Tracking Southern Ocean sea ice extent and frontal positions: Novel techniques based on oxygen isotope and Mg/Ca analyses of foraminifera

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Understanding the drivers of glacial-interglacial carbon dioxide (CO2) cycles is one of the most important questions in the field of paleoclimatology. It has long been known that Earth’s ice sheets expand when atmospheric CO2 is low and melt as CO2 rises, but the underlying climate mechanisms remain unclear. Expanded sea ice coverage in the Southern Ocean around Antarctica may have limited the release of carbon from the deep ocean during natural cold periods such as the Last Glacial Maximum (LGM; ~20,000 years ago). Greater sea ice extent and northward movement of oceanic fronts may have also led to carbon storage in the deep ocean. Reliable paleoclimate records from the Southern Ocean are thus essential to understanding the controls on atmospheric CO2 over long timescales. The proposed study will develop new methods to produce such records, using chemical analyses of microscopic fossils from marine sediment cores. The educational impact of the proposed work includes support for a graduate student and an undergraduate at the University of Connecticut. The proposed work will focus on developing a new proxy for sea ice extent based on the oxygen isotope ratios (d18O) of benthic and planktonic foraminifera. Sea ice formation in the Southern Ocean is characterized by near-freezing surface conditions and negative buoyancy forcing, which creates a unique vertical profile in d18O. Preliminary data from the published literature shows that the d18O difference between recent planktonic and benthic foraminifera yields an estimate of sea ice extent consistent with modern observations. We also plan to develop a novel proxy for subtropical front position based on foraminiferal magnesium/calcium (Mg/Ca) and d18O analyses. Contemporary measurements of sea-surface temperature and the d18O of seawater capture the position of the subtropical front, implying that reconstructions based on Mg/Ca and d18O can be used to track frontal position in the geologic past. Motivated by these initial results, we propose to use a transect of cores from ~55ºS to 35ºS in the Atlantic sector of the Southern Ocean to: 1) estimate sea ice extent and surface buoyancy forcing during the LGM, and 2) reconstruct the position of the subtropical front during the LGM to constrain inter-basin exchange between the Atlantic and Indian Oceans. The proposed work could transform our understanding of the Southern Ocean by adding two new methods to the paleoclimate toolkit. The d18O sea ice proxy will complement existing diatom-based methods and it can be used in regions where diatoms are sparse. Similarly, development of the subtropical front technique will lay the groundwork for studies of the deglaciation when changes in frontal position have been invoked to explain rising atmospheric CO2. Given the relevance of the techniques to research questions across a range of timescales, we anticipate our results will be of broad interest to the paleoclimate community.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.