Mediterranean Outflow Water dynamics across the middle Pleistocene transition based on a 1.3 million-year benthic foraminiferal record off the Portuguese margin

Abstract

Reconstruction of the Mediterranean Outflow Water (MOW) dynamics in the geologic past contributes to exploring its influence on the North Atlantic Ocean circulation and global climate. The middle Pleistocene transition (MPT) is one of the most important characteristics of Quaternary climate change, but until now, research on the MOW variability during this time interval has been limited. To decipher the MOW variability over the last 1.3 myr, this work presents the first continuous and high-resolution (similar to 2 kyr) foraminiferal stable oxygen and carbon isotopes and benthic foraminiferal records from Integrated Ocean Drilling Program (IODP) Site U1391, which was drilled off the Portuguese margin in the Northeast Atlantic Ocean and is currently bathed by the lower MOW core. Based on spectral analysis results of the foraminiferal delta O-18 record, the MPT peaked from 0.9 to 0.7 Ma. The 'epibenthos group' was previously found attached to elevated substrates only within the MOW current and more abundant at higher current velocities, so the relative abundance of the 'elevated epibenthos group' was adopted as an indicator of MOW strength in this study. The MOW indicator suggests that the MOW strengthened during MIS 1, MIS 5, MIS 7, MIS 9, MIS 11, MIS 13, MIS 15, MIS 17, early MIS 18 to late MIS 19, MIS 21, MISs 23-25, MISs 28-30, early MIS 34 to MIS 35, early MIS 36 to late MIS 37 and MIS 40 with a similar to 100 kyr cycle. Superimposed on the long-term variability of the MOW current, the similar to 41-kyr and similar to 20-kyr cyclic fluctuations are also evident, whereby the similar to 41-kyr cycle is more significant. The results of cross-spectral analyses indicate that the similar to 20-kyr cyclic variation in the MOW intensity was drived by the precession forcing; the similar to 41-kyr cyclic variation in the pre- MPT MOW dynamics and the similar to 100-kyr cyclic variation in the post-MPT MOW dynamics were strongly controlled by the Earth's obliquity. The variation in the abundance of the elevated epibenthos group was dominated by the similar to 41-kyr cycle prior to the MPT and by the similar to 100-kyr cycle after the MPT, which suggested there was a MPT-related shift in the MOW dynamics. Benthic delta C-13 record of the last 0.56 myr shows precessional (23-kyr) and glacial-interglacial (similar to 100 kyr) cycles, indicating cyclic variations in bottom water ventilation. Enhanced ventilation coincides with precession maxima and reduced ventilation with precession minima. Bottom water was well ventilated by MOW during the interglacial periods and by GNAIW during the glacial periods, and pooly ventilated during the glaical terminations. (C) 2020 Elsevier Ltd. All rights reserved.