Dissolved inorganic carbon cycling in the offshore Amazon River plume and the western tropical North Atlantic ocean
Cooley, Sarah Rebecca
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The Amazon River discharges low-salinity, low-carbon water into the western tropical North Atlantic (WTNA), where the resultant plume stretches thousands of kilometers offshore and creates a sink for atmospheric carbon dioxide (CO2). The presence of a carbon sink in tropical oceans is noteworthy because tropical waters typically release CO2 to the atmosphere; the relative contributions of physical mixing and biological production must be understood, however, before the potential for changes in the magnitude of the sink can be determined. Biological production and subsequent export is the only process in tropical waters that can lead to a long-term sequestration of atmospheric carbon. This dissertation combines observational data and modeling studies to quantify the major inorganic carbon fluxes through the WTNA, so that the Amazon plume-related carbon sink can be understood mechanistically. I first distinguish between physical and biological effects on CO2 in the offshore plume using observational data, and find that biological drawdown is greatest in association with diazotroph-containing diatoms. Then I determine the relative importance of seasonality in plume supply or processing on the offshore plume-related atmospheric carbon sink, and conclude that although supply of inorganic carbon to the Amazon plume does vary through the year, offshore biological processing year-round has the greatest capacity to alter the magnitude of the sink, although its impact may vary depending on meteorological control. Finally, I examine the role of mixing at the edge of the plume, to understand the ultimate fate of the plume-related deficit in inorganic carbon. Vertical mixing is too small to completely eliminate plume-related inorganic carbon deficits, and horizontal advection and mixing must occur to bring plume water to nonplume conditions in the observed lifetime of the plume. As a result, the Amazon River plume-associated deficit may reduce WTNA inorganic carbon efflux beyond the borders of its low-salinity plume.