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1.
Located in the uplands of the Valley and Ridge physiographic province of Pennsylvania, the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) is a tectonically quiescent, first-order catchment developed on shales of the Silurian Rose Hill Formation. We used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical/chemical weathering fluxes. About 25 m of bedrock was also drilled to estimate parent composition. Depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate-weathering front. Overlying this, extending from ∼6 m below the bedrock-soil interface up into the soil, is the feldspar dissolution front. In the soils, depletion profiles for K, Mg, Si, Fe, and Al relative to the bedrock define the illite and chlorite reaction fronts. When combined with a cosmogenic nuclide-derived erosion rate on watershed sediments, these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite (1-5 × 10−17 mol m−2 s−1) and illite (2-9 × 10−17 mol m−2 s−1) than observed in the laboratory. Mineral reactions result in formation of vermiculite, hydroxy-interlayered vermiculite, and minor kaolinite. During weathering, exchangeable divalent cations are replaced by Al as soil pH decreases.The losses of Mg and K in the soils occur largely as solute fluxes; in contrast, losses of Al and Fe are mostly as downslope transport of fine particles. Physical erosion of bulk soils also occurs: results from a steady-state model demonstrate that physical erosion accounts for about half of the total denudation at the ridgetop and midslope positions. Chemical weathering losses of Mg, Na, and K are higher in the upslope positions likely because of the higher degree of chemical undersaturation in porewaters. Chemical weathering slows down in the valley floor and Al and Si even show net accumulation. The simplest model for the hillslope that is consistent with all observations is a steady-state, clay weathering-limited system where soil production rates decrease with increasing soil thickness.  相似文献   

2.
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 cm−2 ka−1. 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.  相似文献   

3.
How and how fast do hillslope soils form as the landscape’s morphology changes over time? Here results are shown from an ongoing study that simultaneously examines the morphologic and geochemical evolution of soil mantled hillslopes that have been exposed to distinctively different denudation history. In Northern Sierra Nevada, California, the authors are investigating a tributary basin to the Middle Fork Feather River. A major incision signal from the river is well marked in a knickpoint within the tributary basin which stretches from its mouth to the Feather River at an elevation of ～700 m to the plateau at an elevation of ～1500 m. Hillslopes are significantly steeper below the knickpoint. The area’s total denudation rates are currently being constrained using cosmogenic radio nuclides, but a previous study suggested an order of magnitude difference in total denudation rates below and above the knickpoint. When compared with topographic attributes calculated from LIDAR data, physical erosion rates can be modeled as a linear function of ridge top curvature. Surprisingly, over the wide range of total denudation rates, soil thicknesses do not vary significantly until a threshold point where soil mantled landscapes abruptly shift to bedrock dominated landscapes. Bioturbation by tree falls appear to buffer soil thickness over the wide range of physical soil erosion rates. From three hillslopes with different physical erosion rates, the concentrations of Zr, which were considered conserved during dissolution and leaching, were determined and used as a proxy for the degree of mass losses via chemical denudation. There is a general trend that colluvial soils along the hillslopes with lower physical erosion rates are enriched in fine size fractions, Zr, and pedogenic crystalline Fe oxides. Likewise, the saprolites show greater degrees of chemical denudation at the sites above the knickpoint, presumably because of the saprolites’ longer turnover time in the slowly eroding landscapes. In the two steep hillslopes below the knickpoint, no significant or systematic topgraphic trends were found for soil geochemistry. However, soils show increasing Zr enrichment in the downslope direction in the hillslope above the knickpoint, which suggests a critical denudation rate beyond which soils’ turnover time is too short to develop a geochemical catena. As detailed CRN-based soil production rates and catchment scale denudation rates are acquired, the data will be combined with a mass balance model to calculate the rates of chemical denudation and weathering in soils and saprolites along the denudation gradient.  相似文献   

4.
Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.  相似文献   

5.
The Yarlung Tsangpo-Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo-Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO2 consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo-Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km−2 yr−1 and the silicate cation flux more than 34 tons km−2 yr−1. In total, chemical weathering in this area consumes 15.2 × 105 mol CO2 km−2 yr−1, which is twice the Brahmaputra average. These data show that 15-20% of the total CO2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na+ and Cl− fluxes throughout southern Tibet, comprising more than 50% of all Na+ in some stream systems. Carbonate weathering generates 80-90% of all dissolved Ca2+ and Mg2+ cations in much of the Yarlung Tsangpo catchment.  相似文献   

6.
Recent studies of chemical weathering of andesitic-dacitic material on high-standing islands (HSIs) have shown these terrains have some of the highest observed rates of chemical weathering and associated CO2 consumption yet reported. However, the paucity of stream gauge data in many of these terrains has limited determination of chemical weathering product fluxes. In July 2006 and March 2008, stream water samples were collected and manual stream gauging was performed in watersheds throughout the volcanic island of Dominica in the Lesser Antilles. Distinct wet and dry season solute concentrations reveal the importance of seasonal variations on the weathering signal. A cluster analysis of the stream geochemical data shows the importance of parent material age on the overall delivery of solutes. Observed Ca:Na, HCO3:Na and Mg:Na ratios suggest crystallinity of the parent material may also play an important role in determining weathering fluxes. From total dissolved solids concentrations and mean annual discharge calculations we calculate chemical weathering yields of (6-106 t km−2 a−1), which are similar to those previously determined for basalt terrains. Silicate fluxes (3.1-55.4 t km−2 a−1) and associated CO2 consumption (190-1575 × 103 mol km−2 a−1) determined from our study are among the highest determined to date. The calculated chemical fluxes from our study confirm the weathering potential of andesitic-dacitic terrains and that additional studies of these terrains are warranted.  相似文献   

7.
Quantifying long-term rates of chemical weathering and physical erosion is important for understanding the long-term evolution of soils, landscapes, and Earth's climate. Here we describe how long-term chemical weathering rates can be measured for actively eroding landscapes using cosmogenic nuclides together with a geochemical mass balance of weathered soil and parent rock. We tested this approach in the Rio Icacos watershed, Puerto Rico, where independent studies have estimated weathering rates over both short and long timescales. Results from the cosmogenic/mass balance method are consistent with three independent sets of weathering rate estimates, thus confirming that this approach yields realistic measurements of long-term weathering rates. This approach can separately quantify weathering rates from saprolite and from overlying soil as components of the total. At Rio Icacos, nearly 50% of Si weathering occurs as rock is converted to saprolite; in contrast, nearly 100% of Al weathering occurs in the soil. Physical erosion rates are measured as part of our mass balance approach, making it particularly useful for studying interrelationships between chemical weathering and physical erosion. Our data show that chemical weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and chemical weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. One can normalize for variations in physical erosion rates using the “chemical depletion fraction,” which measures the fraction of total denudation that is accounted for by chemical weathering. This measure of chemical weathering intensity increases with increasing average temperature and precipitation in data from climatically diverse granitic sites, including tropical Rio Icacos and six temperate sites in the Sierra Nevada, California. Hence, across a wide range of climate regimes, analysis of chemical depletion fractions appears to effectively account for site-to-site differences in physical erosion rates, which would otherwise obscure climatic effects on chemical weathering rates. Our results show that by quantifying rates of physical erosion and chemical weathering together, our mass balance approach can be used to determine the relative importance of climatic and nonclimatic factors in regulating long-term chemical weathering rates.  相似文献   

8.
This paper investigates the denudation rates in the Quadrilátero Ferrífero, Minas Gerais State (Brazil). The aim is to compare chemical weathering rates from measurements of solute fluxes in rivers and long-term mean erosion rates deduced from in situ-produced cosmogenic 10Be concentrations measured in fluvial sediments. Both water samples and sediments were collected in fifteen stations (checkpoints) located in four hydrographic basins with low anthropogenic perturbations.Depending of the type of substratum, three degrees of chemical denudation rates from water samples are observed: (i) high rates in marbles; (ii) medium rates in schists, phyllites, granites, gneisses and migmatites; (iii) low rates in quartzites and itabirites. Preliminary results of long-term erosion rates deduced from in situ-produced 10Be are comparable with those of chemical rates.  相似文献   

9.
The soils of the Atacama Desert in northern Chile have long been known to contain large quantities of unusual salts, yet the processes that form these soils are not yet fully understood. We examined the morphology and geochemistry of soils on post-Miocene fans and stream terraces along a south-to-north (27° to 24° S) rainfall transect that spans the arid to hyperarid transition (21 to ∼2 mm rain y−1). Landform ages are ? 2 My based on cosmogenic radionuclide concentrations in surface boulders, and Ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2.1 My. A chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall. In the arid (21 mm rain y−1) soil, total mass loss to weathering of silicate alluvium and dust (−1030 kg m−2) is offset by net addition of salts (+170 kg m−2). The most hyperarid soil has accumulated 830 kg m−2 of atmospheric salts (including 260 kg sulfate m−2 and 90 kg chloride m−2), resulting in unusually high volumetric expansion (120%) for a soil of this age. The composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in-soil chemical transformation and loss. Long-term rates of atmospheric solute addition were derived from the ion inventories in the driest soil, divided by the landform age, and compared to measured contemporary rates. With decreasing rainfall, the soil salt inventories increase, and the retained salts are both more soluble and present at shallower depths. All soils generally exhibit vertical variation in their chemistry, suggesting slow and stochastic downward water movement, and greater climate variability over the past 2 My than is reflected in recent (∼100 y) rainfall averages. The geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold: in wetter soils, the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years, while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion. The specific geochemistry of hyperarid soils is a function of atmospheric sources, and is expected to vary accordingly at other hyperarid sites. This work identifies key processes in hyperarid soil formation that are likely to be independent of location, and suggests that analogous processes may occur on Mars.  相似文献   

10.
The Narmada River in India is the largest west-flowing river into the Arabian Sea, draining through the Deccan Traps, one of the largest flood basalt provinces in the world. The fluvial geochemical characteristics and chemical weathering rates (CWR) for the mainstream and its major tributaries were determined using a composite dataset, which includes four phases of seasonal field (spot) samples (during 2003 and 2004) and a decade-long (1990-2000) fortnight time series (multiannual) data. Here, we demonstrate the influence of minor lithologies (carbonates and saline-alkaline soils) on basaltic signature, as reflected in sudden increases of Ca2+-Mg2+ and Na+ contents at many locations along the mainstream and in tributaries. Both spot and multiannual data corrected for non-geological contributions were used to calculate the CWR. The CWR for spot samples (CWRspot) vary between 25 and 63 ton km−2 year−1, showing a reasonable correspondence with the CWR estimated for multiannual data (CWRmulti) at most study locations. The weathering rates of silicate (SilWR), carbonate (CarbWR) and evaporite (Sal-AlkWR) have contributed ∼38-58, 28-45 and 8-23%, respectively to the CWRspot at different locations. The estimated SilWR (11-36 ton km−2 year−1) for the Narmada basin indicates that the previous studies on the North Deccan Rivers (Narmada-Tapti-Godavari) overestimated the silicate weathering rates and associated CO2 consumption rates. The average annual CO2 drawdown via silicate weathering calculated for the Narmada basin is ∼0.032 × 1012 moles year−1, suggesting that chemical weathering of the entire Deccan Trap basalts consumes approximately 2% (∼0.24 × 1012 moles) of the annual global CO2 drawdown. The present study also evaluates the influence of meteorological parameters (runoff and temperature) and physical weathering rates (PWR) in controlling the CWR at annual scale across the basin. The CWR and the SilWR show significant correlation with runoff and PWR. On the basis of observed wide temporal variations in the CWR and their close association with runoff, temperature and physical erosion, we propose that the CWR in the Narmada basin strongly depend on meteorological variability. At most locations, the total denudation rates (TDR) are dominated by physical erosion, whereas chemical weathering constitutes only a small part (<10%). Thus, the CWR to PWR ratio for the Narmada basin can be compared with high relief small river watersheds of Taiwan and New Zealand (1-5%) and large Himalayan Rivers such as the Brahmaputra and the Ganges (8-9%).  相似文献   

11.
Numerous studies of weathering fluxes have been carried out on major world rivers during the last decade, to estimate CO2 consumption rates, landscape evolution and global erosion rates. For obvious logistical reasons, most of these studies were based on large scale investigations carried out on short timescales. By comparison, much less effort has been devoted to long term monitoring, as a means to verify the temporal variability of the average characteristics, their trends, and the representativeness of short-term investigations. Here we report the results of a three-year survey (November 2000 to December 2003) of the major and trace element composition of dissolved and suspended matter in the lower Rhone River (France), the largest river of the Mediterranean area. Subsurface water samples were collected in Arles, about 48 km upstream of the estuary, twice a month routinely, and at higher frequency during flood events.During each flood event, the suspended particulate matter (SPM) show the usual trend of clockwise hysteresis with higher SPM concentrations on the rising limb of the flood than at the same discharge on the falling limb. We show that the annual average SPM flux of the Rhone River to the Mediterranean Sea (7.3 ± 0.6 × 106 tons yr−1) was largely controlled by the flood events (83% of the solid discharge occurred in less than 12% of the time), and that the precision on the total output flux depends strongly on the precise monitoring of SPM variations during the floods.The chemical composition of water and SPM are characterized by the predominance of Ca2+ due to the abundance of carbonate rocks in the Rhone watershed. Chemical budgets have been calculated to derive the contributions of atmospheric deposition, carbonate, silicate and evaporite weathering, and anthropogenic inputs. The chemical weathering rate of carbonates is estimated to be 89 ± 5 t km−2 yr−1 compared to 14.4 ± 3 t km−2 yr−1 from silicates. By contrast, the physical erosion rate of silicates is about 51 t km−2 yr−1 against 19 t km−2 yr−1 for carbonates.The steady-state model of Gaillardet et al. (1995) has been applied to the chemical composition of dissolved and solid products. The results show that the Rhone River currently exports much less material than produced at steady-state by weathering in its watershed. The sediment flux inferred from the steady-state calculation (21-56 × 106 t yr−1) is on the same order as that estimated in literature for the 19th and the beginning of the 20th centuries. This imbalance may suggest that the Rhone is under a transient erosion regime following climate change (i.e. significant decrease of the flooding frequency since the beginning of the 19th century). On the other hand, the imbalance may also be due to the trapping of alluvion by the numerous dams on the river and its tributaries.Our data corroborate with previous studies that suggest a strong coupling between chemical and physical erosion fluxes, during the hydrological seasonal cycle of the Rhone River. The correlation between physical and chemical transport rates is, however, clearly different from that reported for global annual averages in large world rivers.  相似文献   

12.
Concentrations of major ions, Sr and 87Sr/86Sr have been measured in the Gomti, the Son and the Yamuna, tributaries of the Ganga draining its peninsular and plain sub-basins to determine their contribution to the water chemistry of the Ganga and silicate and carbonate erosion of the Ganga basin. The results show high concentrations of Na and Sr in the Gomti, the Yamuna and the Ganga (at Varanasi) with much of the Na in excess of Cl. The use of this ‘excess Na’ (Na∗ = Nariv − Clriv) a common index of silicate weathering yield values of ∼18 tons km−2 yr−1 for silicate erosion rate (SER) in the Gomti and the Yamuna basins. There are however, indications that part of this Na∗ can be from saline/alkaline soils abundant in their basins, raising questions about its use as a proxy to determine SER of the Ganga plain. Independent estimation of SER based on dissolved Si as a proxy give an average value of ∼5 tons km−2 yr−1 for the peninsular and the plain drainages, several times lower than that derived using Na∗. The major source of uncertainty in this estimate is the potential removal of Si from rivers by biological and chemical processes. The Si based SER and CER (carbonate erosion rate) are also much lower than that in the Himalayan sub-basin of the Ganga. The lower relief, runoff and physical erosion in the peninsular and the plain basins relative to the Himalayan sub-basin and calcite precipitation in them all could be contributing to their lower erosion rates.Budget calculations show that the Yamuna, the Son and Gomti together account for ∼75% Na, 41% Mg and ∼53% Sr and 87Sr of their supply to the Ganga from its major tributaries, with the Yamuna dominating the contribution. The results highlight the important role of the plain and peninsular sub-basins in determining the solute and Sr isotope budgets of the Ganga. The study also shows that the anthropogenic contribution accounts for ?10% of the major ion fluxes of the Ganga at Rajmahal during high river stages (October). The impact of both saline/alkaline soils and anthropogenic sources on the major ion abundances of the Ganga is minimum during its peak flow and therefore the SER and CO2 consumption rates of the river is best determined during this period.  相似文献   

13.
In this study we evaluate the dynamics of the biophile element phosphorus (P) in the catchment and proglacial areas of the Rhône and Oberaar glaciers (central Switzerland). We analysed erosion and dissolution rates of P-containing minerals in the subglacial environment by sampling water and suspended sediment in glacier outlets during three ablation and two accumulation seasons. We also quantified biogeochemical weathering rates of detrital P in proglacial sedimentary deposits using two chronosequences of samples of fresh, suspended, material obtained from the Oberaar and Rhône water outlets, Little-Ice-Age (LIA) moraines and Younger Dryas (YD) tills in each catchment. Subglacial P weathering is mainly a physical process and detrital P represents more than 99% of the precipitation-corrected total P denudation flux (234 and 540 kg km−2 yr−1 for the Rhône and Oberaar catchments, respectively). The calculated detrital P flux rates are three to almost five times higher than the world average flux. The precipitation-corrected soluble reactive P (SRP) flux corresponds to 1.88-1.99 kg km−2 yr−1 (Rhône) and 2.12-2.44 kg km−2 yr−1 (Oberaar), respectively. These fluxes are comparable to those of tropical rivers draining transport-limited, tectonically inactive weathering areas.In order to evaluate the efficiency of detrital P weathering in the Rhône and Oberaar proglacial areas, we systematically graded apatite grains extracted from the chronosequence in each catchment relative to weathering-induced changes in their surface morphologies (grades 1-4). Fresh apatite grains are heavily indented and dissolution rounded (grade 1). LIA grains from two 0-10 cm deep moraine samples show extensive dissolution etching, similar to surface grains from the YD profile (mean grades 2.7, 3.5 and 3.5, respectively). In these proglacial deposits, the weathering front deepens progressively as a function of time due to biocorrosion in the evolving acidic pedosphere, with mechanical indentations on grains acting as sites of preferential dissolution. We also measured iron-bound, organic and detrital P concentrations in the chronosequence and show that organic and iron-bound P has almost completely replaced detrital P in the top layers of the YD profiles. Detrital P weathering rates are calculated as 310 and 280 kg km−2 yr−1 for LIA moraines and 10 kg km−2 yr−1 for YD tills. During the first 300 years of glacial sediment exposure P dissolution rates are shown to be approximately 70 times higher than the mean global dissolved P flux from ice-free continents. After 11.6 kyr the flux is 2.5 times the global mean. These data strengthen the argument for substantial changes in the global dissolved P flux on glacial-interglacial timescales. A crude extrapolation from the data described here suggests that the global dissolved P flux may increase by 40-45% during the first few hundred years of a deglaciation phase.  相似文献   

14.
Sediment fluxes from high standing oceanic islands (HSIs) such as New Zealand are some of the highest known [Milliman J. D. and Syvitski J. P. M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol.100, 525-544]. Recent geochemical work has suggested that along with their extremely high physical weathering yields, many New Zealand watersheds also have very high chemical weathering yields. In New Zealand, the magnitude of both the physical and chemical weathering yields is related to the lithology of the watershed. Most of the previous work on this topic has been undertaken in Southern Alps watersheds of schist and greywacke and in East Cape watersheds of semi-consolidated marine sediments and greywacke. We recently sampled North Island watersheds in the Taranaki and Manawatu-Wanganui regions which have been subjected to volcanism since the Miocene. We sampled watersheds that contain both volcanic and sedimentary rocks. A series of water and sediment samples was collected and analyzed for major, minor and trace elements. This was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies. Our results reveal distinct chemical signatures for the different regions. Waters draining the Taranaki region volcanics are significantly enriched in K+, and depleted in Ca2+ and Sr2+ compared to waters draining the Manawatu-Wanganui region volcanics, which also traverse expanses of sedimentary siltstones and mudstones. The Ca2+ and Sr2+ depletions may reflect the relative absence of CaCO3 in the Taranaki region watersheds. In addition, sediment samples from the Taranaki region show significant enrichment in Ti, Al, Ca, Fe, Mn, Mg, Ca, and P and depletion in Si and Rb compared to those of the Manawatu-Wanganui region. From total dissolved solids concentrations and mean annual water discharge, we calculate chemical weathering yields of 60-240 tons km−2 a−1. These weathering yields fall within the middle to upper range of those previously documented for the Southern Alps (93-480 tons km−2 a−1) and East Cape (62-400 tons km−2 a−1). Calculated silicate weathering yields of 12-33.6 tons km−2 a−1 and CO2 consumption of 852-2390 × 103 mol km−2 a−1 for the rivers draining the Taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on HSIs. CO2 consumption is found to be within the range previously measured for the basaltic terrains of the Deccan Traps (580-2450 × 103 mol km−2 a−1) and Réunion Island(1300-4400 × 103 mol km−2 a−1). Our calculated chemical weathering yields demonstrate the importance of HSIs, particularly those with volcanic terrains, when considering global geochemical fluxes.  相似文献   

15.
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 ΔV 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 fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 Å, 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 × 10−14 mol biotite m−2 s−1. 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 μm 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 × 10−13 mol hornblende m−2 s−1: 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.  相似文献   

16.
We present a weathering mass balance of the presently glaciated Rhône and Oberaar catchments, located within the crystalline Aar massif (central Switzerland). Annual chemical and physical weathering fluxes are calculated from the monthly weighted means of meltwater samples taken from July, 1999 to May, 2001 and are corrected for precipitation inputs. The meltwater composition issuing from the Oberaar and Rhône catchments is dominated by calcium, which represents 81% and 55% of the total cation flux respectively (i.e. 555 and 82-96 keq km−2 yr−1). The six to seven times higher Ca2+ denudation flux from the Oberaar catchment is attributed to the presence of a strongly foliated gneissic zone. The gneissic zone has an elevated calcite content (as reflected by the 4.6 times higher calcite content of the suspended sediments from Oberaar compared to Rhône) and a higher mechanical erosion rate (resulting in a higher flux of suspended sediment). The mean flux of suspended calcite of the Oberaar meltwaters during the ablation period is 7 times greater than that of the Rhône meltwaters. Taking the suspended calcite as a proxy for the total (including sub-glacial sediments) weathering calcite surface area, it appears that the available surface area is an important factor in controlling weathering rates. However, we also observe an increased supply of protons for carbonate dissolution in the Oberaar catchment, where the sulphate denudation flux is six times greater. Carbonic acid is the second important source of protons, and we calculate that three times as much atmospheric CO2 is drawn down (short term) in the Oberaar catchment. Silica fluxes from the two catchments are comparable with each other, but are 100 kmol km2 yr−1 lower than fluxes from physically comparable, non-glaciated basins.  相似文献   

17.
Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm−3) may promote the dissolution of silicate phases through complexation of Al3+ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO2 m−2 year−1) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO2 m−2 year−1) suggesting a decrease in marine weathering over time. The production of CO2 through reverse weathering in surface sediments (4.22-15.0 mmol CO2 m−2 year−1) is about one order of magnitude smaller than the weathering-induced CO2 consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO2 consumption through marine silicate weathering estimated here as 5-20 Tmol CO2 year−1 is as high as the global rate of continental silicate weathering.  相似文献   

18.
The watershed in the southern Jiangxi Province (Jiangxi Province is called simply Gan) (SGW) and the watershed in the central Guizhou Province (Guizhou Province is called simply Qian) (CQW) are two subtropical watersheds of the Yangtze River in China. Both watersheds have similar latitudes and climate, but distinct differences in basin lithology. These similarities and differences provide a good natural laboratory in which to investigate weathering processes and Sr end-members in river waters. This work aims to identify and contrast the sources, fluxes and controls on Sr isotopic composition in the river waters of these two areas. Results showed that the 87Sr/86Sr in the SGW waters ranged from 0.716501 to 0.724931, with dissolved Sr averaging 27 μg l− 1. Rhyolites and granites are two major sources for the dissolved Sr. The SGW waters receive 42% of their Sr from silicates weathering, 32% from carbonates and 3.2% from evaporites. 87Sr/86Sr in the CQW waters has a lesser variation from 0.707694 to 0.710039, but higher Sr contents (average of 208 μg l− 1). Dolomite, limestone and dolomitic limestone are major sources of Sr in the waters. The CQW waters receive 69% of their Sr from carbonates, 1.7% from silicates and 0.9% from evaporites. The chemical erosion rate and Sr flux in the CQW are 122 t km− 2 a− 1 and 0.079 t km− 2 a− 1, respectively, which are higher than those of the SGW (56 t km− 2 a− 1 and 0.021 t km− 2 a− 1, respectively). These data suggest that the intensive carbonates weathering occurred in the karstic area in the upper-reach of the Yangtze River exert great influence on the high Sr concentration and low Sr isotopic ratios in the River.  相似文献   

19.
The Rio Solimões/Amazonas (Amazon River) and its major tributaries have been analyzed for U-series nuclides. 238U-234U-230Th-226Ra disequilibria have been measured in the dissolved (<0.2 μm) and suspended loads (>0.2 μm) as well as bed sands. U-series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering. Moreover, while the dissolved load records present-day weathering, suspended particles integrate the erosion history over much longer time scales (>100 ka). Lowland rivers are characterized by long time scales of chemical erosion (?100 ka) resulting in a high weathering intensity. Moreover, exchange between suspended particles and the dissolved load may explain the U-series signature for these rivers. By combining U-series and Pb isotopes in suspended particles, we show that erosion in the Rio Madeira basin occurred as a multi-step process, whereby the pristine continental crust was eroded several hundreds of Ma ago to produce sediments that have then been integrated in the Cordillera by crustal shortening and are currently eroded. In contrast, recent erosion of a pristine crust is more likely for the Rio Solimões/Amazonas (<10 ka). The suspended particles of the rivers draining the Andes (Solimões/Amazonas, Madeira) suggest time scales of weathering ranging between 4 and 20 ka. This indicates that suspended particles transported by those rivers are not stored for long periods in the Andean foreland basin and the tropical plain. The sediments delivered to the ocean have resided only a few ka in the Amazon basin (6.3 ± 1 ka for the Rio Amazonas at Óbidos). Nevertheless, a large fraction of the sediments coming out from the Andes are trapped in the foreland basin and may never reach the ocean. Erosion in the Andes is not operating in steady state. U-series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady-state erosion and that is a consequence of soil destruction greater than production. By relating this observation to the short time scales of weathering inferred for the Andes (a few ka), it appears that the erosion regime has been recently perturbed, resulting in high denudation rates. A possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies. Our results indicate that erosion responds rapidly to high-frequency climatic fluctuations.  相似文献   

20.
CO2 consumption by chemical weathering is an integral part of the boundless carbon cycle, whose spatial patterns and controlling factors on continental scale are still not fully understood. A dataset of 338 river catchments throughout North America was used to empirically identify predictors of bicarbonate fluxes by chemical weathering and interpret the underlying controlling factors. Detailed analysis of major ion ratios enables distinction of the contributions of silicate and carbonate weathering and thus quantifying CO2 consumption. Extrapolation of the identified empirical model equations to North America allows the analysis of the spatial patterns of the CO2 consumption by chemical weathering.Runoff, lithology and land cover were identified as the major predictors of the riverine bicarbonate fluxes and the associated CO2 consumption. Other influence factors, e.g. temperature, could not be established in the models. Of the distinguished land cover classes, artificial surfaces, dominated by urban areas, increase bicarbonate fluxes most, followed by shrubs, grasslands, managed lands, and forests. The extrapolation results in an average specific bicarbonate flux of 0.3 Mmol km−2 a−1 by chemical weathering in North America, of which 64% originates from atmospheric CO2, and 36% from carbonate mineral dissolution. Chemical weathering in North America thus consumes 50 Mt atmospheric CO2-C per year. About half of that originates from 10% of the area of North America.The estimated strength of individual predictors differs from previous studies. This highlights the need for a globally representative set of regionally calibrated models of CO2 consumption by chemical weathering, which apply very detailed spatial data to resolve the heterogeneity of earth surface processes.  相似文献   

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