Publisher: Copernicus Publications
Subjects: Ecology, 550 Earth sciences & geology, QH540-549.5, QE1-996.5, 530 Physics, QH501-531, Geology, Life
The marine cycle of calcium carbonate (CaCO3) is an important
element of the carbon cycle and co-governs the distribution of carbon
and alkalinity within the ocean. However, CaCO3 export fluxes and
mechanisms governing CaCO3 dissolution are highly uncertain.
We present an observationally constrained, probabilistic assessment of
the global and regional CaCO3 budgets. Parameters governing
pelagic CaCO3 export fluxes and dissolution rates are sampled
using a Monte Carlo scheme to construct a 1000-member ensemble
with the Bern3D ocean model. Ensemble results are constrained by
comparing simulated and observation-based fields of excess dissolved
calcium carbonate (TA*). The minerals calcite and aragonite are
modelled explicitly and ocean–sediment fluxes are considered. For
local dissolution rates, either a strong or a weak dependency on CaCO3 saturation is assumed.
In addition, there is the option to have saturation-independent dissolution above the saturation horizon.
The median (and 68 % confidence
interval) of the constrained model ensemble for global biogenic CaCO3 export is 0.90
(0.72–1.05) Gt C yr−1, that is within the lower half of
previously published estimates
(0.4–1.8 Gt C yr−1). The spatial pattern of
CaCO3 export is broadly consistent with earlier
assessments. Export is large in the Southern Ocean, the tropical
Indo–Pacific, the northern Pacific and relatively small in the
The constrained results are robust across a range of diapycnal mixing coefficients and, thus,
ocean circulation strengths. Modelled ocean circulation and transport
timescales for the different set-ups were further evaluated with CFC11
and radiocarbon observations. Parameters and mechanisms governing
dissolution are hardly constrained by either the TA* data or the
current compilation of CaCO3 flux measurements such that model
realisations with and without saturation-dependent dissolution achieve
skill. We suggest applying saturation-independent dissolution rates in
Earth system models to minimise computational costs.
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