Orbital Variability and Evolution of Subantarctic Surface Waters in the Pliocene Epoch

Abstract:

The Pliocene Epoch is regarded as a time of great surface ocean warming in the Northern Hemisphere, but the thermal history in the Southern Hemisphere, where a large ice sheet prevailed, remains unknown. DSDP Site 594 monitors subantarctic surface waters at present and as such, sea surface temperature (SST) at this site may provide our best quantitative indication to date of Pliocene ocean temperatures in the vital region that interacted with the Antarctic ice sheet.

We present a continuous, high resolution (~2 kyr) SST record for the time span from ~5.6 to 2.6 Ma, based on alkenone unsaturation from deep sea drilling project Site 594. The SST record from Site 594 allows us to investigate the Southern Ocean thermal history (magnitude and timing) for the whole Pliocene Epoch for the first time. Average long-term SST at Site 594 is only slightly above present day SST at this location, in agreement with sparse available SST reconstructions in other regions of the Southern Ocean. Our findings suggest that the Southern Hemisphere mid to high latitudes were on average contrastingly colder than the rest of the Pliocene world. This "average Pliocene coolness" may be related to the Antarctic cryosphere, suggesting Antarctic ice volume similar to late Pleistocene.

We also find important trends in this SST record, which include a distinct long-term (_≥ orbital) long wave pattern of pronounced warming and cooling events along a gradual cooling background (~1 °C/Myr). The evolution of long wave patterns shown here is connected to pervasive eccentricity-related changes, contrasting previous assumptions of obliquity dominance during the Pliocene Epoch. The eccentricity response we find clearly dominates the variance of not only Site 594, but also other available reconstructions, and is likely related to non-linear feedbacks triggered by precession. The long wavelength features shown in this work suggest that the time leading up to a bipolar cryosphere was not dominated almost monocyclically by the 41-kyr variance in the Pliocene.