Paleoceanographic changes in the Northern Benguela region inferred from the planktonic foraminifera assemblages during the Pleistocene

The Benguela Upwelling System (BUS), situated in the Southeastern Atlantic Ocean, is known as the most productive wind-driven coastal system on Earth. The BUS is boarded to the north and to the south by the Angola Benguela Front (ABF) and the Agulhas retroflection region, respectively. The ABF is a permanent thermal frontal feature located at around 16°S, which separates the cool Benguela Ocean Current (BOC) from the northernmost Angola warm water masses (e.g., the South Atlantic Central Waters; SACW). The Agulhas retroflection region, instead, represents an area, in the southernmost sector of the BUS, where the Agulhas current (AgC) retroflects towards the Indian Ocean, due to westerlies' stress curl. However, eddies of the warm and saline Agulhas waters can access the southern Atlantic Ocean and subsequently mix within the Benguela current.

In this work, statistical analyses (cluster and PCA analyses, Globorotalia truncatulinoides coiling ratio, and the Agulhas Leakage Efficiency Index) were applied to the planktonic foraminifera assemblages in 94 samples from Holes U1575A and U1576A, collected during International Ocean Discovery Program (IODP) Expedition 391. Precisely, the sites were drilled along the Tristan-Gough-Walvis Ridge (TGW) seamount track, in the northernmost sector of the BUS, at increasing distance from the Namibian continental margin. The investigated record corresponds to the Early to Late Pleistocene, which comprises the Early-Middle Pleistocene Transition (EMPT). The EMPT is an interval in the Earth history spanning from 1.40 to 0.40 Myrs and characterized by prominent glacial to interglacial sea surface temperature (SST) variations.

Our results provide new understandings on the regional paleoceanographic changes occurring within the northern part of the BUS, based upon the ecology and distribution of the planktonic foraminiferal assemblages. Specifically, changes in the assemblage composition during the Pleistocene permitted the characterization of the different water masses (BOC, SACW; AgC) and their interplay, as well as the reconstruction of the local variations of the thermocline in the BUS. The interaction between the water masses mentioned above, produces changes in the BUS such as the fluctuations of the ABF and the influx of the AgC waters from the Agulhas retroflection area.

Moreover, we further examined the possible correlations between the paleoceanographic variability and climatic events (e.g., Benguela Niño/Niña-like states and deglaciations phases), which occurred since the onset of the EMPT.