Abstract

In this study we use the southern Benguela upwelling system to investigate the role of nutrient and carbon stoichiometry on carbonate dynamics in eastern boundary upwelling systems. Six months in 2010 were sampled along a cross-shelf transect. Data were classified into summer, autumn, and winter. Nitrate, phosphate, dissolved inorganic carbon, and total alkalinity ratios were used in a stoichiometric reconstruction model to determine the contribution of biogeochemical processes on a parcel of water as it upwelled. Deviations from the Redfield ratio were dominated by denitrification and sulfate reduction in the subsurface waters. The N:P ratio was lowest (7.2) during autumn once anoxic waters had formed. Total alkalinity (TA) generation by anaerobic remineralization decreased pCO2 by 227 μatm. Ventilation during summer and winter resulted in elevated N:P ratios (12.3). We propose that anaerobic production of TA has an important regional effect in mitigating naturally high CO2 and making upwelled waters less corrosive.

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Marine Carbonate fluxes in the southern Benguela: A schematic showing the magnitude of processes contributing to DIC and TA fluxes. Black numbers represent DIC and gray TA. The solid/dashed/dotted lines represent the thermocline and its intensity. Increases in DIC throughout all seasons were largely due to aerobic remineralization (RM). Large TA gains in autumn were due to benthic processes: denitrification (DN), sulfate reduction (SR), and calcite dissolution (CD). Strong primary production (PP) in summer reduced the surface DIC, while calcification (CL) in autumn resulted in decreased TA.

Marine Carbonate fluxes in the southern Benguela: A schematic showing the magnitude of processes contributing to DIC and TA fluxes. Black numbers represent DIC and gray TA. The solid/dashed/dotted lines represent the thermocline and its intensity. Increases in DIC throughout all seasons were largely due to aerobic remineralization (RM). Large TA gains in autumn were due to benthic processes: denitrification (DN), sulfate reduction (SR), and calcite dissolution (CD). Strong primary production (PP) in summer reduced the surface DIC, while calcification (CL) in autumn resulted in decreased TA.

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