U-series Disequilibrium in Weddell Sea Sediments: Rates of Iceberg Discharge and Impact on Global Oceans

Columbia University, New York NY

Investigators

Abstract

U-SERIES DISEQUILIBRIUM IN WEDDELL SEA SEDIMENTS: RATES OF ICEBERG DISCHARGE AND IMPACT ON GLOBAL OCEANS ABSTRACT Knowledge of the timescales of sedimentary processes is a fundamental pre-requisite for the study and understanding of the interplay between tectonics, climate and landscape evolution. Nevertheless, the vast majority of studies of geological archives focus on the age of sediments in the context of their deposition, in large part because sediment residence and transport time cannot be readily measured by absolute means. Here, the PIs propose to implement a novel approach termed ?comminution dating? (DePaolo et al., 2006), to determine the transport time of fine clastic sediments produced by Antarctic subglacial erosion and deposited in the Weddell Sea over the last ~250 ka. The transport time for this material probably varies between glacial-interglacial stages, and is related to the flux of iceberg discharge into the Southern Ocean, a process that is controlled by climate, and on longer timescales by tectonics. Thus, it is also related to the relative contribution of the Antarctic Ice Sheet to global sea level changes. The proposed study focuses on U-series disequilibrium in detrital material, which as shown in previous studies, is a measure of weathering and transport time. The time elapsed since the initial grinding of the bedrock by glacial abrasion that produces rock flour, including the transport process and age of deposition, is defined as the ?comminution age? of the sediment, and is determined by the relative degree of 234U-depletion in the samples. Preliminary results show significant 234U-depletion in the proposed sediments, whereby the radioactive decay (activity) ratio of parent and daughter uranium isotopes is less than unity (i.e., (234U/238U)<1). The results, which relate to several sub-disciplines in Earth Sciences, including geochronology, geochemistry, and paleoceanography, will provide conceptual and quantitative constraints on the rates of subglacial weathering in Antarctica during the late Quaternary, and their relation to climate changes and sea level changes. Furthermore, the results will provide the possibility to evaluate the impact of continental weathering on the evolution of marine (234U/238U) ratios, which is of importance in the context of U-Th dating methodology of corals. Another product of this study will be an archive of the long-term variations in the Weddell Sea authigenic end member composition, an important contributor to marine deepwater.