Abstract
A 7000 year-long cirque glacier reconstruction from South Georgia, based on detailed analysis of fine-grained sediments deposited downstream in a bog and a lake, suggests continued presence during most of the Holocene. Glacier activity is inferred from various sedimentary properties including magnetic susceptibility (MS), dry bulk density (DBD), loss-on-ignition (LOI) and geochemical elements (XRF), and tallied to a set of terminal moraines. The two independently dated sediment records document concurring events of enhanced glacigenic sediment influx to the bog and lake, whereas the upstream moraines afford the opportunity to calculate past Equilibrium Line Altitudes (ELA) which has varied in the order of 70 m altitude. Combined, the records provide new evidence of cirque glacier fluctuations on South Georgia. Based on the onset of peat formation, the study site was deglaciated prior to 9900±250 years ago when Neumayer tidewater glacier retreated up-fjord. Changes in the lake and bog sediment properties indicate that the cirque glacier was close to its maximum Holocene extent between 7200±400 and 4800±200 cal BP, 2700±150 and 2000±200 cal BP, 500±150-300±100 cal BP, and in the 20th century (likely 1930s). The glacier fluctuations are largely in-phase with reconstructed Patagonian glaciers, implying that they respond to centennial climate variability possibly connected to corresponding modulations of the Southern Westerly Winds.
Highlights
The accelerated melting of glaciers in the Southern Ocean (Cook et al, 2010; Scambos et al, 2014; Koppes et al, 2015) manifests rapid climate change over the last decades (Gille, 2002, 2008; Vaughan et al, 2003; Böning et al, 2008)
We argue that the input of minerogenic sediments to the lake is positively correlated with glacier size (Section Reconstructing the Glacier)
We argue that Diamond glacier likely advanced in the 1930s, which is the case for cirque and small valley glaciers across the northeastern coast of South Georgia (Hayward, 1983; Gordon and Timmis, 1992; Gordon et al, 2008)
Summary
The accelerated melting of glaciers in the Southern Ocean (Cook et al, 2010; Scambos et al, 2014; Koppes et al, 2015) manifests rapid climate change over the last decades (Gille, 2002, 2008; Vaughan et al, 2003; Böning et al, 2008). Anthropogenic forcing probably explains parts of this regional-scaled glacier retreat, strong regional feedback mechanisms still predominate (Thompson et al, 2011; Wang and Cai, 2013). Many of these mechanisms can be traced back to the Southern Westerly Winds, which are closely linked to the Southern Ocean’s uptake of CO2 through their effect on upwelling of carbon-rich deep water, and have the potential to affect climate globally (Lenton and Matear, 2007; Le Quéré et al, 2007; Hodgson and Sime, 2010; Sigman et al, 2010). Moraine chronologies do not capture retreat phases or length of events, and cannot exclude the possibility that moraines have been erased by subsequent glacier advances of equal or larger magnitude (Balco, 2009)
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