Tropical circulation intensification and tectonic extension recorded by Neogene terrestrial d18O records of the western United States

Ran Feng1,2, Christopher J. Poulsen1, Martin Werner3 1Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA 2National Center for Atmospheric Research, Boulder, Colorado 80303, USA 3Alfred Wegener Institute for Polar and Marine Research (AWI) Bussestraße 24, D-27570 Bremerhaven, Germany

ABSTRACT Pacific Coast Ranges and Sierra Nevada, through progressive rainout Terrestrial water isotope records preserve a history of hydrologi- of the heavy isotopologues (e.g., Mulch, 2016). Elevation changes (in cal cycling that is influenced by past climate and surface topography. time and space) are calculated by scaling proxy d18O and dD differences d18O and dD records from authigenic minerals of the western United by modern observed precipitation isotope lapse rates (e.g., Poage and States display a long-term increase during the Neogene in the vicinity Chamberlain, 2002). of the Sierra Nevada and the central Rocky Mountains (Rockies), but d18O values of early Cenozoic (55–28 Ma) records are strongly nega- a smaller increase or decrease in the northern Great Basin. Interpre- tive within the Great Basin, a signal that has been interpreted to reflect the tations of these isotopic trends require quantitative estimates of the presence of a high plateau of 3–4 km elevation across this region (Horton influence of climatic and environmental changes on d18O and dD of soil et al., 2004; Horton and Chamberlain, 2006; Mix et al., 2011; Feng et al., water. Here we use a coupled atmosphere-land model with water-iso- 2013; Mulch et al., 2015). Differences in proxy d18O between the Great topologue tracking capabilities, ECHAM5-JSBACH-wiso, to simulate Basin and the adjacent northern Sierra Nevada (Mulch, 2006; Henry and precipitation and d18O responses to elevation-independent changes in Faulds, 2010; Cassel et al., 2014) and central Rocky Mountains (Rock- Neogene geography, equator to pole temperature gradient (EPGRAD), ies) (Sjostrom et al., 2006) are small, suggesting that these two mountain grassland expansion, and tropical Pacific sea surface temperatures. ranges were ramps of the high plateau. In this scenario, subsequent Basin 18 18 Both precipitation and soil water d O (d Osw) respond strongly to and Range extension within the Great Basin lowered the landscape to Neogene strengthening of the EPGRAD, but weakly to other forcings. its modern elevations (~1.5–2 km), while the northern Sierra Nevada An increase in EPGRAD leads to significant drying and18 O enrich- and central Rockies remained relatively high. Great Basin extension is ment (3‰–5‰) of soil water over the northern Sierra Nevada and supported by structural evidence of widespread extensional deformation central Rockies as a result of Hadley circulation strengthening and throughout this region (Dickinson, 2006) and by the presence of thin crust enhanced coastal subtropical subsidence. These large-scale circulation within the Basin and Range province in comparison to the surrounding changes reduce inland moisture transport from the Pacific Ocean and Sierra Nevada and central Rockies (Chulick and Mooney, 2002). 18 18 18 Gulf of Mexico. Our simulated d Osw responses could explain 50%– Although d O changes (proxy dD values are converted to d O values; 100% of the proxy d18O increases over the Sierra Nevada and central see Table DR3 in the GSA Data Repository1) in Neogene (since ca. 23 Ma) Rockies, suggesting that climate change rather than surface subsid- records from the Great Basin are broadly consistent with the inferred ence may have been the dominant climate signal in d18O records in extension of this region (e.g., Horton et al., 2004), records from areas these regions. On the contrary, d18O responses to climate changes are surrounding mountains of Sierra Nevada (Horton and Chamberlain, 2006) small in the Great Basin, indicating that the observed d18O increase and the central Rockies (Fan et al., 2014) suggest a Neogene elevation over this region was likely a direct response to surface subsidence history that is inconsistent with the Eocene reconstructions and structural with elevation losses of 1–1.5 km. Adding this elevation loss to current evidence of Neogene extension. These records exhibit a d18O increase of Great Basin elevations reveals the former existence of a uniformly up to 8‰, a change that is substantially greater than the increase in the high plateau extending from the Sierra Nevada to the central Rock- Great Basin (e.g., Horton et al., 2004) and, if interpreted solely in terms ies prior to Neogene extension. This revised elevation history brings of paleoelevation, would signify greater elevation loss in these regions Neogene d18O and dD paleoaltimetry of the western United States in than across the Great Basin. One explanation for the seemingly incom- accordance with independent lines of structural evidence and early patible elevation histories is that Neogene d18O records were influenced Cenozoic elevation reconstructions. by factors other than elevation. Mix et al. (2013) and Chamberlain et al. (2014) proposed that western North American d18O increased as a result INTRODUCTION of changing hydrological balance due to Neogene replacement of forests Cenozoic mountain-building events, most notably the Sevier and by grasslands. However, reconstructions of phytoliths in the fossil record Laramide orogenies and subsequent Basin and Range extension, shaped indicate that grassland expansion in the western U.S. likely occurred prior western North America, creating the numerous mountain ranges and to the Neogene, in the late Oligocene–early Miocene (Strömberg, 2011). extensive topographic relief that mark the surface today. Paleoaltimetry Environmental changes other than elevation and grassland expansion estimates of past surface elevations have been widely derived from geo- may also have contributed to the d18O increase. The Neogene climate logical proxies, assuming that the proxies preserve an accurate signal that underwent a major thermal restructuring that included a strengthening translates quantitatively to elevation. In western North America, stable of the zonal sea surface temperature (SST) gradient by 2–6 °C primarily oxygen and hydrogen compositions (d18O and dD) of terrestrial sediments and organic material are frequently used as proxies for the isotopic com- 1 GSA Data Repository item 2016330, additional methods, discussions, Figures positions of ancient surface and soil waters, which display strong negative DR1–DR8, and Tables DR1–DR4, is available online at http://www.geosociety​ .org​ ​ correlations with elevation in many regions, including the present-day /pubs​/ft2016.htm or on request from [email protected].

GEOLOGY, November 2016; v. 44; no. 11; p. 1–4 | Data Repository item 2016330 | doi:10.1130/G38212.1 | Published online XX Month 2016 ©GEOLOGY 2016 Geological | Volume Society 44 | ofNumber America. 11 For | www.gsapubs.orgpermission to copy, contact [email protected]. 1 across the tropical Pacific (e.g., Zhang et al., 2013) and an increase in seasonal dry soil conditions (Fig. DR4). Furthermore, Neogene proxy 18 the meridional SST gradient by 5–8 °C (Goldner et al., 2014) associated d Om changes (with variable record lengths) are standardized into changes with drawdown of atmospheric CO2 and the expansion of high-latitude over 10 m.y. intervals by scaling coefficients of least square regressions 18 glaciation (Raymo, 1994). Concomitantly, hydrological conditions across of individual time series of proxy d Om (Table DR3; Fig. DR5). western North America underwent a marked transition from a moist state during the early Neogene to its present-day semiarid state. Evidence from CLIMATE MODEL RESULTS fossil leaves (Pound et al., 2012), soil chemistry (Retallack et al., 2002), Annual mean temperatures across continental North America (14°– and herbivore mammal hyposdonty (Eronen et al., 2012) indicate a large 56°N, 47°–141°W) differ by -3.1, -0.54, -0.7, and -0.14 °C between reduction in precipitation of 102 cm/yr across western North America. NG-EPGRAD, NG-ELNINO, NG-GEO, NG-GRASS, and the CNTL. Enhanced surface evaporation and reduced soil moisture associated with Correspondingly, the coldest month mean temperature varies by -5.7, 18 18 this regional aridification event may have increased soil waterd O (d Osw) -0.45, 0.24, and 0.93 °C and the regional annual moisture budget (pre- (e.g., Poage and Chamberlain, 2002; Horton and Chamberlain, 2006; Fan cipitation minus evaporation) changes by -74, 0.22, -36, and 0.01 mm/ et al., 2014; Chamberlain et al., 2014). The cause of regional aridification yr. Collectively, an increase in the EPGRAD leads to the strongest dry- is not known with certainty, but may be linked to large-scale Neogene ing and cooling, greater annual temperature range, and best match with climate change. Previous studies have demonstrated that variations in Neogene precipitation changes recorded by proxies across the western 18 atmospheric CO2 (Poulsen and Jeffery, 2011) and tropical SST distribu- United States (Fig. DR6). Increasing EPGRAD leads to d O changes of 18 tion (Winnick et al., 2013) can lead to substantial changes in hydrological ~2‰–5‰ (Fig. 1A, shown for d Oc), about twice the magnitude of the cycling and the stable isotopic composition of continental precipitation. The objective of this study is to identify the influence of climate change on soil water and precipitation d18O in order to better understand the con- 50N tributions of global climate change, as opposed to elevation change, to Neogene proxy d18O changes. Our results have implications for interpreta- tions of the hydrological and elevation history of western North America. 40N

MODEL, DATA DESCRIPTION, AND EXPERIMENTS A B We use the coupled atmosphere–land surface model, ECHAM5- 30N JSBACH-wiso to quantify d18O responses of soil and meteoric water to 50N Neogene environmental changes. ECHAM5-JSBACH-wiso is a three- dimensional global climate model with an isotope-tracking module that simulates both equilibrium and kinetic fractionation of water isotopologue 40N 18 species H2O and HDO during phase transitions in the atmosphere (Wer- ner et al., 2011) and on land (Haese et al., 2013). All experiments were run at a spectral T63 resolution (~1.9° × 1.9° horizontally) with 31 verti- C D 30N cal levels. At this resolution, the model simulates reasonable present-day 120W 110W 100W 120W 110W 100W 18 18 distributions of temperature, precipitation amounts, d O in precipitation Simulated Δδ Oc(‰) Significance level -5 -3 -1 135 (d18O ), and d18O across the western United States (Figs. DR1A–DR1C α ≤ 0.1 α > 0.1 p sw Proxy Δδ18O (‰ per 10 Ma) and DR2A in the Data Repository). We evaluate in our simulations four <-1 ≤-1 - <1 ≤1 - <3 ≤3 - <5 ≥5 aspects of Neogene (NG) environmental change, including: (1) increase 6 50N SN GB of the equator to pole surface temperature gradient (NG-EPGRAD), (2) CR 4 Hg MB intensification of the tropical Pacific zonal SST gradient (NG-ELNINO), (‰) IB m CR WG (3) changes in geography and surface elevation outside the North Amer- O 2 40N ESN NG Nb

18 MNP ica (NG-GEO) compiled by Herold et al. (2008), and (4) expansion of EB

Δδ 0 EPB FLV grassland coverage over North America (NG-GRASS). We run a control RB experiment (CNTL) with present-day boundary conditions, 280 ppm CO E F 2 -2 30N level, and present-day levels of other trace gases. For each aspect, we then Apr Dec Aug 120W 110W 100W Elevation (km) run a sensitivity experiment with the Neogene boundary condition sub- 0.5112.5 2.5 3 stituted for the associated present-day boundary conditions in the model. 18 Figure 1. Proxy (Dd Om, filled markers) and simulated (converted to Climate and isotopic responses to individual Neogene environmental 18 18 18 carbonate d O, d Oc) Neogene d O changes and the contribution of changes are reported as differences between 15-model-year averages 18 enhanced EPGRAD (equator to pole temperature gradient) to Dd Om 18 18 of the control and sensitivity experiments. d O values are weighted by across the western United States. Simulated changes are mean d Oc monthly precipitation. Detailed descriptions of boundary conditions and differences of the fall season (September to November) between control and early Neogene (NG) sensitivity cases. A: NG-EPGRAD. maps for each experiment are provided in Figure DR3. B: NG-ELNINO (intensification of the tropical Pacific zonal sea sur- In order to facilitate data-model comparison, we convert simulated face temperature gradient). C: NG-GEO (changes in geography and 18 18 18 d Osw to mineral d O (d Om) (Table DR2). Due to primary contact and surface elevation outside the North America). D: NG-GRASS (grass- hydration of volcanic glass and smectite with surface water, d18O is land expansion coverage over North America). E: Simulated seasonal m 18 18 18 cycle of d O responses (converted to mineral values, d Om) [CNTL converted from simulated annual d Osw using model annual temperature 18 (control experiment) – NG-EPGRAD] at proxy sites with d Om trend and assuming equilibrium fractionation between soil water and miner- significant above 90% confidence level. CR—central Rocky Moun- als. Considering the warm and dry season bias of carbonate formation tains; GB—Great Basin; SN—Sierra Nevada. F: Residuals of proxy 18 18 18 (Breecker et al., 2009) and the formation depth of pedogenic carbonate Dd Om (Dd Oelev) after removing d Om responses to enhanced EPGRAD 18 18 18 ( 18O ) ( 18O = 18O – 18O ). MB—Montana basins; (>50 cm), we convert simulated d O to d O of carbonate (d O ) using Dd EPGRAD Dd elev Dd m Dd EPGRAD sw c WG—western Great Plains; Nb—Nebraska; MNP—Middle and North fall season (September, October, November) soil temperatures averaged Park; Hg—Hagerman; EB—Elko Basin; IB—Ibapah Badland; ESN— between depths of 78 cm and 6.98 m (the bottom depth of modeled soil eastern Sierra Nevada; FLV—Fish Lake Valley; EPB—El Paso Basin; process). The fall season was chosen because it represents warm and peak RB—Rainbow Basin.

2 www.gsapubs.org | Volume 44 | Number 11 | GEOLOGY 18 18 response to the other environmental factors (≤2‰) (Figs. 1B–1D). d O lack of d Osw changes across the continental interior region of the Great increases across the Sierra Nevada and central Rockies but changes little Basin in response to the increasing EPGRAD. within the Great Basin (Fig. 1A), a pattern that is consistent with Neogene The strengthening of subtropical high pressure is not limited to North 18 proxy d Om changes. America, but is prevalent throughout the subtropical regions. These The 18O enrichment of soil water due to intensification of the EPGRAD changes reflect large-scale strengthening of the Hadley circulation (HC), is greatest during the fall season but persists throughout most of the year defined as the overturning mass flux between 30°S and 30°N, by ~23% 18 11 in the Sierra Nevada and central Rockies (Fig. 1E). The d Om responses (3.8 × 10 kg/s total) in response to intensification of the EPGRAD (Fig. at these locations are ~4‰ and 3‰, accounting for 50%–100% of the DR8). The greater meridional thermal gradient drives stronger surface 18 18 d O increase reconstructed from minerals (Fig. 1F). In contrast, the d Om convergence over the tropical ocean (Fig. 2). The strengthened conver- responses are smaller in the northern Great Basin and become negative gence destabilizes the marine boundary layer and enhances tropical moist during the fall and winter (Fig. 1E). Our model results suggest that an convection. The enhanced latent heat release from condensation (Fig. 18 increase in EPGRAD contributed little to Neogene proxy d Om changes DR8) warms the surrounding air and fuels even stronger updrafts. This in the Great Basin (Fig. 1F). process strengthens the upward branch of the HC, resulting in stronger 18 The fall season d Osw responses correspond to strengthening of boreal compensating subsidence and intensification of high pressure throughout summertime (June, July, August) tropospheric subsidence, and strength- the subtropics (Fig. 2). ening of the subtropical high-pressure system over the eastern Pacific In comparison to the results of Mix et al. (2013) and Chamberlain et 18 and western Atlantic during the boreal summer (Fig. 2). In the eastern al. (2014), our simulated d Osw responses to grassland expansion (CNTL Pacific, anomalous subtropical high pressure enhances surface northerly minus NG-GRASS) are small (<2‰) across the western United States, 18 flow along the Pacific coast of the western North America. Air masses partly due to the lack of explicit model treatment of vertical d Osw profile transported by these northerly winds sink over the coast of western and rooting depths of grassland and forest. Meanwhile, our simulations North America following isentropic surfaces that slope downward from feature responses of surface albedo (as much as 30% fractional increase) the pole to the equator. These descending air masses stabilize the marine and water balance (only ~10% increase in evapotranspiration to precipi- 18 boundary layer, preventing moisture transport into the free troposphere tation ratio) to grassland expansion that minimize the d Osw responses and toward the coastal southwestern United States, and resulting in dry- and may have been overlooked and overestimated by the previous studies ing over this region. Similarly, anomalous high pressure extending from (see the Data Repository). the Atlantic coast toward northeastern North America induces a broad band of subsidence along its southern and southwestern flank due to the DISCUSSIONS AND IMPLICATIONS strengthening of northeasterly winds. Both subsidence and northeasterly wind anomalies counteract the northward moisture transport by south- Implications for Neogene Great Basin Elevations and Climate easterly winds from the Gulf of Mexico, leading to aridification of the History central Rockies. To estimate western North American elevation changes from Neogene 18 18 Aridification over the Sierra Nevada and Central Rockies explains d Om records, we subtract our simulated d Om responses to Neogene 18 18 18 the simulated O enrichment in soil water. In addition to a reduction strengthening of the EPGRAD (Dd OEPGRAD) from proxy Neogene d O 18 18 18 in precipitation, both evapotranspiration and rainwater evaporation (as changes (Dd Om) to isolate the elevation-dependent d O signal (Dd Oelev 18 18 18 18 indicated by an increase in the d O of summer precipitation by ~3‰) = Dd Om – Dd OEPGRAD). Modeled d Om responses to other Neogene increase over this region, and contribute to the 18O enrichment in soil water. environmental changes (tropical Pacific SSTs, grassland expansion, and Changes in lower tropospheric pressure (800–600 hPa) and subsidence geographic and topographic changes) are neglected because they are diminish away from the Pacific and Atlantic coasts, which explains the small (-2.7‰–0.5‰), and comparable to intersample variability of the 18 proxy d Om. 18 Our Dd Oelev estimate shows minimal changes (<1‰) in the central Key changes in wind direction Rockies and in the vicinity of the northern Sierra Nevada (Fig. 1F); we argue that these minimal changes indicate that these regions underwent 60N only small elevation adjustments (<1 km) during the Neogene. Our inter- pretation of the isotopic data is consistent with independent paleoelevation estimates that these regions had attained near-modern elevations prior to 30N the early Neogene and possibly in the late Eocene (e.g., Mulch, 2006; Cassel et al., 2014; Fan et al., 2014). 18 Dd Oelev remains positive at several sites in the southern Sierra Nevada 0 and negative in the northern Great Basin (Fig. 1F). These records may have recorded local elevation adjustments (Kent-Corson et al., 2013) and associated changes in regional atmosphere circulation (in the case 30S of the southern Sierra Nevada) (e.g., Mulch, 2016). Alternatively, they may reflect anomalous local hydrological responses to Neogene climatic changes, the details of which are not adequately resolved in our simula- 60S 2 m/s tions (see the Data Repository). 18 0 90E 180 90W 0 Across the northern Great Basin, we assume that positive Dd Om (3‰–5‰) mainly reflects elevation changes. By applying a preexten- -80 -40 04080 sion lapse rate of ~3‰/km (Feng et al., 2013), surface elevations of the Changes in geopotential height (m) northern Great Basin are estimated to have decreased ~1–1.6 km during the Neogene. Adding this elevation reduction to present-day elevations Figure 2. Summer (June to August) responses of 700 hPa geopotential­ height (m, shaded), winds and subsidence (cross-hatched, enhanced (~1.6–2 km) brings preextension elevations close to those proposed for subsidence) to Neogene strengthening of EPGRAD (equator to pole the Eocene (~3 km) based on independent stable isotope (Mix et al., 2011; temperature gradient). Chamberlain et al., 2012; Feng et al., 2013) and fossil leaf paleoaltimetry

GEOLOGY | Volume 44 | Number 11 | www.gsapubs.org 3 estimates (Wolfe et al., 1998; Feng and Poulsen, 2016). This result indi- Horton, T.W., Sjostrom, D.J., Abruzzese, M.J., Poage, M.A., Waldbauer, J.R., Hren, cates that major extension of the northern Great Basin likely occurred M., Wooden, J., and Chamberlain, C.P., 2004, Spatial and temporal variation of Cenozoic surface elevation in the Great Basin and Sierra Nevada: American during the Neogene, accompanied by relatively small elevation changes of Journal of Science, v. 304, p. 862–888, doi:​10​.2475​/ajs​.304​.10​.862. the topographic ramps of the northern Sierra Nevada and central Rockies. Kent-Corson, M.L., Barnosky, A.D., Mulch, A., Carrasco, M.A., and Chamberlain, Paleo-d18O records from orogenic regions have often been used as C.P., 2013, Possible regional tectonic controls on mammalian evolution in paleoaltimeters. Our study lends support to previous studies (e.g., Poulsen western North America: Palaeogeography, Palaeoclimatology, Palaeoecology, and Jeffery, 2011; Mix et al., 2013; Chamberlain et al., 2014) that dem- v. 387, p. 17–26, doi:​10​.1016​/j​.palaeo​.2013​.07​.014. Mix, H.T., Mulch, A., Kent-Corson, M.L., and Chamberlain, C.P., 2011, Cenozoic onstrate the need to consider additional environmental factors when inter- migration of topography in the North American Cordillera: Geology, v. 39, preting these records. Furthermore, we emphasize here that hydrological p. 87–90, doi:​10​.1130​/G31450​.1. changes preserved in d18O records of the western United States may cap- Mix, H.T., Winnick, M.J., Mulch, A., and Chamberlain, C.P., 2013, Grassland ture large-scale circulation changes due to long-term Neogene cooling. expansion as an instrument of hydrologic change in Neogene western North America: Earth and Planetary Science Letters, v. 377–378, p. 73–83, doi:​10​ .1016​/j​.epsl​.2013​.07​.032. ACKNOWLEDGMENTS Mulch, A., 2006, Hydrogen isotopes in Eocene river gravels and paleoelevation of This study was supported by National Science Foundation EAR grant 1019420 the Sierra Nevada: Science, v. 313, p. 87–89, doi:10​ ​.1126​/science​.1125986. to to C. Poulsen. We are grateful for constructive comments from three review- Mulch, A., 2016, Stable isotope paleoaltimetry and the evolution of landscapes ers, discussions on model parameters and limitations with Barbara Haese at the and life: Earth and Planetary Science Letters, v. 433, p. 180–191, doi:10​ .1016​ ​ Institute of Geography, Universität Augsburg, and discussion of simulation results /j​.epsl​.2015​.10​.034. with members of the Climate Change Research group of University of Michigan. 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