New planktonic foraminifera-derived transfer function for the South Atlantic Ocean: Palaeoceanographic implications for the Brazil- Malvinas Confluence

Abstract. Planktonic foraminiferal assemblages are extensively used for reconstructing sea surface temperature through the application of transfer functions. Nonetheless, it has been observed that several parameters present throughout the water column also influence compositional changes within these assemblages. Selection of driving factors and evaluation of transfer function performances are method-specific processes that require the combination of prior ecological knowledge and objective variable selection approaches. In this study, we compiled a 171 core-top samples dataset of planktonic foraminifera and productivity-related variables to quantify the relationship between the assemblages and modern productivity conditions in the South Atlantic Ocean. Multivariate statistical analyses revealed that planktonic foraminiferal species were related to austral summer nitrate, explaining an independent and significant proportion of variance in the species data. We evaluated different prediction models, and estimated their performances considering spatial autocorrelation. The calibration model Weighted Averaging with tolerance downweighting and inverse deshrinking (WATOL_inv) with h-block cross validation showed a regression coefficient of r²_cv = 0.938, with a root-mean-square error of prediction RMSEP = 1.578 սmol l^-1. The resulting transfer function was applied then to sediment core GeoB2806-4 (~37° S – 53° W; 3500 m) in order to reconstruct variations of summer nitrate concentration during the Holocene. Our reconstructed summer nitrate shows a general decreasing trend from early to mid-Holocene associated with increased biological uptake, and a later increase of it towards the late Holocene. We suggest that changes in summer surface nitrate concentration are linked to the latitudinal shifts of the Brazil-Malvinas Confluence. Understanding the displacement of the Confluence, and the associated shifts in the upper layers’ nutrient availability, is crucial to evaluate the implications of these changes on the local to regional ecosystem dynamics and trophic structure, particularly when considering future climate projections.