An assessment of ocean margin anaerobic processes on oceanic alkalinity budget

Abstract

Recent interest in the ocean's capacity to absorb atmospheric CO2 and buffer the accompanying “ocean acidification” has prompted discussions on the magnitude of ocean margin alkalinity production via anaerobic processes. However, available estimates are largely based on gross reaction rates or misconceptions regarding reaction stoichiometry. In this paper, we argue that net alkalinity gain does not result from the internal cycling of nitrogen and sulfur species or from the reduction of metal oxides. Instead, only the processes that involve permanent loss of anaerobic remineralization products, i.e., nitrogen gas from net denitrification and reduced sulfur (i.e., pyrite burial) from net sulfate reduction, could contribute to this anaerobic alkalinity production. Our revised estimate of net alkalinity production from anaerobic processes is on the order of 4–5 Tmol yr−1 in global ocean margins that include both continental shelves and oxygen minimum zones, significantly smaller than the previously estimated rate of 16–31 Tmol yr−1. In addition, pyrite burial in coastal habitats (salt marshes, mangroves, and seagrass meadows) may contribute another 0.1–1.1 Tmol yr−1, although their long‐term effect is not yet clear under current changing climate conditions and rising sea levels. Finally, we propose that these alkalinity production reactions can be viewed as “charge transfer” processes, in which negative charges of nitrate and sulfate ions are converted to those of bicarbonate along with a net loss of these oxidative anions.

The impact of sedimentary alkalinity release on the water column CO<sub>2</sub> system in the North Sea

“…Whereas this estimate agrees well with an independent estimation by Chen (2002) (16-31 Tmol yr −1 ), Hu and Cai (2011a) obtained a much smaller flux of 4-6 Tmol yr −1 . Note that all these three studies are based on different assumptions about the underlying processes that are generating A T .…”

“…Finally, based on modeled nitrogen and carbon budgets for the northwestern North Atlantic continental shelf, Fennel (2010) suggested that shelf sediments must be an important source of alkalinity. By contrast, Hu and Cai (2011a) estimated a much smaller mean alkalinity flux from global coastal sediments based on the upscaling of local denitrification and sulfate reduction rates. They concluded that sedimentary alkalinity fluxes are generally too low to significantly affect the alkalinity budget of the global ocean, but also stated that sedimentary anaerobic processes could be important on regional scales as an alkalinity release mechanism.…”

“…Note that the coupling of ammonification, nitrification and subsequently denitrification does not lead to net production of A T . Only if nitrate is derived from the overlying water column does one obtain a net production of A T in the pore water (Hu and Cai, 2011a).…”

“…Whereas the A T flux estimated by Chen (2002) is mainly generated by sulfate reduction with zero contribution of carbonate dissolution, Krumins et al (2013) acknowledged carbonate dissolution as a benthic A T source. The estimate of Hu and Cai (2011a) is based on anaerobic A T generation alone; hence, possible contributions by carbonate dissolution are not taken into account. Furthermore, these latter authors treat the coastal sediment-water column system as a single system, while the other two papers only consider the sediment.…”

Is Ocean Acidification an Open-Ocean Syndrome? Understanding Anthropogenic Impacts on Seawater pH

“…Net alkalinity gain is, however, only achieved during anaerobic processes that involve a permanent loss of remineralization products, e.g. through denitrification or pyrite burial (Hu and Cai 2003). Vegetation modifies sediment biogeochemical processes and, thus, affects the pH and alkalinity of pore waters.…”