Absolute temperature seasonality from skeletal carbonates&#8212;Techniques and limitations of oxygen- and clumped isotope analyses
Authors: de Winter Niels , Witbaard Rob , Ullmann Clemens , Soerensen Anne , Thibault Nicolas , Müller Inigo , Kocken Ilja , Claeys Philippe , Ziegler Martin 
Published: March 2020

&lt;p&gt;The carbonate skeletons of marine organisms are unique archives for high-resolution climate reconstructions. Well-preserved specimens potentially allow for seasonal to even daily scale variability reconstructions of climate and environment&amp;#160; in deep time (pre-Quaternary), providing otherwise unavailable snapshots of climate variability during greenhouse periods (e.g. Steuber et al., 2005; Ivany et al., 2008; de Winter et al., 2017). However, uncertainties on past seawater compositions hamper use of the popular stable oxygen isotope ratio (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O) as proxy for paleotemperature reconstructions. The use of the independent carbonate clumped isotope (&amp;#916;&lt;sub&gt;47&lt;/sub&gt;) paleothermometer, which is insensitive to changes in seawater composition, on these promising fossil archives is complicated because of sample size limitations (Fernandez et al., 2017; Bernasconi et al., 2018).&lt;/p&gt;&lt;p&gt;In an attempt to circumvent these issues and use the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O and &amp;#916;&lt;sub&gt;47&lt;/sub&gt; measurements jointly for accurate seasonal reconstructions of temperature and seawater isotope composition, we present a novel data reduction approach that combines &amp;#916;&lt;sub&gt;47&lt;/sub&gt; measurements of small (~100 &amp;#181;g) serially sampled aliquots to estimate summer and winter temperatures in mollusk shell records. When applied on &amp;#916;&lt;sub&gt;47&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O measurements in the same specimens, combined with accurate shell chronologies, this approach reconstructs seasonal differences in temperature and seawater composition in a coastal site from the Campanian (Late Cretaceous) high-latitudes.&lt;/p&gt;&lt;p&gt;To test the robustness of these reconstructions, we apply different approaches of combining &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O and &amp;#916;&lt;sub&gt;47&lt;/sub&gt; data on a wide range of simulated data representing various scenarios of variability in growth rate, temperature and sea water composition typical for the natural shallow marine environments of carbonate-producers. This approach tests how choices such as sampling resolution and the method of data collection and reduction influence the accuracy and reproducibility of (paleo)seasonality reconstructions in these scenarios.&lt;/p&gt;&lt;p&gt;Finally, we present preliminary data of &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O and &amp;#916;&lt;sub&gt;47&lt;/sub&gt; analyses on bivalve specimens grown under controlled temperature conditions that allow us to calibrate the techniques above for temperature reconstructions. Together, these investigations pave the way for accurate, high-resolution climate reconstructions in deep time. These reconstructions provide valuable information on the dynamics of greenhouse climates, against which climate models can be compared to improve predictions of future climate.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Bernasconi, S. M., et al. Geochemistry, Geophysics, Geosystems, 19(9), 2895&amp;#8211;2914, 2018.&lt;/p&gt;&lt;p&gt;Fernandez, A. et al. Geochemistry, Geophysics, Geosystems, 18(12), 4375&amp;#8211;4386, 2017.&lt;/p&gt;&lt;p&gt;Ivany et al. Geological Society of America Bulletin, 120(5&amp;#8211;6), 659&amp;#8211;678, 2008.&lt;/p&gt;&lt;p&gt;Steuber, T. et al. Nature, 437(7063), 1341&amp;#8211;1344, 2005.&lt;/p&gt;&lt;p&gt;de Winter, N. J. et al. Palaeogeography, Palaeoclimatology, Palaeoecology, 485, 740&amp;#8211;760, 2017.&lt;/p&gt;