Strong altitudinal control on the response of local glaciers to Holocene climate change in southwest Greenland
Introduction
Recent worldwide observations show that local glaciers and ice caps (GICs) are responsible for ∼25% of the total global mean sea level rise (1993–2010), equivalent to 0.86 mm yr−1 (Church et al., 2013). The current contribution from the GICs in Greenland is 0.08 ± 0.03 mm yr−1 (Bolch et al., 2013). Projections of future mass loss from Greenland GICs range between 2116 ± 129 and 3907 ± 108 Gt by 2098 C.E. for conservative mid-range scenarios, which amounts to a mean global sea level rise between 5.80 ± 0.4 to 11.2 ± 0.3 mm (Machguth et al., 2013). The main driver of the observed changes in glacier mass balance has been shown to be increased summer temperatures; whereas winter precipitation contributed less significantly (Bolch et al., 2013, Machguth et al., 2013). Longer time series based on aerial photographs and other historical records also demonstrate a strong correlation between glacier fluctuations and changes in summer air temperature (Weidick, 1959, Weidick, 1963, Weidick, 1968, Gordon, 1981, Bjørk et al., 2012, Leclercq et al., 2012, Weidick et al., 2012).
On a decadal to millennial time scale, little is known about the GICs fluctuations in Greenland (Kelly and Lowell, 2009). Most information about long-term GIC fluctuations is derived from radiocarbon dating of reworked organic remains such as molluscs or plant macrofossils in historical moraines, radiocarbon dating of dead vegetation melting out of glaciers, or ice core data showing that the ice extent prior to the Little Ice Age (LIA) was smaller than at present (Weidick, 1963, Weidick, 1968, Sudgen, 1972, Kelly, 1980, Ingolfsson et al., 1990, Hjort, 1997, Landvik et al., 2001, Madsen and Thorsteinsson, 2001, Knudsen et al., 2008, Möller et al., 2010, Lowell et al., 2013, Schweinsberg et al., 2017). However, knowledge remains sparse of GICs fluctuations in Greenland and whether they survived past warmer conditions than today, e.g. the Holocene Thermal Maximum (HTM) ∼8-5 cal. ka BP and the Medieval Climate Anomaly (MCA) ∼1200-950 C.E. Only a few available studies have provided continuous records of Holocene glacier fluctuations in east Greenland (Lowell et al., 2013, Levy et al., 2014, Balascio et al., 2015) and west Greenland (Schweinsberg et al., 2017). These records show that local GICs were significantly reduced and most likely completely absent during the HTM. The number of well constrained mountain glacier records is extremely low compared to the total number of 20,281 GICs (∼90.000 km2) in Greenland (Rastner et al., 2012) and is in sharp contrast to the long tradition of recording past mountain glacier fluctuations and climate change in, for example, Iceland (Larsen et al., 2011a, Striberger et al., 2012, Schomacker et al., 2016), Scandinavia (Karlen, 1981, Nesje et al., 2000, Bakke et al., 2010), the European Alps (Ivy-Ochs et al., 2009), and North America (Menounos and Clague, 2008, Barclay et al., 2009, Briner et al., 2009).
In this study, we use sediment cores from three proglacial lakes to track upvalley Holocene glacier fluctuations in Kobbefjord, southwest Greenland and investigate whether surface summer temperatures and winter precipitation variability are synchronous with the observed changes at centennial scale.
Section snippets
Geographical setting
Kobbefjord (Danish name), or Kangerluarsunnguaq (Greenlandic name) is located ∼20 km from the capital Nuuk in southwest Greenland (Fig. 1). The fjord is ∼17 km long, 0.8–2.0 km wide, and is part of the Godthåbsfjord system (Mikkelsen et al., 2008). The climate in Kobbefjord is classified as low Arctic with a mean annual air temperature of 0.7 °C (2008–2010), ranging from 10.7 °C in (July) to −30 °C (January). The total annual precipitation is 838–1127 mm (2008–2010), and an average of 25–50% of
Methods
We use three proglacial threshold lakes – Badesø, Langesø, and Lake IS21 (informal names) – to constrain the ice marginal fluctuations of the local GICs in Kobbefjord, southwest Greenland (Fig. 2). The proglacial lakes are located adjacent to small ice caps, and by analyzing the sediment cores it is possible to constrain the glacier activity and determine if glaciers were completely absent during the Holocene (e.g. Karlen, 1981, Nesje and Dahl, 1993, Bakke et al., 2005, Briner et al., 2010).
Lake IS21
Lake IS21 (informal name; 64.17°N, 51.34°W) is located at 674 m a.s.l. between Kobbefjord and Godthåbsfjord (Fig. 2). It receives meltwater from the small Qasigiannguit ice cap (0.7 km2), which is located south of the lake between 680 and 1000 m a.s.l. (Fig. 2) (Machguth et al., 2016). A 55 cm long core was retrieved at 22 m water depth. The core, IS21, was subdivided into three units – a lower and upper silty clay unit and a middle organic-rich gyttja unit (Fig. 3). The lower unit (55-49 cm)
Discussion
Our results from lake sediment records in Kobbefjord show a variable response of GICs to Holocene climate changes (Fig. 7). This is surprising as the only differences between the three GICs are slight variations in size and altitudinal location. However, there is a clear correlation between the modest differences in GIC altitude in Kobbefjord and glacier sensitivity, in particular during the last ∼5–6 ka when temperatures have been gradually declining. Below, GIC history is discussed in
Conclusion
In this study, we used three sediment cores from proglacial lakes in Kobbefjord, southwest Greenland, to generate a high-resolution record of GIC fluctuations over the last ∼9.3 ka. We found a clear correlation between GIC altitude and glacial response. Our data revealed that GICs were present in Kobbefjord until ∼7.9 cal. ka BP after which they completely melted away due to high summer surface temperatures. After a prolonged period with no GICs, renewed glacier growth occurred at ∼5.5 cal. ka
Acknowledgements
This study was supported by Arctic Research Centre (ARC), Aarhus University. NKL was supported by a Villum Young Investigator Programme. AAB was supported by the Danish Counsil for Independent Research, grant DFF-610800469 and by the Inge Lehmann Scholarship from the Royal Danish Academy of Science and Letters. TAD and EJ were supported by the North Water Project (NOW.KU.DK) funded by the Velux Foundation, the Villum Foundation and the Carlsberg Foundation of Denmark. We would like to thank
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