Abrupt changes in high-latitude nutrient supply to the Atlantic during the last glacial cycle

The supply of nutrients to the low-latitude thermocline is largely controlled by intermediate- depth waters formed at the surface in the high southern latitudes. Silicic acid is an essential macronutrient for diatoms, which are responsible for a signifi cant portion of marine carbon export production. Changes in ocean circulation, such as those observed during the last deglaciation, would infl uence the nutrient composition of the thermocline and, therefore, the relative abundance of diatoms in the low latitudes. Here we present the fi rst record of the silicic acid content of the Atlantic over the last glacial cycle. Our results show that at intermediate depths of the South Atlantic, the silicic acid concentration was the same at the Last Glacial Maximum (LGM) as it is today, overprinted by high silicic acid pulses that coincided with abrupt changes in ocean and atmospheric circulation during Heinrich Stadials and the Younger Dryas. We suggest these pulses were caused by changes in intermediate water formation resulting from shifts in the subpolar hydrological cycle, with fundamental implications for the nutrient supply to the Atlantic

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°S, 42.51°W, 2,225 m water depth) aiming to unravel changes in terrigenous sediment input and bottom water conditions during the last ∼184 ka at the western South Atlantic middepth. The Al/Si, Fe/κ 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 δ13C 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)

Warmer western tropical South Atlantic during the Last Interglacial relative to the current interglacial period

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Tropical Atlantic stratification response to late Quaternary precessional forcing

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