Background to the voyage
Background to the voyage
The Drake Passage, between the tip of Chile and the West Antarctic Peninsula, is the narrowest constriction for the Antarctic Circumpolar Current (ACC), and contains the world's strongest oceanic current. The wind driven ACC connects the three major ocean basins allowing transport of heat, freshwater and carbon, and down-welling in the Southern Ocean forms a significant component of global deep water. As a result the Southern Ocean plays an important part in the global overturning circulation system and in modulating global climate. Not only are modifications in the nutrient content, sea-ice cover and current strength of the ACC likely to have influenced past climate, but also the ecology and evolution of deep-water organisms in this area. Close faunal similarities have been found between the tip of South America (Magellan province) and the Antarctic shelves in both the fossil record and live benthic species. However, since the opening of the Drake Passage, some 30 million years ago the transport of benthic larvae across the strong ACC has been limited, creating a biogeographical barrier to species dispersal.
Reconstructing the oceanography in the Southern Ocean has been challenging in the Drake Passage region for a number of reasons. The most common way of making marine climate records is to focus on the changing chemistry of carbonate foraminifera in sedimentary cores. However the shallow Carbonate Compensation Depth in the Southern Ocean means that delicate foraminiferal carbonate is often dissolved away before it can accumulate on the seafloor. Paleoceanographic reconstructions have, therefore, typically focused on siliceous diatoms. A further problem particularly affecting the Drake Passage is that strong currents scour away sediments before they can accumulate, leaving bare rock exposed.
The hard skeletons from deep-sea corals form a unique, and under utilised record of the deep ocean, both in the recent and distant past. A number of species of deep-water scleractinian corals (both solitary and colonial) have been found on the Chilean and Antarctic shelves, and from fracture zones and seamounts that lie within the Drake Passage. A total of 24 species of azoozanthellate scleractinians are known from Antarctic waters, with seven of these species being exclusively from the fossil record. Although we know that deep-sea corals have inhabited this important region for many thousands of years there has been no focused attempt to visualize, characterize or collect specimens from deep-water coral habitats either within the Drake Passage or Scotia Sea.