Carbon dynamics of the deglacial and contemporary ocean inferred from radiocarbon measurements in foraminifera, seawater and atmospheric carbon dioxide
PhD: University of Colorado Boulder, 2016.
Late Pleistocene atmospheric CO2 concentrations varied by ~90 ppm, rising with Antarctic and global temperatures during deglaciations. These natural variations were smaller and slower than the present CO2 increase, caused by anthropogenic emissions, which is driving the current transition to the warm Anthropocene. Because the ocean is the largest carbon reservoir that exchanges readily with the atmosphere, most explanations of the glacial-interglacial variations and predictions of future CO2 concentrations involve mechanisms that mediate that exchange. A critical region for such exchange is the Southern Ocean (SO), where carbon-rich deep water is upwelled to the surface by westerly winds that may be responsive to past and present warming. In this dissertation, I use radiocarbon (14C) measurements as a tracer to investigate the ocean’s role in controlling atmospheric CO2 during the last deglaciation and the past two decades.
Previously documented intervals of anomalously low 14C activity (∆14C) in the deglacial (18-11 ka BP) mid-depth ocean coincide with rising CO2 and decreasing ∆14C in the atmosphere, possibly tracing the re-emergence of aged carbon sequestered in the deep ocean during the preceding glacial period. I combined new 14C measurements in foraminifera from marine sediment cores near Baja California with published data to reconstruct regional gradients of ∆14C during deglaciation. The results appear to constrain the source of aged carbon to the SO, via the Equatorial Pacific. I also present new 14C measurements in air sampled since 2006 from Drake Passage in the SO. Transiently high CO2 concentrations correlate with low ∆14C and dominant modes of atmospheric variability, suggesting that increases in wind-driven upwelling drive more deep ocean carbon into the atmosphere, temporarily reducing the local net ocean carbon sink. Finally, I estimate rates of surface ocean ∆14C change since the 1990s using published datasets. The results imply that anthropogenic carbon, previously absorbed at high southern latitudes, is now re-emerging in the low latitude ocean. In summary, evidence presented in this dissertation suggests that SO upwelling, during both deglacial and contemporary periods of global warming, can act as a positive feedback in the coupled climate-carbon system by shifting deep ocean carbon into the atmosphere.