Oligocene-early Miocene paradox of <i>p</i> CO ₂ 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, the decline in the phytoplankton carbon isotopic fractionation (εp) from the early Oligocene to Miocene time interval, interpreted as a long term CO2 decline, differs from the apparent long term stability in climate indicators like benthic oxygen isotopes. Here, we produce two new long-term records of εp 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 trend of εp decline occurring during this interval. Rapid 3 ‰ declines are found from 27 to 24.5 million years ago (Ma) and 24 to 22.5 Ma, and minimum εp is attained at 19 Ma. Between 29.7 and 28.7 Ma at IODP 1406, a 20–30 ky sampling resolution reveals orbital scale 100 kyr cyclicity in εp. Making use of alkenone-based sea surface temperature (SST) estimates and benthic δ18O estimates extracted from the same samples, we perform a direct comparison with εp to evaluate the relationship with climate. We observe that across the long Oligocene to early Miocene interval, εp is positively correlated to SST only at the Southern Ocean Site 1168, but not with SST at the North Atlantic Site 1406. Accounting for the temperature-driven growth rate or cell size effects on εp does not lead to stronger correlations between εp and benthic δ18O nor stronger correlations between εp and SST at Site 1406. Moreover, at orbital timescale, the relationship between εp and benthic δ18O, albeit weak, implies greater ice volume or colder deep ocean at higher CO2. Despite the persistence of climate paradox, the reproducible trends in three widely separated sites, which experienced contrasting temperature evolution and likely experienced different variations in nutrient availability, suggest that a common CO2 forcing is likely the dominant control on the long term trends in εp. Changing ocean heat transport to the North Atlantic may contribute to the observed decoupling of long term εp and SST in this location.