https://doi.org/10.1130/G46058.1 Manuscript received 30 January 2019 Revised manuscript received 26 March 2019 Manuscript accepted 27 March 2019 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online XX Month 2019 Early Younger Dryas glacier culmination in southern Alaska: Implications for North Atlantic climate change during the last deglaciation Nicolás E. Young1, Jason P. Briner2, Joerg Schaefer1, Susan Zimmerman3, and Robert C. Finkel3 1Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA 2Department of Geology, University at Buffalo, Buffalo, New York 14260, USA 3Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA ABSTRACT ongoing climate change event, however, leaves climate forcing of glacier change during the last The transition from the glacial period to the uncertainty regarding its teleconnections around deglaciation (Clark et al., 2012; Shakun et al., Holocene was characterized by a dramatic re- the globe and its regional manifestation in the 2012; Bereiter et al., 2018). organization of Earth’s climate system linked coming decades and centuries. A longer view of Methodological advances in cosmogenic- to abrupt changes in atmospheric and oceanic Earth-system response to abrupt climate change nuclide exposure dating over the past 15+ yr circulation. In particular, considerable effort is contained within the paleoclimate record from now provide an opportunity to determine the has been placed on constraining the magni- the last deglaciation. Often considered the quint- finer structure of Arctic climate change dis- tude, timing, and spatial variability of climatic essential example of abrupt climate change, played within moraine records, and in particular changes during the Younger Dryas stadial the Younger Dryas stadial (YD; 12.9–11.7 ka) glacier response to the YD stadial. Here, we use (YD; 12.9–11.7 ka) within the North Atlantic has received significant attention (Alley, 2000; 19 new high-precision cosmogenic 10Be expo- region. Whereas the YD is clearly expressed Buizert al., 2014); yet the expression of the YD sure ages to report an updated chronology of in some climate archives, the record of moun- in records of glacier change, both throughout moraines deposited during the Mount Waskey tain glacier change through the YD remains the North Atlantic Ocean region and beyond, (Alaska) advance, which broadly dates to around enigmatic and has elicited debate concerning remains ambiguous. the time of the YD, as originally reported by the overall magnitude and seasonality associ- The relatively brief duration of the YD, Briner et al. (2002). ated with YD temperature change. Here, we coupled with its abrupt transitions as expressed report 19 new 10Be ages from a location in the in ice-core and marine-sediment archives, re- SETTING Pacific sector—the Ahklun Mountains, south- quires high-precision absolute chronological In the Mount Waskey region (Fig. 1B), ern Alaska—that constrain the age of a late- tools for dating moraines and glacial-sediment prominent, well-preserved moraines are located glacial terminal moraine to 12.52 ± 0.24 ka, records. Many glacial chronologies lack the ~4 km down-valley of the local cirque glacier in the middle of the YD stadial. Our new 10Be reso lu tion required to attribute glacier advances complex and mark what is likely a readvance ages, combined with additional Northern to YD climate change (e.g., Gosse et al., 1995; of the valley glacier during overall retreat, as Hemisphere records of glacier change, re- Kelly et al., 2008), thereby inhibiting our ability marked by the crosscutting relationship between veal that glacier culminations occurred in the to constrain the spatio-temporal climatic foot- the Mount Waskey moraines and ice-flow fea- early and/or middle YD, followed by glacier print of YD climate change. However, as newer tures in the main east-west–trending trunk val- recession through the remainder of the YD. and more precise glacier chronologies become ley (Briner et al., 2002). The Mount Waskey Widespread early-to-middle Younger Dryas available, there is increasing evidence that some moraines are, notably, clast-supported features, glacier culminations imply modest summer glacier advances in the Northern Hemisphere, and they impound a lake; a macrofossil-based cooling (i.e., seasonality) that briefly punctu- including Norway (Andersen et al., 1995), Scot- radiocarbon age from basal lake sediments con- ated an overall warming trend through the YD land (Bromley et al., 2018), and Greenland (Jen- strains the age of the Mount Waskey moraines stadial. This pattern of glacier culminations nings et al., 2014), culminated during the early to >11,010 ± 250 cal yr B.P. (Levy et al., 2004). occurring in the early-to-middle YD followed or middle YD rather than at the abrupt termi- Briner et al. (2002) originally presented seven by retreat through the remainder of the YD nation of the cold event. Moreover, the appar- 10Be ages from Mount Waskey moraine boulders largely mimics the pattern of YD temperature ently limited response of Greenland mountain and a neighboring equivalent moraine with a change displayed in Greenland ice cores. glaciers, which were located directly adjacent to mean age of 11.31 ± 0.71 ka, after excluding the canonical record of extreme YD temperature two older outliers (n = 5; see the GSA Data INTRODUCTION change (i.e., Greenland ice cores), highlights the Repository1). Earth’s climate is abruptly changing, and likely role of seasonality in YD climate change The clast-supported structure of the Mount glaciers worldwide are in decline (Roe et al., (Denton et al., 2005). There also remains uncer- Waskey moraines encourages exceptional mo- 2017). The relatively short duration of today’s tainty regarding the global versus hemispheric raine stability and largely circumvents issues 1GSA Data Repository item 2019204, materials, methods, and 10Be age calculations, is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org. CITATION: Young, N.E., et al., 2019, Early Younger Dryas glacier culmination in southern Alaska: Implications for North Atlantic climate change during the last deglaciation: Geology, v. 47, p. 1–5, https:// doi .org /10 .1130 /G46058.1 Geological Society of America | GEOLOGY | Volume 47 | Number 6 | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4678802/g46058.pdf by University at Buffalo Libraries user on 07 May 2019 159.5˚W 12.58 ± 0.22 M1 AB12.52 ± 0.24 12.39 ± 0.36 M3 M2 Alaska 12.56 ± 0.25 12.25 ± 0.25 12.49 ± 0.40 11.58 ± 0.24 12.47 ± 0.22 11.66 ± 0.23 Ahklun 11.34 ± 0.17 9.42 ± 0.20 Mnts. Gulf of Alaska 12.60 ± 0.27 11.75 ± 0.25 12.76 ± 0.32 11.64 ± 0.22 12.26 ± 0.24 11.59 ± 0.30 12.04 ± 0.25 Waskey Lake Fig. 1B 59.8˚N 59.86˚N 10.39 ± 0.25 gap in DEM Arolik Lake coverage Waskey Lake Ahklun Mountains M1 12.52 ± 0.07 ka Ahklun Mountains M3 12.09 ± 0.44 ka SW Alaska 100-m interval topographic contour N 25 km N 159.21˚W 1 km Figure 1. During the Last Glacial Maximum (LGM), the Ahklun Mountains (southern Alaska) hosted an ice cap with outlet glaciers fill- ing major valleys and spreading out onto adjacent lowlands; deglaciation from maximum LGM ice limit was under way by ca. 22 ka (Kaufman et al., 2011, 2012; Briner et al., 2017). A: Late Wisconsin maximum ice extent in Ahklun Mountains (red shading), with loca- tions of Mount Waskey field area and Arolik Lake. B: Waskey Lake field area with mapped moraines (M) and individual 10Be ages (ka; 1σ analytical uncertainty only; Table DR1 [see text footnote 1]). DEM—digital elevation model. that have plagued 10Be chronologies in geomor- in the Data Repository). The 10Be ages from the trine summer-temperature–sensitive proxy data, phically unstable environments, such as Alaska inner moraine range from 12.76 ± 0.32 ka to Kaufman et al. (2010) provided evidence from (Briner et al., 2005). Moreover, since the initial 11.58 ± 0.24 ka and have a mean age of 12.09 ± several additional sites in southern Alaska show- 10Be measurements of Briner et al. (2002), the 0.44 ka (n = 6) after excluding one younger ing early YD cooling. Offshore, Gulf of Alaska cosmogenic isotope community has made re- outlier (9.42 ± 0.20 ka). Three 10Be ages from (GOA) planktonic δ18O values also capture a markable improvements in: (1) isolating 10Be boulders inboard of the innermost recessional YD signal (Praetorius and Mix, 2014), and a from quartz, (2) accelerator mass spectrometric moraine are 11.75 ± 0.25 ka, 11.64 ± 0.22 ka, complementary record of GOA alkenone-based measurement of the 10Be/9Be ratio (Rood et al., and 11.59 ± 0.30 ka (mean = 11.66 ± 0.08), and ocean temperatures also reveals pronounced YD 2013), and (3) precisely constraining 10Be refer- one erratic boulder up-valley of Waskey Lake cooling, with lowest temperature occurring in ence production rates (e.g., Balco et al., 2009; has a 10Be age of 10.39 ± 0.25 ka (Fig. 1B). the middle YD (Fig. 3; Praetorius et al., 2015). Young et al., 2013; Putnam et al., 2019). Com- We cannot rule out that changes in winter snow- bined, the Mount Waskey moraines provide an DISCUSSION fall contributed to the oscillation of the Mount opportunity to develop a rare, and precise, cen- Including the production-rate uncertainty Waskey glacier. However, pollen records indi- tennial- to millennial-scale record of late-glacial (1.8%; Young et al., 2013), our 10Be ages re- cate that southern Alaska, including the Ahklun ice extent in Alaska using 10Be.