Carbonate compensation dynamics

Abstract

[1] Carbonate saturation (z sat ) and compensation (z cc ) depths change with deep-ocean acidification and basification. We present simple, explicit, mechanistic formulas for the positions of these two critical depths. In particular z cc is expressed as a function of the mean dissolved carbonate ion concentration of the deep ocean, [CO 3 ] D , the supply of dissolvable CaCO 3 , F c , and the dissolution rate constant at the sediment-water interface, k c , which we show to be essentially mass-transfer controlled. Calculations reveal that z sat and z cc are today some $0.9 km apart and will rise and separate by as much as 1.7 km with acidification; conversely, if [CO 3 ] D increases, z sat and z cc will deepen, but their separation will asymptote to $0.7 km. Citation: Boudreau, B. P., J.

Sedimentary Diagenesis, Depositional Environments, and Benthic Fluxes

“…When carbonate particles are dispersed in undersaturated solutions, dissolution kinetics are high order, with n ¼ 3.5-4 for calcites and aragonite, reflecting progressive change in the dominant dissolution mechanism at particle surfaces as a function of saturation state (Keir, 1980;Morse et al, 2007;Walter and Morse, 1985). Under diffusion-dominated sedimentary conditions, dissolution kinetics can apparently be reasonably approximated by n ¼ 1 (Boudreau et al, 2010;Hales, 2003;Hales and Emerson, 1997) or 1-2 (Green and Aller, 2001). The reaction rate coefficients are strong functions of particle size (reactive surface area) and specific compositions and sources (Keir, 1980;Morse, 1978).…”

“…[17] (Boudreau et al, 2010). Figure 19 The global distribution of sedimentary biogenic CaCO 3 on a weight percentage basis reflects the relative rates of CaCO 3 , C org , and terrigenous sediment supply, and the dissolution of CaCO 3 in both the water column and surface sediments (water and sediment transport-reaction conditions).…”

Balancing the Oceanic Calcium Carbonate Cycle: Consequences of Variable Water Column Ψ

“…By this interpretation, most CaCO 3 dissolution occurs above 1,000 m. CaCO 3 dissolution in the water column above the calcium carbonate saturation depth (often called the lysocline; see Boudreau et al 2010) is contrary to long-held beliefs dating back to the Challenger Expedition. However, the trend we observe and our interpretation of it are consistent with the growing body of recent evidence (e.g., Milliman and Droxler 1996;Milliman et al 1999;Troy et al 1997;Feely et al 2002Feely et al , 2004) that most CaCO 3 dissolution occurs in water shallower than 1,000 m. The calcium carbonate saturation depth apparently marks the depth above which, rather than below which, most water-column dissolution of CaCO 3 occurs.…”

Deep-Sea Sediments

“…Deep-ocean water temperature is low and relatively constant, but pressure causes the calcite solubility product constant to double between 2,000 and 5,000 m water depth (Boudreau et al, 2010). Deeper parts of the ocean lack CaCO 3 because of elevated CaCO 3 dissolution.…”

“…The solubility of calcite increases with increasing pressure, decreasing temperature, and decreasing dissolved carbonate ion, [CO 3 ] ¼ (Boudreau et al, 2010). Deep-ocean water temperature is low and relatively constant, but pressure causes the calcite solubility product constant to double between 2,000 and 5,000 m water depth (Boudreau et al, 2010).…”