Figure 7 : Summary of global climate through the mid-to-late Miocene. (a) Sea surface temperature estimates from Sunbird-1, fellow low latitude ODP sites 850 (Zhang et al. , 2014) and 761 (Sosdian and Lear, 2020), mid latitude Northern Hemisphere ODP site 1021 (LaRiviere et al., 2012), and mid-latitude Southern Hemisphere site 1125 (Herbert et al., 2016), and high-latitude Northern Hemisphere ODP Site 982 (Herbert et al. , 2016). ODP Site 761 data is displayed on an alternative axis as SST anomalies relative to the baseline average from 16.0 – 15.5 Ma. (b) pCO2reconstructions, with Y axis on a log scale, of Sosdian et al.(2018) applying the CCD reconstruction of Pälike et al. (2012) and the δ11BSW scenario ofGreenop et al. (2017), and Stoll et al. (2019) applying temperature estimates from Bolton et al. (2016) and Zhang et al. (2013). Confidence intervals (95%) are displayed as dashed lines and error bars respectively. (c) Composite benthic δ18O record showing data that have been smoothed by a locally weighted function over 20 kyr (blue curve) and 1 Myr (red curve) (Westerhold et al. , 2020). Blue, yellow, and gray panels indicate intervals of ice sheet expansion across the Mid Miocene Climate Transition (MMCT) associated with CO2 decline, the steepening of latitudinal temperature gradeints in the absence of a CO2 trend, and the Late Miocene Cooling (LMC).
5 Conclusions
Our Sunbird-1 sea surface temperature estimates from LA-ICP-MS Mg/Ca analyses are in good agreement with those using the δ18O paleo-thermometer on glassy foraminifera, supporting the use of LA-ICP-MS micro-analysis across multiple specimens for reconstructing paleotemperatures. This analytical technique has allowed the reconstruction of reliable Mg/Ca derived paleotemperatures using foraminifera whose bulk trace element ratios demonstrate diagenetic contamination by authigenic coatings. This finding opens the potential for Mg/Ca paleothermometry on other challenging time intervals, and locations, where contaminant coatings have previously inhibited the geochemical analysis of primary foraminiferal calcite. We present new sea surface temperature records from planktic foraminiferal Mg/Ca for the south west Indian Ocean between 13.5 Ma and 9.5 Ma. Absolute estimates of 24-31⁰C suggest that sea surface temperature was relatively constant through the interval, although our record also suggests two intervals of regional cooling and freshening of surface waters at 11.8 and 10.7 Ma. The late Miocene represented a key interval in the transition of Earth’s climate to its modern state, including the development of stronger latitudinal temperature gradients. Our new temperature record suggests that different mechanisms may have been responsible for this cooling. The initial cooling from ~10 Ma at mid to high latitudes in both hemispheres was not associated with significant cooling at low latitudes. On the other hand, the late Miocene cooling between ~7.5 and 5.5 Ma was global in nature and associated with a drawdown in pCO2. Further work should therefore explore the mechanisms responsible for the enhanced polar amplification between 10 and 7.5 Ma, and the possibility of carbon cycle feedbacks contributing to the subsequent late Miocene Cooling.
Acknowledgments, Samples, and Data
This study uses samples from the Sunbird-1 core provided by BG-Group. All data from this study can be found in Table 1 and Supplementary Tables S1 to S11,and aredeposited in the Zenodo online data repository http://doi.org/10.5281/zenodo.4472994. We thank Alexandra Nederbragt and Anabel Morte-Rodeñas for laboratory assistance. We thank the reviewers and editor for their insightful comments that improved the manuscript. This research was supported by NERC iCASE studentship BW/22003105 (M.G.N.), and NE/L009633/1 grant to C.H.L.
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