An evaluation of radiogenic isotopes as tracers of ocean circulation and sediment transport: modeling, seawater, and sediment studies.

摘要

The role of ocean circulation in climate change is an important open question in paleoceanographic research. The Nd isotope composition of authigenic (in situ forming) sediments has emerged as a circulation tracer with the potential to help to resolve this ongoing debate. The use of Nd isotopes for paleoceanographic reconstructions assumes that water masses are tagged with distinct isotope compositions and that downstream from their source areas changes in the isotope ratio reflects water mass mixing. This use of Nd isotopes assumes that they behave like a conservative tracer (i.e. as a "quasi-conservative tracer"). However, the degree to which Nd is a "well-behaved" tracer, given the complexities of circulation in the ocean, is unclear. In the first chapter of this thesis we put the current day debate in context with the early work on Nd isotopes in the ocean and in authigenic sediments. In the second chapter we use an ocean general circulation model to test some simple scenarios to address the processes governing the behavior of Nd in the modern ocean. The results of this study are consistent with quasi-conservative behavior, except for the deep Pacific, which requires an internal source of Nd.;Another important question regarding the behavior of Nd in the ocean is the degree to which exchange between seawater and sediments at ocean margins affect Nd isotope ratios. Margin exchange holds the potential to overprint the distinct isotope compositions of water masses with the isotope signal of local margin sediments, thus modifying the original water mass isotope signature and preventing its use as a quasi-conservative tracer. We measured seawater samples in depth profiles from the Agulhas Current region near southern Africa to test for the presence of boundary exchange and conservative water mass mixing in this region with strong boundary currents. Our analyses suggest that the surface ocean is generally consistent with input from margin sediments, but the deep ocean waters are consistent with conservative mixing of major global end-member water masses. These results suggest that Nd behaves like a quasi-conservative tracer and that boundary exchange does not occur along this margin.;The trace element contents and radiogenic isotope ratios of the terrigenous elastic fraction of marine sediments can be used to trace the sedimentation processes that arc influenced by climate (e.g. ocean circulation and mineral dust production). We report trace and major element analyses, grain size distributions, and Sr and Nd isotope ratios in the fine fraction (63mm) of terrigenous marine sediments. The results suggest that sediment deposited on the Mid-Atlantic Ridge (MAR) downwind from the Sahara reflect average Saharan dust, based on grain size patterns that are similar to Pacific eolian dust and narrow ranges of Nd and Sr isotope compositions that are consistent with modern aerosols.;Samples from the Eastern Atlantic Basin display a much more variable Nd and Sr isotopes compositions and grain sizes that are consistent with hemipelagic sedimentation, which may reflect redistribution along the ocean margin, or local dust sources that have not reached equilibrium transport in the atmosphere. Samples on the western flank of the MAR also display grain size distributions that are similar to Pacific eolian dust, but are inconsistent with any possible dust source area. These results suggest that the grain size distribution can aid the identification of eolian sediments, but they cannot identify eolian sediments without supporting evidence from geochemical tracers.;Samples from the sub-polar North Atlantic suggest that a significant amount of ice-rafted detritus is transported across the MAR during the Holocene. This pattern is stronger during the Last Glacial Maximum (LGM). A transition from Nd and Sr isotope ratios that are more like volcanic islands (such as Iceland) during the Holocene to values that are more like the continental crust surrounding the North Atlantic basin during the LGM suggests that transport of sediment from the surrounding continents to this basin was increased during the LGM. A comparison of sediments from the western Atlantic basin and bottom waters from the western Atlantic nepheloid layer suggests that the Nd contained in the terrigenous fraction of marine sediments may be isolated from interacting with seawater.