South Atlantic deep-sea temperature evolution across the Pliocene-Pleistocene from clumped isotope thermometry
Authors: Domínguez Valdés Elena , Kocken Ilja , Agterhuis Tobias , Müller Inigo , Bode Noa , Kroon Dirk , Lourens Lucas , Ziegler Martin 
Published: July 2022

&lt;p&gt;The deep ocean comprises a large and relatively stable heat reservoir in the climate system. Hence deep sea temperature (DST) records constitute an important pillar of knowledge in the study of past climate states. Furthermore, a promising link exists between DSTs and global mean surface temperatures (GMSTs) [1]. DST reconstructions are nevertheless hampered by idealistic assumptions surrounding the interpretation of widely-used proxies like the ratio of oxygen isotopes on benthic foraminifera (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;b&lt;/sub&gt;). One such uncertain variable key in the analysis of &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;b&lt;/sub&gt; is the isotopic composition of paleo-seawater (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sw&lt;/sub&gt;). Deconvolving this signal into temperature and ice-volume components across the last 5 Ma is non-trivial. Attempts to resolve it have often relied on independent temperature constraints from foraminiferal Mg/Ca ratios that also involve added uncertainties regarding variations in ocean chemistry through time, as well as the need for species-specific calibrations.&lt;/p&gt;
&lt;p&gt;Carbonate clumped isotope thermometry (&amp;#916;47) can aid in overcoming these limitations since it is based on thermodynamic principles that govern the abundance of &lt;sup&gt;13&lt;/sup&gt;C-&lt;sup&gt;18&lt;/sup&gt;O bonds within the crystal lattice, therefore granting it independence from estimations surrounding the composition of the precipitating fluid [2]. Recent clumped isotope DST records of the Eocene [3] and the Miocene [4] reveal significantly warmer ocean temperatures than traditionally accepted [5], suggesting a re-evaluation of the interpretation of &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;b&lt;/sub&gt; records in the geological past.&lt;/p&gt;
&lt;p&gt;Here we present &amp;#916;47-based DST constraints of the last 5 Ma obtained from benthic foraminifera of ODP Site 1264 in the South Atlantic Ocean. Reconstructions covering the Pliocene-Pleistocene transition shed light on the global climatic change that followed the mid-Pliocene warm period and culminated in full glaciation of the Northern Hemisphere. In particular, our Pliocene DST data lies in closer agreement with recent GMSTs estimations [6] than &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;b&lt;/sub&gt;-based DST would imply.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;[1] Valdes, P. J., et al. (2021). Climate of the Past, 17(4), 1483-1506.&lt;/p&gt;
&lt;p&gt;[2] Eiler, J.M. (2007), Earth Planet. Sci. Lett. 262, 309-327.&lt;/p&gt;
&lt;p&gt;[3] Agterhuis, T., et al. (2022), Commun Earth Environ 3, 39&lt;/p&gt;
&lt;p&gt;[4] Modestou, S. E., et al. (2020) Paleoceanography and Paleoclimatology 35, e2020PA003927.&lt;/p&gt;
&lt;p&gt;[5] Westerhold, T., et al. (2020), Science, 369, 1383&amp;#8211;1387&lt;/p&gt;
&lt;p&gt;[6] McClymont, E. L., et al. (2020) Climate of the Past, 16(4), 1599-1615.&lt;/p&gt;