The causes of alkalinity variations in the global surface ocean

Abstract

Human activities have caused the atmospheric concentration of carbon dioxide (CO2) to increase by 120 ppmv from pre-industrial times to 2014. The ocean takes up approximately a quarter of the anthropogenic CO2, causing ocean acidification (OA). Therefore it is necessary to study the ocean carbonate system, including alkalinity, to quantify the flux of CO2 into the ocean and understand OA. Since the 1970s, carbonate system measurements have been undertaken which can be analyzed to quantify the causes of alkalinity variation in the surface ocean. A tracer of the oceanic calcium carbonate cycle (Alk*) was created by
removing alkalinity variation caused by other processes: evaporation and precipitation, river input, and the biological production and dissolution of organic matter. The remaining variation is similar to the distribution of the major nutrients with low values in the tropical surface ocean and values 110 μmol kg-1 and 85 μmol kg-1 higher in the Southern Ocean and the North Pacific respectively. The causes of longitudinal Alk* gradients in the Pacific Ocean were then analyzed. The results indicate that outcropping of isopycnals and upwelling of water enriches Alk* in the subpolar North Pacific and along the North American margin. On the other hand, the eastern equatorial upwelling appears to be from depths too shallow to contain enhanced Alk*. Two algorithms to predict alkalinity were then created for the surface Pacific with r values between predictions and measurements of 0.970 for the entire Pacific Ocean algorithm and 0.991 for the North Pacific eastern margin. A method using in-situ Alk* and velocity measurements to estimate calcium carbonate export was developed. This method estimates a summer and autumn export in the Southern Ocean of 0.31 Pg C yr-1 with the majority occurring around the Polar and Subantarctic Fronts. The Alk* tracer is shown to be a useful tool which could be improved by further research into riverine alkalinity inputs and the influence of sea ice formation on alkalinity.

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

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