Variability and control of the surface ocean carbonate system observed from ships of opportunity

Abstract

The surface ocean plays an important role in the marine carbon cycle linking the atmosphere and the deep ocean. There are substantial variations in the surface ocean carbonate system in different environments and on various time scales, resulting from the interactions of various physical and biogeochemical processes. In this study, the use of Ships of Opportunity (SOO, as carriers of automatic underway measuring systems and platforms for sample collections) was promoted to enhance the surface ocean observing capacity to provide in-situ observations with better temporal resolution and spatial coverage in a cost- effective way (Chapter 2). The functionality, reliability and accuracy of an automatic pCO2 sensor were assessed extensively under various field and laboratory conditions (Chapter 3). The sensor proved to be suitable for long-term onboard and in-situ measurements, while its uncertainty is largely determined by the reference used for calibration. Observations made from two SOOs were examined to better understand the variability and control of the surface ocean carbonate system (Chapter 4, 5).

The spatial variability of alkalinity in different marine environmental settings were investigated focusing on the influences of physical and biogeochemical processes on the alkalinity-salinity relationship (Chapter 4). By using salinity-normalized alkalinity as an indicator, the TA addition or removal processes were examined in the open ocean regime in the Atlantic, Pacific and Indian Ocean (mainly controlled by precipitation and evaporation), in the western North Atlantic margin, eastern North Pacific and Mediterranean Sea (additional alkalinity inputs from rivers, currents or the Black Sea), and in the Red Sea (alkalinity removal by CaCO3 precipitation). In coastal regions, a regional-specific term for zero salinity end member should be considered in salinity normalization practice, and care should be taken when use the abstract intercept of alkalinity-salinity regression to estimate the river water end member (only works reliably in river-dominated systems).

The temporal variations of the surface carbonate system and air-sea CO2 flux in the Northeast Atlantic (Bay of Biscay) were examined on seasonal to interannual time scales (Chapter 5). The seasonal variability of DIC (single annual peak in winter, shaped by the winter increase due to deep convection followed by the spring biological drawdown) is different to the seasonal cycle of pCO2 (double annual peaks in winter and summer, determined by the competing effects of temperature and nonthermal processes on pCO2). A comparative study shows a latitudinal transition of pCO2 seasonality in the North Atlantic: from the temperature-dominated oligotrophic subtropical gyre to the subpolar region where pCO2 is dominated by changing concentrations of DIC. Located in the transition zone, pCO2 in the mid-latitude Bay of Biscay shows a double-peak distribution in its annual cycle: the summer peak is dominated by temperature increase while the winter peak results from the dominant convection effect. The interannual biogeochemical changes in the Bay of Biscay were found to be closely related to the varying intensity of winter mixing, with higher seasonal amplitudes of DIC and nutrients observed in cold years in response to negative phases of the North Atlantic Oscillation. An increase in annual mean seawater pCO2 was observed from 2002 to 2010 associated with decreased rates of oceanic CO2 uptake.

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ORCID for T. Tyrrell: orcid.org/0000-0002-1002-1716

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