Late Holocene glacier reconstruction reveals retreat behind present limits and two‐stage Little Ice Age on subantarctic South Georgia

Abstract

Observational data show that climate in the Southern Ocean region is rapidly changing. However, past the instrumental period, our understanding of climate variability in the region is limited by a scarcity of high‐resolution palaeoclimate records. Alpine glaciers, present on many Southern Ocean islands, may provide such data because changes in their mass balance, extent and erosion rates often mark a response to climate shifts. Rock flour, the fine‐grained fraction of the glacial erosion process, is suspended in meltwater streams and transferred into the sediments of downstream lakes, continuously recording glacier variations. Here, we utilize this relationship to present a reconstruction of the Late Holocene glacier history of subantarctic South Georgia, using sediments from the glacier‐fed Middle Hamberg Lake. To fingerprint a glacial erosion/size signal, we used titanium counts, validated against changes in sediment density and grain size, allowing a continuous reconstruction of glacier variations over the past ∼1250 years. Together with local moraine evidence and supporting evidence from other Southern Hemisphere glaciers on New Zealand and in Patagonia, our findings reveal a series of consecutively diminishing Late Holocene advances. In addition to a glacier maximum before 1250 cal a BP, these include a two‐stage Litle Ice Age with advances around 300 and 120 cal a BP, in line with evidence from southern Patagonia. In addition, we present evidence for an unreported retreat behind present limits around 500 cal BP. Copyright © 2017 John Wiley & Sons, Ltd.

Cirque Glacier on South Georgia Shows Centennial Variability over the Last 7000 Years

“…This glacier advance is also evident from the bog record, where a 4.5 cm thick silt layer with high χbulk values was deposited at 450 ± 200 cal BP. The LIA is also captured by Van Der Bilt et al (2017) at 300 cal BP (Figures 11A,B) and widely documented on the island (e.g., Clapperton et al, 1978Clapperton et al, , 1989aGordon and Timmis, 1992;Roberts et al, 2010). Clapperton et al (1989b) estimated an average ELA depression of 50 m during the LIA on the northeast coast of South Georgia.…”

“…Cirque and valley glaciers were at its most advanced position in the 1930s, while larger valley and tidewater glaciers reached their maximum glacier extent in the 1970s. Such a glacier advance is also documented for the Hamberg glacier by Van Der Bilt et al (2017). Furthermore, during the recession phase after the twentieth century advance, many cirque glaciers deposited annual moraines (Gordon and Timmis, 1992), such as the ones observed in the innermost moraine cluster.…”

“…The energy in the hydrological system of the valley is high, as indicated by the coarser grain fractions and the input of terrestrial organic material, and the contribution of precipitation vs. glacier advance to the increased χbulk values are difficult to disentangle. However, based on the sensitive response of present cirque glaciers on South Georgia (Van Der Bilt et al, 2017), we find it likely that Diamond glacier advanced within short time after 2700 ± 150 cal BP. The change in the sediment records coincides with a shift in climatic conditions, which is recorded in various proxy records from South Georgia.…”

“…Since we combine two different measures of magnetic susceptibility in the lake, we standardize the χbulk and κbulk results ( Figure 11A). To account for glacier response time, we apply 30-year bins in the presentation of the glacier reconstruction following Van Der Bilt et al (2017).…”

“…At present, only two continuous reconstructions of past glacier change exist: Rosqvist and Schuber (2003) provides a sediment record from Block lake (Husvik, Figure 1) based on gray-scale density, loss-on-ignition and grain-size spanning from 7400 to 1000 cal BP ( Figure 9C), while Van Der Bilt et al (2017) reconstruct the last 1250 years with a record from the Middle Hamberg lake (south of Grytviken, Figure 1) based on titanium counts validated against sediment density and grain size ( Figure 9B). Both studies suggest centennial-scaled variability, although we do not know whether these records are representative for South Georgia.…”