Seawater above the carbonate saturation horizon is chemically oversaturated, yet repeated evidence - most directly a high-precision excess-calcium signal of 13±513\pm5 µmol kg−1^{-1} in the 200-800 m layer of the South China Sea - shows that calcium carbonate dissolves there, at a rate near 0.5 mmol m−2^{-2} d−1^{-1} with a lateral calcium export of (0.8±0.3)×1012(0.8\pm0.3)\times10^{12} mol yr−1^{-1} to the Pacific. The open problem is to explain and verify this shallow-depth (supralysocline) dissolution: to identify, with direct process-level evidence, the mechanism or mechanisms by which CaCO3 dissolves in oversaturated seawater (microbially mediated low-pH microenvironments inside particles and fecal pellets, dissolution in grazer guts, or other pathways); to determine the vertical location and magnitude of dissolution, euphotic zone versus subsurface, disentangled from riverine and surface confounders; and to quantify the integrated contribution of water-column dissolution in marginal seas and its lateral export to open-ocean alkalinity budgets and CO2 buffering - together with the enabling requirements of correcting non-carbonate alkalinity effects and intercalibrating high-precision Ca2+Ca^{2+} measurements.

Background

The ocean's calcium carbonate (CaCO3CaCO_3) cycle is a first-order control on the marine carbon system: calcifying organisms produce CaCO3CaCO_3 in the surface ocean, the particles sink, and their dissolution returns dissolved calcium and alkalinity to the water, influencing how much anthropogenic CO2 the ocean can absorb. Whether a water sample favors dissolution is summarized by the saturation state Ω\Omega, the ratio of the ion product [Ca2+][CO32−][Ca^{2+}][CO_3^{2-}] to the mineral's solubility product; waters with Ω>1\Omega>1 are oversaturated. The depth below which Ω\Omega falls below 1 for calcite or aragonite (the more soluble polymorph) defines the saturation horizon, and the transition zone of increasing seafloor dissolution is the lysocline. The conventional picture places most water-column CaCO3CaCO_3 dissolution below the saturation horizon, typically deeper than 1000 m. Total alkalinity (TAlk, the excess of proton acceptors over donors in seawater, dominated by bicarbonate and carbonate) increases by two equivalents per mole of CaCO3CaCO_3 dissolved, so alkalinity anomalies corrected for nitrate (potential alkalinity, PTA = TAlk + NO3 + PO4, which removes the alkalinity change caused by organic-matter remineralization) have traditionally been used to infer dissolution. That inference is confounded, however, by processes that change alkalinity without carbonate reactions - denitrification, dissolved organic bases, and benthic alkalinity generation - and by transport of preformed (advected) signals.

Dissolved calcium is in principle a cleaner tracer: Ca2+Ca^{2+} varies by only about 1% around roughly 10,200 µmol kg−1^{-1}, and on decadal timescales is almost solely controlled by CaCO3CaCO_3 reactions, but detecting the small anomalies requires high-precision methods such as potentiometric EGTA titration (EGTA is a chelator that binds calcium), standardized against IAPSO standard seawater (the International Association for the Physical Sciences of the Ocean's certified reference material for seawater). Using such measurements at the SEATS station (South East Asian Time-series Study, northern South China Sea, SCS), and removing the preformed Ca2+Ca^{2+} carried into the basin from the West Philippine Sea using an empirical calcium-potential temperature relation, Cao and Dai (2011) found a 13±513\pm5 µmol kg−1^{-1} excess in the 200-800 m layer, where waters are still oversaturated with respect to both calcite and aragonite. Converting the excess with an intermediate-water residence time of about 50 years gave an in-situ shallow dissolution rate of roughly 0.5 mmol m−2^{-2} d−1^{-1} (consistent with the CaCO3CaCO_3 loss between sediment traps at 124 m and 620 m in the basin), and multiplying by the 1.9±0.4 Sv subsurface outflow through the Luzon Strait (1 Sv, or sverdrup, equals 10610^{6} m3^{3} s−1^{-1}) gave a calcium export of (0.8±0.3)×1012(0.8\pm0.3)\times10^{12} mol yr−1^{-1}, about 10% of the estimated global marginal-sea calcium input to the open ocean. This joined an older puzzle: Milliman et al. (1999) had proposed biologically mediated CaCO3CaCO_3 dissolution above the chemical lysocline, and microelectrode studies had shown that marine snow and fecal pellets can host persistent, strongly depressed-pH microenvironments created by microbial respiration (Alldredge and Cohen, 1987), while digestion in the guts of grazing zooplankton offers a second biological pathway. Counterarguments exist too: Friis et al. (2006) showed that alkalinity anomalies above the saturation horizon can be produced by transport of dissolved carbonate from depth, so tracer-based shallow-dissolution estimates are ambiguous. What is missing is a process-level account - direct observations or controlled experiments linking microenvironments or gut chemistry to the measured Ca2+Ca^{2+} and alkalinity increases - and a determination of where in the upper water column the dissolution occurs and how globally representative the marginal-sea signal is.

Problem Statement

The problem, as posed by the source and still open, is to give a mechanistically explained, observationally verified account of shallow-depth (supralysocline, i.e. above the lysocline) CaCO3CaCO_3 dissolution in marginal seas - where it happens, why it happens, and what it contributes to the ocean's alkalinity budget. All of the following linked questions serve this single objective:

  1. Mechanism. Identify, with direct process-level evidence, the cause or causes of CaCO3CaCO_3 dissolution in waters oversaturated with respect to calcite and aragonite: does it arise from localized low-pH microenvironments created by microbial oxidation of organic matter inside fecal pellets or on particle surfaces, from digestive (gut-mediated) dissolution during grazing, from other biological or non-biological pathways, or from a combination - and can the candidate mechanism be shown in the field to account quantitatively for the measured Ca2+Ca^{2+} and alkalinity increases (in the SCS, the 13±513\pm5 µmol kg−1^{-1} excess and the associated rate and export fluxes)?

  2. Vertical location and magnitude. Determine where in the water column this dissolution occurs and how large it is - in particular whether significant dissolution takes place in the euphotic zone as well as in the subsurface (200-800 m) layer of the South China Sea - controlling for the confounders that affect surface Ca2+Ca^{2+} (riverine input, biological uptake, other surface processes), and quantify each layer's fraction of the total water-column CaCO3CaCO_3 loss.

  3. Tracer validity. Establish how much of the observed calcium and potential-alkalinity signals is produced or consumed by non-carbonate processes (denitrification, dissolved organic bases, benthic alkalinity generation) in marginal-sea versus open-ocean settings, so that calcium- and alkalinity-based dissolution estimates can be reconciled; and resolve the intercomparability of high-precision Ca2+Ca^{2+} measurements (the ~63 µmol kg−1^{-1} systematic offset between datasets attributed to IAPSO standard batch differences) so that small anomalies from different laboratories can be compared globally.

  4. Global significance. Determine how representative the South China Sea result is for other marginal seas, and quantify the integrated contribution of water-column shallow-depth dissolution in marginal seas and its lateral export to the open ocean's alkalinity inventory and CO2 buffering capacity.

The mechanism, vertical-location and budget results (items 1, 2 and 4) depend on the item 3 corrections and intercalibration. The mechanism must account for the observed in-situ Ca2+Ca^{2+} and alkalinity increases, and the dissolution budget must be attributed to an identified cause.

References

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