Meridional shifts in the northern boundary of the South Atlantic Subtropical Gyre during Termination V and MIS 11: a multiproxy approach

Subtropical gyres contribute significantly to climate regulation by constituting the main pathways for energy redistribution between low and high latitudes. The South Atlantic Subtropical Gyre (SASG) operates in a region that is critical to the Atlantic energy balance. Its northern boundary, defined by the southern branch of the South Equatorial Current (sSEC), constitutes an important interhemispheric connection for heat and salt exchange. The sSEC bifurcates in the western tropical South Atlantic, giving rise to the Brazil Current, which transports warm and saline tropical waters southward, and the North Brazil Current, which transports heat and salt northwestward. In addition to acting as a linkage between both Atlantic subtropical gyres, the North Brazil Current constitutes an essential part of the upper branch of the Atlantic Meridional Overturning Circulation. Recently, observational data have recorded a reduction in the intensity of heat and salt transport toward the North Atlantic, along with a southward displacement of the SASG. These phenomena are likely influenced by the progressive weakening of the Atlantic Meridional Overturning Circulation, detected since the late 20th century and projected to continue in the coming decades. The lack of long-term oceanic records with adequate spatial coverage for the South Atlantic basin prevents a more complete understanding of the trends and impacts associated with SASG displacements. Here we investigate meridional changes in the position of the northern boundary of the SASG during Termination V and Marine Isotope Stage 11, through a multiproxy approach to reconstruct upper-ocean water-column stratification from a sediment core in the western tropical South Atlantic. To this end, relative abundance counts of the planktonic foraminifer species Globorotalia truncatulinoides (dextral and sinistral) and stable oxygen isotope (δ¹⁸O) analyses of G. truncatulinoides (dextral) and Globigerinoides ruber albus have been conducted. Due to the deeper apparent calcification depth of G. truncatulinoides, the difference in the δ¹⁸O signal of both species (Δδ¹⁸Otrunca-ruber) functions as an indicator of thermocline depth. The strong association of G. truncatulinoides with regions of deep thermocline allows the establishment of a relationship between variations in species abundance and changes in the stratification of the upper ocean. Since deep thermocline conditions can be interpreted as a signature of the presence of both Atlantic subtropical gyres, the proxies employed allow tracking meridional shifts in the SASG. Preliminary results are promising and suggest that the northern boundary of the SASG varied meridionally on millennial and orbital timescales. Mg/Ca ratio analyses will be performed on both species to reconstruct surface and subsurface temperatures, as well as to discriminate the individual roles of temperature and salinity in upper-ocean stratification.