PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2015GL067108 Key Points: • Beryllium-10 ages show deglaciation during the Younger Dryas in central East Greenland • The inner Milne Land Stade moraines of the Greenland ice sheet and a local glacier date to ~11.4 ka • Both ice masses responded in unison to late glacial and early Holocene climate conditions
Supporting Information: • Supporting Information S1 Correspondence to: L. B. Levy,
[email protected]
Citation: Levy, L. B., M. A. Kelly, T. V. Lowell, B. L. Hall, J. A. Howley, and C. A. Smith (2016), Coeval fluctuations of the Greenland ice sheet and a local glacier, central East Greenland, during late glacial and early Holocene time, Geophys. Res. Lett., 43, 1623–1631, doi:10.1002/2015GL067108. Received 3 DEC 2015 Accepted 22 JAN 2016 Accepted article online 28 JAN 2016 Published online 17 FEB 2016
Coeval fluctuations of the Greenland ice sheet and a local glacier, central East Greenland, during late glacial and early Holocene time Laura B. Levy1,2, Meredith A. Kelly2, Thomas V. Lowell3, Brenda L. Hall4, Jennifer A. Howley2, and Colby A. Smith5 1
Department of Geoscience, Høegh-Guldbergs Gade 2, Aarhus University, Aarhus, Denmark, 2Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire, USA, 3Department of Geology, University of Cincinnati, Cincinnati, Ohio, USA, 4 School of Earth and Climate Sciences and Climate Change Institute, University of Maine, Orono, Maine, USA, 5Geological Survey of Sweden, Uppsala, Sweden
Abstract We present a 10Be chronology of late glacial to early Holocene fluctuations of a Greenland ice sheet outlet glacier and the adjacent Milne Land ice cap in central East Greenland. Ages of boulders on bedrock indicate that both ice masses receded during the Younger Dryas (YD), likely due to rising summer temperatures. Since Greenland ice core records register cold mean annual temperatures throughout the YD, these ice-marginal data support climate conditions characterized by strong seasonality. The ice sheet outlet glacier and ice cap deposited inner Milne Land Stade moraines at 11.4 ± 0.8 ka and 11.4 ± 0.6 ka, respectively (mean moraine ages and 1σ uncertainties). Based on the coeval moraine ages, we suggest that both ice masses responded to climate conditions acting on the ice margins, specifically ablation. Moreover, the ice sheet responded sensitively (i.e., on the same time scale as a small ice cap) to climate conditions.
1. Introduction The mechanisms that influenced abrupt cold events such as the Younger Dryas (YD; 12.9–11.7 ka) and the Preboreal Oscillation (PBO; 11.4–11.27 ka) remain unresolved. Well documented by Greenland ice core records, these events are marked by significant cold conditions over Greenland and rapid terminations [e.g., Dansgaard et al., 1989; Björck et al., 1996, 1997; Severinghaus et al., 1998; Alley, 2000]. One hypothesis for the cause of abrupt cold events such as the YD is that extensive Arctic sea ice influenced strong seasonality in the North Atlantic region, specifically very cold winters and only moderately cold summers [Denton et al., 2005; Broecker, 2006; Buizert et al., 2014]. A critical test of this “seasonality hypothesis” is provided by past mountain glacier extents on Greenland, which are interpreted to reflect primarily to past summer temperatures [e.g., Oerlemans, 2001, p. 128; Alley, 2003; Denton et al., 2005; Dyurgerov and McCabe, 2006]. However, past glacier extents on Greenland dating to YD and PBO times are rare [Jennings et al., 2006; Kelly et al., 2008; Miller, 2008; Hall et al., 2008, 2010; Rinterknecht et al., 2014]. This is, in part, due to much of the Greenland ice sheet being located offshore until early Holocene time [Funder et al., 2011]. The Scoresby Sund region (~71°N, 26°W) of central East Greenland is one of only a few locations where the margins of the Greenland ice sheet and glaciers independent of the ice sheet (hereinafter referred to as local glaciers) were located at least partially on land by late glacial time. In this region, the Milne Land Stade (MLS) moraines mark a significant readvance or stillstand during deglaciation from the Last Glacial Maximum [e.g., Funder, 1970, 1978; Hall et al., 2008, 2010; Kelly et al., 2008; Alexanderson and Håkansson, 2014]. Here we present a 10Be chronology of the fluctuations of a Greenland ice sheet outlet glacier and a local glacier in Milne Land in the western Scoresby Sund region (Figure 1) during late glacial and early Holocene time. We show that both ice masses experienced recession during YD time, prior to the deposition of the inner MLS moraines. Ages of the inner MLS moraines deposited by both the Greenland ice sheet outlet glacier and local glacier indicate readvance or stillstand during Preboreal time, possibly during the PBO.
2. Background ©2016. American Geophysical Union. All Rights Reserved.
LEVY ET AL.
In the Scoresby Sund region, large belts of moraines known as the MLS moraines are located between the fjord mouths and the historical drift [e.g., Funder, 1970]. Two distinct sets of moraines, known as the inner and outer MLS moraines, have been defined in the region and are associated with raised shorelines between
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Figure 1. (a) Location of the Scoresby Sund region in central East Greenland (red box). (b) Landsat satellite image of the Scoresby Sund region showing location of the field area in southeastern Milne Land (white box), as well as locations mentioned in the text. (c) Composite WorldView-1 and WorldView-2 satellite images of the field area showing the inner MLS moraines of 10 both the Greenland ice sheet outlet glacier and the Milne Land ice cap and Be sample locations. White arrows indicate the former flow path of the Greenland ice sheet through Fønfjord and the Milne Land ice cap outlet glacier through Vinkeldal.
90 and 132 meters above sea level (m asl) (with local variations throughout the region) [Funder, 1978]. Prior research has dated these moraines to late glacial and early Holocene time, based on radiocarbon ages of shells in associated raised-marine terraces [Sugden and John, 1965; Funder, 1970, 1972, 1978; Funder et al., 1994; Funder and Hansen, 1996]. More recently, Hall et al. [2008] applied radiocarbon dating of shells in raised-marine terraces associated with the MLS moraines and developed a relative sea level curve for Kjove Land in the northwestern Scoresby Sund region (Figure 1). Based on extrapolation from the relative sea level curve, Hall et al. [2008] indicated that the outer MLS moraines were deposited prior to 12.4 cal kyr B.P. Beryllium-10 ages of moraine boulders in nearby Gurreholm Dal (Figure 1) suggest an age of 13.6–12.4 ka for the outer MLS moraines (age ranges are the mean age-youngest age [Kelly et al., 2008] and have been recalculated using the Arctic 10Be production rate [Young et al., 2013] and scaling after Lal [1991] and Stone [2000]). However, many moraine boulder ages were influenced by 10Be inherited from a prior period of exposure. Further work by Hall et al. [2010] in nearby Schuchert Dal (Figure 1) indicated that ice receded from the outer MLS moraines by 12.3–11.9 cal kyr B.P. Radiocarbon dating of shells and 10Be dating of moraine boulders suggest ages of 11.3–11.0 cal kyr B.P. (range of ages) [Hall et al., 2008] and 12.1–11.1 ka (mean age to youngest age) [Kelly et al., 2008] for the inner MLS moraines. Farther afield in Jameson Land, 10 Be and optically stimulated luminescence dating yield ages of ~11.7–10.6 ka for moraines formed by the Liverpool Land advance [Alexanderson and Håkansson, 2014], suggesting that these moraines are broadly correlative with the inner MLS moraines elsewhere in the Scoresby Sund region.
3. Methods We applied geomorphic mapping and 10Be dating to reconstruct MLS ice extents in an area known as Arabertoppe in southeastern Milne Land, the type locality for the MLS moraines (Figure 1). We conducted mapping using aerial photographs taken in 1985 and WorldView-1 and WorldView-2 satellite images with a resolution of 0.5 m. We checked our maps with detailed fieldwork and obtained samples for 10Be dating LEVY ET AL.
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of boulders on moraines and of perched boulders and glacially scoured bedrock distal to the moraines. We sampled the surfaces of large (>1 m high) boulders on moraine crests or perched on bedrock as well as bedrock surfaces using a hammer and chisel or the “drill-and-blast” method of Kelly [2003]. Beryllium-10 samples were processed in the cosmogenic nuclide laboratories at Dartmouth College and Lamont-Doherty Earth Observatory of Columbia University and measured at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory (see supporting information for further methodology). We calculated all 10Be ages using the CRONUS-Earth online calculator [Balco et al., 2008] with the Arctic production rate [Young et al., 2013] and time-invariant scaling (“St”) after Lal [1991] and Stone [2000]. Beryllium-10 ages calculated using other scaling schemes deviate by
