Increased zonal &#948;13C gradient in the deep South Atlantic after the Mid-Brunhes Transition
Authors: Ballalai João , Santos Thiago , Nascimento Rodrigo , Venancio Igor , Piacsek Patrícia , Dias Bruna , Belem André , Costa Karen , Vázquez Riveiros Natalia , Albuquerque Ana Luiza 
Published: March 2022

&lt;p&gt;The climate system experienced several periodic oscillations over the last ca. 800 ka known as glacial-interglacial (G-IG) cycles. Disruptions of the global carbon cycle were evident on this time scale, promoting fluctuations in the atmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentration leading to global climate variability. In the more recent interglacials, both Antarctic temperatures and atmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentrations are significantly higher than in the previous &amp;#8220;lukewarm interglacials&amp;#8221; (ca. 800 &amp;#8211; 430 ka) before the Mid-Brunhes Transition (MBT). Changes in the Atlantic Meridional Overturning Circulation (AMOC) and deepwater formation rate around Antarctica have been invoked to explain a 30 ppm increase in the atmospheric CO&lt;sub&gt;2 &lt;/sub&gt;&amp;#173;during post-MBT interglacial periods. Deepwater variability is tightly coupled to the ventilation of CO&lt;sub&gt;2 &lt;/sub&gt;in the Southern Ocean by atmospheric and oceanic connections, contributing to carbon storage in the deep ocean and the atmospheric CO&lt;sub&gt;2&lt;/sub&gt;. Here, we present a new 770 ka benthic foraminifera &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C record from sediment core GL-854 retrieved from the western South Atlantic (WSA) at 2200 m water depth. We compare our record with published &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C data from the eastern margin to investigate the zonal gradient variability of the North Atlantic Deep Water (NADW) in the deep South Atlantic basin. WSA &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C variability and absolute values strongly mimic the North Atlantic mid-depth record at the NADW formation region. This similarity is interpreted as NADW preferentially carrying a modified signal through the deep western boundary current towards the WSA (rather than towards the eastern margin) after the MBT. The &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C gradient based on the difference between benthic foraminifera &lt;em&gt;C. wuellerstorfi &lt;/em&gt;from both margins (&amp;#916;&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;w-e&lt;/sub&gt;) gradually increases after a transitional period between ca. 400 ka to 300 ka towards the Holocene. We suggest that the mechanism behind this long-term increasing trend on the &amp;#916;&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;w-e&lt;/sub&gt; record post-MBT is the result of enhanced production of North Component Water due to Agulhas Leakage intensification driven by reduced sea-ice extent after the MBT. Furthermore, reduced sea-ice extent decreases the Antarctic Bottom Water density and formation in the Southern Ocean, contributing to the deepening of the AMOC during post-MBT interglacial periods. Our interpretation proposes a framework connecting sea-ice and ocean-atmosphere dynamics to deepwater geometry within the South Atlantic basin, which ultimately contributed to the&amp;#160;climate change observed across the MBT.&lt;/p&gt;