Variability of oxygenation index in the Southwestern South Atlantic Ocean during Heinrich Stadial I based on geochemical proxies

Oxygen Minimum Zones (OMZ) are water layers characterized by low oxygen saturation state in response to a complex interplay of biological, chemical, and physical processes. Modern OMZs are typically found along the western side of continents (i.e., Arabian Sea, Eastern Pacific Ocean, Eastern Tropical Atlantic Ocean, and the Southeast South Atlantic Ocean). Low oxygen conditions can however be more widely prevalent in shallower, continental shelf environments. Here, we study the evolution of bottom water oxygen conditions of Brazil’s continental shelf Santos Basin from the Southeast Atlantic. The Santos Basin which is located near the Cabo Frio upwelling system. We reconstructed bottom water oxygen conditions using the enhanced Benthic Foraminifera Oxygen Index (EBFOI) using samples from the Santos Basin Slope core C4-GC-2 ( 25°51.519’S/ 45°30.685’W, 395 m water depth). These data are integrated with mineralogical analysis and oxygen and carbon stable isotope data from the benthic foraminifera Cibicidoides spp. The age model was constructed based on four radiocarbon dating samples, which covers most part of the Heinrich Stadial 1 (HS1, 17.8–15.7 kyr). From 16.8 kyr to 16 kyr the continental shelf of Santos Basin was characterized by low oxic conditions as revealed by relatively low EBFOI values (1.9-5.3). The mineralogical analysis from the studied core revealed the presence of pyrite during this time interval, which together with geochemical proxy signatures, indicates low oxygenation of bottom-water conditions, with the development of localized anoxic microenvironments within the sediments. Notably, at 15.8 kyr marine oxygenation decreased to suboxic conditions (EBFOI = -20.3). Elevated δ¹⁸O values indicate cold conditions during HS1, likely associated with intensified upwelling, while low δ¹³C values are comparable to those recorded in Eastern Pacific intermediate waters during the same interval. The dominance of low-oxygen tolerant benthic foraminifera suggests reduced bottom-water oxygenation at ~395 m depth, consistent with a shoaling or expansion of the regional OMZ rather than methane seepage. These conditions were likely sustained by poor ventilation during HS1, limiting benthic foraminiferal diversity.

Keywords: Benthic Foraminifera Assemblage, Isotope Geochemistry, Oxygen Minimum Zone