H41H-01 08:00h
Linkages Between Physical Erosion and Chemical Weathering, Measured by Cosmogenic Nuclides and Geochemical Mass Balance
We have recently shown how rates of physical erosion and chemical weathering can be measured over 1,000- to 10,000-year time scales, using cosmogenic $^{10}$Be measurements coupled with the bulk elemental composition of regolith and its parent rock. We have used these methods to measure long-term rates of physical erosion and chemical weathering for 42 sites, encompassing widely varying climates and denudation rates. Across these sites, mean annual temperatures vary from 2 to 25 $\deg$C, average annual precipitation spans a 20-fold range (from 22 to 420 cm/yr), and denudation rates vary by 32-fold (from 23 to 755 t km$^{-2}$ yr$^{-2}$). Long-term chemical weathering rates range from 0 to 173 t km$^{-2}$ yr$^{-2}$, in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. Our measurements show that, for a given precipitation and temperature regime, chemical weathering rates increase proportionally with fresh material supply rates. We refer to this as ``supply-limited'' weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. We consider several mechanisms that may be responsible for supply-limited weathering, and for the resulting strong correlation between rates of chemical weathering and physical erosion. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.
H41H-02 INVITED 08:15h
Atmospheric CO$_{2}$ Consumption in Uplifting Mountain Ranges: New Insight From the New Zealand Southern Alps
Rates of physical erosion and chemical weathering in uplifting mountain ranges are generally higher than the rates observed in tectonically stable regions. This observation has led to the hypothesis that orogenic events lead to global cooling over geologic time scales by accelerating the rate of atmospheric CO$_{2}$ drawdown from silicate weathering. However, recent studies of rivers draining the rapidly uplifting Himalaya Mountains have demonstrated that much of the chemical weathering flux is dominated by carbonate dissolution, which does not influence long-term atmospheric CO$_{2}$ levels. To examine if carbonate weathering dominates in other orogenic environments, we have undertaken investigations of rivers draining the New Zealand Southern Alps, which present a largely unexplored setting for systematically examining tectonic controls on the carbon cycle. In particular, we quantified rates of physical erosion and both silicate and carbonate weathering across a gradient of variable uplift rates but constant bedrock composition. We also compared the findings to global mean values as well as to data for major world rivers in other tectonic and climatic settings. Rapid uplift in the western Southern Alps elevates mechanical erosion rates by a factor of ~13 relative to those on the tectonically stable eastern side. Similarly, the average chemical weathering rate is ~5 times higher on the western compared to eastern side of the mountain range. However, because the proportion of stream-water Ca$^{2+}$ and Mg$^{2+}$ from the weathering of trace hydrothermal calcite increases as the rate of mechanical erosion increases, the long-term atmospheric CO$_{2}$ consumption rate on the western side is only ~2 times higher than that on the eastern side and only ~1.5 times higher than the global mean value. These data demonstrate that tectonic uplift in the New Zealand Southern Alps accelerates physical erosion and chemical weathering rates but does not greatly enhance the rate of long-term atmospheric CO$_{2}$ consumption, because mechanical erosion effectively maintains low ratios of silicate to carbonate weathering. The highest ratios of silicate to carbonate weathering occur in the eastern Southern Alps, where landscape stabilization leads to the depletion of carbonate from developing soil profiles. Data for major world rivers (including Himalayan rivers) yield a consistent interpretation. We find that landscapes subject to either intense mechanical erosion or cool temperatures experience the lowest ratios of silicate to carbonate weathering. By comparison, landscapes experiencing modest mechanical erosion and warm temperatures have the highest ratios of silicate to carbonate weathering. The strong similarity between river chemistry in the Southern Alps and the Himalaya Mountains suggests that these results are generally applicable to understanding the relationship between uplift and chemical weathering. We therefore conclude that mountain building increases atmospheric CO$_{2}$ consumption rates by only a factor of ~2, which is much smaller than previous estimates. Because the area of uplifting mountain ranges is small relative to the total area of the continents, we suggest that stable landscapes with overall lower chemical weathering rates but high ratios of silicate to carbonate weathering may exert a more significant influence on long-term atmospheric CO$_{2}$ levels.
H41H-03 08:30h
Formation Rate of Saprolitic Regolith in the Southern Blue Ridge Mountains, U.S.A., From Solute Geochemical Mass Balance: Equality With Erosion Rates and Implications for Landscape Evolution
Primary-mineral destruction rates during weathering of high-grade metasedimentary rocks in two low-order forested watersheds of the southern Appalachian Blue Ridge Mountains (U.S.D.A. Forest Service Coweeta Hydrologic Laboratory, western North Carolina, U.S.A.), are determined by combining balanced stoichiometric weathering reactions (based on mineralogical observations) with stream solute flux data for major elements and REE (in mol/ha/yr) over a period-of-record of several decades. The rate at which the weathering front descends into the deeper fresh rock (volume of fresh rock converted to saprolite/ha/yr, leaving saprolitic regolith above) is determined by combining the weathering rate with the modal abundance (determined from petrographic analysis) and molar volume of each mineral, and assuming isovolumetric weathering of rock to saprolite. Weathering of plagioclase feldspar in a watershed underlain by bedrock of the Coweeta Group occurs at a rate corresponding to the depletion of plagioclase from approximately 26 meters of rock (beneath a unit area of landscape) per million years (m/Ma). Corresponding values for garnet and biotite are 61 and 28 m/Ma, respectively. Corresponding rates for plagioclase, garnet and biotite in a watershed underlain by bedrock of the Otto Formation are 21, 24 and 28 m/Ma, respectively. Most saprolitization rates determined from present-day solute fluxes are between 20-30 m/Ma and spatially uniform among several metamorphic rock units. If the landscape were in dynamic equilibrium and the weathering profile were steady-state (constant thickness over time), the volume rate of landscape reduction rate by physical erosion and sediment export would equal the saprolitization rate (25±5 m/Ma). This is essentially identical to the time-averaged erosion rate for the past 10$^{4}$-10$^{5}$ years recently determined for the nearby Great Smoky Mountains using cosmogenic radionuclides. The excellent accord between the modern chemical saprolitization rate and the long-term average erosion and denudation rate is consistent with the suggestion that the southern Blue Ridge landscape is in dynamic equilibrium.
H41H-04 08:45h
Chemical Weathering Processes and Mass Losses on an Actively Eroding Hill Slope
Chemical weathering rates for landscapes are difficult to quantify because the timescales over which weathering occurs are often unknown. In this study, we use timescales defined by prior cosmogenic nuclide analyses and a suite of geochemical measurements to calculate weathering rates in saprolite and soil and to determine how these rates vary across an eroding hill slope. We also estimate the relative contributions of solute and erosional mass loss to landscape lowering. Analyses were conducted on a soil-mantled hill slope developed on a granodiorite pluton in southern NSW, Australia. Between $\sim$ 35% and 55% of total mass loss from the hill slope occurs in solution. Saprolite at the soil-saprolite boundary is less weathered at the ridge than at distance from the ridge. The calculated flux of silica from the saprolite is 5 tons km$^{-2}$ yr$^{-1}$ and does not vary with distance from the ridge or overlying soil thickness. With the exception of Ca and Na, rates of loss of most major elements in solution are lower in the saprolite (prior to bioturbation) than in the soil. Ca and Na are preferentially lost from the saprolite. Within the soil, major element fluxes initially increase as soils are transported down slope. Most elemental fluxes reach a maximum between 20 and 40 m of transport, and then decrease slightly. For example, soil silica weathering fluxes increase from 6 tons km$^{-2}$ yr$^{-1}$ at the ridge to a maximum of ~9 tons km$^{-2}$ yr$^{-1}$ at transport distances between 20 and 40 m, and then decline to 8 tons km$^{-2}$ yr$^{-1}$ in soils that have been transported greater than 40 m. The soils are geochemically stratified. Soil regions close to the soil-saprolite boundary are less weathered than those at distance from the soil-saprolite boundary. This suggests that the soil column never becomes thoroughly vertically mixed as soils move down slope. By the time soils have been transported from the convex ridge crest to the concave swale, they have lost more than 50% of their original mass to chemical weathering. The findings of this study indicate that erosional transport and solute processes make comparable contributions to landscape lowering at this site, and that transport-distance dependent solute loss may partially explain deviations between observations of hillslope curvature and predictions of curvature based on linear creep transport.
H41H-05 09:00h
The Topographic Control of Chemical Weathering in Hillslope Soils
Chemical weathering drives biogeochemical cycles from local to global scales, and has the power to regulate the earth's climate on geological time scales. However, little is known of the spatial variation in weathering on hillslopes, and the mechanisms behind those variations. This study addresses the topographic control on soil chemical weathering on convex uplands. We developed a process-based mass balance model that integrates chemical mass losses with physical sediment transport. We applied the model along a $\sim$60 meter long, semi-arid eucalyptus-grassland savanna hillslope underlain by granodiorite, in the southwestern Australian Highlands. Measurements of soil elemental chemistry, cosmogenic isotope-based saprolite-to-soil conversion rates, and a fine scale topographic survey provided model data. The soil weathering rates varied from the losses of $\sim$35 g m$^{-2}$ yr$^{-1}$ on the ridge to the net gains of $\sim$28 g m$^{-2}$ yr$^{-1}$ at the lowest portion of the slope. A net chemical mass loss occurred in all soils along the entire slope, decreasing from $\sim$65 % near the convex ridge to $\sim$35 % at the base of the slope. The mechanism for the apparent discrepancy between spatially constrained weathering rates and net weathering losses (relative to saprolite) is that as sediment moves faster with an increasing slope gradient in the downslope direction, soils eroded from upslope positions pass quickly through the downslope zones of chemical gains, which are able to only partially replenish the pre-weathered soil material. Differences in chemical mobility of elements, and biological nutrient demand, significantly modified the spatial redistribution of elements released by weathering: P and Ca, relative to Si, Al, and Fe, were preferentially retained, particularly within an apron of relatively high fertility that mantled the hillslope base. Finally, when chemical weathering losses were subtracted from the overall sediment mass balance, the slope-dependent physical soil transport rate was reduced by half, and the resulting physical transport was found to increase nonlinearly with increasing slope gradients. In conclusion, this study integrates chemical weathering and physical transport on convex uplands, complementing recent watershed scale analyses and providing a spatial perspective of both weathering and elemental redistribution on hillslopes that have applications to ecology as well as geochemistry.
H41H-06 09:15h
The Relationship Between Carbonate Weathering, Hydrothermal Activity, and Strontium Isotopes of River Waters in Steady State Orogens
We present a model for steady-state chemical weathering. This model is applicable to the case where uplift provides a constant supply of fresh rock to the bottom of the weathering zone that is balanced by physical erosion from the surface. This condition of steady-state chemical weathering should exist in steady state orogens, at certain temporal and spatial scales. We have applied this model to the weathering of granitic composition rocks that contain trace amounts of calcite to examine the relationship between calcite weathering and rock uplift. Trace amounts of hydrothermal calcite are ubiquitous in rocks of granitic composition exposed in active collisional mountain belts. These calcite veins form as a result of hydrothermal systems that occur in areas of high uplift rates and surface elevation. The calcite veins are often enriched in 87Sr which complicates interpretation of the relationship between the marine strontium isotope record, silicate weathering, and long-term drawdown of carbon dioxide. The strontium isotope composition of weatherable material will be strongly influenced by the abundance of calcite and the uplift rate. The strontium isotope composition of rock at the Earth's surface will be set by the strontium isotopic composition of the constituent minerals at their closure temperature and the length of time it takes for the rock to move from the closure depth to the weathering zone. Our calculations show that calcite dominates the flux of strontium in active orogens, and holds the strontium isotope composition of river waters essentially constant over a range of tectonic conditions from active orogens to temperate cratons. In steady state orogens the strontium flux and the 87Sr/86Sr values should vary in an inverse fashion and this close coupling provides information on the types of orogens supplying strontium to the oceans.
H41H-07 09:30h
Tectonic and Climatic Controls on Silicate Weathering
Chemical weathering of silicate minerals is thought to provide the temperature-sensitive feedback that regulates global climate on geological timescales. However it has proved hard to distinguish the temperature sensitivity of weathering rates from other factors such as physical erosion rates, rainfall and vegetation. Even the premise that weathering rates depend on temperature has been contested. Previous interpretations of weathering in small catchments have produced conflicting results and do not explain the scatter that emerges at larger basin scales. Here we present a new compilation of chemical and physical erosion rates in small catchments and show that the overall variation in chemical weathering rates with physical erosion rates, rainfall, and temperature is predictable from consideration of the limiting relationships. The data fit with an activation energy between 50 and 100 kJ/mol, a linear dependence on rainfall and a square-root dependence on erosion rate. The analysis confirms the concepts of "transport" and "weathering" limited regimes. Chemical weathering rates are ultimately controlled by a combination of climatic and tectonic processes with the more rapidly eroding terrains providing the temperature-sensitive feedback which regulates long-term climate change.
H41H-08 INVITED 09:45h
Validity of the Steady State Approach of Erosion in Large River Systems
At the scale of a drainage basin, secondary products of chemical weathering reactions accumulate in soils and reservoirs. They are periodically removed by physical erosion processes and exported to the ocean. The question to known whether the flux of exported material equals the flux of newly formed material by chemical reactions is that of the "steady state" of erosion. This question is typical a matter of time scale at which the mass budget of erosion is considered: are erosion processes in equilibrium at the scale of a hydrological cycle? Of tens of years? At a geological time scale? To address this issue, we will review the use of geochemical mass budgets based on elemental ratios or isotopic ratios. The largest rivers will be considered with special emphasis on the Congo, Amazon and Mackenzie rivers. Different hypothesis on the mean composition of the upper crust have to be made when elemental ratios are used. Using uranium series disequilibrium, bedrock composition is much more constrained and theoretical suspended sediments concentrations can be predicted. It appears that most of the largest drainage basin are not in steady state and export more sediments than predicted based on a geochemical mass budget approach. If the storage of suspended sediments that occur in most of the largest rivers is taken into account, the discrepancy between predicted and measured sediment yields is even greater. What is the cause of this disequilibrium? Is there a global explanation or is there a specific reason for each drainage basin? We will address this question in terms of response time of physical erosion to climatic or tectonic events in large river basins.