Strontium isotope compositions of river waters as records of lithology-dependent mass transfers : the Garonne river and its tributaries (SW France)

The relation of lithology in the drainage basin to the dissolved load of the Garonne river and its main tributaries, in southwestern France, was evaluated by determining 87Sr/86Sr ratios, and concentrations of major and trace elements during a 2-year-long survey. In the upper drainage basin, the Garonne river waters were isotopically varied at two observation points: 0.71131+/−0.00030 (2σ) for 84+/−18 ppb (2σ) and 0.71272+/−0.00044 for 86+/−10 ppb, respectively. In the lower drainage basin, the Garonne river waters were isotopically identical at three observation points at 0.71020+/−0.00024 for 125+/−22 ppb. By contrast, the tributaries (Lot, Truyère, Aveyron, Arriège, Gers and Salat) are widely varied in their 87Sr/86Sr ratios and Sr concentrations from 0.70836+/−0.00049 to 0.71058+/−0.00057, and from 18+/−8 to 280+/−116 ppb. The Sr isotope ratios and concentrations suggest a dominant supply of two reservoirs of Sr, one of which is with low 87Sr/86Sr ratios and high Sr contents that is typically characteristic of carbonate rocks, and the other with high 87Sr/86Sr ratios and low Sr concentrations that is characteristic of felsic rocks. Locally as in the Lot waters draining the Massif Central and within the Pyrénées mountains, a third source of Sr from mafic rocks may be involved. Mass-balance calculations based on the mean 87Sr/86Sr ratios and contents of the dissolved Sr, and on the mean discharges integrating the 2-year survey, suggest that contribution of the silicate reservoir amounts 3–8% of the total dissolved Sr flux. Mass-balance calculations also suggest that variation in the supply of Sr from either of the two major reservoirs does not exceed the analytical uncertainty at about +/−5%. The 87Sr/86Sr ratios of HCl and NH4Cl leachates of suspended loads of the Garonne river are different from that of the associated dissolved Sr. This leaching-related supplementary Sr represents less than 10% of the total amount of Sr transported by the Garonne waters. The Sr isotope characteristics of the leachates are probably records of an intermediate pedogenic episode in the weathering-erosion process occurring in the Garonne drainage basin

Impact of nitrogen fertilizers on the natural weathering-erosion processes and fluvial transport in the Garonne basin

Knowledge of the impact of N-fertilizers on the weathering-erosion processes of soils in intensively cultivated regions is of prime importance. Nitrification of NH4− fertilizers produces HNO3 in the basin of the Garonne river, enhancing soil degradation. Their influence on the weathering rates was determined by calculating the consumption rate of atmospheric/soil CO2 by soil weathering and erosion, and its contribution to the total dissolved riverine HCO3−. This contribution was found to be less than 50% which corresponds normally to a complete carbonate dissolution by carbonic acid, suggesting that part of the alkalinity in the river waters is due to carbonate dissolution by an acid other than carbonic acid, probably HNO3

Evidence for carbon sequestration by agricultural liming

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95356/1/gbc1382.pd

Landscape-level controls on dissolved carbon flux from diverse catchments of the circumboreal

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 26 (2012): GB0E02, doi:10.1029/2012GB004299.While much of the dissolved organic carbon (DOC) within rivers is destined for mineralization to CO2, a substantial fraction of riverine bicarbonate (HCO3−) flux represents a CO2 sink, as a result of weathering processes that sequester CO2 as HCO3−. We explored landscape-level controls on DOC and HCO3− flux in subcatchments of the boreal, with a specific focus on the effect of permafrost on riverine dissolved C flux. To do this, we undertook a multivariate analysis that partitioned the variance attributable to known, key regulators of dissolved C flux (runoff, lithology, and vegetation) prior to examining the effect of permafrost, using riverine biogeochemistry data from a suite of subcatchments drawn from the Mackenzie, Yukon, East, and West Siberian regions of the circumboreal. Across the diverse catchments that we study, controls on HCO3− flux were near-universal: runoff and an increased carbonate rock contribution to weathering (assessed as riverwater Ca:Na) increased HCO3− yields, while increasing permafrost extent was associated with decreases in HCO3−. In contrast, permafrost had contrasting and region-specific effects on DOC yield, even after the variation caused by other key drivers of its flux had been accounted for. We used ionic ratios and SO4 yields to calculate the potential range of CO2 sequestered via weathering across these boreal subcatchments, and show that decreasing permafrost extent is associated with increases in weathering-mediated CO2 fixation across broad spatial scales, an effect that could counterbalance some of the organic C mineralization that is predicted with declining permafrost.Funding for this work was provided through NSF-OPP-0229302 and NSF-OPP-0732985. Additional support to S.E.T. was provided by an NSERC Postdoctoral Fellowship.2013-02-2

Fractionation of rare earth elements in plants: a study of radish plants grown in separate soils of calcium smectite and illite clay minerals under a laboratory condition

International audienc

Plant decay as a major control of river dissolved potassium: A first estimate

International audienc

Fractionation of rare-earth elements in plants during experimental growth in varied clay substrates

International audienc

Mobilization and Redistribution of Elements in Laterites of Semail Ophiolite, Oman: A Mass Balance Study

Several samples of laterites were collected from four paleosol profiles, Ibra, East Ibra, Al-Russayl, and Tiwi representing the vertical lithological variation within each profile. The mineralogical and geochemical composition of laterites in every section revealed differences in thickness and redistribution of elements reflecting different conditions of weathering processes. Elemental mass balance was calculated for every profile relative to the parent rock. The results indicated redistribution of elements from the surface to deeper zones with an enrichment of elements in the saprolite and oxide zones. Among the different sections, the profile of East Ibra composite 1 and 2 is characterized by high concentration of all elements compared to the other profiles. Sc/Fe ratio in different zones indicates low values for the profile of Tiwi profile 1, Ibra profile and Al-Russayl composite 2 and 3 profile due to the significant enrichment of Fe in these zones independently of redox conditions. Large fluctuations characterize Th/U ratios and reflect redox condition more reduced in Tiwi area than in East Ibra and Al-Russayl areas