Multi‐proxy estimates of sea surface temperature and CO₂ in the Western Atlantic during the Late Miocene

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-ε_p-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 ε_p 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. New alkenone ε_p CO₂ estimates at 926 yield generally similar CO₂ levels (400 ppm ± 100 ppm) as the new and published δ¹¹B-based CO₂ records (500 ppm ± 100 ppm) for the site, and are similar to published alkenone ε_p CO₂ records (500 ppm ± 100 ppm) from the South Atlantic ODP Site 1088. However, over the 7.3 to 7.8 Ma interval, the CO₂ values from ε_p are 100–200 ppm lower than other records, which may reflect uncertainties in estimation of δ¹³C_DIC from planktic foraminifera or variations in algal physiology. We evaluate which proxy indicators can best predict variations in algal physiology which may bias the ε_p-based CO₂ reconstructions in this interval at Site 926.