B23B-1264
Anthropogenic alteration of riverine water and carbonate fluxes
A new high resolution 100 year data set from the mouth of the Mississippi river indicate a clear increase in dissolved inorganic carbon (DIC) concentration and fluxes in the mid 20th century. A response in the flux lagged behind the DIC concentration shift by a couple of decades due to a two decade period of low flow. The increase in flux is driven by a recovery from acidification and an acceleration of water and carbonate fluxes from agricultural watersheds.
B23B-1265
Carbonate Mineral Weathering Contributions to the HCO3- Flux from Headwater Mid-latitude Streams in the Face of Increasing Atmospheric CO2
As anthropogenic liberated CO2 increases in the atmosphere, landscape level responses of the carbon cycle to perturbations associated with global warming are likely to be observed in carbonate bearing regions. Within physically open weathering environments, carbonate (calcite and dolomite) mineral solubility is proportional to pCO2 and inversely proportional to temperature, with the solubility of dolomite progressively greater than calcite below 25°C. Changes in weathering zone CO2 occur as CO2 drawdown is increased due to CO2 fertilization effects on plant growth, to warmer mean annual temperatures, or to land use changes. The rise in weathering zone CO2 will significantly augment the open system solubility of carbonate minerals and increase the DIC content of surface waters (unconfined groundwaters and rivers). The thermodynamic relationships between calcite and dolomite indicate the further need to examine the role of dolomite on the global riverine DIC budget. On a continental scale, the global weathering budget indicates the importance of northern hemisphere landmasses to riverine fluxes of Ca2+, Mg2+ and DIC as HCO3-. The results of a hydrogeochemical study of carbonate mineral equilibria and weathering fluxes for headwater streams within the Danube, the James and the St. Lawrence River Basins is presented. Available long-term geochemical and discharge data along with detailed catchment geochemical views of surface water and soil weathering zones were determined to examine the historical and current contribution of carbonate weathering to the geochemical fluctuations of the these headwater regions and the ability of these watersheds to maintain current conditions in the facing of increasing CO2. In order to gauge how these streams with variable climates, land use practices, lithologies, and weathering zone thicknesses compare to each other, river runoff and HCO3- concentrations are normalized to catchment area. The resulting carbonate weathering intensity on a global scale, shows the study regions exceeding the world average by factors of between 2 to 20. Within each stream, variability of HCO3- concentrations are minimal over a wide range of discharges indicating that carbonate weathering is not limited by solubility. A closer look at dolomite weathering contributions estimated from riverine Mg2+ fluxes exceeds the world average by factors between 2 to 15. Our results indicate that both calcite and dolomite mineral weathering within temperate zone watersheds will be able to carry an increased flux of HCO3- to the ocean as global atmospheric CO2 increases. In addition this work reinforces the significant contribution of dolomite weathering to the global HCO3- flux.
B23B-1266
Response of carbonate weathering to environmental gradients
Chemical weathering of rocks and the subsequent carbonate deposition in the Ocean are the most important processes regulating the CO2 content of the atmosphere on geological timescale. Although research into weathering has mainly focused on silicate weathering, the dynamics of the carbonate reservoir is of prime interest to understand and reconstruct the global carbon cycle. Because of the size of this reservoir, any imbalance between oceanic precipitation and chemical weathering of carbonate will result in CO2 excursions. In addition, the fast dissolution of carbonate rocks classically leads to calcite-oversaturated river water, suggesting that the weathering of carbonate is limited by environmental conditions. In order to decipher what controls carbonate weathering, we focused on river chemistry in a well-drained pure carbonate area exhibiting strong but coupled altitude, temperature, runoff and vegetation gradients, the Jura Mountains, France. Associated to those gradients, we observe a two-fold decrease in the chemical weathering of carbonate with the increasing elevation. As the climatic variability cannot explain this weathering gradient, we used the ecological and hydrological ASPECTS model (Rasse et al. 2001) to reconstruct the influence of vegetation on weathering reactions through the production of CO2 in soils. Simulations allowed us to reconstruct the daily soil partial pressure of CO2 as well as the hydrological cycle. We show that carbonate weathering can be explain by a difference in soil CO2 productivity directly linked to the change in vegetation species as a function of altitude. In the Jura Mountains, the dissolution of carbonate as well as the weathering rates are mainly controlled by the vegetation which influences both the carbon and water cycle in soils. This study demonstrates that the chemical weathering is strongly sensitive to the ecosystem dynamics and suggests that carbonate weathering could quickly react to global change.
B23B-1267
Climatically Driven Loss of Calcium in Steppe Soil.
Analysis of archived soil samples collected in Kemannaya Steppe Preserve in 1920, 1947, 1970, and fresh samples collected in 1998 indicated that the native steppe soil lost 17-28 kg m-2 of calcium in the form of carbonates in 1970-1998. Here we demonstrate that the loss of calcium was caused by fundamental shift in the steppe hydrologic balance. Previously unleached soils where precipitation was less than potential evapotranspiration are now being leached due to increased precipitation and, possibly, due to decreased actual evapotranspiration. Because this region receives low levels of acidic deposition, the dissolution of carbonates involves the consumption of atmospheric CO2. Our estimates indicate that this climatically driven terrestrial sink of atmospheric CO2 is ~ 2.2 – 8.0 g C m-2 yr-1. In addition to the net sink of atmospheric carbon, leaching of pedogenic carbonates significantly amplified seasonal amplitude of CO2 exchange between atmosphere and steppe soil. http://www.albany.edu/faculty/alapenis/research/Natpark/index.html
B23B-1268
Mineral Occurrence, Translocation, and Weathering in Soils Developed on Four Types of Carbonate and Non-carbonate Alluvial Fan Deposits in Mojave Desert, Southeastern California
Soil geomorphology and mineralogy can reveal important clues about Quaternary climate change and geochemical process occurring in desert soils. We investigated (1) the mineral transformation in desert soils developed on four types of alluvial fans (carbonate and non-carbonate) under the same conditions of climate and landscape evolution; and (2) the effects of age, parent materials, and eolian processes on the transformation and translocation of the minerals. Four types of alluvial-fan deposits along the Providence Mountains piedmonts, Mojave Desert, southeastern California, USA were studied: (1) carbonate rocks, primarily limestone and marble (LS), (2) fine-grained rhyodacite and rhyolitic tuff mixed with plutonic and carbonate rocks (VX), (3) fine- to coarse- grained mixed plutonic (PM) rocks, and (4) coarse-grained quartz monzonite (QM). These juxtaposed fan deposits are physically correlated in a small area (about 20 km by 15 km) and experienced the same climatic changes in the late Pleistocene and Holocene. The soils show characteristic mineral compositions of arid/semiarid soils: calcite is present in nearly all of the samples, and a few of the oldest soils contain gypsum and soluble salts. Parent material has profound influence on clay mineral composition of the soils: (1) talc were observed only in soils developed on the volcanic mixture fan deposits, and talc occurs in all horizons; (2) palygorskite occur mainly in the petrocalcic (Bkm) of old soils developed on the LS and VX fan deposits, indicating pedogenic origin; (3) chlorite was observed mainly in soils developed on VX fan deposits (all ages) and on some LS deposits, but it is absent in soils developed on PM and QM fan deposits; and (4) vermiculite was common throughout soils developed on plutonic rock fan deposits. These mineralogical differences suggest that minerals in the soils are primarily inherited from their parent materials and that mineral weathering in this area was weak. Except the abundance of palygorskite, soils developed on alluvial fans with different ages (4,000 to 200,000 yrs old) did not show other distinct mineralogy difference as a function of age or soil development, which supports the weak weathering of the soils. The results suggest that the clays in the argillic horizons are primarily derived from the accumulation of desert dust, and with time, are translocated into subsoil horizons. The pedogenic accumulation of dust is a soil-geomorphic process common to the Mojave Desert, as well as other deserts in the world.
B23B-1269 INVITED
Anthropogenic Increase Of Soil Erosion In The Gangetic Plain Revealed By Geochemical Budget Of Erosion
Tectonic and climatic factors are the key natural variables controlling the erosion through complex interactions. Nonetheless, over the last few hundred years, human activity also exerts a dominant control in response to extensive land use. The geochemical budget of erosion allows the balance between the different erosion processes to be quantified. The chemical composition of river sediment results from the chemical composition of the source rock modified by (1) weathering reactions occurring during erosion and (2) physical segregation during transport. If erosion is at steady state, the difference between the chemical composition of source rocks and that of river sediments must therefore be counterbalanced by the dissolved flux. However, climatic variations or anthropic impact can induce changes in the erosion distribution in a given basin resulting in non steady state erosion. Using a mass balance approach, the comparison of detailed geochemical data on river sediments with the current flux of dissolved elements allows the steady state hypothesis to be tested. In this study, we present a geochemical budget of weathering for the Ganga basin, one of the most densely populated basin in the world, based on detailed sampling of Himalayan rivers and of the Ganga in the delta. Sampling includes depth profile in the river, to assess the variability generated by transport processes. Himalayan river sediments are described by the dilution of an aluminous component (micas + clays + feldspars) by quartz. Ganga sediments on the other hand correspond to the mixing of bedload, similar to coarse Himalayan sediments, with an aluminous component highly depleted in alkaline elements. Compared with the dissolved flux, the depletion of alkaline elements in Ganga sediments shows that the alkaline weathering budget is imbalanced. This imbalance results from an overabundance of fine soil material in the Ganga sediment relative to other less weathered material directly derived from Himalaya. Based on the average composition of the suspended load and of floodplain soils, we estimate that 250x106 t/yr i.e. 5 t/ha/yr is eroded from soil surfaces of the Ganga floodplain. This enhanced soil erosion is likely triggered by intense deforestation and change in land use due to increasing human activity in the basin.
B23B-1270
Assessing the Impact of Floodplains in Sedimentary Transport in the Amazon Basin Using the Chemistry of Riverine Suspended Load and Bedload
Large rivers integrate over time and space the geologic events affecting their catchment. But, as dynamic systems, they also have their own characteristic time-scale. Large rivers such as the Amazon are thus likely to record important geologic events such as glacial-interglacial alternance or tectonic uplift. It has been demonstrated by several independent methods that most of the Earth's largest rivers are not eroding at steady-state, exporting amounts of material different from what has been inferred from chemical, geomorphological and tectonic studies. Floodplains can act as major sediments export filters, and could be one of the systems making rivers not to erode continents at steady-state. Nevertheless, the exact impact on riverine sedimentary storage and transfer, as well on weathering, are still a matter of debate. To assess the impact of floodplains, we propose a simple model for the Amazon basin main rivers (Amazon, Madeira and Solimões), based on the inversion of data on total and elemental riverine fluxes. This inversion method has been widely used (e.g. in Gaillardet et al., 1999) to solve geochemical issues. From the assumption of an equilibrium between erosion in the Andes, floodplain deposition/erosion, and export by the river, a set of equations is obtained and inverted, the most largely unknown parameters being deposition/erosion in floodplains and the bedload export flux. The inversion method will allow to better constrain these fluxes. A priori values are needed for all the parameters of the equations, as well as for their uncertainties. Particulate and elemental fluxes are estimated through integration of particulate concentration and composition, and water velocity and composition, over the water column of the rivers near their mouth. The chemical composition of the floodplain river sediments is constrained by the litterature (e.g. by Martinelli et al., 1993) and by correlations obtained on analyzed suspended sediments and bedload sands of the river. The composition of the drained continental crust is taken from litterature models or composite rocks, following the tributary. Large uncertainties are applied to the most unknown parameters. The results of the inversion process show that :
B23B-1271
Estimating the impact of natural and anthropogenic changes on the transfer of elements from the continent to the ocean at the large watershed scale in a tropical environment
Continental weathering of silicate rocks has been recognized as a major driver of climatic changes at the geological timescale. New evidences and preliminary modelling works [1] show that the impact of continental weathering on the geochemical cycles and climate might be non negligible at shorter time scales, from 102 to 104 years, with for instance an increase in total continental weathering by 12 percent from the LGM towards the present day [2]. Continental weathering at large scale (106 km2) is generally described through parametric laws (see for instance [3]), linking weathering rates to mean annual air temperature and continental runoff. These laws are masking numerous other parameters, implicitly included, such as the role of vegetation and physical erosion, and limits the ability of those models to predict changes in weathering due to anthropogenic impacts. A coupled model of biospheric and weathering processes has been developped to estimate the transfer of elements to the ocean originating from tropical watersheds. The BERNI model is the result of the coupling of LPJ - dynamic global vegetation model [5] with the WITCH model [6] that estimates dissolution/precipitation of minerals in the soil environment from kinetic laboratory laws. Here we present results obtained for the Orinoco watershed. Output of the coupled model includes major ion concentrations that are compared to available data, and CO2 consumption through weathering for a tropical environment. The impact of land use changes is tested, as well as the response of weathering to climatic change at continental scale. [1] Aumont et al. (2001) GBC15, 393-405. [2] Burton K.W. et al. (2006) EGU General Assem., Vienna [3] Dessert C. et al. (2003) Chem. Geol.202, 257-273. [4] Edmond et al. (1995) GCA59, 3301-3325; Millot et al. (2002), EPSL196, 83-98. [5] Sitch S. et al. (2003) Global Change Biology 9, 161-185. [6] Goddéris et al. (2006) GCA70, 1228-1147.
B23B-1272
Riverine Re and Mo Inputs to Seawater
We present Re, Mo, and major ion concentration data for a variety of natural fluids as an update to previous geochemical studies (Bertine, 1970; Colodner et al., 1993). The majority of samples are rivers, though data on rain and hydrothermal fluids are also presented. If the data represent natural fluxes, previous assessments of the world average river Re concentration are only ~¼ of the true value, necessitating a shorter marine residence time for this element. Alternatively the data may indicate that the surface Re cycle is anthropogenically perturbed on a global scale. Average Re concentration in world rivers has been previously assessed at 2.1 pM (Colodner et al., 1993) based on a subsample of 4 rivers (Amazon, Orinoco, Ganges, and Brahmaputra) which together account for about 23% of the global river water flux. We believe this estimate to be too low due to the disproportionately large influence of the Re-poor Amazon, which accounts for 75% of the flux considered, though it is only 17% of the global flux. Our larger sample set of more than 50 rivers, accounting for >30% of the global river water flux, supports this as does consideration of the correlative Re vs. SO22-, and Re vs. Σcations relationships first noted by Dalai et al. (2002) for Himalayan rivers. On this basis we believe the average global river water Re concentration to be about 4x higher than the value given by Colodner et al. (1993). This would shorten the estimated marine residence time of Re from 750 ka to about 175 ka. Alternatively, the higher estimate of world river average Re concentration may reflect an anthropogenic perturbation in the global Re cycle. Initial assessment of Re in rivers was heavily biased by the Amazon and Orinoco which accounted for 88% of total water flow considered (Colodner et al.,1993); these two rivers are considered to be relatively pristine. In contrast our evaluation encompasses rivers such as the Mississippi, and Hudson, which are more anthropogenically influenced and have much higher Re levels, often greatly in excess of seawater concentrations of 40 pM. Mississippi Re concentrations of more than 90 pM (Walker et al., 2006) and Hudson concentrations exceeding 200 pM are observed. At more local scales in areas of known human influence, such as in the heavily irrigated South Platte River valley or the acid mine pit in Butte MT, dissolved Re concentrations are higher still, 1,200 pM and 11,000 pM respectively. Previous studies of marine Re have observed an imbalance, with sinks exceeding sources by up to two-fold (Morford and Emerson, 1999). If our data better represent the natural Re flux, this imbalance persists, but now requires additional sinks; shallow sub-oxic margin sediments are a possibility that has been not quantitatively considered to this point. The shorter residence time of 175 ka resulting from this assessment implies that Re should respond more rapidly to changes in sources and sinks than previously thought.
B23B-1273
Upscaling Weathering With an Earth System Model of Intermediate Complexity
How can we model weathering on a global scale without using parametric laws? On local scales, weathering can be modeled based on information on soil profiles, lithologies and vegetation types. On regional scales, catchment area characteristics need to be specified since topography and lithologies along the river play a key role in the final cation and alkalinity fluxes of rivers into oceans. On the global scale ocean biogeochemistry and the fractionation between pelagic and demersal waters has changed during Earth history, adding a whole other set of challenges to deep paleo carbon cycle modeling. We propose a set of simple but mechanistically inspired parameterizations for modeling the variability of weathering with a spatially explicit Earth System Model of Intermediate Complexity (EMIC). With a focus on the biospheric influence on local scales and carbonate/silicate lithologies as controls on river chemistry on regional scales, we sum up weathering input to oceans on the global scale. Finally, we test the sensitivity of our model to climate variability and anthropogenic land use change.