Holocene mountain glacier history in the Sukkertoppen Iskappe area, southwest Greenland
Introduction
Progressive cooling throughout the Holocene in the Northern Hemisphere, primarily driven by a gradual reduction in summer insolation, has been punctuated by multi-decadal to millennial-scale climate variability (e.g., Mayewski et al., 2004; Wanner et al., 2008, 2011; Marcott et al., 2013; Solomina et al., 2015; Kobashi et al., 2017). Numerous studies document short-lived Holocene climate perturbations in terrestrial and marine archives in the Arctic and North Atlantic regions (e.g., Kobashi et al., 2011; Larsen et al., 2012; Miller et al., 2013a; b; Geirsdóttir et al., 2013; Axford et al., 2009; Balascio et al., 2015; Schweinsberg et al., 2017), yet the spatio-temporal patterns of these climate changes and the forcing mechanisms that drive this variability remain under debate. Recent summaries (Wanner et al., 2008, 2011) suggest that a complex interaction of factors likely influenced Holocene climate, including Atlantic Meridional Overturning Circulation (AMOC) variability (McManus et al., 2004), meltwater forcing (Clark et al., 2001), solar irradiance (Bond et al., 2001; Wiles et al., 2004), explosive volcanism (Miller et al., 2012; Geirsdóttir et al., 2013; Kobashi et al., 2017), and internal unforced dynamics (Trouet et al., 2009), with increasing evidence for abrupt (Mayewski et al., 2004) and periodic (Denton and Karlén, 1973; Bond et al., 2001) climate changes. Additional high-resolution archives of climate fluctuations with vast spatial coverage are necessary to discern the leading mechanisms driving sub-millennial Holocene climate variability (Mayewski et al., 2004).
Although contemporary observations are critical for understanding the current mechanisms driving glacier behavior, geological reconstructions of mountain glacier and ice cap (GIC) fluctuations that extend beyond the instrumental period place constraints on the magnitudes of glacier responses to climate forcings, and illustrate the spatio-temporal variability of past climate. GIC are ideal for paleoclimatic reconstructions because they respond rapidly to small changes in glacier mass balance, and their records often preserve small-scale climatic signals, which are useful indicators of regional and global climatic changes (Oerlemans, 2005; Bakke et al., 2005). GIC reconstructions from the North Atlantic region demonstrate that GIC record centennial-scale climate variability superimposed on the millennial-scale insolation-driven cooling trend (Larsen et al., 2012; Geirsdóttir et al., 2013; Balascio et al., 2015). The non-linear nature of these sub-millennial-scale changes reflects both complex interactions in response to declining insolation, and the presence of additional climate drivers or strong local to regional feedbacks operating on varying timescales (Larsen et al., 2012; Geirsdóttir et al., 2013). Past centennial-scale variations in GIC extent have been linked to changes in ocean circulation in West and East Greenland (Balascio et al., 2015; Levy et al., 2014; Schweinsberg et al., 2017) and Iceland (Larsen et al., 2012). However, changes in ocean circulation have been attributed to solar forcing (Bond et al., 2001; Moffa-Sanchez et al., 2014; Jiang et al., 2015) and some modeling studies have confirmed that AMOC can switch between distinct modes in response to a small external forcing, such as solar variability (Jongma et al., 2007). Alternatively, late Holocene GIC fluctuations in Iceland (Larsen et al., 2011; Geirsdóttir et al., 2013), Baffin Island (Anderson et al., 2008; Miller et al., 2012), and on Disko island, West Greenland (Jomelli et al., 2016), suggest that periods of cooler temperatures (and glacial advances) are influenced by explosive volcanism and associated sea-ice/ocean feedbacks. Recently, volcanic forcing has been postulated as a driver of Holocene temperature fluctuations reconstructed from Greenland ice cores (Kobashi et al., 2017). In general, many studies postulate that sub-millennial scale Holocene climate variability was likely driven by a combination of mechanisms, yet regional discrepancies illustrate the ongoing need for additional continuous sub-millennial scale glacier and paleoclimate archives.
Few Holocene glacier records with centennial-scale resolution exist because in locations such as Greenland, extensive glacier advances during the last few hundred years commonly destroyed the geomorphic evidence of former glacier activity earlier in the Holocene (Gibbons et al., 1984; Kelly and Lowell, 2009). The majority of information on past local glacier activity in Greenland is fragmentary and primarily concerned with the timing of maximum extent and rates of twentieth century retreat (e.g., Citterio et al., 2010; Bjørk et al., 2012; Rastner et al., 2012; Bolch et al., 2013). Thus, little is known about local glacier evolution throughout the Holocene (Kelly and Lowell, 2009). Only a few studies have provided continuous records of Holocene mountain glacier fluctuations in East Greenland (Lowell et al., 2013; Levy et al., 2014; Balascio et al., 2015) and West Greenland (Fig. 1; Larsen et al., 2017; Schweinsberg et al., 2017).
In this study, we reconstruct Holocene GIC fluctuations at the centennial-scale in southwestern Greenland to 1) investigate the synchrony of GIC response to Holocene climate variability, and 2) explore climate forcing mechanisms that may have driven local glacier change during the Holocene. To achieve these objectives, we reconstruct GIC change in the Sukkertoppen region of southwest Greenland using proglacial lake sediment analysis, radiocarbon dating of formerly ice-entombed in situ moss, and cosmogenic nuclide exposure dating of erratics (in situ 10Be) and bedrock (in situ 14C). We compare our glacier reconstructions with previously published local glacier records in the North Atlantic region and nearby Greenland Ice Sheet (GrIS) margin chronologies. Combined, the reconstructions reported here provide a comprehensive view of GIC change throughout the Holocene in the Sukkertoppen region of southwest Greenland, and complement the instrumental records by providing a longer temporal context within which to interpret the magnitude and rate of recent GIC changes (Marcott et al., 2013).
Section snippets
Study area
Local glaciers in the study region include the Sukkertoppen Iskappe (∼2000 km2, average thickness of ∼300 m) and associated valley glacier outlets, a second large ice cap, Qaarajuttoq (∼2000 km2), and the nearby ice caps and mountain glaciers on uplands from Søndre Isortoq to Søndre Strømfjord (Fig. 1, Fig. 2; Weidick et al., 1992; Kelly and Lowell, 2009). The region is located between the coastline of Davis Strait and the western GrIS margin located ∼200 km to the east. Sukkertoppen Iskappe
Previous work in the Sukkertoppen region
Previous work in the Sukkertoppen region has primarily focused on reconstructing fluctuations of the GrIS margin from the Last Glacial Maximum (LGM; 26-19 ka; Clark et al., 2009) to present (Funder et al., 2011) with few studies on local glacier change. In our field area, the LGM ice margin likely extended to the continental shelf edge ∼100 km offshore from the present coastline (Funder et al., 2011; Vasskog et al., 2015; Winsor et al., 2015b), and GIC across the Sukkertoppen area coalesced
Sediment coring and analysis
During summer 2014, we cored two proglacial lakes (“Crash” and “Gnat” lakes; informal names; Fig. 2, Fig. 3) to capture a continuous history of Holocene mountain glacier change in the Sukkertoppen region. We recovered sediment cores from each lake basin using a modified Universal Coring system (Nesje, 1992) deployed from an inflatable raft. A handheld GPS and a tape measure were used to determine the locations and water depths of the coring sites in each lake (Table 1), and bathymetric data
Lacustrine sediment records
Three sediment cores were recovered from Gnat Lake (Fig. 6, Table 1, Table 2). Sediment cores 14GNT-B3, 14GNT-D2, and 14GNT-E2 are 58.5, 93.0, and 86.5 cm-long, respectively, and display similar downcore stratigraphy but with varying unit thicknesses (Fig. 6). Cores 14GNT-B3 and 14GNT-D2 are composed of a basal unit of gray mineral-rich sediment, middle units of laminated organic-rich sediment, and a top unit of light gray mineral-rich sediments that grade to yellow (Fig. 6). Basal mineral-rich
Fluctuations of glaciers in the Baffin Bay region
Here, we summarize our GIC reconstructions from the Sukkertoppen region and compare them with records of GIC and GrIS margin fluctuations from elsewhere around Greenland and Baffin Bay. Following regional deglaciation of the area ∼10 ka (Lesnek and Briner, 2018), a brief interval of increased mineral-rich input shows that GIC retreat during the early Holocene may have been interrupted by an episode of glacier advance at ∼9 ka, coeval with the deposition of nearby ice sheet moraines (Qátqatsiaq
Conclusions
Our multi-proxy glacier-size reconstruction illustrates that GIC in the Sukkertoppen Iskappe area experienced centennial-scale variations superimposed on the millennial-scale insolation-driven net increase in glacier size during the Holocene. The Crash Lake record reveals an ice cap advance at ∼9 ka that is coeval with GrIS moraines in the study area (Lesnek and Briner, 2018) and in the Disko Bugt region (Young et al., 2011, 2013b), implying that both the western GrIS margin and GIC responded
Acknowledgements
We would like to thank CH2M Hill Polar Field services for logistical support and the United States 109th Air Lift Wing Air National Guard. We are grateful to PolarTREC for supporting Christina Ciarametaro for research opportunities on Greenland. We thank A. Lesnek for field assistance, S. Cronauer, S. Clements, and C. Sbarra for contributions to lab work in the University at Buffalo Cosmogenic Isotope Laboratory, and C. Beel and B. Goehring for assistance with lab work at Purdue University and
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