Summary
An eleven-box model of the ocean-atmosphere subsystem of the global carbon
cycle is developed to study the potential contribution of continental rock
weathering and of oceanic sedimentation to variations of atmospheric CO2
pressure over glacial-interglacial time scales. The model is capable of
reproducing the present-day distributions of total dissolved inorganic
carbon, total alkalinity, phosphate, d13C, and D14C
between the various ocean basins, as well as the partial pressure of atmospheric
CO2. A simple sedimentation scheme drives carbonate deposition
and dissolution at the sea-floor as a function of the depths of carbonate
and aragonite lysoclines in each ocean basin considered (Atlantic, Antarctic
and Indo-Pacific). Carbonate accumulation on the shelf is also taken into
account.
Three different methods are used to calculate histories for the evolution
of CO2 consumption by continental rock weathering processes,
with special emphasis on silicate weathering. The first method relies on
the marine 87Sr/86Sr isotopic record. We find that
this record does not represent a very strong constraint, due to the large
spread of the
87Sr/86Sr ratios of waters draining
silicate terrains. It is possible to construct a silicate weathering history
that reproduces both the strontium isotopic record and the glacial-interglacial
CO2 signal. This weathering history implies that CO2
consumption by silicate rock weathering was about 120% higher during glacial
than during interglacial time.
The second approach is based upon the marine Ge/Si record. Taking the
major uncertainties in the knowledge of the Ge and Si cycles into account,
several histories for the evolution of the riverine dissolved silica fluxes
are calculated from this record. The investigation of the systematics between
riverine dissolved silica and bicarbonate fluxes under different weathering
regimes leads us to the tentative conclusion that, although there is no
correlation between dissolved silica and total bicarbonate concentrations
in the major rivers, there may exist a negative correlation between weathering
intensity and the ratio of bicarbonate derived from silicate weathering
alone to dissolved silica. With this correlation as a working hypothesis,
it is possible to interpret the dissolved silica fluxes in terms of equivalent
CO2 consumption rates. The calculated histories indicate that
glacial rates of CO2 consumption by chemical silicate rock weathering
could have been twice, and possibly up to three times and a half, as high
as they are today. When used to force the carbon cycle model, they are
responsible for glacial-interglacial pCO2 variations
in the atmosphere of typically 50-60 ppm and up to 95-110 ppm. These variations
are superimposed to a basic oscillation of 60 ppm generated by the model,
mainly in response to coral reef buildup and erosion processes. The total
pCO2 signal has an amplitude of about 80-90 ppm and up
to 125-135 ppm. Although these large amplitudes indicate that silicate
weathering processes should be taken into account when studying glacial-interglacial
changes of CO2 in the atmosphere, it also raises new problems,
such as too high CO2 levels during the period from 110-70 kyr
B.P.
In the third approach, the glacial-interglacial histories for the consumption of CO2 and the resulting transfer of bicarbonate to the ocean are calculated from the erosion model GEM-CO2}. The required variations of the continental runoff are derived from four different GCM climatologies. We find that the CO2 consumption and river bicarbonate fluxes were about 20% higher at the last glacial maximum than at present. The exposed shelf accounts for a large fraction of the calculated LGM flux, overcompensating the 20% decrease of the two fluxes over the continent. The constructed weathering scenarios still produce pCO2 variations of about 60 ppm between glacial and interglacial times, but the contribution from variable silicate weathering to this signal is now reduced to only 12+/-5 ppm.
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