Dr. William Gray will give two talks at this Thursday's Speakers Club.  The first talk is entitled "Release of CO₂ from the subarctic Pacific Ocean over the last deglaciation" and the second talk is entitled "The effects of temperature and salinity on Mg incorporation in planktonic foraminifera G. ruber (white): results from a global sediment trap Mg/Ca database."  Both talks are in Webb Hall 1100.

1. Release of CO₂ from the subarctic Pacific Ocean over the last deglaciation

The ~90 ppmv increase in atmospheric CO₂ over the last deglaciation was most likely driven by changes in the way the high latitude ocean regulates the exchange of dissolved inorganic carbon (DIC) between the ocean and atmosphere, however the mechanisms involved in this repartitioning remain unclear. The subarctic Pacific Ocean is currently a significant source of CO₂ to the atmosphere due to mixing of DIC-rich intermediate waters into the surface ocean and incomplete nutrient utilisation. It is hypothesised that during the last glacial period, greater stratification in the subarctic Pacific reduced CO₂ outgassing and increased DIC storage within the deep ocean, and that during deglaciation a breakdown in ocean stratification returned this deeply sequestered DIC back to the surface ocean and atmosphere.

After demonstrating paired Li/Ca and Mg/Ca ratios in benthic foraminifera Uvigerina peregrina can be used to reconstruct past changes in bottom water temperature and carbonate ion concentration [CO₃^2-] with a coretop dataset, I will present a deglacial record of [CO₃^2-] and bottom water temperature from the intermediate to deep subarctic Pacific Ocean. The data indicate a decrease in [CO₃^2-] within the deep waters of the subarctic Pacific of 12 µmol/kg during glacial times, equivalent to an ~80 µmol/kg increase in DIC. Over deglaciation, an increase in [CO₃^2-], coeval with warmer bottom waters and an increase in benthic radiocarbon (?¹⁴C), denotes a transfer of DIC out of the deep subarctic Pacific Ocean. Boron isotopes measured on planktonic foraminifera N. pachyderma (s) in the same core show a large drop in surface ocean pH at ~15 ka, demonstrating a significant release of CO₂ to the atmosphere at this time.

2. The effects of temperature and salinity on Mg incorporation in planktonic foraminifera G. ruber (white): results from a global sediment trap Mg/Ca database

Mg/Ca in Globigerinoides ruber is arguably the most important proxy for sea surface temperature (SST) in tropical and sub tropical regions, and as such guides our understanding of past climatic change in these regions. However, the sensitivity of Mg/Ca to salinity is debated; while analysis of foraminifera grown in cultures generally indicates a sensitivity of 3 – 6% per salinity unit, core-top studies have suggested a much higher sensitivity of between 15 – 27% per salinity unit, bringing the utility of Mg/Ca as a SST proxy into dispute. Sediment traps circumvent the issues of dissolution and post-depositional calcite precipitation that hamper core-top calibration studies, whilst allowing the analysis of foraminifera that have calcified under natural conditions within a well constrained period of time. Previously published sediment trap/plankton tow G. ruber (white) Mg/Ca data were collated, and new Mg/Ca data were generated from a sediment trap located in the highly-saline tropical North Atlantic, close to West Africa. Calcification temperature and salinity were calculated for the time interval represented by each trap/tow sample using World Ocean Atlas 2013 data. The resulting dataset comprises ~250 Mg/Ca measurements (in the size fraction 150 – 350 µm), that span a temperature range of 18 – 28 °C and 33.6 – 36.7 PSU. Multiple regression of the dataset reveals a temperature sensitivity of 7.2 ± 0.4% per °C (p < 1*10^-15) and a salinity sensitivity of 4.5 ± 1% per salinity unit (p = <1*10^-4). Application of this calibration has significant implications for both the magnitude and timing of glacial-interglacial temperature changes when variations in salinity are accounted for.