weathering
Streams of the Montane Humid Tropics. Treatise on Geomorphology
Wed, 08/03/2011 - 11:00 — leonmiScatena F.N., Gupta A., 2011. Streams of the Montane Humid Tropics. Treatise on Geomorphology. Editors E. Wohl. Academic Press, San Diego Ca. Vol 9. in press April 2011
Abstract:
Tropical montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastal
regions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not
been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive
2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfalls
that alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these
streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic
landforms that can be solely attributed to their low-latitude locations. While they are relatively understudied, an emerging
view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a high
frequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic
processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to
transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in
other fluvial systems.
Long‐term patterns and short‐term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed
Wed, 08/03/2011 - 10:44 — leonmiShanley, J. B., W. H. McDowell, and R. F. Stallard (2011), Long‐term patterns and short‐term dynamics of stream
solutes and suspended sediment in a rapidly weathering tropical watershed, Water Resour. Res., 47, W07515,
doi:10.1029/2010WR009788
Abstract:
The 326 ha Río Icacos watershed in the tropical wet forest of the Luquillo Mountains,
northeastern Puerto Rico, is underlain by granodiorite bedrock with weathering rates
among the highest in the world. We pooled stream chemistry and total suspended sediment
(TSS) data sets from three discrete periods: 1983–1987, 1991–1997, and 2000–2008.
During this period three major hurricanes crossed the site: Hugo in 1989, Hortense in
1996, and Georges in 1998. Stream chemistry reflects sea salt inputs (Na, Cl, and SO4),
and high weathering rates of the granodiorite (Ca, Mg, Si, and alkalinity). During rainfall,
stream composition shifts toward that of precipitation, diluting 90% or more in the largest
storms, but maintains a biogeochemical watershed signal marked by elevated K and
dissolved organic carbon (DOC) concentration. DOC exhibits an unusual “boomerang”
pattern, initially increasing with flow but then decreasing at the highest flows as it becomes
depleted and/or vigorous overland flow minimizes contact with watershed surfaces.
TSS increased markedly with discharge (power function slope 1.54), reflecting the erosive
power of large storms in a landslide‐prone landscape. The relations of TSS and most
solute concentrations with stream discharge were stable through time, suggesting minimal
long‐term effects from repeated hurricane disturbance. Nitrate concentration, however,
increased about threefold in response to hurricanes then returned to baseline over several
years following a pseudo first‐order decay pattern. The combined data sets provide insight
about important hydrologic pathways, a long‐term perspective to assess response to
hurricanes, and a framework to evaluate future climate change in tropical ecosystems.
Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico
Thu, 07/28/2011 - 14:52 — ckassPett-Ridge J. C., Derry L. A. and Kurtz A. C. (2009) Sr isotopes as
a tracer of weathering processes and dust inputs in a tropical
granitoid watershed, Luquillo Mountains, Puerto Rico. Geochim.
Cosmochim. Acta 73, 25–43.
Abstract:
Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains
of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides
new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on
major elements alone. 87Sr/86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs.
Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is
2100 ± 700 mg cm2 ka1. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence
of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall
short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based
on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase
and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/86Sr
ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination
of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be
identified
Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing
Wed, 07/27/2011 - 12:36 — ckassBuss HL, Sak PB, Webb SM, Brantley SL. 2008. Weathering of the Rio
Blanco quartz diorite, Luquillo Mountains, Puerto Rico: coupling
oxidation, dissolution, and fracturing. Geochimica et Cosmochimica
Acta 72: 4488–4507.
Abstract:
In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing
a 0.2–2 m thick zone of partially weathered rock layers (2.5 cm thickness each) called rindlets, which form concentric
layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive DV of
reaction builds up elastic strain energy. The rates of spheroidal fracturing and saprolite formation are therefore controlled by
the rate of the weathering reaction.
Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fractureinducing
reaction. This reaction occurs with an expansion in d (001) from 10.0 to 10.5A ˚ , forming ‘‘altered biotite”.
Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using
the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of
8.2 1014 mol biotite m2 s1. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray
microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 lm resolution revealed
that oxidized zones within individual biotite crystals are the first evidence of alteration of the otherwise unaltered
corestone.
Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding
the rindlet–saprolite interface, hornblende dissolves to completion at a rate of 6.3 1013 mol hornblende m2 s1: the
fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution
rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are
able to describe the process by which the quartz diorite bedrock disaggregates and forms saprolite. In the corestone, biotite
oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase
and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary
minerals. The thickness of the resultant saprolite is maintained at steady state by a positive feedback between the denudation
rate and the weathering advance rate driven by the concentration of pore water O2 at the bedrock–saprolite
interface.
Controls on major solutes within the drainage network of a rapidly 3 weathering tropical watershed
Wed, 07/27/2011 - 12:29 — ckassBhatt, M. P., and W. H. McDowell (2007), Controls on major solutes within the drainage network of a rapidly weathering
27 tropical watershed, Water Resour. Res., 43, XXXXXX, doi:10.1029/2007WR005915.
Abstract:
Surface water chemistry in the main stem and source points of the Rio Icacos basin
7 (Luquillo Experimental Forest, Puerto Rico) was studied to investigate the factors
8 regulating spatial variability in major solutes in a rapidly weathering landscape. We
9 sampled along the main stem as well as at small source points at high elevation where
10 fresh bedrock is frequently exposed, and at low elevation in the floodplain/colluvial
11 plain of the main stem. Concentrations of silicon, alkalinity, and the sum of base
12 cations were lower at the source points than in the main stem, and were lowest in low-
13 elevation source points. Calcium and sodium were the dominant cations at all sampling
14 points after sea-salt correction, reflecting the weathering of plagioclase feldspar
15 throughout the basin. The partial pressure of carbon dioxide (pCO2) tended to be higher,
16 and HCO3
concentrations were lower, in the low-elevation source points than at other
17 positions in the landscape. When coupled with the relatively low concentrations of Si and
18 base cations, this suggests that the availability of primary reactive minerals, rather than
19 carbonic acid concentrations, limits weathering in these low-elevation sources.
20 Mechanical denudation appears to enhance chemical weathering rates not only by
21 refreshing reactive mineral surfaces but also by contributing carbon dioxide from the
22 decomposition of organic-rich material in landslides, which occur frequently. The spatial
23 variability of major solutes appears to depend primarily on the availability of fresh primary
24 reactive minerals, carbon dioxide concentrations, and hydrolysis conditions.
REDUCTION OF BEDROCK BLOCKS AS CORESTONES IN THE WEATHERING PROFILE: OBSERVATIONS AND MODEL
Tue, 07/26/2011 - 16:25 — ckassFletcher RC, Brantley SL. 2010. Reduction of bedrock blocks as corestones in the weathering profile: observations
and model. Am. J. Sci. 310:131–64
Abstract:
the Espiritu Santo and Mameyes rivers within the Luquillo Experimental
Forest (Puerto Rico) are interpreted as corestones, reduced from initial joint-bounded
bedrock blocks by subsurface weathering. Maximum corestone size, expressed as the
geometric mean of the three dimensions, S 3 abc, shows a smooth envelope when
plotted against elevation. We postulate that, for each catchment, they represent in situ
corestones within a stratified weathering profile, many tens of meters in thickness, that
has been subsequently exhumed by younger erosion. We formulate a simplified
one-dimensional model for reduction in corestone size within a steady-state weathering
profile that incorporates: (i) vertical fluid transport of the reactant and the soluble
products of chemical weathering; (ii) linear kinetics of corestone reduction; and,
subsequently, (iii) erosion. The rate of advance of a steady-state weathering profile is a
statement of the mass balance between entering reactants and weathering components,
here idealized as H and albite. The mathematical relations, tie the laboratory-determined rate constant for dissolution of albite (k) to a generalized
kinetic constant for the rate of decrease (K) in corestone diameter to the advance rate
of the weathering profile (V ). The last parentheses contain an effective roughness at
the scale of the weathering profile, where S0 is the maximum size of initial bedrock
blocks, inferred to be set by initial bedrock fracture spacing, and 3L* is the profile
thickness. The laboratory scale roughness value, , is the ratio of the surface area
accessed by BET analysis to that of the corestone grain scale. In the model, erosion is
not coupled with weathering, although the presence of corestones of finite size, SE>0,
exiting at the erosional surface may be postulated to affect the erosional flux. The
thickness of the corestone weathering profile derived for the model for the distance
between bedrock and a corestone-free saprolite cap is approximately This expression is the product of the effective pH buffering-adjusted input reactant
flux per unit area times a stoichiometeric factor linking this to net albite dissolution,
divided by the rate of corestone size reduction at the input concentration of protons.
Further, the profile thickness scales with the input “particle” size, S0. The model fit,
which yields the ratio is consistent with a rate constant for albite dissolution that lies between laboratorymeasured
and field-estimated values. Sensitivity to the reaction order of albite dissolution
with respect to H, N, is small, except near the base of the profile. This model
yields insights into the relationship between fracture spacing and the evolution of
particle size and chemistry in weathering profiles.
