Oligocene-early Miocene paradox of pCO₂ inferred from alkenone carbon isotopic fractionation and sea surface temperature trends

Abstract. Atmospheric carbon dioxide decline is hypothesized to drive the progressive cooling over the Cenozoic. However, at multimillion-year timescales during the Oligocene to Miocene time interval the existing reconstructions, most based on the phytoplankton carbon isotopic fractionation (ε_p) proxy, differ from what is expected to drive the climate observations.

Here, we produce two new long-term records of ε_p over the Oligocene to early Miocene time interval from widely separated locations at IODP Site 1406 and ODP 1168 and increase the resolution of determinations at the equatorial Atlantic ODP 925. These new results confirm a global footprint of ε_p shift occurring during this interval. Abrupt 3 ‰ declines are found from 27 to 24.5 Ma and 24 to 22.5 Ma, and minimum ε_p is attained at 19 Ma. Between 28.7 and 29.7 Ma at IODP 1406, a higher resolution sampling reveals orbital scale 100 kyr cyclicity in ε_p. Making use of alkenone-based sea surface temperature (SST) estimates and benthic δ¹⁸O estimated extracted from the same samples, we perform a direct comparison with ε_p to evaluate the relationship with climate dynamics. We observe that across the long Oligocene to early Miocene interval the two sites’ relationships contrast with what is expected if CO₂ was the main driver of ε_p and average earth surface temperature evolution was registered at the local surface SST and global benthic δ¹⁸O. Moreover, at orbital timescale, ε_p and benthic δ¹⁸O appear to follow an inverse relationship, although located within the multimillion-year period with the strongest direct correlation between these variables (>25.5 Ma). To evaluate the physiological, non- CO₂ influences on ε_p, we use modern cultures to evaluate the impact of changing cell size and growth rate on the trends in ε_p. Although at specific time intervals, those drivers seem to explain part of the ε_p divergence with SST or benthic δ¹⁸O, most periods remain largely divergent, particularly the late Oligocene warming. We infer that a common CO₂ forcing is likely the dominant control on the coherent temporal trends in ε_p at widely separated sites, which experienced contrasting temperature evolution and likely experienced different variations in nutrient availability. While CO₂ changes likely caused significant changes in radiative forcing, SST variation at the examined sites may have been conditioned by regional heat transport, and the relationship between benthic δ¹⁸O and ε_p could reflect variable phasing between ice growth and global temperature.