High sea-surface temperatures during the early Aptian Oceanic Anoxic Event 1a in the Boreal Realm

The early Aptian was characterized by the widespread occurrence of anoxia in the oceans, known as Oceanic Anoxic Event (OAE) 1a. Intense degassing from submarine volcanic plateaus presumably resulted in high atmospheric CO2 concentrations, culminating in greenhouse conditions. OAE 1a can, therefore, be considered as a past “natural experiment” important to understand the future evolution of our climate. Paleotemperature estimates for OAE 1a are, however, predominantly based on bulk oxygen isotopes, which are susceptible to diagenetic overprinting, while TEX86 paleotemperature estimates are limited in number or derived from stratigraphically poorly constrained sections. Here we reconstructed for the first time sea-surface water temperatures (SSTs) based on the TEX86 paleothermometer from an OAE 1a section from the middle northern latitudes (39°N paleolatitude). We find a SST rise starting prior to OAE 1a and reaching a maximum during the event, with SSTs of ∼31–34 °C, 4–9 °C higher than those of older Hauterivian–lower Aptian sediments from the same sedimentary basin. The end of OAE 1a is marked by relatively lower SSTs of ∼30 °C. These observations are supported by belemnite-based oxygen-isotope (δ18OBel) data and calcareous nannofossils. Our integrated data set clearly indicates that “super greenhouse” conditions prevailed during OAE 1a at northern latitudes. SSTs are similar to those estimated for coeval low-latitudinal sites, suggesting that an equable warm climate, with reduced latitudinal gradients, characterized the early Aptian.