CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales

This work was supported by NERC Standard Grant NE/N003861/1 to J.W.B.R. and L.F.R., a NOAA Climate and Global Change VSP Fellowship to J.W.B.R, NERC Standard Grant NE/M004619/1 to AB and JWBR, a NERC Strategic Environmental Science Capital Grant to A.B. and J.W.B.R., Marie Curie Career Integration Grant CIG14-631752 to AB, an ERC consolidator grant to L.F.R., NSF grant OCE-1503129 to J.F.A., and NERC studentships to B.T. and E.L.The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial–interglacial CO2 change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere1. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region2. However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH—and thus the CO2 chemistry—of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean’s lower overturning cell, we find a close relationship between ocean pH and atmospheric CO2: during intervals of low CO2, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO2, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO2, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO2 change, and show that deep-ocean CO2 release can occur as a dynamic feedback to rapid climate change on centennial timescales.PostprintPeer reviewe

Uranium distribution and incorporation mechanism in deep-sea corals : implications for seawater [CO32–] proxies

This research received funding from NSF grant OCE-1737404 awarded to JA and China Scholarship Council Ph.D. Scholarship 201508020007 awarded to SC. JR was supported by ERC Starting Grant 805246 OldCO2NewArchives.A conservative element in seawater, uranium is readily incorporated into the aragonitic skeletons of scleractinian corals, making them an important paleoclimate archive that can be absolutely dated with U-Th techniques. In addition, uranium concentrations (U/Ca ratios) in corals have been suggested to be influenced by the temperature and/or carbonate ion concentration of the ambient seawater based on empirical calibrations. Microsampling techniques have revealed strong heterogeneities in U/Ca within individual specimens in both surface and deep-sea corals, suggesting a biological control on the U incorporation into the skeletons. Here we further explore the mechanism of uranium incorporation in coral skeletons with the deep-sea species Desmophyllum dianthus, an ideal test organism for the biomineralization processes due to its relatively constant growth environment. We find a negative correlation between bulk coral U/Ca and temperature as well as ambient pH and [CO32–] that is consistent with previous studies. By sampling the growth bands of individual corals, we also find a twofold change in U/Ca within individual corals that is strongly correlated with the δ18O, δ13C, and other Me/Ca ratios of the bands. A similar correlation between U/Ca and stable isotopes as well as other Me/Ca ratios are observed in bulk deep-sea coral samples. With a numerical coral calcification model, we interpret the U/Ca-stable isotope correlation as a result of changes in uranium speciation in response to internal pH elevations in the extracellular calcifying fluid (ECF) of the corals, and suggest that the Ca2UO2(CO3)3(aq) complex, the dominant U species in seawater, may be the major species incorporated into the coral skeleton. Therefore, the correlation between U/Ca and ambient [CO32–] is likely a result of the response of the biomineralization process, especially the magnitude of internal pH elevation, to the growth environment of the corals. Our data suggest overall lower alkalinity pump rates in corals from low saturation seawater compared to those from high saturation seawater, and possible increases in Ca2+ supply from active pumping relative to seawater transport in response to the environmental stress of low saturation.Publisher PDFPeer reviewe