Possible overestimation of shallow‐depth calcium carbonate dissolution in the ocean

Abstract

[1] We argue that diagnostics of excess dissolved calcium carbonate (TA*) above the saturation horizon cannot be unambiguously interpreted in terms of local in situ dissolution. We examine a three-dimensional numerical model of global ocean circulation and biogeochemistry with explicit representation of the formation and dissolution of calcium carbonate. In particular, dissolution is only allowed to occur below the saturation horizon. The model qualitatively captures the observed basin-and global-scale patterns of alkalinity and TA* as well as the relationship between TA* and chlorofluorocarbon age in the thermocline. The existence of TA* above the saturation horizon in the model can only be explained in terms of transport subsequent to dissolution. The model study does not rule out the possibility of shallow-depth calcium carbonate dissolution but suggests that dissolution rates derived from tracer observations have not adequately accounted for the influence of transport.

A probabilistic assessment of calcium carbonate export and dissolution in the modern ocean

“…There are in fact several tracer-based studies reporting CaCO 3 dissolution above the saturation horizon of bulk seawater (Barrett et al, 2014;Feely et al, 2002Feely et al, , 2004Sabine et al, 2002b;Chung et al, 2003). In a modelling study, Friis et al (2006), nevertheless, demonstrated that the method which is often employed to derive these upper ocean dissolution rates (Berelson et al, 2007, see Discussion section on TA * CFC age method), might not be applicable, because this method neglects physical transport and mixing of alkalinity. It is therefore still debated where and how fast settling CaCO 3 particles are dissolved in the water column.…”

“…Equation (ueq kg −1 ) Mean RMSE from Global (mol m −3 ) B3D (mol m −3 ) Gruber et al (1996) Gruber et al (1996) is the original, global regression based on two explanatory variables (based on their 1996 database). Feely et al (2002) and Friis et al (2006) are both taken from Sabine et al (2002b), also including temperature as an additional explanatory variable and fitted to a Pacific subset. Global B3D 2V is a global regression with two explanatory variables (based on WOA09 Locarnini et al, 2010;Antonov et al, 2010;Garcia et al, 2010a, b) on the Bern3D grid (32 × 40 × 41 grid cells).…”

Inorganic carbon and pH levels in the Rockall Trough 1991–2010

“…Where A T data was not available (1991,1994,2009) To investigate the vertical distribution of A T in the water column without the effect of salinity, it was normalised to a salinity of 35 as described by Friis et al (2003) and O 2 are estimated it increases to ±5µmol kg -1 . Since ∆C T-abio is independent of the effects of the biological pump, it can therefore be assumed that any changes in ∆C Tabio within a water mass are due to increasing levels of anthropogenic CO 2 .…”

“…A number of studies have found that dissolution of CaCO 3 can occur above the saturation horizon (e.g. Chen, 2002;Feely et al, 2004;Milliman et al, 1999); however it may not be a significant process in the North Atlantic (Friis et al, 2006a;Friis et al, 2006b). Aragonite is more soluble than calcite and the aragonite saturation horizon (ASH) occurs at shallower depths, ~3500m deep in the North Atlantic .…”

A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor

“…Chung et al [2003] and Feely et al [2002] have suggested that much of the excess alkalinity in the upper waters could be due to the dissolution of particles within the upper water column. Friis et al [2006] suggested that the excess alkalinity in the upper water column could be derived from dissolution of CaCO3 from the sediment. The role of river-derived alkalinity on these estimates is also uncertain, but potentially important [e.g., Lee et al, 2006].…”