Plio-pleistocene nutrient dynamics and export productivity in the California and Benguela Current upwelling systems

Multiple paleoceanographic geochemical proxies were measured in sediment cores from the Benguela and California Current Upwelling Systems to investigate the variability of export production and nutrient dynamics in two geographically disparate but oceanographically similar settings. Four late Pliocene upwelling records are compared to distinguish local and global controls utilizing organic geochemical, trace elemental, and δ13C_org and δ15N measurements combined with biogenic component accumulation rates. This multi-proxy approach creates a comprehensive view of carbon production, burial and oxidation rates, by combining organic carbon and carbonate carbon accumulation rates with redox sensitive metals, and of nutrient dynamics, utilizing δ15N to examine nitrate sources and supplies.; Cyclic variations in total organic carbon, CaCO₃ and opal accumulation rates are related to changes primarily in upwelling intensity and secondarily in the concentration of nutrients in the upwelled waters. Organic carbon accumulation rates, as a first order evaluation of export productivity, average ∼0.3 g/cm2ky under the California Current and ∼0.4 g/cm2ky under the Benguela Current. These late Pliocene fluxes are high and moderate respective to the long-term organic carbon records on each margin.; δ15N, opal and P data provide evidence for sub-surface water mass intrusion into both the California and Benguela margins during the late Pliocene. The addition of Southern Ocean water, rich in silica and nitrate, generated the Matuyama opal maxima with opal accumulation rates up to 9 g/cm2ky under the Benguela Current. Ventilation by North Pacific Intermediate Water on the California margin contributed a distinct source of nitrate and relatively low concentrations of phosphate. Overall, δ 15N values averaged 2.3‰ lower than modern values and sea surface temperatures were 5–10°C warmer than today in both hemispheres and ocean basins, hinting at global connections, related to ocean circulation and climate change that cannot be overprinted by local processes.