Late Glacial to Holocene history of the Mediterranean Outflow. Evidence from benthic foraminiferal assemblages and stable isotopes at the Portuguese margin

Abstract

Records of benthic foraminiferal assemblage variations and benthic δ 13C along 12 sediment cores from the western Iberian Margin, between 36° and 42°N at water depths from 820 to 3580 m, are used to monitor fluctuations of the Mediterranean Outflow Water (MOW) during the past 30 ka. The chronostratigraphy of the cores is based on planktonic δ 18O records, 14C AMS-dating, and the recognition of Heinrich Events H1 through H4. Increased abundances of suspension feeding benthic foraminifers, denoted as ’Epibenthos Group‘, closely match areas where the recent MOW core layers impinge on the continental slope at 800 and 1300 m water depth, and near-bottom current velocities are enhanced. Elevated ‘Epibenthos Group’ abundances, increased benthic δ 13C, and sedimentological evidence for winnowing and erosion are found in glacial sections up to the earliest Termination I in cores at water depths between 1600 and 2200 m off southern Portugal. The combined evidence reveals enhanced current activity at these depths due to a deep glacial MOW. The MOW advection at the Portuguese margin during the last Glacial was about 700 m deeper than today, conceivably forced by increased MOW density due to higher salinity and colder temperatures of Mediterranean waters. The deep MOW current gradually decreased in strength and shoaled to 1300 m water depth during the Termination and early Holocene. A shallow MOW core layer became active with the onset of Termination I at depths between 600 and 1000 m. Both the shallow and deep MOW current culminated during the Younger Dryas period. The present flow pattern with two MOW core layers centred at 800 and 1300 m water depth was established between 7.5 and 5.5 ka.