New approaches to study the marine carbon cycle
Fassbender, Andrea Jayne
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Seven years of in situ salinity and carbon dioxide (CO2) measurements from two moorings located in the North Pacific Ocean were used to evaluate transformations in mixed-layer total alkalinity (TA) and dissolved inorganic carbon (DIC) using a time-dependent mass-balance approach. Results from the Kuroshio Extension Observatory (KEO) and Ocean Station Papa (OSP) moorings indicate that the physical input of carbon is primarily counterbalanced by biological processes over the annual cycle. At KEO these processes are highly coupled, while at OSP they are somewhat decoupled due to alternative limiting nutrient supply mechanisms. The annual net community production (NCP) at KEO was 4.5 ± 2.2 mol C m-2 yr-1 and at OSP was 1.9 ± 0.5 mol C m-2 yr-1. Although NCP is higher at KEO, it occurs over ~4 months in spring and has little influence on the air-sea CO2 exchange, while production at OSP occurs over ~7 months from winter to summer and has a large influence on sea surface CO2 throughout the year. In addition, the low buffer capacity of waters at OSP makes it 50% easier for biology to reduce CO2 levels than at KEO, which contributes to the larger role of biology in atmospheric CO2 uptake at OSP. Discovery of the complex interplay and timing of physics, biology and chemistry in these regions highlights the value of high-frequency, continuous observations. This new dual-carbon-tracer approach makes it possible to study the drivers of ocean carbon cycling remotely, but relies upon the calculation of DIC from in situ salinity and CO2 data when the direct measurement of DIC would be more ideal. To improve in situ carbon cycle study capabilities, a novel DIC sensor with an accuracy of ±5 µmol kg-1 was designed for extended-duration (>year) surface ocean monitoring. Results from two field tests of the prototype sensor indicate that direct measurement of DIC is ~90% more accurate than calculating DIC from collocated and contemporaneous measurements of pH and pCO2. This sensor now makes it possible to directly measure the surface ocean carbon inventory and to conduct high-accuracy carbon cycle studies autonomously over the entire annual cycle.
- Oceanography