Chemistry Reference
In-Depth Information
structure and functioning of the ecosystem. The monographs of De Angelis (1992) and
Valiela (1995) illustrate this close integration of ecology and biogeochemistry.
At the other end of the spectrum, the biogeochemical dynamics of the global earth
system respond to geophysical forcings on time scales of millions of years and longer.
Plate tectonics continuously rearrange the physical setting in which the biosphere
functions and evolves, by shaping the morphology of ocean basins and modifying the
position and elevation of the continents. Equally important, plate tectonics control the
intensity of chemical exchanges between the earth's surface environment and the
underlying lithosphere (e.g., Berner 1990).
The major bioessential elements, including carbon and the nutrient elements N, P, S,
Si, K and Ca, are efficiently recycled within terrestrial and marine ecosystems
(Schlesinger 1997). As a consequence, on a yearly basis, only very small fractions of
these elements escape through removal to the lithosphere. Therefore, on relatively short
time scales, say, less than 1000 years, the lithosphere is a minor sink in most
biogeochemical cycles. At these time scales, modeling efforts tend to focus on the
redistribution of chemical constituents among the atmosphere, hydrosphere and
biosphere. A good example is provided by global carbon cycle models used in predicting
the future fate of anthropogenic CO
2
emissions to the atmosphere. How much of this CO
2
remains in the atmosphere is, in the short term, mainly determined by transfer of CO
2
from the atmosphere to the ocean, and to vegetation plus soils on land (e.g., Sarmiento
and Gruber 2002).
With the passing of time, however, the cumulative loss of bioessential elements to
the lithosphere becomes significant. For most major nutrient elements, the principal
escape route is incorporation in marine sedimentary deposits. Unless somehow
compensated, burial in ocean sediments would ultimately deplete the surface reservoirs of
nutrients, resulting in the collapse of biological activity on earth. Marine turnover times
(Equation 3) of limiting nutrients, relative to removal by sediment burial, provide rough
estimates of the time scale over which such a collapse would take place. These turnover
times are on the order of 10
4
to 10
5
years.
Fortunately, the loss of bioessential elements to the lithosphere is countered by their
release by volcanic outgassing and chemical weathering of rocks. Hence, the latter
processes are essential for the continued survival of life on geological time scales. From
the point of view of the global cycles of bioessential elements, we can therefore
distinguish between long and short time scales, depending on whether the lithosphere is a
significant sink and source, or not. The distinction between short and long times is not a
sharp one and varies from one element to the other. It is safe to state, however, that for
time spans
10
4
years chemical exchanges with the lithosphere become a key factor
controlling biogeochemical cycles.
With the exception of N, the lithosphere is the largest reservoir of the major nutrient
elements. Sediments and rocks contain orders of magnitude more C, P, S, Fe, Si, Ca, and
K than the atmosphere, hydrosphere, pedosphere and biosphere combined (e.g., Garrels
and Mackenzie 1971; Drever et al. 1988; Chameides and Perdue 1997; Reeburgh 1997;
Mackenzie 1998). The transit times of these elements through the lithosphere are
similarly orders of magnitude longer than their turnover times at the earth surface. On
average, it takes several hundreds of millions of years for non-volatile materials buried in
marine sediments to be exposed on land by plate tectonic processes. Once incorporated in
the lithosphere, bioessential elements may follow a variety of different pathways,
however (Fig. 2). As a result, transit times through the lithosphere exhibit a broad range
of values.
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