What Drives Mediterranean Outflow Water Variability during the Mid-Pleistocene Transition and Early Pleistocene at IODP Site U1387 in the Gulf of Cadiz?
Friday, 19 December 2014
The Mediterranean Outflow Water (MOW) forms extensive contourite drift deposits along the Iberian margin, especially in the Gulf of Cadiz, and injects heat and salt into the intermediate depths of the North Atlantic. The sediments recovered during IODP Expedition 339 allow studying MOW’s history throughout the Pleistocene and Pliocene and thus under varying climate forcing. Here we present centennial-to-millennial scale proxy records for surface water and MOW variations as recorded at IODP Site U1387, drilled into the Faro Drift, which is formed by the upper MOW core. We focus our study on the early to middle Pleistocene with special attention on the Mid-Pleistocene Transition (MPT) when the dominant climate cyclicity changed from 41 kyr to 100 kyr. Surface water and MOW proxy records show millennial-scale stadial/ interstadial oscillations on top of the glacial/ interglacial cycles. Planktonic and benthic δ18O records are tightly coupled highlighting the constant exchange between the (sub)surface waters and the MOW. Low benthic δ13C values during deglacial and peak interglacial periods, coinciding with insolation maxima, reveal a poorly ventilated upper MOW core and a causal link between MOW ventilation and sapropel formation in the Mediterranean Sea. Better ventilation was recorded during glacial and stadial intervals, often in association with the formation of contourites. During the warmer MIS contourites, often more pronounced than their glacial counterparts, were formed during the stadial(s) following the peak interglacial period when northern hemisphere summer insolation was low. Thus, changes in the upper MOW core are tightly coupled to summer insolation with poor ventilation occurring during insolation maxima and higher current velocity marking insolation minima. This insolation forcing reveals a close link between MOW and Mediterranean Sea climate conditions.