Exploring proxy-proxy and proxy-model (dis)agreements during the leadup to the Miocene Climatic Optimum in the Southern Ocean

High reconstructed temperatures and pCO₂ concentrations during the Miocene Climatic Optimum (MCO; ~17-15 Ma) make it a possible analog for future warm climates. Proxy sea surface temperature (SST) reconstructions often indicate warm high latitudes, with a relatively small latitudinal temperature gradient. However, Earth system models generally do not yield the relatively flat latitudinal temperature gradients given by proxy reconstructions, and instead have cooler polar regions and somewhat warmer tropics. In tropical regions, this proxy-model disagreement may be due to habitat depths below the surface mixed layer, where the proxy would record temperatures from deeper, cooler waters, whereas the model output is simply SST. At high latitudes, however, the proxy-model disagreement cannot be fully explained by habitat depth or seasonal temperature, leaving two possibilities: one, that the models do not fully capture the Earth System; or two, that proxies are impacted by widespread non-thermal effects.

In an attempt to elucidate matters, we investigated clumped isotopes in coccoliths (cocco-Δ₄₇) at Southern Ocean sites 1168 (South Tasman Sea) and 751 (Kerguelen Plateau) and compare these results to previously published biomarker-based temperature proxies (U^K’₃₇ and TEX₈₆) at Site 1168. Clumped isotope samples were screened for good preservation, and contain little or no diagenetic carbonate. Unlike most temperature proxies, cocco-Δ₄₇ values are independent of seawater chemistry and do not exhibit species- or strain-specific offsets, instead yielding an absolute growth temperature. Additionally, coccoliths and alkenones are derived from the same organisms, and should be directly comparable. During peak warmth (~16.5 Ma), cocco-Δ₄₇ values yield temperatures of 12.0 ± 2.8 and 7.3 ± 2.8 °C at Sites 1168 and 751, respectively, and agree well with latitudinal averages of climate model output. At Site 1168, previously published U^K’₃₇ and TEX₈₆ yield much higher temperatures (27 °C SST and 21 °C 0-200 m temperatures, respectively; Guitián and Stoll P&P, 2021; Hou et al. Clim. Past, 2023). The difference in proxy temperature estimates is large and cannot be reconciled with modern ranges in seasonal SST or photic zone habitat depth temperature. Mixing effects and physiological influences will be further explored.