Calcite dissolution rates in seawater: Lab vs. in-situ measurements and inhibition by organic matter

Abstract

Ocean acidification from fossil fuel burning is lowering the mean global ocean saturation state (Ω = [Ca^(2+)][CO_3^(2−)]/K_(sp)′), thus increasing the thermodynamic driving force for calcium carbonate minerals to dissolve. This dissolution process will eventually neutralize the input of anthropogenic CO_2, but the relationship between Ω and calcite dissolution rates in seawater is still debated. Recent advances have also revealed that spectrophotometric measurements of seawater pHs, and therefore in-situ Ωs, are systematically lower than pHs/Ωs calculated from measurements of alkalinity (Alk) and total dissolved inorganic carbon (DIC). The calcite saturation horizon, defined as the depth in the water column where Ω = 1, therefore shifts by ~5–10% depending on the parameters used to calculate Ω. The “true” saturation horizon remains unknown. To resolve these issues, we developed a new in-situreactor and measured dissolution rates of ^(13)C-labeled inorganic calcite at four stations across a transect of the North Pacific Ocean. In-situ saturation was calculated using both Alk-DIC (Ω_((Alk, DIC))) and Alk-pH (Ω_((Alk, pH))) pairs. We compare in-situ dissolution rates with rates measured in filtered, poisoned, UV-treated seawater at 5 and 21 °C under laboratory conditions. We observe in-situ dissolution above Ω_((Alk, DIC)) = 1, but not above Ω_((Alk, pH)) = 1. We emphasize that marine carbonate system equilibria should be reevaluated and that care should be taken when using proxies calibrated to historical Ω_((Alk, DIC)). Our results further demonstrate that calcite dissolution rates are slower in-situ than in the lab by a factor of ~4, but that they each possess similar reaction orders (n) when fit to the empirical Rate = k(1-Ω)^n equation. The reaction orders are n < 1 for 0.8 < Ω < 1 and n = 4.7 for 0 < Ω < 0.8, with the kink in rates at Ω_(crit) = 0.8 being consistent with a mechanistic transition from step edge retreat to homogenous etch pit formation. We reconcile the offset between lab and in-situ rates by dissolving calcite in the presence of elevated orthophosphate (20 μm) and dissolved organic carbon (DOC) concentrations, where DOC is in the form of oxalic acid (20 μm), gallic acid (20 μm), and D-glucose (100 μm). We find that soluble reactive phosphate has no effect on calcite dissolution rates from pH 5.5–7.5, but the addition of DOC in the form of D-glucose and oxalic acid slows laboratory dissolution rates to match in-situ observations, potentially by inhibiting the retreat rate of steps on the calcite surface. Our lab and in-situ rate data form an envelope around previous in-situ dissolution measurements and may be considered outer bounds for dissolution rates in low/high DOC waters. The lower bound (high DOC) is most realistic for particles formed in, and sinking out of, surface waters, and is described by R_((mol cm-2 s-1)) = 10^(–14.3±0.2)(1-Ω)^(0.11±0.1) for 0.8 < Ω < 1, and R_((mol cm-2 s-1)) = 10^(–10.8±0.4)(1-Ω)^(4.7±0.7) for 0 < Ω < 0.8. These rate equations are derived from in-situ measurements and may be readily implemented into marine geochemical models to describe water column calcite dissolution.