Deep water mass geometry in the south east Atlantic across the Mid-Pleistocene Transition: bathimetric vs oceanographic controls
Authors: Pena Leopoldo D. , Jaume-Seguí María 
Published: March 2020

&lt;p&gt;&lt;span&gt;The Mid-Pleistocene Transition (MPT, ~1.3-0.7 Ma) &lt;span&gt;is one of the most drastic&lt;/span&gt; &lt;span&gt;climatic transition in the recent climatic history of our planet. During this transition, &lt;/span&gt;glacial-interglacial variability shifted from 41- to 100-ka cycles, without notable changes in the orbital forcing. Internal forcing mechanisms in Earth&amp;#8217;s climate likely shifted the system towards particularly more extreme glacial periods. A decrease in the atmospheric CO&amp;#8322; contemporary to a severe &lt;span&gt;weakening&lt;/span&gt; of the Atlantic deep-ocean circulation around 900 ky suggests that weakened deep-ocean circulation facilitated the capture of CO&amp;#8322; into the deep ocean and thus contributed to the switch towards more intense and longer glacial periods.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;ODP Site 668B, in the deep eastern equatorial Atlantic, has been previously used to reconstruct the atmospheric CO&amp;#8322; evolution across the MPT using boron isotopes in surface dwelling foraminifera. Here we present new high resolution proxies from the same site covering the last 2 Ma. In particular, benthic foraminifera stable isotopes and trace elements (B/Ca, Mg/Ca, Cd/Ca), &lt;span&gt;as well as&lt;/span&gt; Nd isotope data (&amp;#949;Nd) from Fe-Mn encrusted foraminifera shells. Using the newly improved chronology based on benthic foraminifera stable isotopes we show that o&lt;span&gt;ur &lt;/span&gt;&lt;span&gt;new &lt;/span&gt;&amp;#949;&lt;span&gt;Nd data covaries substantially with the &lt;/span&gt;&lt;span&gt;atmospheric &lt;/span&gt;&lt;span&gt;pCO&amp;#8322; data and shows a glacial-interglacial variability through the entire record, with &lt;/span&gt;&amp;#949;&lt;span&gt;Nd values matching typical glacial-interglacial range values in the North-Atlantic basin (~-11 to ~-14). &lt;/span&gt;&lt;span&gt;Between&lt;/span&gt;&lt;span&gt; ~1 to 2 Ma, when the 41-ka-cycles were dominant, &lt;/span&gt;&amp;#949;&lt;span&gt;Nd data also covaries with carbonate ion saturation index (&lt;/span&gt;&amp;#916;&lt;span&gt;CO&amp;#8323;&amp;#178;-) &lt;/span&gt;&lt;span&gt;as derived from the new B/Ca data&lt;/span&gt;&lt;span&gt;, Bottom Water Temperatures (BWT, &lt;/span&gt;&lt;span&gt;Mg/Ca&lt;/span&gt;&lt;span&gt;) and, with &lt;/span&gt;&lt;span&gt;deep ocean nutrient content (&lt;/span&gt;&lt;span&gt;phosphate &lt;/span&gt;&lt;span&gt;derived from Cd/Ca)&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;R&lt;/span&gt;&lt;span&gt;esults &lt;/span&gt;&lt;span&gt;indicate&lt;/span&gt;&lt;span&gt; a higher fraction of warmer, less corrosive and nutrient-poor northern-sourced waters (higher BWT, higher &lt;/span&gt;&amp;#916;&lt;span&gt;CO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;3&lt;/span&gt;&lt;/sub&gt;&lt;sup&gt;&lt;span&gt;2-&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;, lower Cd/Ca, lower &lt;/span&gt;&lt;span&gt;&amp;#949;Nd&lt;/span&gt;&lt;span&gt;) reaching the deep-equatorial Atlantic during interglacial periods compared to glacial periods. &lt;/span&gt;&lt;span&gt;Interestingly&lt;/span&gt;&lt;span&gt;, this covariation does not stand after ~&lt;/span&gt;&lt;span&gt;0.9&lt;/span&gt;&lt;span&gt;Ma. Even though &lt;/span&gt;&amp;#949;&lt;span&gt;Nd and BWT data suggest an increas&lt;/span&gt;&lt;span&gt;ed contribution&lt;/span&gt;&lt;span&gt; of southern-sourced waters to the site during glacial periods after &lt;/span&gt;&lt;span&gt;0.9Ma&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;as shown&lt;/span&gt;&lt;span&gt; by a gradual decrease in glacial BWT (&gt;1&amp;#176;C) and increasing glacial &lt;/span&gt;&amp;#949;&lt;span&gt;Nd values (~1&lt;/span&gt;&amp;#949;&lt;span&gt; units), &lt;/span&gt;&lt;span&gt;both B/Ca and Cd/Ca show a distinctive low frequency variability superimposed to the glacial-interglacial variability. These oscillations can be interpreted as infiltrations and/or overflows &lt;/span&gt;&lt;span&gt;of southern-sourced waters &lt;/span&gt;&lt;span&gt;across the mid-ocean ridge in&lt;/span&gt;&lt;span&gt;to the &lt;/span&gt;&lt;span&gt;SE Atlantic &lt;/span&gt;&lt;span&gt;basin that do not &lt;/span&gt;&lt;span&gt;completely&lt;/span&gt;&lt;span&gt; follow glacial-&lt;/span&gt;&lt;span&gt;interglacial&lt;/span&gt;&lt;span&gt; periodicity. We propose that bathymetrical constrains exert a control on the chemistry of the deep waters in the deep eastern equatorial Atlantic with potential impacts on global climate.&lt;/span&gt;&lt;span&gt; Partially isolated sub-basins such as the SE Atlantic could have effectively acted as carbon reservoirs over longer time scales than glacial-interglacial changes.&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;