Reconstructing atmospheric CO2 concentration in the Late Miocene is crucial for understanding the relationship between greenhouse gas concentrations and climate change in a warmer-than-modern world. Both δ11B-based and alkenone-ep-based CO2 estimates feature uncertainties due to poorly constrained past seawater chemistry, and algal physiological processes, respectively. Additionally, both proxies estimate CO2[aq], so they require reliable surface ocean temperatures to calculate solubility and atmospheric CO2. To evaluate proxy coherence, in this study we generate new records of alkenone ep and δ11B, from the western Tropical Atlantic ODP Site 926 during the Late Miocene. We provide surface ocean temperature estimates from coccolith clumped isotope thermometry, alkenone undersaturation ratios, and planktonic foraminiferal Mg/Ca ratios. The warm temperatures estimated from our new clumped isotope records, together with alkenone temperatures >29°C, confirm warm tropics, and provide constraints on the assumptions of seawater Mg/Ca and dissolution corrections for foraminiferal Mg/Ca SST estimates. The new alkenone ep CO2 estimates at 926 yield generally similar CO2 as the new and published δ11B-based CO2 records for the site, and are similar to published alkenone ep CO2 records from the South Atlantic ODP Site 1088. However, over the 7.3 to 7.8 Ma interval, the CO2 values from ep are lower than other records. We evaluate which proxy indicators can best predict variations in algal physiology which may bias the ep-based CO2 reconstructions in this interval at Site 926.

Calcareous benthic foraminifera can develop pores in their test wall to facilitate gas exchanges (e.g., O2, CO2) with the surrounding seawater. The patterns of these pores, i.e., porosity, pore density, and pore size, can vary based on environmental factors, including bottom water dissolved oxygen concentration (BWDO). Specifically, some species react with increased test porosity to lower BWDO levels, highlighting their proxy potential for reconstructing past BWDO concentrations. To validate this proxy in the Southeast Pacific (SEP), the pore patterns of six benthic foraminifera species were compared with BWDO. The specimens, dated Holocene to modern, were collected from surface sediment samples along the SEP (12°–44°S) from 24 to 3,252 m water depth. Porosity, pore density, and pore size were measured on the umbilical and spiral sides, including all visible chambers, and the penultimate and antepenultimate chambers (PAC) on both sides. Our study reveals species-specific pore pattern responses to BWDO, with most species increasing their test porosity to adapt to lower BWDO. Notably, Cibicidoides species show increased porosity on the umbilical side, whereas Planulina species do so primarily on the spiral side. This indicates that using the pore patterns of several species together is not recommended in the SEP. Instead, focusing on individual species provides stronger inverse correlations between BWDO, porosity, and pore density, especially for C. wuellerstorfi. These findings support the use of benthic foraminifera pore patterns as reliable indicators for reconstructing quantitively past BWDO, with an error range down to ±5–20 µmol/kg.