A late Miocene seasonality and wildfire record from northern Siberia utilising novel speleothem proxies

<p>The Miocene provides one of the best analogues for near future anthropogenic warming — with atmospheric CO<sub>2</sub> concentrations similar, or slightly higher than present, warmer global temperatures (Steinthorsdottir et al., 2021), and a summer ice-free Arctic (Stein et al., 2016). Yet discrepancies still persist between the proxy record and model reconstructions (Steinthorsdottir et al., 2021), highlighting the need for chronologically well constrained and sensitive proxy records to aid our understanding of the underlying forcings of Miocene palaeoclimate and regional environmental response to climatic changes. Particularly sparse proxy coverage in the Siberian Arctic (Popova et al., 2012; Pound et al., 2012; Steinthorsdottir et al., 2021) hampers reconstruction of Miocene temperatures and hydrological dynamics in the northern hemisphere, despite the region being home to the globe’s largest extent of continuous permafrost – a key climate tipping element likely to play a significant role in future climate trajectories (Steffen et al., 2018).</p><p>Here we use U/Pb dated speleothem samples from Taba Bastaakh (72°15' N, 126°56' E), situated on the eastern bank of the river Lena in northern Siberia, to gain insights into climatic conditions during the Tortonian. The calcitic speleothems most likely formed under vadose conditions and have been U/Pb dated to 8.7 ± 0.6 Ma. Our multiproxy speleothem study utilises conventional (ẟ<sup>13</sup>C, ẟ<sup>18</sup>O, and trace elements) and novel (lignin and levoglucosan biomarkers and ẟ<sup>13</sup>C of non-purgeable organic carbon) environmental indicators to derive information on atmospheric circulation, local hydrology, wildfire occurrence, and vegetation regime. Macroscopically visible layers align with cyclic isotopic shifts of ca. 0.8 ‰ in ẟ<sup>13</sup>C (-9.8 ‰ to -8.6 ‰) and 1.6 ‰ in ẟ<sup>18</sup>O (-16.6 ‰ to -15 ‰). Oxygen isotope compositions are similar to those of southern Siberia in the modern day – indicative of a warmer, strongly seasonal environment. Carbon isotopes suggest a large organic component.</p><p>Stable isotopes have been measured at NICEST lab Northumbria University, biomarkers at JGU Mainz, ẟ<sup>13</sup>C NPOC at the University of Bern, and U/Pb dating in the Oxford geochronological lab.</p><p> </p><p><strong>References</strong></p><p>Popova et al. (2012). Palaeoclimate evolution in siberia and the Russian far east from the oligocene to pliocene - evidence from fruit and seed floras. <em>Turkish Journal of Earth Sciences</em>, <em>21</em>(2), 315–334.</p><p>Pound et al. (2012). Global vegetation dynamics and latitudinal temperature gradients during the Mid to Late Miocene (15.97-5.33Ma). <em>Earth-Science Reviews</em></p><p>Steffen et al. (2018). Trajectories of the Earth System in the Anthropocene. <em>Proceedings of the National Academy of Sciences of the United States of America</em>, <em>115</em>(33), 8252–8259.</p><p>Stein et al. (2016). Evidence for ice-free summers in the late Miocene central Arctic Ocean. <em>Nature Communications</em>, <em>7</em>.</p><p>Steinthorsdottir et al. (2021). The Miocene: The Future of the Past. <em>Paleoceanography and Paleoclimatology</em>, <em>36</em>(4).</p>