Abstract
Despite the important roles of shallow-water sediments in global biogeochemical cycling, the effects of ocean acidification on sedimentary processes have received relatively little attention. As high-latitude cold waters can absorb more CO2 and usually have a lower buffering capacity than warmer waters, acidification rates in these areas are faster than those in sub-tropical regions. The present study investigates the effects of ocean acidification on sediment composition, processes and sediment-water fluxes in an Arctic coastal system. Undisturbed sediment cores, exempt of large dwelling organisms, were collected, incubated for a period of 14 days, and subject to a gradient of pCO2 covering the range of values projected for the end of the century. On five occasions during the experimental period, the sediment cores were isolated for flux measurements (oxygen, alkalinity, dissolved inorganic carbon, ammonium, nitrate, nitrite, phosphate and silicate). At the end of the experimental period, denitrification rates were measured and sediment samples were taken at several depth intervals for solid-phase analyses. Most of the parameters and processes (i.e. mineralization, denitrification) investigated showed no relationship with the overlying seawater pH, suggesting that ocean acidification will have limited impacts on the microbial activity and associated sediment-water fluxes on Arctic shelves, in the absence of active bio-irrigating organisms. Only following a pH decrease of 1 pH unit, not foreseen in the coming 300 years, significant enhancements of calcium carbonate dissolution and anammox rates were observed. Longer-term experiments on different sediment types are still required to confirm the limited impact of ocean acidification on shallow Arctic sediment processes as observed in this study.
Highlights
Because about one third of anthropogenic CO2 emissions has been stored in the oceans since the industrial revolution [1], surface seawater pH has already declined by,0.1 unit compared with pre-industrial values [2,3] and it is projected to decrease by 0.36 unit by the end of the century [3]
A one-way ANOVA was used to test the effects of seawater pressure of CO2 (pCO2) on denitrification rates
Most of the parameters and processes investigated within this study showed no relationship with the overlying seawater pH
Summary
Because about one third of anthropogenic CO2 emissions (from fossil fuel, cement production and land-use changes) has been stored in the oceans since the industrial revolution [1], surface seawater pH has already declined by ,0.1 unit compared with pre-industrial values [2,3] and it is projected to decrease by 0.36 unit by the end of the century [3]. Acidification rates in the Arctic Ocean have shown to be faster that in sub-tropical regions [8] with some areas, as in the Canada basin and the Chukchi Sea, already experiencing undersaturated conditions [9,10] due to a combination of sea-ice melting and anthropogenic CO2 penetration. These effects of freshening and increased carbon uptake in response to sea-ice retreat due to global change make Arctic Ocean surface waters the area in the world that will experience the largest pH declines in the coming decades [11]. River runoff to Arctic shelves causes a significant decrease of alkalinity and calcium ions, and lowers saturation states with respect to aragonite and calcite [12]
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