British Antarctic Survey
South Scotia Ridge and South Orkney Plateau
Geological and geophysical surveys of the South Scotia Ridge
The southern Scotia Sea is located in a unique location for studying environmental change for two reasons. First, seafloor ridges like the South Scotia Ridge, and the sub-Antarctic islands act as topographic barriers to ocean currents moving from the Weddell Sea to the South Atlantic, with basin areas providing some routes through for cold dense waters from the Weddell Sea. Second, glaciers and ice caps on the islands themselves are particularly sensitive to climatic change (known as “sentinels” of change) because they are located in the transition zone between the isolated, cold, Southern Ocean and the better-connected, warmer mid-latitude oceans. Thus, the aims of this project are two-fold:
(i) to understand how and when the South Scotia Ridge formed and its impact on ocean circulation;
(ii) to investigate the last glaciation and retreat of the South Orkney ice cap and its sensitivity to climate variability.
By analysing the composition and geochemistry of basement rocks from the South Scotia Ridge we can determine the age of the topographic highs, for example, did these banks form as part of a Cretaceous-aged granite batholith belt identified in Tierra del Fuego (Eagles, 2006) with similar a magnetic anomaly signature? Also, by retrieving seafloor cores from the tops of the highs we can analyse conditions at the seafloor (from things like isotopes and the species of microfossils) back through time. We do this to look for past changes in ocean conditions including: variability in influence of the Antarctic Circumpolar Current vs. Weddell Sea waters, and variability in primary productivity and sea-ice extent during recent West Antarctic Ice Sheet expansions, as well as during the warm interglacials. This work complements IODP Leg 382 (scheduled 2019), to drill sediments deposited in the basins bordered by Bruce and Pirie banks.
The approach in our second work area, the South Orkney Plateau (SOP), is to perform geophysical surveys to map glacial landforms and sediments on the seafloor in large trenches or troughs that we believe were carved by fast-flowing ice during the last glaciation (c. 20 000 years ago). For example, suites of small (<5 m high) moraines, which can only be identified by high-resolution 3D mapping of the seafloor and sub-surface sediments, indicate “episodic” retreat of grounded ice back towards the landmasses. By taking seafloor cores targeted to sample the transition between subglacial and postglacial sediments we can add a chronology of South Orkney ice cap retreat to the record provided by the glacial landforms; longer cores can be used to identify significant climatic/oceanographic changes that may have driven the retreat. This work builds on previous cruises to the area (e.g. Dickens et al., 2014) and addresses a significant data gap on the southwestern SOP where the deglacial history is virtually unknown.
During DY088 we will deploy seafloor dredges from the RRS Discovery to sample volcanic material from the slopes of Discovery Bank as the prime target, and Bruce and Pirie Banks as secondary targets, and we will recover gravity cores on the tops of the highs. On the SOP we aim to survey in “Signy Trough” (south of Signy Island) and retrieve sediment cores both along the trough and on either side of a significant mid-shelf bank interpreted as the extent of the South Orkney ice cap during the last glacial. The geoscientific expedition team is includes hard-rock geologists, geophysicists and palaeoceanographers, as well as several students who are “first-timers” at sea!
Check the BAS Twitter feed here for more information during the cruise.
The cruise blog can also be found here