry conditions prevailed until one or more highstands between ~18.3 and ~17.7 cal kyr BP.

Subsequent highstands occurred at ~17.4 and ≥16.7-16.3 cal kyr BP, and transgressions to 1,271 m occurred at ~15.5 and ~15.2 cal kyr BP. A period of intensified ground-water discharge from ~14.4-14.1 cal kyr BP culminated in a highstand at ~14.2 cal kyr BP. A relatively dry interval at 14.0±0.1 cal kyr BP preceded a brief transgression to 1,271 m, at ~13.9 cal kyr BP, and again at ~13.1 cal kyr BP. Following an interval of drier conditions from

13.0±0.1 cal kyr BP, a highstand at ~12.9 cal kyr BP was followed by regression of the lake and formation of marshes, behind the beach ridge, drying up as early as ~12.75 cal kyr BP.

Following the Younger Dryas, a brief but regionally extensive humid interval (~10.8±0.1 cal kyr

BP) marked the transition to the dry interglacial conditions of the Holocene epoch.

SUMMARY OF REGIONAL PALEO-LAKE RECORDS

For comparison to paleo-Lake Cochise, paleo lake-level information from the southwestern U.S. is available from the Estancia, San Agustin, and San Luis basins of New

Mexico. The only dated paleoshorelines in the region include those formed by paleo-Lake

Cloverdale, in southwestern New Mexico, and paleo-Lake Cochise, in Willcox basin.

Unfortunately, the other records are based on interpretation of lake level from proxies, anchored by dates from aquatic carbonate with unknown or poorly constrained 14C reservoir effects.

In the San Agustin record (Phillips et al., 1992), transgressions to progressively lower elevations occurred at ~26, ~23, and ~20.5 cal kyr BP. These dates were interpolated from an age model that is sparsely dated and untested for reservoir effects, and are therefore unreliable.

In comparison, the age model for the Estancia core record is based mostly on 14C dates from aquatic biota, with much higher sampling resolution than for the San Agustin record. There,

Allen and Anderson (2000) sampled live Artemia cysts yielding a date of 130±65 14C yr BP.

They interpreted this as evidence for a negligible hard-water effect, but did not account for the

“bomb effect” in the sampling year (1990), when fMC was ~1.16 (Levin and Kromer, 2004).

Assuming complete exchange between modern DIC and atmospheric CO2, a correction of

~1,300 14C yr would apply to a date of 13,000 14C yr.

Radiocarbon reservoir uncertainties aside, the faunal, geochemical, and sedimentological proxies in the deep-lake muds of paleo-Lake Estancia record a series of maximum freshening events between >25 and ~18.3 cal kyr BP, followed by an extreme lowstand from ~18.2 to ~16.9 cal kyr BP, and another highstand at ~16.8 cal kyr BP. Transgressions at ~16.2 and ~15.7 cal kyr BP are constrained by 14C dating of nearshore sediments (Allen and Anderson, 2000). The

frequency of freshening events, each with an identical signature in the proxy records, suggests that Estancia rose to the same level each time and was therefore not geohydrologically closed.

Because the highest paleoshoreline in the basin (~1,890 m) is ~40 m below the lowest topographic outlet, it is certain that the lake did not overflow. However, this highest shoreline traces to a solution-enlarged fracture zone along the southeastern hydrologic divide with

Tularosa Basin (Hawley, 1986), suggesting that ground-water underflow limited highstand elevation. Thus, while the approximate timing of these freshening events is known, the relative paleohydrologic intensity of each highstand cannot be determined. On the other hand, the up- section change in the mud’s mineralogical composition from calcareous to gypsiferous provides unambiguous evidence for a shift from fresh-water to alkaline conditions, induced by a significant drop in lake level.

Within the prominent shoreline berm of Paleolake Cloverdale, Unit L1 yielded reliable dates of ~23,800±1,400 cal yr BP on carbonized wood residue (Krider, 1998), and 21,760±640 cal yr BP on humates extracted from another piece yielding no residue. The shoreline berm of paleo-Lake Cochise yielded five reliable charcoal dates, spanning 16.6-16.1 cal kyr BP (Waters,

1989). Geomorphic studies of shoreline deposits in the San Agustin basin constrain the highest known elevation of paleo-Lake San Agustin to 2,120 m (Powers, 1939). More recent work on paleo-Lake San Agustin (Hill and Holliday, 2011) suggests that by ~19 cal kyr BP it had regressed to the lowest-lying and largest subbasin (Horse Spring). Following this, lake-level declined irregularly as marshy conditions persisted in the adjacent/higher subbasin, reaching as high as 2,105 m by ~13.2 cal kyr BP. The lake was alkaline by ~12.3 cal kyr BP and dried up during the middle Holocene.

REGIONAL CORRELATION

Early Deglacial (19-15.0 Cal ka BP)

The earliest highstand of paleo-Lake Cochise, from ~19.2-18.9 cal kyr BP, slightly preceded the initial AMOC slow down. Further, it coincided with a highstand of paleo-Lake

Estancia and the onset of warming and deglaciation in the northern high latitudes. Packrat midden assemblages record cooler/wetter conditions in the southeastern Basin and Range at the same time (Holmgren et al., 2003, 2006). One or more highstands between ~18.3 and ~17.7 cal kyr BP coincided with the sudden re-advance of the southern Laurentide ice sheet beginning at

~18.3 ka (Dyke et al., 2002), overlapping with a wet interval in Playas Valley during the first part of the LEL. This suggests the existence of a sharp climatic boundary at 34oN during the first part of the LEL.

A depositional phase of the 1,274 m shoreline of paleo-Lake Cochise at ~17.4 cal kyr BP, and another between ≥16.6 and ~16.1 cal kyr BP, both overlap with events recorded in midden assemblages from Playas Valley, including the descent of Pinus edulis below ~1,500 m at ~17 cal kyr BP and the local presence of aquatic vegetation between ~17.2 and 16.0 cal kyr BP

(Holmgren et al., 2003). A transgression to 1,271 m at ~15.5 cal kyr BP coincided with the onset of a regional warming/drying trend between ~15.6 and 13.1 cal kyr BP, inferred from midden records (Holmgren et al., 2003; Holmgren et al., 2006). The highstand at ~15.2 cal kyr BP coincided with the transition from glacial to post-glacial vegetation assemblages in middens from the Pinaleño Mountains (Holmgren et al., 2006). In the San Pedro Valley, the Coro Marl dates from 28-15.2 cal kyr BP at Murray Springs and >42-14 cal kyr BP in Horsethief Draw (Pigati et al., 2009).

A comprehensive paleoshoreline record for paleo-Lake Franklin (40±0.5oN) (Fig. 12), located between mega-Lakes Lahontan and Bonneville, provides a more accurate account of past hydroclimatic change in the northern Great Basin. This is because the large size of either megalake would have influenced basin-scale weather patterns (the “lake effect”) and caused a lag in lake-level response in increased effective moisture. Furthermore, Bonneville overflowed at its highstand (Oviatt, 1997). During MIS2, paleo-Lake Franklin reached its maximum shoreline level of 1,850 m at ~22.6, ~20.0, ~18.3, and ~16.8 cal kyr BP. Notably, the LEL was marked by relatively low levels of Franklin, which reached highstand levels during the LGM as well as upon termination of the LEL (Fig. 12). Also in the northern Great Basin, Jakes Lake experienced high-, mid-, and low-stands at ~16.9, ~16.7, and ~14.5 cal kyr BP, respectively

(Garcia and Stokes, 2006).

Late Deglacial (15.0-10.5 Cal ka BP)

During the late-glacial period, several wet intervals caused transgressions of paleo-Lake

Cochise to ≥1,271 m and/or expansion of valley floor wetlands. Polyak et al. (2012) interpreted increases in 18O and 234U in speleothem FS2 from Ft. Stanton Cave, in southern New Mexico, to reflect regional drying between ~14.5 and ~13 ka. However, speleothem 18O values must be interpreted with great caution, as they reflect multiple non-climatic factors and, potentially, variable chronologic offsets that would arise from long and variable infiltration times. Drying is also suggested by a date from the top of the Estancia lake bed sequence, constraining the timing of its desiccation to ~14.6 cal kyr BP or later. From packrat middens in the southeastern

Arizona, Holmgren et al. (2006) inferred a drying trend from ~15.4-13.1 cal kyr BP. Further, we note the absence of middens dating to between ~15 and ~14 cal kyr BP, and suggest that this

reflects protracted seasonal or perennial drought beginning several centuries before the Bolling-

Allerod. The ~14.2 cal kyr BP highstand coincided with the latest occurrence of Coro Marl deposition in the adjacent San Pedro Valley (Pigati et al., 2009), suggesting that drying ensued abruptly thereafter. This highstand also coincided with a transgression of Owens Lake and emplacement of the lower part of a massive barrier beach deposit in Owens Valley at ~14.3 cal kyr BP (Orme and Orme, 2008). However, most Great Basin paleo-Lake records do not show late Deglacial-age highstands, other than a highstand of paleo-Lake Chewaucan (42oN) at ~13.7 cal kyr BP (Licciardi, 2001).

Hydrologic conditions from 14-13 ka in the southeastern U.S. are poorly understood.

Haynes (1991) interpreted stratigraphy at the Murray Springs Clovis site in the San Pedro Valley as evidence for a late Bolling-Allerod drying trend, punctuated by episodes of enhanced spring discharge. Evidence for this is the “Clovis channel” sand, constrained by charcoal dates of ~12.7 and ~15.5 cal kyr BP, overlying the Coro Marl and underlying a black clay deposited in a cienega (the “Black Mat”). In the Jornada del Muerto basin in south-central New Mexico, paludal muds intercalated with fluvial sands in Chupadera Draw yielded bulk organic matter 14C dates of ~13.7-13.1 cal kyr BP. Holliday et al. (2009) interpreted the organic-rich deposits as evidence for reductions in stream flow resulting from a shift to drier conditions. Because the organic deposits are indicative of ground-water discharge, this interpretation rests on the assumption that the sands aggraded in a perennial, and not ephemeral, stream. Furthermore, according to Darcy’s Law, a stream’s flow rate decreases as its cross-sectional area increases— provided that the hydraulic gradient and hydraulic conductivity remain constant. Thus, if these conditions are met, a rise in water table would result in a reduction in flow rate, potentially leading to deposition of finer-grained materials in parts of the channel. However, because

changes in the hydraulic gradient cannot be ruled out on the basis of available evidence, and because the fluvial sands were possibly transported by an ephemeral stream, the Chupadera

Draw sediment record remains equivocal.

Finally, late Deglacial-age packrat middens from southeastern Arizona and southwestern

New Mexico yielded dates of ~13.9, ~13.7, and ~13.1 cal kyr BP. Although the midden assemblages indicate drier conditions relative to older middens (all >15 ka), we interpret them as evidence of wet episodes during a relatively dry period. This interpretation rests on the following logic. Prolonged drought corresponds to increased foraging range, also increasing exposure to predation; in turn, this would suppress midden production, since packrats tend to be solitary and inhabit a single midden in the course of a lifetime (Holmgren et al., 2006). Another possibility is preservation bias, but this is unlikely as it would favor the construction of resilient middens during drier periods. Our interpretation of the midden chronology is consistent with the

Cochise record, which shows that the lake transgressed at ~13.9 and ~13.1 cal kyr BP. In conclusion, several lines of evidence suggest a shift to, and maintenance of, drier conditions after

15 ka, punctuated by several intense and regionally extensive wet episodes.

Decreases in 18O values in the Cave of the Bells and Ft. Stanton speleothem records at

12.9 cal kyr BP suggest a return to wetter and/or colder conditions in the region, coinciding with

Heinrich event H0 and the start of the Younger Dryas stadial. The concordance in timing of a major (~2‰) drop in 18O in both records with abrupt cooling over Greenland shortly after ~13 ka (e.g., Rasmussen et al., 2006) points toward the chronological fidelity of these speleothem records. Furthermore, this lowering of values in both records is interpreted by Asmerom et al.

(2010) and Wagner (2010) to reflect a decrease in temperature and/or increase in the proportion of Pacific-derived winter precipitation. In southeastern Arizona, the Black Mat was emplaced

between ~12.8-11.6 cal kyr BP, providing a well-dated record of sustained ground-water discharge during the Younger Dryas stadial. Further, one of three Younger Dryas-age middens from the nearby Peloncillo Mountains shows a rebound in P. edulis at ~12.4 cal kyr BP, which followed its decline after ~15.4 cal kyr BP and brief disappearance at ~13.9 cal kyr BP, and preceded its final disappearance sometime after ~12.1 cal kyr BP, suggesting a brief return to pre-Bolling-Allerod conditions (cooler/wetter) between ~12.4 and ~12.1 cal kyr BP. Low sampling density may well account for the absence of this feature in the other two Younger

Dryas-age middens; thus, certainty of this conclusion depends on replication of this feature in future midden studies (Betancourt et al., 2001; Holmgren et al., 2003; Holmgren et al., 2006).

To existing paleohydrologic records of the Younger Dryas from the southeastern Basin and Range we add a highstand at ~12.9 cal kyr BP. This transgression of paleo-Lake Cochise formed a lagoon in the topographic depression behind the 1,274 m beach berm at Locality C.

Upon regression, marsh sedimentation commenced and continued through ~12.75 cal kyr BP.

The final documented episode of enhanced ground-water discharge along the 1,271 m scarp occurred around ~10.9 cal kyr BP. Shortly after this, extensive valley floor wetlands were replaced by dry playa deposition and a few springs and seeps that persisted into the historic period.

REGIONAL HYDROLOGIC RESPONSES TO NORTH ATLANTIC CLIMATE FORCINGS

Depending on the indicator record under consideration, paleoclimatologists have defined the beginning of deglaciation at different points between 19 and 17.5 ka. To better resolve the timing of this important transition and regional hydrologic responses worldwide, we must first consider the paleoceanographic evolution of H1 in the North Atlantic. This is an

essential first step in formulating explanations for terrestrial paleohydrologic changes in western

North America and elsewhere during the last Deglacial interval. Evidence from paleo-shorelines and -wetlands in Willcox basin shows that southeastern Arizona experienced rapid jumps in effective moisture at several points during the interval of AMOC slowdown and collapse between ~19 and ~14.6 cal kyr BP. All appear to coincide with a change in AMOC intensity or sea-surface temperatures corresponding to changes in IRD concentration and/or salinity in the

North Atlantic, indicating that the freshening events that triggered changes in AMOC and sea- surface temperatures were caused by ice berg and meltwater discharges.

During the last glacial period, abrupt and often synchronous climate changes throughout both hemispheres coincided with these freshening events and/or associated ice sheet changes

(e.g., Barker et al., 2009; Denton et al., 2010; Stager et al., 2011), thus accounting for terrestrial changes that were not in rhythm with orbital cycles. Comparison of speleothem records from the southern Basin and Range with marine and ice-core records from the northern high latitudes shows that interstadial warming in the northern Atlantic ocean corresponded to warming over

Greenland, and changes in precipitation 18O values, whereby Asmerom et al. (2010) and

Wagner et al. (2010) infer drying and/or warming from 18O increases, and elevated moisture and/or cooling from 18O decreases. While this relationship holds for the last glacial period over stadial-interstadial cycles, the details of terrestrial hydrologic “responses” to oceanographic perturbations occurring over hundreds to thousands of years remain unclear. Paleoshorelines allow for detailed reconstruction of geographic variability in the hydrologic cycle of western

North America during the last major Heinrich event, a requirement for understanding the large- scale atmospheric mechanism(s) that governed regional hydrologic cycles.

Early Deglaciation and H1

The most oft-cited dates for H1 come from sediment cores in the North Atlantic IRD belt, in which the basal age of the H1 IRD layer is constrained by 14C dates on foraminifera tests, calibrating to between ~17.8 and ~16.8 cal kyr BP (summarized in Munroe and Laabs, 2012).

Additionally, core SU-118 from the subtropical Northeast Atlantic contains two IRD layers with basal ages of ~17.9 and ~16.8 cal kyr BP, with peak deposition estimated at ~17.5 and ~16.0 cal kyr BP, respectively (Bard et al., 2000). Notably, the younger layer does not appear in most records from the main IRD belt, and the accuracy of its estimated age is uncertain due to a potentially variable marine reservoir effect, currently estimated at 400 14C yr. Using 231Pa/230Th in Bermuda Rise sediment cores as a proxy for AMOC intensity, McManus et al. (2004) document (1) a slowdown beginning at ~19 cal kyr BP, (2) near-halt or shutdown from ~17.5-

16.0 cal kyr BP, and (3) abrupt resumption at ~14.6 cal kyr BP.

From this, it is apparent that AMOC slowdown at ~19 cal kyr BP—coinciding with the

~19.2-18.9 cal kyr BP highstand—began well before the conventionally defined start of H1 in the main IRD belt, and that it resumed abruptly long after the end of H1. To explain what initiated the AMOC slowdown, Stanford et al. (2011a) invoked small-scale ice rafting in the

Nordic Seas beginning at ~19 cal kyr BP induced by high latitude warming, which in turn caused gradual sea-surface freshening and cooling at ~18.3 cal kyr BP. This change triggered the well- documented re-advance of the British/Irish and southern Laurentide ice sheets at ~18.3 cal kyr

BP (Bowen et al., 2002; Dyke et al., 2002; Clark et al., 2012), coinciding with abrupt reduction in the rate of Deglacial sea-level rise (Hanebuth et al., 2009) and a highstand of paleo-Lake

Cochise. Maximum dilution of North Atlantic surface waters from ~17.4-16.7 cal kyr BP

coincided with the resumption and/or intensification of glacial retreat worldwide (Dyke et al.,

2002 and sources therein) and a highstand of paleo-Lake Cochise.

Freshwater pooling in the Nordic Seas sustained AMOC collapse between 16.7 and 15.1 cal kyr BP, during which time the Nordic Seas experienced peak IRD deposition (Stanford et al.,

2011a). Peak deposition coincided with deposition of the second IRD layer in the subtropical

Northeast Atlantic. The ≥16.6-16.1 cal kyr BP highstands of paleo-Lake Cochise coincided with the first part of the AMOC collapse. Outflow of freshwater from the Nordic Seas began at ~15.5 cal kyr BP, corresponding to intense cooling in the northern hemisphere subtropics and high latitudes (Alm, 1993) and a highstand of paleo-Lake Cochise. After ~15.1 cal kyr BP, increasing

North Atlantic salinity enabled formation of North Atlantic Deep Water, in turn leading to abrupt resumption of the AMOC at ~14.6 cal kyr BP (Stanford et al., 2011a). A brief transgression of paleo-Lake Cochise occurred simultaneously with the initial rise in North Atlantic salinity.

Termination 1 and Late Deglaciation

Onset of the continuous Deglacial sea-level rise at ~17 cal kyr BP was synchronous with the AMOC collapse, occurring in several phases, owing to both Heinrich events and precursor meltwater pulses. The most rapid rises occurred during meltwater pulses 1a and 1b (MWP1a and

MWP1b, respectively). Based on analysis of existing sea-level chronologies, Stanford et al.

(2011b) concluded that MWP1a occurred between ~14.3 and 12.8 cal kyr BP, and that a hiatus in the Sunda Shelf core log between ~13.8 and ~13.4 cal kyr BP corresponds to the fastest sea-level rise within MWP1b; MWP1a occurred from 11.5-8.8 cal kyr BP, with peaks in the rate of sea- level rise at ~10.9 and 9.5 cal kyr BP. The contemporaneity of wet phases in Willcox basin with these changes offers compelling evidence of North Atlantic forcing.

CONCLUSIONS

The chronostratigraphy, sedimentology, fauna, and stable isotope geochemistry of shoreline and wetland deposits along the 1,274-1,271 m paleoshoreline complex in Willcox basin

(32oN) document moisture balance changes in southeastern Arizona from late MIS 3 to the early

Holocene. Between >37 and ~20 ka, paleo-Lake Cochise remained below 1,271 m or was absent, with wetter conditions and possibly lowstands from ~32-30 and at ~23 ka. A series of well-documented lake transgressions occurred from ~19.2-18.9 ka (1,274 m), on one or more occasions between ~18.3 and ~17.7 ka (~1,274 m), at ~17.4 ka (1,274 m), from ≥16.6-16.1 ka

(1,274 m), and at ~15.5 ka (1,271 m), ~15.2 ka (1,271 m), ~14.2 ka (1,274 m), ~13.9 ka (1,271 m), ~13.1 ka (1,274 m), and ~12.9 ka (1,274 m). Finally, the onset of early Holocene aridification followed a brief wet interval from ~10.9-10.7 ka.

In contrast to nearby paleo-Lake Estancia (35oN) in north-central New Mexico, paleo-

Lake Cochise remained dry or at low levels during the LGM—probably because of its small size and comparatively low elevation. Confirming the status of paleo-Lake Cochise’s area between

~18.9 and ~16.6 cal kyr BP—following the contemporaneous transgression of both lakes—is essential for establishing whether early Deglacial-age transgressions of Cochise and Estancia were synchronous or anti-phased. Nonetheless, transgressions of paleo-Lake Cochise were in rhythm with Estancia and many paleolakes throughout western North America during numerous pluvial episodes between ~16.6 and ~15.1 ka, during the H1 stadial. At ~17.5 ka, the AMOC shutdown followed the initiation of H1b by as much as 800 cal yr and preceded the Great Basin pluvial maximum by ca. 500 cal yr. This indicates that these events did not share a single

trigger, thus reflecting the complex nature of Heinrich events related to interaction of the ocean, cryosphere, and atmosphere.

Shoreline deposits document transgressions of paleo-Lake Cochise during the Bolling-

Allerod interstadial and Younger Dryas stadial. The first two of these occurred at ~14.2 and

~13.9 ka, during the first part of the Bolling-Allerod, coinciding with the culmination of MWP1b and retreat/lowering of the southern Laurentide ice sheet. The ~13.1 ka highstand slightly preceded another at ~12.9 ka that was contemporaneous with H0 and the abrupt onset of stadial conditions in the North Atlantic region. Finally, a wet interval at ~10.9 cal kyr BP coincided with a peak in the rate of sea-level rise during MWP1b, as global CO2 levels approached pre- industrial levels. Termination of this wet episode marked the end of the glacial-interglacial transition and onset of Holocene aridity.

ACKNOWLEDGMENTS

Support for this work came in part through funding from the Comer Science and

Education Foundation, NSF-EAR 1103379 (Global Change) to JQ, USGS Award No.

G10AC00421 to AK and VTH, a Gladys Cole research grant awarded to VTH, the Argonaut

Archaeological Research Foundation, and graduate research scholarships awarded to AK from the H. Wesley Peirce Foundation, GSA, Sigma Xi, the UA College of Science, and Chevron-

Texaco. The Amerind Foundation provided logistical support. Special thanks to staff at the

Arizona AMS facility: Tim Jull assisted with sample costs, and Todd Lange, Rich Cruz, and Lori

Hewitt provided technical support. David Dettman conducted all isotope analyses, Jordon Bright performed ostracode identifications, and Art Vokes assisted with snail identifications. Jay Quade made comments and structural revisions through the Results section which, along with additional

comments from Vance Holliday and Andy Cohen, greatly improved the quality of this manuscript. Finally, Jim Grim and family made this study possible by granting unlimited access to the study sites.

REFERENCES

Adams, D.K. and Comrie, A.C., 1997. The North American Monsoon. Bulletin of the American Meteorological Society 78(10):2197-2213.

Alm, T., 1993. ØvreÆråsvatn—A Palynostratigraphy of a 22,000 to 10,000 BP Lacustrine Record on Andøya, Northern Norway. Boreas 22:171-188.

Antevs, E., 1938. Postpluvial Climatic Variations in the Southwestern Bulletin of the American Meteorological Society 19:190-193.

Asmerom, Y., Polyak, V.J., Burns, S.J., 2010. Variable Winter Moisture in the Southwestern United States Linked to Rapid Glacial Climate Shifts. Nature Geosciences 3:114-117.

Ballenger, J.A.M., Mabry, J.B., 2011. Temporal Frequency Distributions of Alluvium in the American Southwest: Taphonomic, Paleohydraulic, and Demographic Implications. Journal of Archaeological Science 38(6):1314-1325.

Bard, E., Rostek, F., Turon, J.-L., Sonzogni, C., 2000.Interhemispheric Synchrony of the Last Deglaciation Inferred from Alkenone Paleothermometry. Nature 385:707-710.

Barker, S., Diz P., Vautravers, M.J., Pike, J., Knorr, G., Hall, I.R., Broecker, W.S., 2009. Interhemispheric Atlantic Seesaw Response during the Last Deglaciation. Nature 457(26):1097- 1102.

Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, C.G., Robinson, S.W., Smith, G.I., Stine, S., 1990.Chronology of Expansion and Contraction of Four Great Basin Lake Systems during the Past 35,000 Years. Palaeogeography, Palaeoclimatology, Palaeoecology 78:241-286.

Benson, L.V., Lund, S.P., Burdett, J.W., Kashgarian, M., Rose, T.P., Smoot, J.P., Schwartz, M., 1998. Correlation of Late Pleistocene Lake-Level Oscillations in Mono Lake, California, with North Atlantic Climate Events: Quaternary Research 49:1-10.

Benson, L.V. and Mifflin, M.D., 1986. Reconnaissance Bathymetry of Basins Occupied by Pleistocene Lake Lahontan, Nevada and California. U.S. Geological Survey Water Resources Investigations Report 85-4262, 14 pp.

Benson, L.V. and Paillet, F.L., 1989. The Use of Total Lakesurface Area as an Indicator of Climatic Change: Examples from the Lahontan Basin. Quaternary Research 32:262-275.

Benson, L.V. and Thompson, R.S., 1987. Lake-Level Variation in the Lahontan Basin for the Past 50,000 Years. Quaternary Research 28:69-85.

Bequaert, J.C. and Miller, W.B., 1973. The Mollusks of the Arid Southwest, with an Arizona Check List: 271pp. University of Arizona Press, Tucson, AZ.

Bowen, D.Q., Phillips, F.M., McCabe, A.M., Knutz, P.C., Sykes, G.A., 2002. New Data for the Last Glacial Maximum in Great Britain and Ireland. Quaternary Science Reviews 21:89-101.

Broecker, W.S., 2009. The Mysterious 14C Decline. Radiocarbon 51(1):109-119.

Broecker, W.S., 1994. Massive Iceberg Discharges as Triggers for Global Climate Change. Science 372:421-424.

Broecker, W.S. and Barker, S., 2007. A 190 Per Mil Drop in the Atmopshere’s 14C during the “Mystery Interval” (17.5 to 14.5 Kyrs). Earth and Planetary Science Letters 256:90-99.

Brown, B. (Ed.), 1982. Desert Plants: Biotic Communities of the American Southwest-United States and Mexico. Boyce Thompson Southwestern Arboretum Special Issue 4 (no. 1-4). University of Arizona Press (Tucson).

Burch, J.B. 1982. Freshwater Snails (Molluska: Gastropoda) of North America. U.S. Environmental Protection Agency Contract No. 68-03-1280.

Clark, D.H. and Gillespie, A.R., 1997. Timing and Significance of Late Glacial and Holocene Glaciation in the Sierra Nevada, California. Quaternary International 38/39:21-38.

Clark, J., McCabe, M.A., Bowen, D.Q., Clark, P.U., 2012. Response of the Irish Ice Sheet to Abrupt Climate Change during the Last Deglaciation. Quaternary Science Reviews 35:100-115.

Colman, S.M., Keigwin, L.D., Forester, R.M., 1994. Two Episodes of Meltwater Influx from Lake Agassiz into the Lake Michigan Basin and their Climatic Contrasts. Geology 22:547-550.

Delorme, L.D., 1967. New Freshwater Ostracodes from Saskatchewan, Canada. Canadian Journal of Zoology 45:357-363.

Delorme, L.D., 1970. Freshwater Ostracodes of Canada. Part 3: Family Candonidae. Canadian Journal of Zoology 48:1099-1127.

Denton, G.H., Anderson, R.F., Toggweiler, J.R., Edwards, R.L., Schaefer, J.M., Putnam, A.E., 2010. The Last Glacial Termination. Science 328:1652-1656.

Denton, G.H., Broecker, W.S., Alley, R.B., 2006. The Mystery Interval 17.5 to 14.5 Kyrs Ago. PAGES News 14:14-16.

Dyke, A.S., Andrews, J.T., Clark, P.U., England, J.H., Miller, G.H., Shaw, J., Veillette, J.J., 2002.The Laurentide and Innuitian Ice Sheets during the Last Glacial Maximum. Quaternary Science Reviews 21:9-31.

Forester, R.M., 1983. Relationship of Two Lacustrine Ostracode Species to Solute Composition and Salinity: Implications for Paleohydrochemistry. Geology 11:435-438.

Forester, R.M., 1991. Ostracode Assemblages from Springs in the Western United States: Implications for Paleohydrology. In Williams, D.D. and Danks, H.V. (Eds.): Arthropods of Springs, with Particular Reference to Canada: Memoirs of the Entomological Society of Canada no. 155, pp. 181-201.

Furtos, N.C., 1933. The Ostracods of Ohio. Ohio Biological Survey Bulletin 29(5):411-524.

García, A.F. and Stokes, M., 2006. Late Pleistocene Highstand and Recession of a Small, High- Altitude Pluvial Lake, Jakes Valley, Central Great Basin, USA. Quaternary Research 65:179- 186. doi:10.1016/j.yqres.2005.08.025.

Gardner, P.M., and Heilweil, V.M., 2009, Evaluation of the Effects of Precipitation on Ground- Water Levels from Wells in Selected Alluvial Aquifers in Utah and Arizona, 1936–2005: U.S. Geological Survey Scientific Investigations Report 2008-5242, 28 pp. Available at http://pubs. er.usgs.gov/sir/2008/5242.

Gilbert, G.K., 1890. Lake Bonneville.U.S. Geological Survey Monograph 1.

Hanebuth, T., Stattegger, K., Bojanowski, A., 2009. Termination of the Last Glacial Maximum Sea-Level Lowstand: the Sunda Shelf Data Revisited. Global and Planetary Change 66:76-84. 100

Hawley, J.W., 1986. Environmental Geology of the Keers Environmental, Inc. Asbestos Disposal Site, Torrance County, New Mexico: New Mexico Bureau of Mines and Mineral Resources Open-File Report 245.

Haynes C.V. Jr., 1991. Geoachaeological and Paleohydrological Evidence for a Clovis-Age Drought in North America and its Bearing on Extinction. Quaternary Research 35:438-450.

Haynes, C.V., Long, A., and Jull, A.J.T., 1987. Radiocarbon Dates at Willcox Playa, Arizona, Bracket the Clovis Occupation Surface. Current Research in the Pleistocene 4:124-126.

Hearty, P.J., O’Leary, M.J., Kaufman, D.S., Page, M., Bright, J., 2004. Amino acid Geochronology of Individual Foraminifer (Pulleniatinaobliquiloculata) Tests, North Queensland Margin, Australia: a New Approach to Correlating and Dating Quaternary Tropical Marine Sediment Cores. Paleoceanography 19:PA4022.

Hill, M.E. and Holliday, V.T., 2011. Paleoindian and Later Occupations along Ancient Shorelines of the San Agustín Plains, New Mexico. Journal of Field Archaeology 36(1):3-20.

Holmgren, C.A., Penalba, M.C., Rylander, K.A., Betancourt, J.L., 2003. A 16,000 14C yr B.P. Packrat Midden Series from the USA-Mexico Borderlands. Quaternary Research 60:319-329.

Horne, F.R., 1993. Survival Strategy to Escape Desiccation in a Freshwater Ostracod. Crustaceana 65(1).

IntCal13 Radiocarbon Calibration Data Set, 2012: http://www.radiocarbon.org/IntCal13.htm

Kaufman, D.S., 2003. Amino Acid Paleothermometry of Quaternary Ostracodes from the Bonneville Basin, Utah. Quaternary Science Reviews 22:899-914.

Kaufman, D.S. and Manley, W.F., 1998. A New Procedure for Determining DL Amino Acid Ratios in Fossils Using Reverse Phase Liquid Chromatography. Quaternary Science Reviews 17(11):987-1000.

Kim, S.T. and O’Neil, J.R., 1997. Equilibrium and Nonequilibrium Oxygen Isotope Effects in Synthetic Carbonates. Geochimica et Cosmochimica Acta 61:3461-3475.

Kosnik, M.A. and Kaufman, D.S., 2008. Identifying Outliers and Assessing the Accuracy of Amino Acid Racemization Measurements for Geochronology: II. Data Screening. Quaternary Geochronology 3(4):328-341.

101

Kowler, A.L., 2013. Lake Shoreline Evidence of Hydrologic Conditions in the Southern Basin and Range Province During the Late Pleistocene and Early Holocene: Paleoclimatic and Archaeological Implications. In Goman, M. (Ed.): Human Environment Interactions–Vol. 2. Springer Briefs in Earth System Sciences.

Krider, P.R., 1998. Paleoclimatic Significance of Late Quaternary Lacustrine and Alluvial Stratigraphy, Animas Valley, New Mexico. Quaternary Research 50:283-289.

Kutzbach, J.E. and Wright Jr., H.E., 1985. Simulation of the Climate of 18,000 yr B.P.: Results for the North American/North Atlantic/European Sector and Comparison with the Geologic Record of North America. Quaternary Science Reviews 4:147-187.

Langer, W. H., Mulvihill, D. A., and Anderson, T. W., 1984, Maps showing ground-water levels, springs, and depth to ground water, Basin and Range province Arizona: U. S. Geological Survey Water Investigations Report 83-4114-B, scale 1:500,000.

14 Levin, I. and Kromer, B., 2004. The Tropospheric CO2 Level in Mid-Latitudes of the Northern Hemisphere (1959-2003). Radiocarbon 46(3):1261-1272.

Licciardi, J.M., 2001. Chronology of Latest Pleistocene Lake-Level Fluctuations in the Pluvial Lake Chewaucan Basin, Oregon, USA. Journal of Quaternary Science 16:545-554.

Long, A., 1966. Late Pleistocene and Recent Chronologies of Playa Lakes in Arizona and New Mexico. Unpublished Ph.D. Dissertation, University of Arizona, Tucson: 144 pp.

Martin, P.S., 1963. Geochronology of Pluvial Lake Cochise, Southern Arizona, II. Pollen Analysis of a 42-meter Core. Ecology 44:436-444.

Meinzer and Kelton, 1913. Geology and Water Resources of the Sulfur Springs Valley, Arizona: U.S. Geological Survey Water-Supply Paper 320, GPO, Washington, D.C.: 231 pp.

McGee, D., Quade, J., Edwards, R.L., Broecker,W.S., Cheng, H., Reiners, P.W., Evenson, N., 2012. Lacustrine Cave Carbonates: Novel Archives of Paleohydrologic Change in the Bonneville Basin (Utah, USA).Earth and Planetary Science Letters 351/352:182-194.

McManus, J.F., Francois, R., Gherardi, J.-M., Keigwin, L.D., Brown-Leger, S., 2004. Collapse and Rapid Resumption of Atlantic Meridional Circulation Linked to Deglacial Climate Changes. Nature 428:834-837.

Meinzer, O.E. and F.C. Kelton. 1913. Geology and Water Resources of Sulphur Spring Valley, Arizona. U.S. Geological Survey Water-Supply Paper 320, 231pp.

Mifflin, M.D. and Wheat, M.M., 1979. Pluvial Lakes and Estimated Pluvial Climates of Nevada. Nevada Bureau of Mines and Geology Bulletin No. 94, 57pp.

Munroe, J.S. and Laabs, B.J.C., 2013. Latest Pleistocene History of Pluvial Lake Franklin, 102

Northeastern Nevada, USA. Geological Society of America Bulletin 125:322-342.

Munroe, J.S. and Laabs, B.J.C., 2012. Temporal Correspondence between Pluvial Lake Highstands in the Southwestern US and Heinrich Event 1. Journal of Quaternary Science 28(1):49-58.

National Elevation Data Set: http://ned.usgs.gov

North American Commission on Stratigraphic Nomenclature, 2005.North American Stratigraphic Code. AAPG Bulletin 89(11):1547-1591.

Oram, Paul III, 1993, Maps showing ground-water conditions in the Willcox basin Graham and Cochise Counties, Arizona—1990: Arizona Department of Water Resources Hydrologic Map Series Report Number 25, 2 Sheets.

Oviatt, C.G., 1997. Lake Bonneville Fluctuations and Global Climate Change. Geology 25(2):155-158.

Oviatt, C.G., Currey, D.R., Sack, D., 1992. Radiocarbon chronology of Lake Bonneville, Eastern Great Basin, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 99:225-241.

Penkman, K.E.H., 2005. Amino Acid Geochronology: a Closed System Approach to Test and Refine the UK Model. Unpublished Ph.D. Thesis, University of Newcastle.

Penkman, K.E.H., Preece, R.C., Keen, D.H., Maddy, D., Schreve, D.S., Collins, M.J. 2007. Testing the Aminostratigraphy of Fluvial Archives: the Evidence from Intra-Crystalline Proteins within Freshwater Shells. Quaternary Science Reviews.

Penkman,K.E.H., Kaufman, D.S., Maddy, D., Collins, M.J., 2008.Closed-System Behaviour of the Intra-Crystalline Fraction of Amino Acids in Mollusc Shells. Quaternary Geochronology 3:2-25.

Pigati, J.S., Rech, J.A., Nekola, J.C., 2010. Radiocarbon Dating of Small Terrestrial Gastropod Shells in North America. Quaternary Geochronology 5:519-532.

Powers, W.E., 1939.Basin and Shore Features of Extinct Lake San Agustin, New Mexico. Journal of Geomorphology 2:345-356.

Quade, J., 1986. Late Quaternary Environmental Changes in the Upper Las Vegas Valley, Nevada. Quaternary Research 26(3):340-357.

Quade, J. and Broecker, W.S., 2009. Dryland hydrology in a Warmer World: Lessons from the Last Glacial Period. European Physical Journal Special Topics 176:21-36.

Quade, J., Forester, R.M., Whelan, J.F., 2003. Late Quaternary Paleohydrologic and Paleotemperature Change in Southern Nevada. Geological Society of America Special Paper 368:165-188.

Quade, J. and Pratt, W.L., 1989. Late Wisconsin Ground-Water Discharge Environments of the Southwestern Indian Springs Valley, Southern Nevada. Quaternary Research 31(3):351-370.

Quade, J., Forester, R.M., Pratt, W.L., Carter, C., 1998. Black mats, Springfed Streams, and Late-Glacial-Age Recharge in the Southern Great Basin. Quaternary Research 49:129-148.

Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Johnsen, S.J., Larsen, L.B., Dahl-Jensen, D., Bigler, M., Röthlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E., Ruth, U., 2006. A New Greenland Ice Core Chronology for the Last Glacial Termination. Journal of Geophysical Research 111:6102.

Rech, J.A., Pigati, J.S., Lehmann, S.B., McGimpsey, C.N., Grimley, D.A., Nekola, J.C., 2011. Assessing Open-System Behavior of 14C in Terrestrial Gastropod Shells. Radiocarbon 53(2):325- 335.

Reimer, P.J., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Cheng H., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Haflidason, H., Hajdas, I., Hatté, C., Heaton, T.J., Hoffmann, D.L., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., Manning, S.W., Niu, M., Reimer, R.W., Richards, D.A., Scott, E.M., Southon, J.R., Staff, R.A., Turney, C.S.M., van der Plicht J., 2013. IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0–50,000 Years Cal BP. Radiocarbon 55(4):1869-1887.

Rinterknecht, V.R., Clark, P.U., Raisbeck, M., Yiou, F., Bitnas, A., Brook, E.J., Marks, L.,

104

Zelcs, V., Lunkka, J.-P., Pavlovskaya, I.E., Piotrowski, J.A., Raukas, A., 2006. The Last Deglaciation of the Southeastern Sector of the Scandinavian Ice Sheet. Science 311:1449-1452.

Robinson, R.C., 1965. Sedimentology of Beach Ridge and Nearshore Deposits, Pluvial Lake Cochise, Southeastern Arizona. Unpublished M.S. Thesis, 111pp. University of Arizona, Tucson.

Russell, I.C., 1889. Quaternary History of the Mono Valley, California. 8th Annual Report of the U.S. Geological Survey, p. 267-394.

Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., and Soil Survey Staff, 2012.Field Book for Describing and Sampling Soils, Version 3.0. Natural Resources Conservation Service (U.S. Department of Agriculture). National Soil Survey Center, Lincoln, NE.

Schreiber Jr., J.R., Pine, G.L., Pipkin, B.W., Robinson, R.C., Wilt, J.C., 1972.Sedimentologic Studies in the Willcox Playa Area, Cochise County, Arizona. In: Reeves Jr., C. (Ed.): Playa Lake Symposium, p.133-184. Texas Tech University, Lubbock.

Stager, J.C., Ryves, D.B., Chase, B.M., F.S.R. Pausata, 2011. Catastrophic Drought in the Afro- Asian Monsoon Region During Heinrich Event 1. Science 331:1229-1301.

Stanford, J.D., Rohling, E.J., Bacon, S., Roberts, A.P.,Grousset, F.E.,Bolshawa, M., 2011a. A New Concept for the Paleoceanographic Evolution of Heinrich Event 1 in the North Atlantic. Quaternary Science Reviews 30:1047-1066.

Staplin, F.L., 1963. Pleistocene Ostracoda of Illinois. Part I. Subfamilies Candoninae, Cyprinae, General Ecology, Morphology. Journal of Paleontology 37:1164-1203.

Stuiver, M., Reimer, P.J., Reimer, R.W., 2012. Calib 7.0 Radiocarbon Calibration Program: http://calib.qub.ac.uk/calib

Tessler, W.L., 1954. Fresh-water Ostracoda from Texas and Mexico. Journal of the Washington Academy of Science 44:138-149.

Von Grafenstein, U., Erlenkeuser, H., Trimborn, P., 1999. Oxygen and Carbon Isotopes in Modern Fresh-Water Ostracod Valves: Assessing Vital Offsets and Autecological Effects of Interest for Palaeoclimate Studies. Palaeogeography, Palaeoclimatology, Palaeoecology 148:133-152.

Waters, M.R., 1989. Late Quaternary Lacustrine History and Paleoclimatic Significance of Pluvial Lake Cochise, Southeastern Arizona. Quaternary Research 32(1):1-11.

Wehmiller, J.F., and Miller, G.H., 2000. Aminostratigraphic Dating Methods in Quaternary Geology. In Noller, J.S., Sowers, J.M., Lettis, W.R. (Eds.): Quaternary Geochronology: Methods and Applications, p. 187-222. American Geophysical Union, Washington D.C.

Western Regional Climate Center: www.wrcc.dri.edu/climatedata.html

Xia, J., Ito, E., Engstrom, D.R., 1997. Geochemistry of Ostracode Calcite, Part 1: An Experimental Determination of Oxygen Isotope Fractionation: Geochimica et Cosmochimica Acta 61:377-382.

106

TABLE CAPTIONS

Table 1. Description of stratigraphic units and constituent facies and inclusions, following standardized terminology. For Munsell Colors, d = dry and m = moist; abbreviations for textures and pedogenic/sedimentologic features, follow Schoeneberger et al. (2012). Unit numbers denote Information in bold describes the fabric of each unit, including individual facies; other descriptions apply to inclusions or sedimentological/pedogenic features.

Table 2. Radiocarbon dates that constrain the age of Groups 1-3 in the Locality A succession are listed. Uncalibrated radiocarbon ages are expressed as the mean and 1σ uncertainty, abbreviated as “14C yr BP”; calibrated radiocarbon dates are expressed as the 2σ range and median probability of possible calendar year ages (before 1950, “present”), abbreviated as “cal yr BP”. Calibrations were generated using Calib 7.0.

Table 3. Faunal assemblages in stratigraphic units from Groups 1-3, including abundances of ostracode taxa and presence/absence of gastropod taxa. For gastropod counts, “F” signifies presence of fragments in the absence of intact shells, and “Tr” signifies trace abundance. For gastropod habitat categories, “T” = terrestrial, “S-A” = semi-aquatic, and “A” = aquatic. Sample numbers correspond to bulk samples depicted in Fig. 4.

FIGURE CAPTIONS

Figure 1. Map of the western U.S. with many of the studied pluvial lakes (deglacial highstand areas are shown in blue), and paleoenvironmental records (open circles).

Figure 2 (after Martin, 1963). Diagram of Willcox basin with regional map inset. Dashed red line delineates the hydrologic divide. The yellowish-tan area represents Willcox Playa, and the area covered by paleo-Lake Cochise at 1,274 m (solid red lines) is colored blue. Locations of stratigraphic profiles shown in Figure 4 are indicated by bold black line segments intersecting the western beach ridge.

Figure 3 (after Waters, 1989). General chronostratigraphy of the 1,274 m beach ridge at Locality B.

Figure 4. Stratigraphic cross-section depicting intercalated shoreline and spring-related deposits exposed in trenches along a transect perpendicular to the shoreline berm at Locality C. Dates shown are the median calibrated 14C ages (Table 2); color-coded boxes denote the depositional environment inferred from ostracode assemblages.

Figure 5. Photos of stratigraphy exposed in the Locality C trenches, showing dated units as follows: (a.) T1, west-facing view. (b.) T2(E), east-facing view. (c.) Southern wall of T2(W), close-up of Units 13 and 15. (d.) Southern wall of T2(W), Units 13, 15, and 21. (e.) Eastern wall of T0, close-up of Units 7,14, and 22.

Figure 6. Stable isotope (18O) plots for biogenic carbonate (fossil gastropods and ostracodes) and cements. Values from C. patzcuaro and C. truncata do not reflect a 1-2‰ vital effect enrichment.

107

Figure 7. Hydrograph of paleo-Lake Cochise. Green lines represent ground-water stands; solid lines indicate confidence in the timing and continuity of a given lake stand; dashed lines indicate uncertainty about the continuity of the paleohydrologic event; blue indicates uncertainty in the lake- level reconstruction whereas red indicates certainty; and dashed vertical lines indicate uncertainty about the depositional environment (i.e., lake vs. spring), allowing for the possibility of a lake, wetlands, or both at/below 1,274 m. The stratigraphic unit corresponding to each paleohydrologic event is labelled above it. Lake-level is based on the highest elevation from which samples of a given age were recovered; for graphical purposes, dates are not plotted according to elevation.

Figure 8. Regional paleoenvironmental stack, including (d.) 18O time series from speleothems in Cave-of-the-Bells (purple) and Ft. Stanton (orange), and hydrographs of paleo-Lakes Cochise (c.), Estancia (b.), Lahontan (a.). Between (c.) and (d.): deposition of the Coro Marl, from ground-water discharge at two well-dated sites in the San Pedro Valley, is indicated by solid green lines.

108

Table 1

Unit Color (dry) Section Zone Structure Texture Modifier Feature Lower Contact Consistence (dry)

UA-1 white (5Y 8/1) T6,T7 all VFi Ma GrV-S marly - UA-2 white (2.5Y 8/1) T5 upr VFi 2CoPr marl clayey va,w UA-2 olive (5Y 5/3) T5 lwr VFi 2CoPr C marly c UA-2 white (2.5Y 6/2) T6,T7 all VFi Ma marl 3CAM (rhizolith) c UA-2 olive (5Y 5/3) T6,T7 all H 3FPr C lenses, balls UA-3 white (10YR 8/1) T6,T7 all VFr 1FPl1VFSBK marl sandy c UA-3 white (2.5Y 8/1) T4,T5 all VFi Ma marl va 1 olive (5Y 5/3) T0,T1,T2 all VFi 3 Co Pr C - lt yellowish brown well-sorted, qtz- T0,T1,T2 all H Ma S va,w 2 (2.5Y 6/4) dominated

olive gray (5Y 5/2) calcareous, well- 3FeM lining root T0,T1,T2 lwr Fi 3 VCo Pr S va/g,w 3 to pale olive (5Y 6/3) sorted, qtz-dominated channels paleo olive (5Y 6/3) T0,T1,T2 upr H - FSC marly 3 olive (5Y 5/3) T0,T1,T2 lenses H - C 3 incl 3/4 white (10YR 8/1) T0,T1,T2 (top H Ma C marly 10cm) 3FeM lining root lt brownish gray channels; T0,T1,T2 all VFi 3 VCo Pr C calcareous d,w 4 (2.5Y 6/2) 3CaM (rhizolith)

4 “ T0,T1,T2 top H - - 3CaN

109

Lower Contact Consistence (Dry)

Unit Color (dry) Section Zone Structure Texture Modifier Feature dark grayish brown top 4 T0 - - - 2MnM (10YR 4/2) 20cm black (10YR 2/1) to lwr 4 very dark brown T1, T2 H - C MnM, FeM(3,ig) d,w 20cm (10YR 2/2) basal white (10YR 8/1) T0,T1,T2 H Ma C marl 4/5 30cm lt brownish gray T0,T1,T2 H 3 VCo,FPr SiC calcareous d,s 5 (2.5Y 6/2) 5 “ T0,T1,T2 incl Fi stratified C <4cm-thick beds lt brownish gray 3 Co Pr  T1 all VFi C c,ig 6 (2.5Y 6/2) 3 M Pl 35-50% Gr (subround) grayish brown 2 M SBK/ 3CaM (root); CaC T0,T1,T2 all SH GrV-C including rip-up clasts va,ig 7 (2.5Y 5/2) 3 M Pl (clasts) and CaN “ T1 basal - stratified S, Si, C lenses 7

7 lt olive gray (5Y 6/2) T0 all Fr 3 VF Pl C calcareous 50-75% CaM (root) 7 “ T1 incl - stratified Gr-CoS white marl; 5% FGr T5,T6 all MH 2 VCo SBK C va,ig (subround) 8 (2.5Y 8/2)

pale brown T3,T4,T5, 5-10% FGr; 1-5% all SH 3 F ABK C c,w T6 CoGr 9 (10YR 6/3)

T3,T4,T5, incl - stratified CoS, Gr lenses 9 T6

pale olive well-sorted; ostracode T2,T3 all Lo Ma SL coquina comprises 3 VF CaN va,w 10A (5Y 6/3) sand fraction

110

Table 1 (cont’d)

Unit Color (dry) Section Zone Consistence (dry) Structure Texture Feature Lower Contact Modifier pale olive 3-5cm-thick beds of T2,T3 all Lo - FS c,w ostracode valves 10B (7.5 YR 6/3)

white T0 all indurated Ma Gr va,w 11 (5YR 8/1) lt yellowish brown T1,T4,T5, all Lo 3 Co Pl VCoS well-sorted 2CaM (rhizolith) a,s 12 (2.5Y 6/4) T6 13 variegated T1,T2,T3 all Lo stratified S, Gr SW-dipping beds va,ig basal 13 - T1,T2,T3 - - - carb clast coatings - 20cm pale olive weakly calcareous, poorly- T1 all Ma SL g,ig 14 (5Y 6/3) indurated sorted unimbricated clasts, 14 “ T1 middle - - Gr-SL - discontin carb coatings lt yellowish brown well-sorted, qtz/feldsp- T2,T3 all Lo stratified S c,s 15A (2.5Y 6/4) dominated Lt brown T2,T3,T5, calcareous, well-sorted, all indurated Ma S g,s 15B (7.5YR 6/4) T6 qtz/feldsp-dominated lt yellowish brown T1 all SH 1 F,M Pr S poorly-sorted va,w 16 (2.5Y 6/4) lt brownish gray T1 all Fr 3 F,M ABK FSL calcareous va,w 17 (2.5Y 6/2) white T2,T3 all indurated Ma GrX-S carb-cemented laminar cap va,ig 18 (5Y 8/1) lt brownish gray T7 all So Ma VFSL calcareous va,ig 19 (10YR 6/2) lt brownish gray weakly T7,SP all So FSL very porous, low D c,ig 20 (10YR 6/2) indurated b

111

Table 1 (cont’d)

Unit Color (dry) Section Zone Consistence (dry) Structure Texture Modifier Feature Lower Contact

lwr 20 “ T7,SP SH - - - 40cm

calcareous; clasts lt grayish brown FGr- T0,T1 all Fr Ma reworked from c,s 21 (10YR 6/2) LFS underlying units OM-rich; common T0,T1,T2, dark brown burrows (hollow and T3,T4,T5, all Fr Ma Gr-FSL d,ig 22 (10YR 4/3) infilled); Gr derived T6 from underlying units

112

Table 2 113

Table 3

114

115

Figure 2 (after Martin, 1963)

116

Locality B stratigraphy

(after Waters, 1989) Figure 3

117

118

Figure 5

119

Ostracodes Gastropods Carbonates

L. ceriotuberosa H. glaucus C. patzcuaro various taxa Succineidae Physidae cement 21 21 21 21 21 21 21 20 20 20 20 20 20 20 19 19 19 19 19 19 19 18 18 18 18 18 18 18 17 17 17 17 17 17 17 16 16 16 16 16 16 16 15 15 15 15 15 15 15 14 14 14 14 14 14 14 13 13 13 13 13 13 13 12 12 12 12 12 12 12 11 11 11 11 11 11 11 10 10 10 10 10 10 10

Unit # Unit 9 9 9 9 9 9 9 8 8 8 8 8 8 8 7 7 7 7 7 7 7 6 6 6 6 6 6 6 5 5 5 5 5 5 5 4 4 4 4 4 4 4 3 3 3 3 L. sappaensis 3 3 3 2 2 2 2 C. truncata 2 2 2 Potamocypris 1 1 1 1 1 1 1 0 0 0 0 0 0 0 -10-8 -6 -4 -2 0 2 4 -10-8 -6 -4 -2 0 2 4 -10-8 -6 -4 -2 0 2 4 -10-8 -6 -4 -2 0 2 4 -10-8 -6 -4 -2 0 2 4 -10-8 -6 -4 -2 0 2 4 -10-8 -6 -4 -2 0 2 4 18  O (‰ VPDB)

Figure 6

120

Figure 7

121

(m) Level Lake

3 10 Calendar Years Before Present

Figure 8

122

APPENDIX C: RADIOCARBON DATING OF CHARCOAL FROM THE EL FIN DEL MUNDO 14 CLOVIS SITE IN NORTHERN SONORA, MEXICO AND IMPLICATIONS FOR AMS C MEASUREMENT IN SMALL SAMPLES

ABSTRACT

Radiocarbon dating of charcoal from the El Fin del Mundo Clovis site, recently discovered in the Sonoran desert of northwestern Mexico, reveals that the site was occupied at

11,560±140 14C yr BP, and possibly earlier. Barring old-wood effects, this result renders the newly discovered occupation contemporaneous with that of the Aubrey Clovis site, ~1,500 km

E-NE in northern Texas—previously considered the oldest Clovis site on the basis of two charcoal A-fraction dates of 11,540±110 and 11,590±90 14C yr BP. In light of small sample masses, special measures were taken to ensure that the Clovis site was dated accurately. These include (1) simultaneous processing of blanks of about the same mass as the samples, enabling independent assessment of background contamination—usually estimated on the basis of the long-term blank average—and (2) paired-fraction dating, allowing us to ascertain the effects of natural contamination under the unlikely scenario that the standard A-B-A pretreatment regimen was not fully effective. Analyses on the mass-specific blanks confirm the accuracy of the reported 14C measurements, but also raise the possibility of slight age overestimation by 60 14C yr (<1σ), whereas paired-fraction results would make the reported date a minimum age. Finally, the reported charcoal dates reflect a correction for accelerator-induced fractionation, known to cause significant and systematic overestimation of 14C age and analytical precision in samples containing less than ~0.2 mgC. Examination of this effect on the apparent age of our charcoal samples constrains age overestimation to <1σ (<100 14C yr); however, concomitant underestimation of error can cause underestimation of the calibrated age range by much larger amounts. Charcoal samples from both Clovis sites were processed by the Arizona AMS Facility,

123

which first recognized and instituted a correction for this effect subsequent to dating of the

Aubrey samples. Therefore, 14C dates from Aubrey and other Clovis sites should be recalculated for samples where the carbon yields were small and therefore potentially affected by accelerator- induced fractionation.

INTRODUCTION

There is general consensus among archaeologists that the Clovis archaeological horizon represents the earliest unambiguously documented techno-complex and human occupation in

North America south of the late-glacial continental ice sheets; however, uncertainty lingers over when the Clovis “culture” emerged, a matter that is linked to understanding the mechanism(s), rate, and geographic pattern of its expansion (Haynes, 2002; Haynes et al., 2007; Waters and

Stafford, 2007; Buchanan and Collard, 2009; Fiedel, 2010; Jennings and Waters, 2014). The majority of Clovis sites date to 11,200-10,700 14C years before present (14C yr BP), equivalent to

13,200-12,700 calendar (cal) yr BP, and two sites provide convincing evidence for significantly earlier occupations. Charcoal samples from both early sites were processed by the Arizona AMS

Facility. First, the Aubrey site in north Texas (Ferring, 2001) yielded charcoal dates of

11,540±110 (AA-5271) and 11,590±90 14C yr BP (AA-5274); the other, the El Fin del Mundo

Clovis site, ~1,500 km W-SW of Aubrey, yielded a single charcoal date of 11,560±140 14C yr

BP (AA-100181A) (Sanchez et al., 2014). Efforts to interpret the significance of these important dates would benefit from a thorough evaluation of the procedures used to obtain them, including sample processing, analysis, and corrections employed in the final age formulation. Here we limit our evaluation to the dating of charcoal at El Fin del Mundo.

124

SITE DESCRIPTION

El Fin del Mundo is situated in an intermontane basin within a chain of volcanic hills in the Sonoran desert of northern Sonora, Mexico, ∼100 km northwest of the capital city of

Hermosillo (Fig. 1). The discovery of Clovis artifacts and Proboscidean remains, following their recent exhumation from the sidewalls of an erosional remnant (Locality 1) within a dry tributary of the Rio Bacoachi, prompted excavations and surveys from 2007-2012. That work revealed buried Clovis artifacts associated with the remains of two Gomphotheres, and Clovis encampments on the surrounding uplands (Sanchez et al., 2014). At Locality 1, five specimens of charcoal were recovered among stone flakes and burned bone fragments within the Clovis level (Stratum 3B) (Fig. 2). Each specimen was comprised of several splinters, ranging from

~0.3-1 mm in diameter and ~1-3 mm in length.

The detailed stratigraphy presented in Sanchez et al. (2014) is summarized as follows.

Erosional remnants, including the Locality 1 sediment bloc (prior to excavation), were created by fluvial incision into the distal portion of a bajada and underlying basin fill. The Gomphothere bone bed, artifacts, and associated strata were only found at Locality 1, where bedrock was buried by three strata (Strata 2-4, bottom to top) (Fig. 2); Strata 3 and 4 were emplaced within a

<100 m-wide channel eroded into the upper ~1 m of Stratum 2, atop of which lay a Mastodon bone bed (Fig. 2). Where exposed, Stratum 2 is a <3 m-thick pebbly sandy clay fining upward into a sandy clay, locally buried by late Pleistocene- and Holocene-age ground-water calcrete.

Stratum 3A is a <1 m-thick, massive, poorly-sorted pebbly sandy clay overlain by Stratum 3B, a poorly-sorted sandy clay. The basal portion of Stratum 4, a <10 cm-thick, discontinuous diatomite, lay unconformably on Stratum 3B. Lastly, Stratum 4 is composed of a <1m-thick greyish brown silty diatomaceous earth, carbonate-cemented in the upper 20 cm. The

125

Gomphothere bones and most artifacts were randomly oriented within the upper 15 cm of

Stratum 3B. This suggests that they were emplaced during initial deposition and subsequently mixed prior to desiccation of the soft sediment. The mixing could have been the result of trampling by humans/animals and/or translocation through cracks prior to burial. Maximum constraint of the age of Stratum 3B, and the stone flakes and bone fragments within it, are provided by radiocarbon dating of intermingled charcoal flecks (Fig. 3).

RADIOCARBON SYSTEMATICS

Background Levels and Process Blanks

Laboratory contamination can occur during wet chemical pretreatment via introduction of solid phase contaminants of sufficient mass and age relative to the sample, although strict measures are taken to minimize this. Additional contamination is introduced during cryogenic purification (“extraction”) of CO2 following sample combustion, when the sample comes into contact with extraneous C—accounting for part of the measured 14C content of a given sample.

The effect of this contamination on a sample’s apparent age depends upon the mass and 14C content of contaminant C relative to the mass and age of the sample. During extraction, both modern and “radiocarbon-dead” contamination is introduced through admixture of CO2 liberated from the sample, with that originating from several possible sources, such as (1) atmosphere, (2) diffusion pump oil, (3) previously extracted samples, adsorbed onto various parts of the line, and

(4) off-gassing of petrochemical-derived O-rings and valves. The Arizona AMS Facility quantifies background levels, expressed as the fraction of modern carbon (fMC) (Donahue et al.,

1990; Burr and Jull, 2010), defined as follows:

126

14 13 ( C/ C)blank (1) fMCblank = 14 13 = ( C/ C)1950 [13C = -25‰]

Specifically, fMCblank is the long-term weighted average of process-blank measurements—as

defined by Donahue et al. (1990), McNichol et al. (2001), and Burr and Jull (2010)—henceforth

referred to as the uncorrected long-term blank average (ULBA). Background levels within the

accelerator are assessed independently with unprocessed graphite “machine” blanks, but are

negligible under normal circumstances.

Small-Sample Effect

The above considerations suffice for assessing background levels in samples ≥1 mgC,

while additional uncertainties impact smaller samples. These uncertainties are collectively

referred to as the “small-sample effect”.

Background levels

The influence of background levels on the 14C content of a sample is inversely

proportional to its mass, causing disproportionate age underestimation and increased scatter in

samples <1 mg, and significantly impacting fMC measurements on samples <0.5 mgC. To

account for this effect, an empirical blank correction can be determined from a suite of analyses

on blanks of different sizes (e.g., Brown and Southon, 1997; McNichol et al., 2001). At the

Arizona AMS Facility, the following relationship is employed to adjust the long-term weighted

fMCblank average as a function of sample mass (m), resulting in the mass-corrected long-term

blank average (CLBA) (Donahue et al., 1990):

fMCblank (1mgC) (2) fMCblank (m) = m

127

Machine-Induced Fractionation (MIF)

Various laboratories have reported fractionation in small samples below 0.2 mgC, resulting in significantly depressed fMC values (e.g., Klinedinst et al., 1994; Brown and

Southon, 1997; Pearson et al., 1998; Santos et al., 2007). This fractionation has been attributed to reduced and variable ion source efficiency, affecting the influence of beam current intensity on the iron catalyst—which is bound with sample C in its reduced form, iron carbide

(“graphite”). In contrast to background contamination, MIF in small samples causes systematic age overestimation and increased age scatter that are inversely proportional to sample mass.

Since blank measurements do not reflect fractionation, analyses of standards of different masses are needed to quantify MIF. At the Arizona AMS Facility, fMC measurements are adjusted using a long-established correction factor (y) for a sample of given mass (x), expressed in mgC.

The correction factor is defined by the following equation, accompanied by an increase in 1σ uncertainty amounting to ±30% of the absolute correction:

(3) y = 0.0104x2 - 0.0215x + 1.0109

METHODS

Charcoal Identification and Subsampling

Five specimens of charcoal were gathered from the upper 15cm of Stratum 3B, found in association with Clovis artifacts and Gomphothere bones (Fig. 2). Examination under binocular microscope revealed preservation of woody tissue structure in only two of these specimens, but their small dimensions (diameter ~0.5 mm, length ~1 mm) precluded taxonomic identification.

Dating efforts were focused on specimens unequivocally identified as charcoal—also the only specimens large enough to produce reliable 14C measurements—FdM#63176 (AA-100182) and

128

FdM#63447 (AA-100181), shown in Fig. 2A and 2B, respectively. We used a stainless steel probe to separate sub-millimeter size charcoal splinters from the enclosing sediments under binocular microscope, resulting in ~1.2 mg per specimen. Samples were subsequently handled at the Arizona AMS Facility per the small-sample protocol documented herein.

Radiocarbon Processing

Both charcoal specimens were pretreated in combustion tubes, and subsequent weighing was avoided so as to minimize sample loss or introduction of particulate contaminants prior to combustion. Samples were pretreated in accordance with a standard A-B-A protocol. The first acid (10% HCl) immersion removes inorganic carbon (bound in carbonate salts) and acid-soluble organic molecules. Immersion in base (2% NaOH) produces a suspension of humic acid (B- fraction) that is subsequently decanted from the treated charcoal (A-fraction), recovered by flocculation upon final acidification with HCl. A final acid immersion prevents uptake of atmospheric CO2 following pretreatment. Following the final HCl immersion, A-fractions were rinsed in deionized water and both fractions dried in vacuo in preparation for combustion.

FdM#63447 (AA-100181) yielded both A- and B-fractions; FdM#63176 (AA-100182) was

100% base-soluble and thus yielded only B-fraction.

Following combustion, CO2 extractions were performed on one the “cleanest” standard vacuum line (i.e., that exhibiting the lowest ULBA) in the facility following CO2 dilution (30 min) and subsequent pump-down (24-48 hr). Three radiocarbon-dead charcoal standards (Rio

Frio Ash), henceforth referred to as “blanks”, were processed prior to the unknowns. This allowed us to independently constrain background levels and ascertain validity of the CLBA for use in the blank correction. For each unknown, we processed at least one small blank of about

129

13 14 the same mass.  C measurements were performed on the same CO2 aliquots targeted for C analysis to avoid unnecessary reduction in sample masses, and resultant decrease in precision.

Age Formulation

Formulae used to calculate 14C ages and errors are described in Burr and Jull (2010), with the addition of the MIF correction (Eq. 3). All dates reported by the Arizona AMS Facility reflect the small-sample corrections as previously described. However, because customers who perform extraction and graphitization on their own equipment must independently assess background levels—and because some AMS labs do not correct for MIF, have not published their correction, or do not report MIF-corrected measurements unless requested—we examine the influence of MIF, along with various blank corrections, on charcoal dates from El Fin del

Mundo.

First, all of the correction schemes examined employ a blank correction based on the (1)

CLBA, (2) ULBA, or (3) small-blank average (SBA), which is calculated from the average of two blanks having masses similar to, and bracketing, that of the unknown. Second, the dates calculated with these blank corrections are compared to their MIF-corrected counterparts, resulting in six corrections schemes, including that normally utilized by the Arizona AMS

Facility. Finally, we used the Calib 7.0 software (Stuiver et al., 2012), based on the IntCal 2013

Data Set (2012) reported in Reimer et al. (2013), to calibrate 14C dates to the 2σ range and median probability of possible calendar year ages (Table 2).

RESULTS

130

Carbon yields and fMC measurements from unknowns and small blanks, and the SBA value determined from the weighted average of the small blanks (Table 1), were used to calculate the date reported by the Arizona AMS facility in addition to those calculated using the

SBA correction scheme (with and without the MIF correction) (Table 2, Fig. 4). Calculations for

FdM#63447 (AA-100181A) range from 11,385-11,610 14C yr BP, with 1σ errors ranging from

±60 to ±140 14C yr; the official date, reported by the laboratory, is 11,560±140 14C yr BP, and the date reported for the corresponding B-fraction (AA-100181B) is 10,650±250 14C yr BP. All of the small blanks were larger than AA-100181B (0.1 mgC), precluding calculation of the SBA.

The date reported date for the other charcoal sample (AA-100182B), also a B-fraction, is

11,880±200 14C yr BP; using the SBA in place of the CLBA results in a date of 11,900±360 14C yr BP. Fig. 4 shows the results of modeling corrected fMC values (Fig. 4A) and apparent ages

(Fig. 4B) for material of known age (11,560 14C yr BP) at different hypothetical sample masses

(<0.9 mgC), using each of the aforementioned correction schemes. Blank corrections are derived either from small blanks (Table 1) or the reported ULBA value of 0.0025±0.0005 (Todd Lange, personal communication).

DISCUSSION

Correction Schemes and Associated Error Propagation

The largest potential cause of inaccuracy in dating of small samples is failure to account for the effect of sample size on fMCblank, potentially resulting in age underestimation beyond the lower limit of the 1σ error envelope (Fig. 4B). This correction can be made either by calculating the CLBA from the ULBA and sample mass, or empirically deriving the relationship between blank mass and fMCblank. For example, use of the ULBA in place of the CLBA to calculate the

131

age of AA-100181A results in underestimation of its age and 1σ error by 220 and ±40 14C yr, respectively. Next, because we did not process modern standards with masses similar to the unknowns (in the same accelerator wheel), we rely exclusively on the long-established MIF correction (Eq. 3); failure to apply this adjustment to the CLBA-corrected fMC value measured from AA-100181A results in overestimation of its mean age by 50 14C yr, and underestimation of its 1σ error by ±15 14C yr (Table 2).

Next, we used small blanks to assess the effect of using CLBA for estimation of fMCblank.

Although the SBA does not provide a statistically robust estimate of fMCblank, it can be used to validate the CLBA. Further, it can also provide insight into the amount and variability of background contamination affecting a batch of similar-age unknowns. For example, use of the

SBA in place of the CLBA to calculate the age of AA-100181A results in a reduction in mean age and 1σ error by 60 and ±15 14C yr, respectively; both calculations overlap at 1σ and are therefore statistically indistinguishable, validating the CLBA. Similarly, utilization of either the

CLBA or SBA to calculate the age of AA-100182B results in dates that are statistically indistinguishable, although in this case the former produces a 1σ error much smaller than the latter by ±160 14C yr. This discrepancy stems from the disparity between blanks used to calculate the SBA, much greater in the case of AA-100182B than for AA-100181A. This is likely due to the significantly smaller size of the lower limiting blank used to calculate the SBA for AA-100182B. However, this assessment is based on measurements from two blanks that were extracted several months apart, and is therefore conjectural.

It is noteworthy that fMCblank measured from V21568 (0.16 mgC) is >1σ less than that from the significantly larger sample V21567 (0.23 mgC), and that the CLBA exceeds both measurements by >1σ. Thus, not only is the SBA lower than the CLBA, but the observed

132

relationship between fMC and sample mass is opposite from expected. This suggests, but does not prove, that background contamination levels were lower and less variable than reflected by the long-term average. Moreover, overestimation of fMCblank by the CLBA for a given extraction batch is consistent with the unusually rigorous procedure we followed in preparing the extraction line. Our results underscore the necessity of processing unknowns in a fashion identical to the blanks incorporated into the CLBA, thereby avoiding systematic overestimation of fMCblank and therefore sample age.

In conclusion, the date reported for AA-100181A is accurate at 1σ, although the abnormally rigorous procedure we used to clean the extraction line may have resulted in slight age overestimation for the dated samples. This would render the single reliable charcoal date from El Fin del Mundo slightly younger than those reported for the Aubrey charcoal samples.

However, the Aubrey samples were processed at the Arizona AMS Facility prior to institution of the MIF correction, potentially rendering their dates overestimated and errors underestimated.

Thus, a direct comparison of charcoal dates from the El Fin del Mundo and Aubrey Clovis sites requires knowledge of the C yields, and B-fraction ages, for the Aubrey charcoal samples. Small masses would render the Aubrey dates and precisions overestimated.

Inter-comparison of Results from Different Charcoal Samples and Fractions

Using the dates reported by the Arizona AMS Facility, we interpret the significance of the 14C ages with respect to the age of associated artifacts and bones in upper Stratum 3B. The

B-fraction extracted from charcoal is comprised of an unknown proportion of indigenous to contaminant humic acids, the latter potentially influencing the 14C content of the A-fraction if not completely removed. Under ideal circumstances, paired dating of the A- and B-fraction allows

133

one to assess the relative age of natural contamination in charcoal. If both fractions are equivalent in 14C content, then any trace quantities of humic acid remaining in the charcoal following pretreatment would have no influence on the 14C content of the pretreated A-fraction.

On the other hand, concern arises if there is a discrepancy between the 14C contents of both fractions.

As previously documented, one of the two charcoal samples (FdM#63447/AA-100181) yielded both an A- and B-fraction, and the other (FdM#63176/AA-100182) yielded only a B- fraction. For FdM#63447, the A-fraction (AA-100181A: 11,560±140 14C yr BP) produced a mean age older than the B-fraction (AA-100181B: 10,650±250 14C yr BP). Both fractions produced mean ages younger than the B-fraction of FdM#63176 (AA-100182B: 11,880±200 14C yr BP), although the dates from AA-100182B and AA-100181A overlap at 1σ. Under the unlikely circumstance that the pretreatment regimen was ineffective, paired-fraction dating of

FdM#63447 would render the A-fraction date of 11,560±140 14C yr BP a minimum age.

Unfortunately, it is impossible to evaluate the reliability of the date from charcoal sample

FdM#63176, since it was 100% base-soluble—resulting in a B-fraction (AA-100182B) containing both the charcoal and contaminants. Although this date overlaps at 1σ with that reported for AA-100181A—the only reliable date from El Fin del Mundo—it might reflect an earlier occupation and/or “old-wood” effect. However, the relatively large error and additional uncertainties associated with the small size and possible contamination of AA-100182B preclude us from resolving this. Finally, the date from AA-100181A might also reflect an old-wood effect, in which case the associated Clovis occupation occurred later than 11,560±140 14C yr BP.

CONCLUSIONS

134

Use of similar-size blanks enables us to confirm the accuracy of 14C measurements on charcoal fragments from the El Fin del Mundo Clovis site, and paired-fraction enables us to confirm the antiquity of charcoal sample FdM#63447 (AA-100181A). Current efforts to improve constrains on the small-sample effect at the Arizona AMS Facility will hopefully result in the improvement of precision for 14C measurements in small samples. This, in turn, would enable archaeologists to place tighter constraints on the emergence of Clovis, as additional sites are discovered and dated in the future. In turn, this would contribute to ongoing efforts to explain the appearance and spread of Clovis in the archaeological record of North America.

Additionally, caution must be taken when comparing, or computing weighted averages of, 14C dates from Clovis-age materials processed at different times and/or by different laboratories; not all laboratories utilize a correction for MIF, and those that do first instituted them several years after the advent of AMS 14C dating. Therefore, 14C dates from Aubrey and other Clovis sites should be recalculated with all of the above considerations in mind, especially where the carbon yields were small and ages therefore potentially affected by accelerator-induced fractionation.

ACKNOWLEDGMENTS

This work was executed in support of archaeological investigations at the El Fin del

Mundo Clovis site, funded by support from: Argonaut Archaeological Research Fund, National

Geographic Society, Instituto Nacional de Antropología y Historia de Mexico, and Archaeology

Southwest. Todd Lange handled all radiocarbon processing and Rich Cruz conducted all AMS analyses. Comments from George Burr, Greg Hodgins, and Todd Lange improved the quality of this manuscript.

135

REFERENCES

Brown, T.A. and Southon, J.R., 1997. Corrections for Contamination Backgrounds in AMS 14C Measurements. Nuclear Instuments and Methods in Physics Research B:123-208.

Buchanan, B. and Collard, M., 2009. Phenetics, Cladistics, and the Search for the Alaskan Ancestors of the Paleoindians: A Reassessment of Relationships among the Clovis, Nenana, and Denali Archaeological Complexes. Journal of Archaeological Science 35:1683-1694.

Burr, G.S., Donahue, D.J., Tang, Yu, Beck, J.W., McHargue, L., Biddulph, D., Cruz, R., Jull, A.J.T., 2007. Error Analysis at the NSF Arizona AMS Facility. Nuclear Instruments and Methods in Physics Research B 259:149-153.

Burr, G.S. and Jull, A.J.T., 2010. Accelerator Mass Spectrometry for Radiocarbon Research. In Gross, M.L. and Caprioli, R. (Eds.): Encyclopedia of Mass Spectrometry vol. 5., pp. 656-669. Elsevier (Amsterdam).

Donahue, D.J., Linick, T.W., Jull, A.J.T., 1990. Isotope-Ratio and Background Corrections for Accelerator Mass Spectrometry Radiocarbon Measurements. Radiocarbon 32(2):135-142.

Ferring, C.R., 2001. The Archaeology and Paleoecology of the Aubrey Clovis Site (41DN479), Denton County, Texas (Center for Environmental Archaeology, University of North Texas, Denton, TX).

Haynes, G., 2002. The Early Settlement of North America: The Clovis Era. Cambridge University Press: New York.

IntCal13 Radiocarbon Calibration Data Set, 2012: http://www.radiocarbon.org/IntCal13.htm

Jennings, T.A. and Waters, M.R., 2014. Pre-Clovis Lithic Technology at the Debra L. Friedkin Site, Texas: Comparisons to Clovis through Site-Level Behavior, Technological Trait-List, and Cladistics Analysis. American Antiquity 79:25-44.

Klinedinst, D.B., McNichol, A.P., Currie, L.A., Schneider, R.J., Klouda, G.A., von Reden, K.F, Verkouteren, R.M., Jones, G.A., 1994. Comparative Study of Fe-C Bead and Graphite Target Performance with the National Ocean Science AMS (NOSAMS) Facility Recombinator Ion Source. Nuclear Instruments and Methods in Physics Research B 92:166-171.

Leonard, A., Castle, S., Burr, G.S., Lange, T., Thomas, J., 2013. A Wet Oxidation Method for AMS Radiocarbon Analysis of Dissolved Organic Carbon in Water. Radiocarbon 55(2-3):545-552.

McNichol, A.P., Jull, A.J.T., Burr, G.S., 2001. Converting AMS Data to Radiocarbon Values: Considerations and Conventions. Radiocarbon 43(2A):313-320.

Pearson, A., McNichol, A.P., Schneider, R.J., von Reden, K.F., Zheng, Y., 1998. Microscale AMS 14C Measurements at NOSAMS. Radiocarbon 40(1):61-75.

136

Santos, G.M., Southon, J.R., Griffin, S., Beaupre, S.R., Druffel, E.R.M., 2007. Ultra Small-Mass AMS 14C Preparation and Analyses at KCCAMS/UCI Facility. Nuclear Instruments and Methods in Physics Research B 259:293-302.

Stuiver, M., Reimer, P.J., Reimer, R., 2012. Calib 7.0 Radiocarbon Calibration program: http://calib.qub.ac.uk/calib

Waters, M.R., Stafford, T.W., Jr., 2007. Redefining the Age of Clovis: Implications for the Peopling of the Americas. Science 315(5815):1122-1126.

137

FIGURE CAPTIONS

Figure 1 (after Sanchez et al., 2014). El Fin del Mundo (FdM) with locations of erosional remnants near the headcut of an arroyo in the Rio Bacoachi drainage system, including Locality 1, the site of archaeological excavations. Inset map shows location of FdM relative to Clovis mammoth kills in the upper San Pedro Valley (SPV), in southeastern Arizona.

Figure 2 (after Sanchez et al., 2014). Schematic cross-section running through Locality 1 and south through Locality 3, then southwest to the uplands, where diagnostic Clovis and later period artifacts were found on the surface. Charcoal marks the Clovis level.

Figure 3 (after Sanchez et al., 2014). (a.): the upper bone bed an associated flakes and charcoal at Locality 1, showing the relation of charcoal (C), bone (B), and flakes (F). (b.): close-up of 1-by- 2 m excavation blocs at the western end of Locality 1, showing the provenience of charcoal specimens (including 14C-dated specimens FdM#63176 and FdM#63447) in relation to one another, and to bone fragments and flakes.

Figure 4. Solid curves represent mean fMC values (4A, top) resulting from different correction schemes applied to hypothetical samples with known age of 11,560 14C yr BP, ranging in mass from 0.05-0.9 mgC. Horizontal dashed lines in both plots represent the results on a 0.23 mgC sample, with corresponding uncertainties represented by matching-color vertical bars. Radiocarbon and equivalent calendar year timescales are shown along y-axes on the left and right, respectively. Results calculated using the uncorrected long-term blank average (ULBA), the ULBA corrected for machine-induced fractionation (MIF), the mass-corrected long-term blank average (CLBA), and the CLBA corrected for MIF are represented by the red, green, blue, and purple lines/curves, respectively; results calculated using the small blank average (SBA) with and without the MIF correction are represented by the black and orange squares, respectively. The ULBA and CLBA corrections are based on a long-term average Fblank value of 0.0025±0.0015.

138

Table 1. Sample information and uncorrected 14C results (F) for unknowns and blanks. ______

Field# Lab# Chem# Extraction Type Fraction mgC 13C F ± (scat) ± (stat) - - V21567 19-Nov-12 blank - 0.23 -22.8 0.0096 0.0008 0.0004 - - V21568 20-Nov-12 blank - 0.16 -22.8 0.0082 0.0004 0.0003 - - V21841 8-Aug-13 blank - 0.10 -22.8 0.0351 0.0015 0.0015 V21567/V21568 (Average) - SBA - - - 0.0088 0.0007 0.0007 V21568/V21841 (Average) - SBA - - - 0.0195 0.0133 0.0133 63447 AA-100181A V21564 19-Nov-12 unknown A 0.23 -24.2 0.2443 0.0024 0.0016 63447 AA-100181B V21564H 8-Aug-13 unknown B 0.08 -24.3 0.2682 0.0023 0.0018 63176 AA-100182B V21565 19-Nov-12 unknown B 0.13 -21.8 0.2417 0.0023 0.0016

139

Table 2. 14C measurements and dates from charcoal samples associated with Clovis artifacts and Gomphothere remains in upper Stratum 3B, Locality 1 at the El Fin del Mundo site. 

 13  CORRECTIONS 14 Cal yr BP range (2σ)  C LAB NO. fMC ± C yr BP ± MIF *BLANK MEDIAN + - (‰ VPDB)

AA-100181A Y CLBA .2372 .0040 11,560 140 13,390 320 275 -24.2 AA-100181A N CLBA .2356 .0037 11,610 125 13,440 285 235 “ AA-100181A Y ULBA .2437 .0030 11,340 100 13,195 210 155 “ AA-100181A N ULBA .2422 .0025 11,385 85 13,230 170 150 “

AA-100181A Y SBA .2389 .0038 11,500 125 13,340 220 235 “ AA-100181A N SBA .2375 .0017 11,550 60 13,385 105 120 “ AA-100181B Y CLBA .2656 .0083 10,650 250 12,485 585 730 -24.3 AA-100182B Y CLBA .2279 .0058 11,880 200 13,735 495 455 -21.8

AA-100182B Y SBA .2272 .0059 11,905 360 13,855 1,175 780 “

Note: Dates reported by the Arizona AMS Facility are shown in purple.  Abbreviations for different types of blank-corrections LBA: weighted long-term blank average from F values measured on blanks ≥1mgC processed at the Arizona AMS Facility in the preceding 12-month period: 0.0025±0.0015. ULBA (Uncorrected LBA) CLBA (mass-Corrected LBA) SBA (small-blank average): weighted average of measured fMC values for blanks having masses close to and bracketing the that of the unknown.  Suffix denotes A-fraction (base-insoluble residue) or B-fraction (base-soluble humate extract). + Calibrations generated with Calib 7.0 software, based on the IntCal13 Radiocarbon Calibration Data Set (2012).

140

Figure 1 (after Sanchez et al., 2014)

141

Figure 2 (after Sanchez et al., 2014)

142

Figure 3 (after Sanchez et al., 2014)

143

Figure 4

144