Hydrodynamic changes in the Gulf of Cadiz during the Early-Middle Pleistocene Transition revealed by benthic foraminifera and radiogenic isotope data

The Early-Middle Pleistocene Transition (EMPT), a global climate event that occurred between 700-1250 thousand years (kyr) ago, was characterized by a drastic change in the deep thermohaline circulation, resulting in more intense and longer-lasting interglacial periods. High-resolution records documenting environmental changes on the ocean seafloor associated with the EMPT in the North Atlantic are still limited. This knowledge is crucial for evaluating and modeling climate variability in the near future. The Gulf of Cadiz (Iberian Margin) is a key region as a gateway between the Mediterranean Sea and the North Atlantic Ocean, being affected at intermediate depths by the Mediterranean Outflow Water (MOW). The MOW plays an important role in modulating the North Atlantic salt budget. Therefore, past climate variability in the Mediterranean region may have affected the MOW intensity and global thermohaline circulation. Thus, the present study aims to understand the environmental parameters influencing the distribution of benthic foraminifera species and their significance in regional oceanographic dynamics. 

Benthic foraminifera inhabit diverse (sub)seafloor environments and respond to factors such as oxygen levels, as well as the quantity and quality of food. Although other factors might influence the assemblage, strong bottom current regimes favor abundances of a group known as the “elevated epifauna”. Previous studies in the Gulf of Cadiz have found that elevated epifauna abundance correlates with MOW intensity in the modern ocean, suggesting it as a potential indicator of MOW intensity in the past.

Here we present results from a high-resolution study of benthic foraminifera assemblage of the period from Marine Isotope Stages (MIS) 26 to 19 at IODP Site U1387 (559 m water depth), drilled into the Faro drift on the southern Portuguese margin. Our study identifies two distinct phases: the first phase (MISs 25 to 22, 959–866 kyr) experienced persistent and intensified MOW flow, as evidenced by an increase in the abundance of elevated epifauna. This phase also exhibits an increase in the abundance such as Globobulimina spp., species that live under oxygen and trophic conditions prevailing at the boundary between dysoxic and anoxic environments, suggesting stronger influence of relatively low oxygen Mediterranean waters. As soon as the MOW intensity declines in the second phase (MIS 21 to MIS 19, 866 – 761 kyr), there is a decrease in the Globobulimina spp. abundance, and an increase in oxygenated-preferred species abundance. We hypothesize that phase I is highly influenced by Mediterranean-sourced waters, whereas phase II improved oxygen conditions indicate potential dominance of Atlantic-sourced waters due to a lesser Mediterranean water contribution. To validate these results, Neodymium isotope analyses (εNd) are being conducted to help distinguishing between the prevailing water masses. Following our hypothesis, we are expecting more positive values during phase I, indicating stronger MOW influence, and more negative values for phase II, suggesting weaker MOW influence and dominantly Atlantic-sourced waters. These findings will further contribute to our understanding of the interplay between climate change and oceanographic dynamics in the Gulf of Cadiz during the EMPT.