Paleoproductivity of the Southern Chilean Margin over the past 30,000 years

Productivity on ocean margins is highly sensitive to climate change, including changes in macro-and micro nutrient inputs from land, upwelling intensity, and nutrient content of upwelled waters. Coastal productivity may also contribute to climate change, through impacts on carbon sequestration, sediment and water-column redox state (and hence rates of denitrification), and production of dimethyl sulfide. Reconstructing margin productivity can thus provide insight into the nature and causes of past climate change. We examined productivity off southern Chile at ODP Site 1233. At 41°S, 838 m depth, the site is at the core of Antarctic Intermediate Water (AAIW). It is at the southern limit of the Peru-Chile upwelling system, where the northern extent of the Antarctic Circumpolar Current (ACC) impinges on the South American continent. Paleoproductivity was reconstructed by normalizing biogenic fluxes to the flux of 230Th. Over the last 30ky, maximum organic carbon, opal and carbonate fluxes occurred during the last glacial interval (26-20 ky BP). A steady decline is seen in carbonate flux from the middle Holocene (~8 ky BP) to present, organic carbon flux increases from the late Holocene (~5 ky BP) to present while opal flux is essentially unchanged from 20 ky BP to present. The pattern of reconstructed productivity is consistent with a more northerly position of the ACC during the last glacial interval, bringing the core of high-nutrient waters to 41°S. However, maximum prodctivity is observed well prior (~5 ky) to the deglacial increase in alkenone- reconstructed SST at this site. The productivity maximum is coincident with a maximum in glacier extent, and in the flux of terriginous material to the site. High glacial productivity may therefore have been supported by enhanced macro or micro-nutrient delivery from land. The pattern of reconstructed productivity implies that the less reducing conditions during the glacial interval, inferred from authigenic metal accumulation, were driven by increased ventilation rather than changes in local productivity.