Paleoceanographic and climatic implications of a new Mediterranean Outflow branch in the southern Gulf of Cadiz

Abstract

The presence of contourite drifts in the southern Gulf of Cadiz (GoC) along the Moroccan margin raises questions about the (re)circulation of Mediterranean Outflow Water (MOW) in the GoC and the origin of the currents depositing them. Here, we compare two cores representative of Iberian and Moroccan contourite drifts, covering the last 22 kyr. Although the whole sequence is contouritic in character, it reflects the interaction of distinctive silty-contourite facies (high flow velocity periods) imbedded in muddy-contourite facies (low flow velocity periods). Evidence from benthic foraminifera δ13C, sortable silt grain-size, oceanographic CTD profiles and numerical simulations, indicate the Mediterranean water mass as the source of the southern contourite deposits. Our data, therefore, suggests an additional branch of upper-MOW veering southwards off the Straits of Gibraltar along the Moroccan margin. During MIS-(Marine Isotope Stage) 2, upper-MOW was a sluggish current while in the Holocene upper-MOW dominated as a fast, semi-steady flow. Throughout the deglaciation, silty contourites associated with higher flow speeds were deposited in the northern and southern GoC during cold events such as Heinrich Stadial 1 (HS1) and the Younger Dryas, forced by global millennial-scale climate variability. Millennial variability also appears to drive the deposition of silty-contourites in the Holocene. We estimated an average duration of 1 ka for the process of depositing a fast contourite unit. The case of silty-contourite I6 (within HS1) allows us to illustrate with extremely high resolution a “rapid” sequential change in circulation, with gradual slow-down of dense Mediterranean water while surface was freshening (HS1), provoking injection of high-salinity intermediate waters (via contour-currents) into the GoC, and hence the North Atlantic. The subsequent brief collapse of dense water formation in the Mediterranean Sea triggered a major increase in sea surface temperatures (10 °C/ka) in the GoC, developing into the next interstadial (Bølling/Allerød). The impact of Mediterranean intermediate waters is manifested here by triggering a substantial rearrangement of intermediate and deep circulation in the North Atlantic, which would have further impacted the Atlantic Meridional Overturning Circulation (AMOC).