Af. White et al., CHEMICAL-WEATHERING IN A TROPICAL WATERSHED, LUQUILLO MOUNTAINS, PUERTO-RICO - I - LONG-TERM VERSUS SHORT-TERM WEATHERING FLUXES, Geochimica et cosmochimica acta, 62(2), 1998, pp. 209-226
The pristine Rio Icacos watershed in the Luquillo Mountains in eastern
Puerto Rico has the fastest documented weathering rate of silicate ro
cks on the Earth's surface. A regolith propagation rate of 58 m Ma(-1)
, calculated from iso-volumetric saprolite formation from quartz diori
te, is comparable to the estimated denudation rate (25-50 Ma(-1)) but
is an order of magnitude faster than the global average weathering rat
e (6 Ma(-1)). Weathering occurs in two distinct environments; plagiocl
ase and hornblende react at the saprock interface and biotite and quar
tz weather in the overlying thick saprolitic regolith. These environme
nts produce distinctly different water chemistries, with K, Mg, and Si
increasing linearly with depth in saprolite porewaters and with strea
m waters dominated by Ca, Na, and Si. Such differences are atypical of
less intense weathering in temperate watersheds. Porewater chemistry
in the shallow regolith is controlled by closed-system recycling of in
organic nutrients such as K. Long-term elemental fluxes through the re
golith (e.g., Si = 1.7 x 10(-8)moles m(-2) s(-1)) are calculated from
mass losses based on changes in porosity and chemistry between the reg
olith and bedrock and from the age of the regolith surface (200 Ma). M
ass losses attributed to solute fluxes are determined using a step-wis
e infiltration model which calculates mineral inputs to the shallow an
d deep saprolite porewaters and to stream water. Pressure heads decrea
se with depth in the shallow regolith (-2.03 m H2O m(-1)), indicating
that both increasing capillary tension and graviometric potential cont
rol porewater;infiltration. Interpolation of experimental hydraulic co
nductivities produces an infiltration rate of 1 m yr(-1) at average fi
eld moisture saturation which is comparable with LiBr tracer tests and
with base discharge from the watershed. Short term weathering fluxes
calculated from solute chemistries and infiltration rates (e.g., Si =
1.4 x 10(-8) moles m(-2) s(-1)) are compared to watershed flux rates (
e.g., Si = 2.7 x 10(-8) moles m(-2) s(-1)). Consistency between three
independently determined sets of weathering fluxes imply that possible
changes in precipitation, temperature, and vegetation over the last s
everal hundred thousand years have not significantly impacted weatheri
ng rates in the Luquillo Mountains of Puerto Rico. This has important
ramifications for tropical environments and global climate change. Cop
yright (C) 1998 Elsevier Science Ltd.