A Multi-Proxy Approach to Unravel Late Pleistocene Sediment Flux and Bottom Water Conditions in the Western South Atlantic Ocean

Magnetic signals in deep-sea sediments have the potential to unravel past continental environmental changes, via changes in primary terrigenous magnetic supply, but also record past marine environmental conditions, via in situ formation of secondary magnetic minerals, particularly when complemented by independent proxies. By combining environmagnetic, geochemical, and siliciclastic grain size data, we investigated marine sediment core GL-1090 (24.92 degrees S, 42.51 degrees W, 2,225 m water depth) aiming to unravel changes in terrigenous sediment input and bottom water conditions during the last similar to 184 ka at the western South Atlantic middepth. The Al/Si, Fe/kappa and siliciclastic grain size data show that terrigenous sediments at this core location derived from the Plata River (southeastern South America). This material was transported northwards by the Brazilian Coastal Current and their delivery to our core site was modulated by sea-level oscillations. Periods of low sea-level were characterized by the input of coarser and more abundant terrigenous sediments. Environmagnetic parameters indicate significant downcore variations in the magnetic domain state, which we interpret as changes in the content of biogenic magnetite following glacial-interglacial cycles. Coeval negative excursions in magnetic grain size and benthic delta C-13 suggests that concentrations of single domain magnetite (possibly magnetotactic bacterial magnetite) vary in response to middepth water ventilation. We suggest that reduced ventilation in the middepth western South Atlantic bottom waters during peak glaciations triggered a decrease in the production of biogenic magnetite. Peak glaciations were, in turn, linked with increases in the residence time of North Atlantic Deep Water (or its glacial counterpart).

Automated summary

The study reveals that sediments in marine sediment core GL-1090 from the southwestern South Atlantic Ocean are primarily sourced from the Plata River in southeastern South America and transported northward by the Brazilian Coastal Current, with delivery modulated by sea-level oscillations. Terrigenous sediment input significantly increased during periods of low sea level, while variations in biogenic magnetite content following glacial-interglacial cycles were observed based on environmagnetic parameters.