Impact of the marine biological pump on atmospheric CO2 uptake in the North Pacific: a study based on basin-wide underway measurements of oxygen/argon gas ratios and pCO2

Abstract

To predict how the ocean will respond to and feed back on increasing atmospheric CO₂ levels and climate change, we need excellent benchmark measurements of ocean primary productivity and organic carbon export to the deep ocean, known as the biological pump. In this dissertation, the contribution of the biological pump to atmospheric CO₂ uptake in the North Pacific was assessed through high-resolution estimates of net community productivity (NCP) and air-sea CO₂ flux. These parameters were determined across the North Pacific on a research cruise and repeated volunteer observation ship transits (2008–2011) by continuous measurement of surface values of the ratio of dissolved oxygen to argon (O₂/Ar) and the partial pressure of CO₂ (pCO₂). These synoptic NCP estimates cover a wider spatial extent at greater resolution (∼4 km) than any previous shipboard basin-scale estimates in the North Pacific. A general "west high, east low" pattern is found across the basin, with the greatest annual NCP in western boundary provinces influenced by the Oyashio and Kuroshio currents (9.1 ± 1.7 and 7.6 ± 3.6 mol C m<super>-2</super> yr<super>-1</super>, respectively) and higher NCP in the western transition zone (4.5 ± 3.3 and 3.7 ± 1.7 mol C m<super>-2</super> yr<super>-1</super>) than in the eastern transition zone (2.3 ± 0.9 and 2.8 ± 1.2 mol C m<super>-2</super> yr<super>-1</super>). NCP is similar in the subarctic west and east provinces at 4.8 ± 2.1 and 4.5 ± 1.5 mol C m<super>-2</super> yr<super>-1</super>, respectively, in contrast with previous results showing higher NCP in the western subarctic gyre than in the Alaska gyre. The impact of biological carbon export and physical supply on the surface dissolved organic carbon (DIC) budget was determined based on the difference between climatological air-sea CO₂ flux and O₂/Ar-based estimates of NCP. In the western subarctic and the coastal eastern transition zone, removal of DIC by NCP over the annual cycle is nearly balanced by physical DIC supply, yielding a relatively small net annual air-sea CO₂ influx in these regions (0.4–0.7 mol C m<super>-2</super> yr<super>-1</super>). In contrast, in the open-ocean transition zone and the eastern subarctic, removal of DIC by annual NCP is ∼2–3x the rate of physical supply of DIC, yielding substantial CO₂ influx (1.4–2.7 mol C m<super>-2</super> yr<super>-1</super>). Thus both biological uptake and physical supply exert significant control on annual atmospheric CO₂ uptake in the North Pacific, and both must be accurately modeled to predict ocean feedbacks on future climate change.