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

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

(Table S2) Age determination of ODP Sites 202-1234 and 202-1235

Antarctic Intermediate Water is, at present, a water mass that brings oxygen to intermediate depths throughout the Southern Hemisphere oceans. Models have suggested that intermediate waters had higher concentrations of oxygen during the last glacial period (Meissner et al., 2005, doi:10.1029/2004PA001083; Liu et al., 2002, doi:10.1029/2001GL013938), consistent with globally reduced denitrification (Galbraith et al., doi:10.1029/2003PA001000) and increased production of Antarctic Intermediate Water (Lynch-Stieglitz and Fairbanks, 1994, doi:10.1029/93PA02446). However, some palaeoceanographic reconstructions (Bostock et al., 2004, doi:10.1029/2004PA001047; Pahnke and Zahn, 2005, doi:10.1126/science.1102163) have indicated that production decreased in the southeast Pacific Ocean at this time. Here we analyse the concentrations of Re and Mn, the sedimentary concentrations of which are controlled by the amount of dissolved oxygen at the sea floor, from three sediment cores located along the Chilean margin for the past 30,000 years. Our results from the cores, which bracket the present-day water-column extent of Antarctic Intermediate Water, show that the depth range of well-oxygenated Antarctic Intermediate Water increased off Chile during the Last Glacial Maximum. Dissolved oxygen content began to decrease approximately 17,000 years ago, coincident with rapid Antarctic warming and a poleward shift of the southern westerly winds (Anderson et al., 2009, doi:10.1126/science.1167441). Our estimates of productivity from accumulation rates of organic carbon and opal do not co-vary with the seafloor oxygen variations, ruling out local control of seafloor oxygenation. We conclude that the data are best explained by a combination of increased oxygenation and increased flux of Antarctic Intermediate Water during the Last Glacial Maximum

Th-normalized fluxes, mass accumulation rates and age models of ODP Sites 202-1233 and 202-1234

Biogenic particle flux was reconstructed using 230-Thorium normalization at two sites on the southern Chile margin. ODP Site 1233 at 41°S, 838 m depth, is at the southern limit of the Peru-Chile upwelling system, where the northern extent of the Antarctic Circumpolar Current impinges on the South American continental margin. ODP Site 1234, at 36°S, 1014 m depth, is located within the core of the coastal upwelling system near the mouths of the Bio Bio and Itata Rivers. At 41°S, opal, lithogenic and carbonate fluxes are greatest during the Last Glacial interval (26-20 ka), carbonate has a secondary peak during the mid Holocene (~8 ka) and organic carbon fluxes increase slightly from 17 ka to the present. At 36°S, large lithogenic fluxes are observed both during the Last Glacial interval and the Holocene, and a maximum in organic carbon flux is observed during the late Holocene (~5 ka) without an accompanying peak in opal flux. These reconstructed fluxes at 36°S and 41°S fit within a larger latitudinal pattern of a poleward increase in the magnitude of opal flux during the glacial period. The pattern of normalized opal flux, opal mass accumulation rate and opal:carbonate ratios is consistent with either i) enhanced supply of Si from the Southern Ocean, as proposed by the Silicic Acid Leakage Hypothesis or ii) enhanced Si and Fe delivery from land, driven by glacial erosion. The pattern of reconstructed export production supports our view that the appearance of more reducing conditions in the sediments upon deglaciation was most likely driven by decreased ventilation, rather than increased local productivity

Geochemistry of ODP Site 202-1233 and river sediments

Bulk sediment chemistry from three Chilean continental margin Ocean Drilling Program sites constrains regional continental erosion over the past 30,000 years. Sediments from thirteen rivers that drain the (mostly igneous) Andes and the (mostly metamorphic) Coast Range, along with existing rock chemistry datasets, define terrestrial provenance for the continental margin sediments. Andean river sediments have high Mg/Al relative to Coast-Range river sediments. Near 36°S, marine sediments have high-Mg/Al (i.e. more Andean) sources during the last glacial period, and lower-Mg/Al (less Andean) sources during the Holocene. Near 41°S a Ti-rich source, likely from coast-range igneous intrusions, is prevalent during Holocene time, whereas high-Mg/Al Andean sources are more prevalent during the last glacial period. We infer that there is a dominant ice-sheet control of sediment sources. At 36°S, Andean-sourced sediment decreased as Andean mountain glaciers retreated after ~17.6 ka, coincident with local oceanic warming and southward retreat of the Patagonian Forest and, by inference, westerly winds. At 41°S Andean sediment dominance peaks and then rapidly declines at ~19 ka, coincident with local oceanic warming and the earliest deglacial sea-level rise. We hypothesize that this decreased flux of Andean material in the south is related to rapid retreat of the marine-based portion of the Patagonian Ice Sheet in response to global sea-level rise, as the resulting flooding of the southern portion of the Central Valley created a sink for Andean sediments in this region. Reversal of the decreasing deglacial Mg/Al trend at 41°S from 14.5 to 13.0 ka is consistent with a brief re-advance of the Patagonian ice sheet coincident with the Antarctic Cold Reversal