Expeditions into the Past: Paleoceanographic Studies in the South Atlantic

The South Atlantic is tightly coupled to the North Atlantic climate amplifying system. At present, enormous amounts of heat are delivered across the equator to the north, with surface and subsurface waters. The return flow occurs at depth, within the coldwater sphere. During the last glacial the Atlantic Heat Conveyor was much less efficient, that is, the North Atlantic heat piracy is a positive feedback on climate change. This positive feedback is an important ingredient in the orbitally driven climate cycles. The current (that is, late Quaternary) conditions in the South Atlantic are the result of a long evolution of climate and geographic boundary conditions, which started with the opening of the basin at the end of the Jurassic and in the early Cretaceous, by continental breakup and seafloor spreading. Todays margins contain the ancient deposits of a narrow trough with restricted access, including evaporites. Warm-ocean sediments accumulated during the Cretaceous, including organic-rich deposits indicative of poorly oxygenated deep waters. Sinking of the sea floor from cooling of the lithosphere, and ridges produced as hot spot tracks (from Tristan da Cunha on the Mid-Atlantic Ridge) determined the main features of the bathymetry. The leitmotifs of Cenozoic evolution are general cooling (from mountain building and associated regression, and from reduction of atmospheric CO₂), the closure of the world-encircling tropical Tethys Ocean and opening of passages in the south, linking ocean basins through a circumpolar Cold Ring. Overall regression and associated polar deepwater production forced new patterns of biogenous deposition which resulted in a large-scale global drop of the Carbonate Compensation Depth (CCD) about 40 million years ago. At the same time, the isotopic ratio in the element strontium in seawater (as captured by calcareous fossils) started a long trend toward more radiogenic values, indicating increased supply of continental material. The major reorganization in deepsea sediments (the Auversian Facies Shift) in the late Eocene is also expressed as the onset of deposition of rather pure pelagic carbonates, with opaline and organic-rich sediments being increasingly restricted to ocean margins. Continued cooling eventually led to large-scale deepwater formation in high latitudes, which is expressed in the first great cooling step in the deep sea, at the end of the Eocene. The second great cooling step saw the buildup of ice on Antarctica, roughly 15 million years ago, presumably after considerable reduction of atmospheric CO₂. The third great cooling step consists of ice buildup around the North Atlantic, a step that moved the system into modern climate dynamics. Concerning the third step, it is commonly surmised that the closing of the Panama Straits was responsible for its timing (about 3 million years ago). We propose (Panama Hypothesis) exactly the reverse: in fact, the emergence of the Isthmus greatly favored North Atlantic heat piracy, so that northern glaciations were delayed by several million years. After initial onset of northern glaciations (7 to 6 million years ago) it took another 3 million years of mountain building and CO₂ reduction to attain sustained glaciations (3 million years ago). This period of delay is the well-know warm period of the early Pliocene. The story of the onset of northern glaciations is further complicated by the fact that cooling first enhances NADW production, before the onset of northern glaciations, and then obstructs it, presumably by reduction of evaporation and by sea ice formation. The identification of the switch point of NADW production, from negative to positive feedback, is vital for the understanding of the ocean’s role in climatic change in the late Neogene.