Coastal ocean CO₂–carbonic acid–carbonate sediment system of the Anthropocene

Abstract

[1] There is little doubt that human activities such as burning of fossil fuels and land use practices have changed and will continue to change the cycling of carbon in the global coastal ocean. In the present study, two biogeochemical box models were used to investigate the consequences of increasing atmospheric CO 2 and subsequent ocean acidification and increasing riverine transport of organic matter and nutrients arising from human activities on land on the global coastal ocean between the years 1700 and 2300. Numerical simulations show that the net flux of CO 2 between coastal ocean surface water and the atmosphere is likely to change during this time from net evasion to net invasion owing to increasing atmospheric CO 2 , increasing net ecosystem production arising from increasing nutrient loading to this region, and decreasing net ecosystem calcification due to lower carbonate ion concentration and subsequent lower surface water saturation state with respect to carbonate minerals. Model calculations show that surface water saturation state with respect to calcite will decrease 73% by the year 2300 under a business-as-usual scenario, which in concert with increasing temperature will cause overall biogenic calcification rate to decrease by 90%. Dissolution of carbonate minerals increased by 267% throughout the model simulation. This increase was in part due to increased invasion of atmospheric CO 2 , but mainly due to greater deposition and remineralization of land-derived and in situ produced organic matter in the sediments, producing CO 2 that caused pore water pH and carbonate saturation state to decrease. This decrease, in turn, drove selective dissolution of metastable carbonate minerals. As a consequence, the relative carbonate composition of the sediments changed in favor of carbonate phases with lower solubility than that of an average 15 mol% magnesian calcite phase. Model projected changes in surface water carbonate saturation state agree well with observations from the Hawaiian Ocean Time series and the calculated air-sea CO 2 exchanged agrees well with a recent independent estimate of this flux derived from measurements from diverse coastal ecosystems scaled up to the global coastal ocean area.

Size-dependent pH effect on calcification in post-larval hard clam Mercenaria spp.

“…Green & Aller 1998, Andersson et al 2006. The rapid equilibrium thermodynamics of the carbonate system and diffusion-dominated transport of solutes in organically rich sediments result in higher concentrations of CO 2 and lower pH of sediment porewater versus overlying water, especially in shallow, well-mixed water bodies (Fig.…”

“…It would therefore be naive to suggest that pH is the only factor that would have significant effects on shell growth in this species and other bivalves, as salinity and temperature also alter calcification (Waldbusser et al 2010). However, the differences in hatchery calcification rates suggest that selective breeding programs may be one possible mitigation strategy for commercial bivalve stocks.Predicting the change in pH in coastal and estuarine waters due to atmospheric CO 2 and other anthropogenic impacts on biogeochemical cycles is a daunting task (Andersson et al 2006, Blackford & Gilbert 2007, Wootton et al 2008. Hard clams and other benthic infaunal calcifiers are ecologically and commercially important to many coastal ecosystems, their sediment habitats are typically more corrosive than overlying waters, and these ecosystems are likely to be altered by increased CO 2 and acidification in complex ways (Andersson et al 2006, Borges & Gypens 2010, Waldbusser et al 2010.…”

Chapter 1 Impacts of the Oceans on Climate Change

“…Even if dissolution of CaCO 3 was taking place at a rate equivalent to the estimated total annual production of CaCO 3 in surface waters, it would still only be partially buffered (estimated maximum 6%). This is especially so as most CaCO 3 production is pelagic and sinks to deeper depths (Andersson et al, 2006). Dissolution from bottom sediments as the lysocline shoals in response to the reduction in pH over the next century and longer will enable the oceans to increase their CO 2 sink.…”

“…Dissolution of CaCO 3 minerals in the surface layers of the oceans acts as a further buffer of pH and carbonate saturation state against acidification from an increasing ocean uptake of atmospheric CO 2 (Andersson et al, 2006). However, ''CaCO 3 dissolution has a negligible impact on atmospheric Author's personal copy pCO 2 or the atmospheric stabilisation of CO 2 emissions'' over the next few centuries (Archer et al, 1998).…”

Calcium carbonate production response to future ocean warming and acidification

“…Several recent modelling studies have been conducted to examine changes in CaCO 3 production rates due to acidification and their effect on the fate of anthropogenic CO 2 (Andersson et al, 2006;Gangstø et al, 2008;Gehlen et al, 2007;Heinze, 2004). Despite significant research attention, however, estimates of the current rate of global CaCO 3 production vary widely: from 0.4 to 1.8 Pg C yr −1 .…”