Variations in Mediterranean Outflow Water and its salt discharge versus Pliocene changes in North Atlantic thermohaline circulation prior and during the onset of major Northern Hemisphere Glaciation, 3.7 – 2.6 Ma
Pliocene changes in Mediterranean Outflow Water (MOW) and its potential influence on northern North Atlantic thermohaline circulation prior and during the onset of Northern Hemisphere Glaciation (NHG; 3.7 – 2.6 Ma) were investigated at westernmost Mediterranean Site ODP 978 (1930 m w.d.) and along the northeast Atlantic continental margin, at Sites DSDP 548 (1250 m w.d.) and ODP 982 (1135 m w.d.). Foraminiferal δ18O records (and geomagnetic events) formed the base for stable isotope stratigraphy which led to a major revision of age control at Site 982. The Nd isotopic composition (εNd) of bottom seawater served to trace back the origin of water masses. Sea surface temperatures (SST) were reconstructed from the alkenone unsaturation index (U￼k'37) and Mg/Ca estimates on planktic shells. Bottom water temperatures (BWT) were estimated from Mg/Ca measured on epibenthic foraminifera. Epibenthic δ13C records were used as tracer of bottom water ventilation. This investigation consists of four interrelated studies. Study 1 provides new insights from proxy intercomparison. Uk'37￼-based vs Mg/Ca-based SST estimates at Site 982 show that SST values derived from alkenones are ~1°C higher than those based on Mg/Ca in planktic foraminifera. However, both proxies report similar climate trends. Over the period of fairly global stable climate between 3.7 and 3.3 Ma both proxies record a large orbital-scale variability that may be linked to changes in the strength and position of the North Atlantic atmospheric pressure systems and the position of the North Atlantic Current (NAC). From ~3.3 to 3.0 Ma northeast Atlantic SST underwent a major drop, a result that may contradict the PRISM concept assuming a time slice of largely constant climate conditions. Study 2 documents a major and long-term Upper Pliocene intensification of the Mediterranean outflow, 3.5 – 3.3 Ma. εNd values higher than -11 to -9 show that MOW spread continuously over the northeast Atlantic throughout the Upper Pliocene and reached up to the Rockall Plateau from 3.4 until 2.7 Ma and after 2.55 Ma. From 3.5 – 3.3 Ma, northeast Atlantic Sites 548 and 982 showed a singular and persistent increase in bottom water salinities (BWS) by ~2 psu and in densities (BWD) by 1 kg m-3, which was matched by a ~1 to 3°C increase in BWT at Sites 548 and 982. This event of increased MOW flow was coeval with a unique and long-term rise in BWS by ~1 psu and in BWD by ~1 kg m-3 of WMDW at West Mediterranean Site 978 which partly forms the source of MOW by turbulent entrainment. These changes were most likely linked to a major aridification in the Mediterranean region following a key change in the African monsoon system. Precisely at the same interval, surface waters of the Alboran Sea showed an increase of nutrient contents (decreasing planktic δ13C). The increase was most likely linked to an enhanced Atlantic inflow of nutrient-enriched surface waters that had to compensate for the enhanced outflow of MOW. Study 3 concerned the onset of major NHG from ~3.0 to 2.7 Ma, when the long-term average BWT and BWS at S. 548 decreased by 3°–4°C and 2 psu, respectively, until 2.82 Ma (MIS G10). However, BWT and BWS at the shallower (and most distal) S. 982 continued to oscillate at a level that was 3°C and 1.5-psu higher than today. Generally, BWD then was slightly higher at S. 982 than at S. 548. Accordingly, it appears that the coeval enhanced production of upper North Atlantic Deep Water (NADW) may have diluted and/or rather displaced the core of the MOW tongue upward in the region off Brittany up to a level shallower than 1250 m w.d. (S. 548), although BWD still was about 0.5-kg m-3 higher than today. Study 4 concerned potential links between the strength of MOW salt discharge and upper NADW formation. The increased advection of salt with MOW from 3.5 – 3.3 Ma did not translate into any trends of better ventilation (δ13C) of Lower NADW and Upper NADW, except for a single potential response of (distal) Upper NADW at Caribbean Sea S. 999. Likewise, the distinct change in MOW advection and salt injection after 2.9 Ma was not linked to any coeval change in the ventilation of NADW. In total, the influence of MOW salt injections into the North Atlantic Meridional Overturning Circulation (MOC) may be too small to produce any significant effects.
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