Climate and carbon cycling across the mid-Pliocene Warm Period

Abstract

<p>The mid-Piacenzian or mid-Pliocene Warm Period (mPWP; 3.264-3.025Ma) is the most recent period of sustained global warmth, which stands in contrast to the more variable and progressively cooler Pleistocene glacial-interglacial climate which followed. Global temperatures were 3˚C higher than the pre-industrial level, with atmospheric carbon dioxide (<em>p</em>CO<sub>2</sub>) reaching up to 410ppmv, making the mPWP the nearest past analogue for future warm climate. Thus, proxy reconstructions that can accurately capture biological response to past and projected <em>p</em>CO<sub>2</sub> are crucial in understanding future climate scenarios.</p> <p>Here we present initial high-resolution climatic variability and carbon cycling records from oxygen and carbon isotopes, assemblage, and morphometry of coccoliths – calcium carbonate exoskeletons produced by marine phytoplankton coccolithophores – from sediments collected during the IODP Expedition 361 at the Mozambique Channel (U1476; 15°49.25′S, 41°46.12′E; 2166m water depth), spanning 2.96 and 3.40Ma at 0.5 to 3.7kyr resolution. Previous studies show that the modern surface waters above Site U1476 are in air-sea exchange CO<sub>2</sub> equilibrium, making Site U1476 an ideal location for <em>p</em>CO<sub>2</sub> reconstructions. We also explore the expressions of coccolith isotopic vital effects from size-separated fractions that have been linked to cell size, growth rate, and calcification degree, providing empirical correlation with aqueous CO<sub>2 </sub>concentrations<sub> </sub>and ultimately, with <em>p</em>CO<sub>2</sub> levels.</p> <p>Preliminary results show strong precession-related 23-kyr cyclicities prior to M2 glaciation. These cycles are associated with negative coccolith fraction δ<sup>18</sup>O (δ<sup>18</sup>O<sub>CF</sub>) excursions coinciding with increasing upper ocean primary productivity resulting from a more vigorous Mozambique Channel Throughflow, forced by precession minima and northern hemisphere summer insolation maxima. A change in orbital configuration in upper ocean temperature and stratification records, from precession to obliquity, occurs after M2, with an overall climate background of 100-kyr glacial-interglacial cycles in upper ocean primary production, indicating commencement of the longer-term 100-kyr cooling trend observed through the Pleistocene. Periodicities at the eccentricity band, often linked to Pliocene ice volume, are shown in the δ<sup>13</sup>C<sub>CF</sub>, supporting prior findings on tight coupling between ice volume and carbon cycle changes, analogous to those recorded during the late Pleistocene.</p>