The North Pacific biological pump: Rates, efficiency and influence on ocean carbon uptake
Palevsky, Hilary Ilana
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The North Pacific features a band of strong atmospheric CO2 uptake at the transition between the subarctic and subtropical gyres, making it a region of particular importance to quantify the rates and mechanisms of ocean carbon cycling. We measured the surface concentration of dissolved inorganic carbon (DIC), the partial pressure of carbon dioxide to determine air-sea CO2 flux, and triple oxygen isotopes and oxygen/argon dissolved gas ratios as non-incubation based geochemical tracers of gross oxygen production (GOP) and net community production (NCP) on sixteen container ship transects across the North Pacific between Hong Kong and Long Beach, CA from 2008-2012. This dissertation assesses the rates and efficiency of biological carbon export and evaluates the relative roles of biology, physical circulation, and temperature-driven solubility changes in driving air-sea CO2 flux throughout the full annual cycle across the North Pacific basin (35°N – 50°N, 142°E – 125°W) by constructing mixed layer budgets that account for physical and biological influences on the measured tracers. The dynamic western basin influenced by the Kuroshio and Oyashio currents experiences a stronger seasonal cycle in DIC, NCP and GOP than the more quiescent eastern basin. During the productive season from spring through fall, GOP and NCP are highest in the Kuroshio region west of 170°E and decrease eastward across the basin. However deep winter mixed layers (>200 m) west of 160°W ventilate ~40-90% of this seasonally exported carbon while only ~10% of seasonally exported carbon east of 160°W is ventilated in winter where mixed layers are <120 m. As a result, despite higher annual GOP in the west than the east, the annual carbon export (sequestration) rate and efficiency decrease westward across the basin from export of 2.3 ± 0.3 mol C m-2 yr-1 east of 160°W to 0.5 ± 0.7 mol C m-2 yr-1 west of 170°E. The annual ocean carbon sink is strongest in the Kuroshio region and decreases eastward across the basin (air-sea CO2 flux of 2.7 ± 0.9 mol C m-2 yr-1 west of 170°E, as compared to 2.1 ± 0.3 mol C m-2 yr-1 east of 160°W). This indicates that carbon export via the biological pump can fully account for annual CO2 uptake in the eastern basin, but less than 20% of annual CO2 uptake in the Kuroshio region, requiring a significant removal of DIC via physical processes in the west. The seasonal timing of NCP as well as its annual magnitude strongly affects the influence of the biological pump on air-sea CO2 flux. Estimates of primary and export production from satellite algorithms and global biogeochemical models provide promising tools to extend analysis over spatial and temporal scales where in situ measurements are unavailable. However, we find that no single satellite algorithm or model can reproduce seasonal and annual geochemically-determined primary production and NCP rates throughout the North Pacific basin, based on comparisons throughout the full annual cycle at time series stations in the subarctic and subtropical gyres and in the basin-wide region sampled by container ship transects. The high latitude regions show large primary production discrepancies in winter and spring and strong effects of deep winter mixed layers on annual NCP that cannot be accounted for in current satellite-based approaches. These results underscore the need to evaluate satellite- and model-based estimates using multiple productivity parameters measured over broad ocean regions and throughout the full annual cycle, including during winter ventilation, in order to accurately estimate the rate and efficiency of carbon sequestration via the ocean’s biological pump.
- Oceanography