Sea surface temperature evolution in the Labrador Sea during the Oligocene

The Oligocene epoch is characterized by a coolhouse climate, paced by several cooling episodes. The few existing proxy records suggest that during the Oligocene the latitudinal temperature gradient increased, when compared to the warmer Eocene. However, the number of Oligocene temperature records from northern latitudes above 50 °N is very limited. The recent study of the Oligocene (ca. 34.5 to 26.5 Ma) succession of the southern Labrador Sea (53 °N; Śliwińska et al. 2023) provided sea surface temperature (SST) derived from both GDGTs and alkenones. A similar temperature range and overall trend are observed in the nearby DSDP Site 112. In both sites, alkenones first appear around 33 Ma. The similarity of both records during the earliest Oligocene (23-25 °C) suggests that the temperatures recorded by both proxies are indicative of surface conditions. Until a temperature minimum is detected by both the proxies at ca. 29 Ma (close to the calcareous nannofossil NP23/NP24 boundary), the 𝑈K37′-derived SST is up to ca.1 °C warmer than suggested by TEX86^H. From around 29 Ma, 𝑈K37′-derived SST becomes significantly colder (2-4 °C) than TEX86-derived SST. Potentially, this may be because the surface conditions, reflected by the 𝑈K37′ index, changed more substantially than subsurface temperatures, which will affect TEX86 to a larger extent. Alternatively, it could indicate that there were shifts in seasonal impacts on these proxies. In order to understand the driving mechanism of the temperature offset, we have extracted vertical profiles for both sites from four simulations in the climate model GFDL CM2.1 adapted to the late Eocene boundary conditions (~38 Ma), following Hutchinson et al. (2019): with connection to the Arctic either closed or open, and the atmospheric CO₂ levels either high (800 ppm) or low (400 ppm). We observe that the simulation with open connection to the Arctic gives warmer sub-surface (ca. 100 to 500 m water depth) than surface temperature, and show overall the best match with our SST trends after 29 Ma in Site 647. On the contrary, the warmest SST (of ca. 20 °C) reproduced by the simulation with the connection to the Arctic closed and high CO₂ values fit well with the absolute SST values derived from the alkenones for the time interval between ca. 29 and 26.5 Ma, but fails to explain the warmer subsurface GDGT-derived temperatures.
References:
Hutchinson et al. 2019. Nature Communications 10(1): 3797.
Śliwińska et al. 2023. Climate of the Past 19(1): 123–40.