Cumberland Bay (South Georgia) glacial evolution during the Holocene

Abstract

Today, South Georgia is heavily glaciated with glaciers often terminating into the ocean via steep-sided bay and U-shaped valleys. However, due to a low number of well-dated climate records from the island, there is uncertainty about how glaciers responded to Holocene climate variability. Here, we reconstruct the glacial evolution of Cumberland Bay, one of the most dynamic glacial systems in the world, from marine sediment core GC673 (ca. 9.7 cal. Kyr BP to Present; ca. 6 m from the nearest land). We used benthic foraminiferal and diatom assemblages, biogenic silica, alkenones and pXRF to infer Holocene glacial evolution in Cumberland Bay. The relative abundance of the benthic foraminifera Fursenkoina fusiformis is interpreted as a proxy for more intense diatom blooms resulting from increased terrestrial runoff associated with the spring-summer melting of glaciers. The F. fusiformis abundance correlates well with diatom concentration accumulation rates and peaks in both proxies correspond to elevated (but low) abundance of sea ice diatom taxa. A sequence of several glacial advancement events can be recognized. The high productivity of the early Holocene is associated with melt associated with the retreat of glaciers into the inner fjord during the early Holocene warm period. Subsequent advancements seem to have two different causes. The first mid-Holocene advancement corresponds to a decrease in alkenone-derived palaeotemperatures from GC673. The two advancements from the late Holocene correspond to increases in published LOI data suggested as indicating increased strength of the South Westerly Winds (SWW) at the latitude of South Georgia which would have increased winter snowfall aiding the growth of glaciers. Our proxies are aligned with and build upon published glacial trends previously constrained with plant macrofossil and pollen evidence from nearby peat bogs and dated glacial moraines. We conclude that the primary driver of productivity at site GC673 were diatom blooms associated with spring/summer melt of glaciers whose growth is partially associated with strengthened SSW.