Convulsed western Mediterranean thermohaline circulation during the last glacial-interglacial transition from neodymium isotope ratios
Authors: Frigola Jaime , Pena Leopoldo D. , Català Albert , Pérez-Asensio Jose Noel , Garcia-Solsona Ester , Goldstein Steve L. , Lirer Fabrizio , Cacho Isabel 
Published: March 2020

&lt;p&gt;Severe changes in the western Mediterranean (WMed) thermohaline circulation occurred during the last major glacial-interglacial transition, mostly due to oceanographic and atmospheric re-adjustments. Changes in water column stratification or deep water convection are controlled by the evaporation-precipitation balance of the basin but also by temperature and the intensity of local winds.&lt;/p&gt;&lt;p&gt;A general freshening of the inflowing Atlantic waters through the Strait of Gibraltar during the central phase of the Heinrich Stadial (HS) 1 due to the intense iceberg melting favoured increased water column stratification in the WMed and reduced deep convection, as observed by synchronous depletions in both the benthic &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C and grain-size signals from a location close to the deep convection source. Post-glacial sea level rise further enhanced stratification of the WMed, with minimum deep convection achieved at a later time, and leading to the formation of the last Organic Rich Layer (ORL) in the Alboran Sea. These stratified conditions in the surface WMed were also favoured by more stable atmospheric conditions related to summer insolation maxima.&lt;/p&gt;&lt;p&gt;The integrated study of benthic foraminifera assemblages and TOC records from three sediment cores distributed between 900 m to 2400 m water depths in the basin corroborate the deterioration of deep water ventilation conditions during this period but also provide a first glimpse on an earlier re-ventilation of the intermediate layer. However, the Deep Western Mediterranean Waters did not re-ventilated until the early Holocene, when deep water convection in the Gulf of Lion was resumed by a major cold event with intensified north-westerly winds.&lt;/p&gt;&lt;p&gt;We review these key changes in the western Mediterranean thermohaline system by means of neodymium isotopic ratios (&amp;#949;&lt;sub&gt;Nd&lt;/sub&gt;) measurements on Fe-Mn encrusted planktonic foraminifera from 4 sediment cores covering a depth profile from 650 mwd to 2400 mwd. Neodymium isotope ratios are used here as a conservative water mass mixing proxy thus allowing to investigate changes of the western Mediterranean thermohaline circulation during the last 22 kyr. Validation of Fe-Mn encrusted foraminifera &amp;#949;&lt;sub&gt;Nd&lt;/sub&gt; as tracer of deep water masses in the Mediterranean has been carried out by means of analysis in core-top samples from depths representing main Mediterranean water masses and comparison with published seawater &amp;#949;&lt;sub&gt;Nd&lt;/sub&gt; values.&lt;/p&gt;&lt;p&gt;Our results show a quite homogeneous &amp;#949;&lt;sub&gt;Nd&lt;/sub&gt; signal for intermediate and deep waters during the end of the last glacial period, prior to the deposition of the ORL, likely suggesting a very well homogenized water column. However, significant differences can be observed between the deepest record and those of intermediate depths during the ORL time interval and the early Holocene (14-6 kyr), pointing to important changes in the western Mediterranean thermohaline circulation and likely suggesting different waters mass sources for deep and intermediate levels.&lt;/p&gt;