Calcium carbonate dissolution in the upper 1000 m of the eastern North Atlantic

Abstract

Recent analyses suggest that considerable CaCO 3 dissolution may occur in the upper water column of the ocean (< 1500 m). This study uses the distribution of particulate calcium from high-resolution suspended matter sampling along the Climate Variability and Predictability/CO 2 Repeat Hydrography A16N transect in 2003 to estimate CaCO 3 dissolution in the top 1000 m of the North Atlantic. Dissolution rates were also approximated using changes in total alkalinity measurements along isopycnal surfaces. Water masses were found to be undersaturated with respect to aragonite at intermediate depths (400-1000 m) in the eastern tropical North Atlantic. The CaCO 3 dissolution rate in this region is estimated to be 0.9 mmol CaCO 3 m À2 d À1, indicating this region is a hotspot for upper water column CaCO 3 dissolution compared to the Atlantic basin as a whole. Dissolution rates calculated from particulate calcium distributions outside of this region were significantly lower (0.2 mmol CaCO 3 m À2 d À1) and are comparable to previous estimates of CaCO 3 dissolution flux for the Atlantic Ocean. The magnitude of upper water column dissolution rates compared to measured surface ocean CaCO 3 standing stocks suggests that biologically mediated CaCO 3 dissolution may be occurring in the top 1000 m of the Atlantic.

Burial-nutrient feedbacks amplify the sensitivity of atmospheric carbon dioxide to changes in organic matter remineralisation

“…It has been suggested that changes in the rate of CaCO 3 dissolution in the upper ocean may be a significant feedback Earth Syst. Dynam., 5, 321-343, 2014 www.earth-syst-dynam.net/5/321/2014/ affecting atmospheric CO 2 concentrations and future climatic changes (Barrett et al, 2014). In our sensitivity experiments, we varied the e-folding dissolution length scale between 2300 and 3500 m, and thereby the amount of CaCO 3 dissolution in near-surface water and the upper ocean.…”

“…Barrett et al, 2014). To this end, we prescribe in a further set of sensitivity simulations a step change in the e-folding dissolution length scale l calc governing the dissolution profile of CaCO 3 particles within the water column (see Eq.…”

“…However, considerable CaCO 3 dissolution may occur in the upper ocean (Berelson et al, 2007) in waters that are saturated with respect to CaCO 3 in the mineral form of calcite or even of aragonite, perhaps due to the dissolution of high-magnesium calcite or due to dissolution in specific microenvironments such as zooplankton guts, fecal pellets or organic aggregates (see e.g. Barrett et al, 2014, and references therein). Here, we do not explicitly take into account such effects, and the dissolution is for example not affected by changes in the calcite saturation horizon (the depth where the concentration of calcium times the concentration of carbonate equals the solubility product of calcite; Mucci, 1983).…”

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

“…On a regional level, only two out of ten regional estimates by Berelson et al (2007) are within our uncertainty ranges; these are the estimates for the tropical Pacific (taken as 5 • N to 5 • S) and the low-and mid-latitude Atlantic (40 • S to 40 • N) (Table 5). High dissolution rates in the range of ∼ 0.1 to 0.4 mmol C m −3 yr −1 are estimated by Barrett et al (2014) for a transect in the upper tropical and northern North Atlantic. These values are much larger than our estimates of order 0.01 mmol C m −3 yr −1 for the upper tropical and northern Atlantic.…”

“…CaCO 3 dissolution may nevertheless still occur in the upper ocean in suitable, undersaturated microenvironments which would be present for instance in the guts of zooplankton, suspended organic aggregates, or fecal pellets (Bishop et al, 1980;Milliman et al, 1999;Jansen and Wolf-Gladrow, 2001). 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.…”