Geoscience Reference
In-Depth Information
circulation change, not only under the effect of climate changes, but
also under tectonic and paleogeographic variations on timescales
ranging from hundreds of millions of years to a few million years.
Despite the regulation that we have just described, such modifications
can lead the ocean to diverge slightly, but permanently, from the
Redfield ratio N/P, thus creating conditions for a Darwinian evolution
where new species are selected for, which promote a new Redfield
ratio. The global data compiled in the NOAA Levitus Atlas in 1994
suggest an overall nitrate/phosphate ratio slightly under 16, which is
perhaps evidence of such a transitory state. In the Earth's distant past,
the variations in oxygen enrichment of the environment have
themselves also contributed to variations in the Redfield ratio, either
by reducing the extent of anoxic zones where the denitrification could
occur or by modifying the average N/P ratio of the cell. For example, it
is possible that the membrane arsenal, a major consumer of
phosphorus, was developed and perfected in the course of evolution,
leading to an increase in the demand for P by the organisms and, thus,
to a decrease in the N/P ratio of the biological demand.
Figure 1.5.
The principle of the regulation of oceanic
photosynthesis across the rapid cycle of major nutrients,
nitrogen and phosphorus. (see color section)
