At the Last Glacial Maximum (LGM; ~20,000 years ago), atmospheric carbon dioxide (CO2) levels were ~30% lower than pre-industrial values. Because most of the carbon in the climate system resides in the deep ocean, it likely plays the primary role in regulating atmospheric CO2 on time-scales of several millennia. Tracers of the ocean circulation suggest the deep ocean was more stratified during the LGM, potentially creating a 'trap' for CO2 in the abyss. Reduced mixing between deep waters may have aided storage of CO2, but data supporting this idea have remained elusive. This research uses a new tracer budget for Antarctic Bottom Water based on the oxygen isotopic composition of microscopic shells in marine sediments.
The aims of the work are two-fold: 1) to evaluate the circulation of Antarctic Bottom Water during the LGM when atmospheric CO2 was low, and 2) to do the same for the last deglaciation as ice sheets melted back and atmospheric CO2 increased. Preliminary analysis suggests that vertical mixing in the deep Atlantic was lower during the LGM. This research tests this result using an expanded set of well-dated sediment cores from the South and North Atlantic. The investigators also evaluate whether tracer gradients, and hence the deep circulation, changed in step with atmospheric CO2 during the last deglaciation. If this were the case, it would support the idea that circulation of the deep ocean plays a primary role in regulating CO2 on glacial timescales.
Funding supports a new faculty member with no prior NSF funding, as well as graduate and undergraduate involvement in research. Results of this study will shed light on the ocean's role in CO2 storage, and will help refine ocean-climate models.
The last major rise in atmospheric carbon dioxide prior to the industrial revolution occurred during the earth's transition out of the last ice age, as the large continental ice sheets melted. This rise in CO2 was one of the primary factors that caused the earth's climate to transition into into the current relatively warm and equitable pre-industrial state. Although paleoclimatologists have known about the CO2 rise for 30 years, its exact cause has remained elusive. One of the leading ideas has been that carbon escaped from the Southern Ocean near Antarctica where deep CO2 rich waters come to the surface. The object of the proposed study was to evaluate whether the oceanic circulation changed in step with the rise in atmospheric CO2 . It has long been assumed that the deep ocean played an important role but direct evidence has remained sparse and controversial. Early results from this project showed that carbon isotopic changes in the deep South Atlantic (~2000 m water depth) displayed a remarkable degree of similarity to atmospheric records, pointing to a strong link between the deep ocean and atmosphere. Our subsequent work has shown that the circulation in the abyssal South Atlantic (below 2500 m) showed little change early in the deglaciation. Instead, it appears that processes above 2000 m were the primary driver of the isotopic variability. By comparing our results with those from the North Atlantic, it appears that the primary driver of changes occurred in that basin, perhaps related to variations in the deep overturning circulation. Although the exact driver remains unclear, the available data point to a North Atlantic trigger for the last major rise in atmospheric CO2 rather than a Southern Ocean source.
