| The analyses of both modern river loads and chemical and physical hillslope denudation
show that rates of weathering and erosion are tightly interlinked. An increase in
silicate weathering through increased mountain erosion should therefore lead to
global cooling by increased drawdown of atmospheric CO2. A five-fold increase of
global sediment delivery was suggested for the last 5 Million years from a global
compilation of sedimentation rates (1), and was also specifically suggested for the
European Alps (2) and the India-Asia collisional area (3). While much research effort is
currently being directed at explaining this increase, we should have been suspicious
because this apparent increase in global erosion rate was not accompanied by a
similar concomitant decrease in atmospheric CO2 as recorded by ocean alkalinity
proxies.
One possibility is that the increase in Neogene sedimentation rates is an artifact
introduced by the discrepant time scale over which sedimentation is measured (4). The older
the age, the longer the observational integration time will be, which in turn include longer
periods of hiatus, and hence decreasing sedimentation rates with geologic time.
An analysis of the records named above for time scale bias indeed suggests the
possibility that erosion in these mountain belts might have been constant throughout the
Neogene.
An independent test of this hypothesis is provided by marine records of the isotopes of
Beryllium (5). Be-10 is a rare radioactive cosmogenic nuclide, is produced mostly in the
atmosphere and introduced to the surface oceans with a flux that can be considered to
be globally uniform when averaged over time scales exceeding those of climate
cycles. Be-9, in contrast, is stable, and enters the oceans from the continents mainly
by river particulates (which are dissolved to some extent in the water column and
during early diagenesis), in the dissolved form, and in minor amounts from dust.
Should the global erosion rates have increased, the isotope ratio of Be-10 to Be-9
would have decreased by roughly the same factor. Over the past 10 My, records of
chemical marine deposits (Fe-Mn crusts and authigenic deep sea sediments) show
no change in this ratio after correction for radioactive decay of Be-10. Therefore,
these records support the hypothesis of constant global erosion and weathering
fluxes.
If this hypothesis is true, neither Late Tertiary mountain building nor Quaternary cooling
affected or was affected by a change in silicate weathering rates. Instead, a more
continuous mechanism is suggested in that subtle ongoing hillslope rejuvenation in any
soil-mantled hillslopes in kinetically-limited settings enable the feedback that stabilizes
atmospheric CO2 and climate levels through silicate weathering. In fact, in steep,
active mountain belts an increase of relief and erosion rates to those that are in
excess of conditions were soils are stable lead to a decrease, not an increase of
weathering rate. But the aim here is not to discount active mountains as premier agents
of CO2 withdrawal. Silicate weathering may still take place within the adjacent
sedimentary basins, and large carbon deposits could also be buried there in the organic
form.
(1) P. Z. Zhang, P. Molnar, W. R. Downs, Nature 410, 891 (2001).
(2) J. Kuhlemann, W. Frisch, B. Székely, I. Dunkl, M. Kázmér, Int J Earth Sci) 91, 818
(2002).
(3) F. Métivier, Y. Gaudemer, P. Tapponier, M. Klein, Geophys. J. Int 137, 280
(1999).
(4) P. M. Sadler, GeoResearch Forum 5, 15 (1999).
(5) F. von Blanckenburg, R. K. O’Nions, Earth and Planet. Sci. Letters 167, 175
(1999). |