Glacial to holocene watermass and continental weathering reconstructions from the Southeast Pacific Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/sj1394896

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  • Sediments of the central Chile margin record changes in ocean circulation and continental erosion associated with large–scale climate change. Here Antarctic– influenced Southern Ocean currents flow equatorward, forming a link between the high– and low–latitude oceans. Part of this link, Antarctic Intermediate Water, is an important conduit that ventilates South Pacific subsurface and mid–depth waters. Differential weathering of the two parallel mountain ranges of Chile has been attributed to latitudinal shifts in the position of the southern westerly wind belt, with an equatorward shift increasing Coast Range rainfall and weathering rates. An increase in Coast Range weathering relative to Andean weathering has been proposed to reduce the delivery of iron to the marginal ocean, with implications for productivity in the Fe– limited Southern Ocean–derived waters. Sediments from three drill sites along the Chile margin (36°S–41°S, water depths ~500–1000 m) monitor changes in the extent and distribution of surface and subsurface ocean currents, as well as provenance of continental weathering products. Concentrations of the redox–sensitive metals Mn and Re in the sediments are controlled by bottom water oxygen. Decreasing concentrations of Mn and increased levels of Re relative to glacial–age concentrations suggest a deglacial poleward retreat of Antarctic Intermediate Water and subsequent expansion of low–oxygen water masses sourced from low latitudes. A suite of elements defines lithogenic contributions to the sediments, and a river sediment dataset provides a link to possible source regions on the continent. Rivers draining the Chilean Coast Range have similar Al concentrations to Andean rivers, but are poorer (relative to Al) in Ca, Mg, and Sr. Although Andean rocks have been hypothesized to be relatively iron–rich, levels of Fe/Al and Ti/Al are indistinguishable among river types. Marine sediments show reduced values of Mg/Al relative to glacial levels after deglaciation, suggesting a switch to a Coast Range source of material. This analysis implies that at latitudes south of 36°S ice sheet retreat after the glacial period caused an overall decrease in the rate of Andean weathering, which overwhelmed any signal of decreased Coast Range weathering driven by the poleward retreat of the westerly winds. In southern Chile (41°S) increased Holocene Ti/Al is interpreted as a switch after deglaciation to a Coast Range source rich in titanium. While weathering patterns, driven mainly by ice sheet changes, are not likely to change in a warming planet, recent observational trends in ocean circulation and intermediate water oxygen content suggest a future expansion of oxygen–minimum zones.
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