Award: OPP-1542962

With support from this award we collected set of sediment cores and seawater samples from expedition 1702 of the research vessel Nathaniel B. Palmer along a transect at 170° W in the Ross Sea sector of the Southern Ocean. In the cores with the highest accumulation rate we found clear evidence for a change in redox status of the sediments at the end of the last ice age approximately 20,000 years ago. Specifically, we found a large peak in the manganese concentration of sediments deposited just after the end of the last ice age. We have combined these results with unpublished results from the equatorial Pacific, together with published results from cores collected along the margin of Antarctica, to show that the deep water of entire the Pacific Ocean and Southern Ocean had much lower oxygen concentrations during the Pleistocene ice ages than is the case today. Lower oxygen concentrations translate directly into increased storage of carbon dioxide. These findings are consistent with a growing body of evidence for greater storage of carbon in the deep sea during the Pleistocene ice ages, thereby explaining the remarkable correlation between atmospheric carbon dioxide and temperature of Antarctica over the last 800,000 years that has been extracted from ice cores. In addition, Columbia undergraduate Kelly-Marie Powell used our cores to test the hypothesis that iron supplied to the iron-starved waters of the Southern Ocean by debris released from melting ice bergs (ice-rafted debris, IRD) would stimulate the growth of phytoplankton and thereby increase the amount of carbon sequestered in the deep sea. She unambiguously falsified this hypothesis by showing that there is no correlation between the abundance of IRD and indicators of biological productivity. From the analysis of seawater samples we found that lateral transport by isopycnal mixing is a large term in the mass budgets of 230Th and 231Pa in the Southern Ocean. This discovery will require that certain conclusions about climate-related changes in ocean circulation based on sedimentary 231Pa/230Th ratios, relying on the assumption that net lateral redistribution of 231Pa occurs primarily or even entirely by advection, be reinterpreted. Under circumstances where is the mass budget of 230Th is affected by the lateral transport, as was found for the Southern Ocean (see above), the use of 230Th as a constant flux proxy for determining sediment accumulation rate has been thought to be compromised. However, PhD student Frankie Pavia found a way to correct for the lateral transport of 230Th and thereby calculate thorium normalized fluxes of dust to this region of the Southern Ocean where dust supply is extremely low and, consequently, iron limitation is thought to regulate the efficiency of the biological carbon pump. Results of this study support the growing body of evidence that the tight correlation between atmospheric carbon dioxide and earth’s climate was caused by greater storage of carbon the deep ocean during the Pleistocene ice ages. This increased storage of carbon must be related in some way to a greater stratification of the deep water masses of the ocean under ice-age environmental conditions. Results from cores collected during this project further demonstrate that fertilization of marine plankton by iron released from melting ice bergs cannot be called on to explain the correlation between carbon dioxide in the atmosphere and Earth’s climate. From the study of seawater samples we have found that lateral mixing supplies a large amount of particle-reactive elements, that is, elements prone to absorption to particles, to the Southern Ocean. This finding will require that certain interpretations of changes in ocean circulation over glacial to interglacial cycles be re-examined. Application of a new approach that accounts for the lateral transport of 230Th allows the well-established thorium normalization approach for determining sediment accumulation rate to be applied in the Southern Ocean despite this lateral transport that we discovered. Last Modified: 08/26/2020 Submitted by: Robert Anderson