Intra-annually-resolved sea surface temperature variability at the onset of the Oligocene icehouse based on Nummulites geochemistry

Corals, otoliths, molluscs, and foraminifera all produce growth-banded shells, which can be subsampled to produce records of intra-annual variation in Earth's past, including seasonality. However, foraminifera remain under-utilised for this purpose, with only a handful of studies to date. In particular, the larger foraminifera, which live for several months or more, remain a largely untapped archive of past intra-annual changes. Here we use laser ablation ICPMS to generate continuous Mg/Ca records along the spiral whorl of Nummulites from the late Eocene and early Oligocene of Tanzania. Using in situ temperature logger data in a modern tropical setting, the intra and inter-test variability of this Mg/Ca palaeothermometer was calibrated utilising corresponding Mg/Ca profiles from the related modern species Heterostegina depressa. Eocene Nummulites proved affected by diagenesis and could not be used for reconstructions. However, nine Oligocene specimens showed excellent preservation enabling us to generate records of intra-test Mg/Ca variability, and transfer this to an estimate of intra/inter-annual palaeotemperature variability using the relationship between these parameters determined from our modern sample site. Our results constrain a mean annual temperature of 29.7 ± 3.9 °C, comparable with the oxygen isotope and TEX86 sea surface temperatures in previous studies from Tanzania, and intra/inter-annual temperature variability of ±2.3–3.0 °C. This is similar to both modern values for the region and sparse existing Oligocene seasonal data from the US. Our records thus contribute to unravelling Oligocene climate, and highlight the potential of larger foraminifera in filling the key seasonality gap in our understanding of past climates.

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