Previous studies have interpreted evidence from South Atlantic sediments (between 40 and 50S latitude) to indicate that, during the last glacial period, there were (1) rapid changes in ocean currents that transport heat from the tropics to polar latitudes, and (2) rapid changes in biological productivity correlated with the changes in ocean currents. This project is designed to test the following hypotheses: 1) Changes in biological productivity were caused by the addition of iron, a growth-limiting essential nutrient, delivered to the region in the form of soil carried by icebergs from southern South America, and 2) The soil that provided the iron also carried a chemical signature (in the form of authigenic neodymium (Nd) isotopes) that introduced an artifact in the evidence used previously to infer past changes in ocean circulation.
To test these hypotheses, researchers will determine if the burial rates of organic carbon (a measure of biological productivity) and of soil from southern South America (identified by its unique composition of Nd and Sr isotopes) are correlated in sediments from cores in the mid South Atlantic at about 45°S. Research will also be conducted on fish teeth from the sediments to measure their authigenic Nd isotope composition because these fossils provide a more reliable measure of the Nd isotope composition of seawater than is obtained by leaching bulk sediments and fish teeth are not influenced by neodymium carried by soil.
Reconstructions of past climate-related changes in the ocean play a critical role in developing both conceptual and numerical models of climate variability that will ultimately serve to make predictions of future climate change, and of its consequences. Consequently, it is vital that these reconstructions be accurate. This is key broader impact of this research project. In addition, this study will both test cause and effect relationships underlying past changes in ocean biology and its role in the global carbon cycle, and test for potential artifacts that may have confused previous interpretations of climate-related changes in ocean circulation.
