Reconstructing sea surface temperature and atmospheric CO₂ across the Tortonian using alkenone εp records from South Atlantic ODP Site 1088

The Tortonian stage (11.6–7.2 Ma) represents a warm interval preceding the Late Miocene Cooling (7.5–5.5 Ma), during which high-latitude temperatures exceeded modern values by up to ~17°C. Atmospheric CO₂ reconstructions during the Tortonian are poorly constrained, with existing proxy records suggesting either high CO2 levels, reaching ~790 ppm at ~11 Ma (Mejia et al., 2017), or more moderate and relatively stable CO₂ conditions.

Here, we present new high-latitude pCO₂ reconstructions based on alkenone carbon isotopic fractionation (ε_p) from Ocean Drilling Program (ODP) Site 1088 in the subantarctic South Atlantic, covering the interval from 11.6 to 9.0 Ma. Combined benthic and bulk carbonate δ¹³C and δ¹⁸O records are used to identify the Tortonian thermal maximum and to guide targeted, higher-resolution sampling. Sea surface temperatures are reconstructed from alkenone Uk′₃₇ ratios using the Bayspline calibration (Tierney et al., 2018), and ε_p is calculated from compound-specific δ¹³C measurements of the C_37:2  alkenone. The isotopic composition of dissolved inorganic carbon is estimated from planktonic foraminiferal (G. bulloides) δ¹³C, accounting for the temperature-dependent fractionation between DIC and aqueous CO₂.

pCO₂ concentrations are then reconstructed using a probabilistic ε_p model (Stoll et al., 2019) that explicitly incorporates coccolithophore cell size, growth rate, and light availability. Coccolith size and thickness distributions are quantified from circular polarized image analyses, while growth rates are inferred from temperature and nutrient availability.