EXPLORING PLIO-PLEISTOCENE OCEAN SURFACE CONDITIONS USING THE ALKENONE ORGANIC PROXIES

Because the observational record of climate change is short relative to the time scales on which many climate system processes operate, it is useful to examine past climate intervals to gain insight into the behavior of critical climate system processes. Of particular interest are past warm climate intervals, which help provide context for potential future climate conditions. The last interval of sustained warmth in Earth history was the Pliocene (~3-5 Ma), during which global mean annual temperatures were 3-4ºC warmer than modern and atmospheric concentrations of carbon dioxide were comparable to modern (~400ppm). Over the past several decades, organic geochemical proxies have greatly augmented our understanding of past warm climate intervals including the Pliocene.

Alkenones, organic compounds produced by a few species of ocean phytoplankton, allow for quantitative estimates of past sea surface temperature (SST) as well as qualitative estimates of past export productivity. These proxies have greatly advanced our understanding of climate conditions during the Pliocene warm period and the subsequent descent into northern hemisphere glaciation due to straightforward analytical procedures, a globally applicable SST calibration, and a limited influence of potentially confounding factors. Here, I share two recent syntheses of Plio-Pleistocene ocean surface conditions, which are primarily based on alkenones. The first synthesis, of SST change, shows that the spatial distribution of temperature during the Pliocene was markedly different than modern, with significantly reduced (4-6ºC) zonal and meridional temperature gradients. The second synthesis, of marine biological export production, reveals a dramatic shift in marine productivity at the end of the Pliocene, with marked producitivity declines in high latitude regions and marked productivity increases in mid to low latitude regions. Both of these syntheses speak to fundamentally different patterns of ocean surface conditions during the Pliocene warm period and highlight the possibility that increases in global average temperatures may be associated with major changes in the pattern of climate and ocean conditions. These studies also highlight the efficacy of organic geochemical approaches in augmenting our understanding of past climates.