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intense and we all picture the luxuriant forests where dinosaurs roamed. Provided enough
nutrients were available, a high pressure of atmospheric CO 2 increased primary produc-
tivity in the ocean. It also enhanced intense weathering, and thus induced strong fluxes of
alkalinity to the ocean and massive limestone sedimentation. The Cretaceous is indeed a
period characterized by remarkable carbonate sedimentation, in particular on continental
shelves.
So how can we be invoking a reduced carbonate precipitation during the high P CO 2
bouts of the Quaternary and apparently the opposite for the Cretaceous? This is actually
a matter of kinetics. The residence time of alkalinity in the ocean is
100 000 years. We
also remember that the mean time of oceanic overturn is 1600 years by today's standards,
i.e. geologically instantaneous. For temperature and P CO 2 fluctuations on this time scale or
faster, such as those imposed by astronomical forcing, the incoming and outgoing fluxes of
alkalinity are not balanced and carbonate precipitation reflects short-term external changes.
For time scales
100 ka, like the steady high P CO 2 conditions of the Cretaceous, steady
state is achieved and what comes around goes around: the abundant carbonate series pre-
cipitated on the seafloor represent the alkalinity lost by the ocean and therefore must have
been compensated for by a corresponding alkalinity flux to the ocean. The conclusion
remains that the Cretaceous was a time of high chemical erosion rate and therefore of
high P CO 2 . Observations on different time scales therefore provide opposite conclusions
on P CO 2 changes.
An alternative explanation of climate control, suggested by the isotope geochemistry of
strontium in seawater, is that of tectonic forcing. In marine carbonates, the 87 Rb/ 86 Sr ratio
is so low that it has barely varied with time since deposition (see (4.15) ) . Consequently,
87 Sr/ 86 Sr remains frozen near its original value and therefore at the 87 Sr/ 86 Sr value of
seawater (from which the limestone is derived) at the time of sedimentation. The reason
for this is that the concentration in the alkali element Rb (whose behavior is very similar to
that of K) is very low compared with that of the alkaline-earth element Sr (whose behavior
is very similar to that of Ca). It has been observed that the 87 Sr/ 86 Sr ratio of seawater as
recorded in marine carbonates has increased since the Cretaceous ( Fig. 9.6 ) . This increase
is evidence of the increased input of continental strontium with its far higher 87 Sr/ 86 Sr
ratio (0.711 on average) than that of strontium derived from the weathering of basalts
(0.702). We will see later that this difference reflects the high Rb/Sr ratio of the continental
crust. This value points to increased erosion, while the evidence of carbon isotopes and the
general cooling of climates and of the ocean since the Cretaceous argue against increased
emissions of volcanic CO 2 . This suggests, then, more efficient erosional processes, and it
has been proposed that the collision between India and Asia, and the resulting uplift of
the Himalayas in the Tertiary, were responsible for the decline in atmospheric CO 2 .The
high elevation of the Himalayas and the monsoon regime that dominates this area favor
particularly intense mechanical erosion. South and southeast Asia therefore account for a
large fraction of the sediment produced in the world. In contrast to the dissolved loads of
rivers flowing into the Atlantic, suspended solids and sediments are carried to the Indian
Ocean (e.g. along the Ganges and Indus) or to the Pacific (e.g. along the Mekong and
Yangtze). This second scenario emphasizes the consumption of CO 2 and explains climatic
cooling by the more intense erosion resulting from the uplift of the Himalayas.
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