Gray, WR; (2015) The role of the North Pacific Ocean in the deglacial CO₂ rise: insights from trace elements and boron isotopes in biogenic carbonates. Doctoral thesis , UCL (University College London).

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Abstract

The ∼90 ppmv increase in atmospheric CO₂ over the last deglaciation was most likely driven by changes in the way the high latitude oceans regulate the exchange of dissolved inorganic carbon (DIC) between the surface and interior ocean, 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 increased stratification of the subarctic Pacific reduced CO₂ outgassing and increased DIC storage within the interior ocean, and over deglaciation this deeply sequestered DIC reservoir was returned back to the surface ocean and atmosphere. This thesis provides the first quantitative reconstruction of changes in DIC storage within the interior North Pacific, and CO₂ outgassing from the subarctic Pacific over the last deglaciation. After demonstrating paired Li/Ca and Mg/Ca ratios in benthic foraminifera Uvigerina spp. can be used to reconstruct past changes in bottom water DIC with a core top calibration, Mg/Ca and Li/Ca ratios were measured in Uvigerina senticosa from marine sediment core MD01-2416, located at 2317m water depth in the subarctic Pacific. The results indicate a ∼200 μmol/kg increase in DIC within the interior North Pacific during glacial times. At ∼16 ka a rapid reduction in DIC, coeval with warmer bottom waters and an increase in benthic ∆¹⁴C, denotes the transfer of DIC out of the deep-intermediate subarctic Pacific Ocean. A reconstruction of surface ocean pCO₂ using boron isotopes measured on planktonic foraminifera Neogloboquadrina pachyderma (s) in the same core shows a ∼90 ppmv decrease during glacial times, and a major release of CO₂ to the atmosphere during deglaciation. These results suggest a significant role for the North Pacific Ocean in the deglacial atmospheric pCO₂ rise.

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