Sack Lunch Seminar (SLS)

Abstract:
The search for reliable proxies of past deep ocean temperature and salinity has proved difficult, thereby limiting our ability to understand the coupling of ocean circulation and climate over glacial-interglacial timescales. Previous inferences of deep ocean temperature and salinity from sediment pore fluid oxygen isotopes and chlorinity indicate that the deep ocean density structure at the Last Glacial Maximum (LGM, ~20,000 years BP) was set by salinity, and that the density contrast between northern and southern sourced deep waters was markedly greater than in the modern ocean. High density stratification could help explain the marked contrast in carbon isotope distribution recorded in LGM aged ocean sediments relative to that we observe today, but what made the ocean's density structure so different at the LGM? Was it actually different?

We investigate both the feasibility and cause of a highly- and salinity-stratified deep ocean at the LGM. With a Bayesian Markov Chain Monte Carlo (MCMC) inverse method we test the robustness of the previously inferred LGM deep ocean density structure, and, using a coupled ocean--sea ice--ice shelf cavity model (MITgcm), we test whether the deep ocean density structure at the LGM can be explained by ice--ocean interactions over the Antarctic continental shelves. We show that the LGM southern source bottom water was saltier than northern source deep water, but the contrasts in deep ocean water masses were likely smaller than previously reported. Our sensitivity experiments in MITgcm indicate that a significant portion of the LGM salinity structure can be explained through lower-than-modern ocean temperature.