HR: 1340h
AN: H43C-0386    [Abstracts]
TI: Sulfur Isotopes in the Rivers of the Mackenzie River Basin: Implication for CO$_{2}$ Consumption
AU: * Calmels, D
EM: calmels@ipgp.jussieu.fr
AF: Laboratoire de G\'{e}ochimie et Cosmochimie, Institut de Physique du Globe de Paris, Universit\'{e}. Paris VII, 4, place Jussieu, Paris, 75252 France
AU: Gaillardet, J
EM: gaillard@ipgp.jussieu.fr
AF: Laboratoire de G\'{e}ochimie et Cosmochimie, Institut de Physique du Globe de Paris, Universit\'{e}. Paris VII, 4, place Jussieu, Paris, 75252 France
AU: Brenot, A
EM: brenot@crpg.cnrs-nancy.fr
AF: Centre de Recherches P\'{e}trographiques et G\'{e}ochimiques - CNRS, 15 rue Notre Dame des Pauvres, B.P. 20, Vandoeuvre-les-Nancy, 54501 France
AU: France-Lanord, C
EM: cfl@crpg.cnrs-nancy.fr
AF: Centre de Recherches P\'{e}trographiques et G\'{e}ochimiques - CNRS, 15 rue Notre Dame des Pauvres, B.P. 20, Vandoeuvre-les-Nancy, 54501 France
AB: Mass budgets of chemical weathering in hydro-systems usually assume that the dissolution of CO$_{2}$ in rain and soil waters provides most of the protons that attack rock minerals. However, the oxidative weathering of reduced species containing sulfur, such as pyrite, can be a significant source of protons. On a global scale, the origin of sulfate in rivers is still unclear. At least three possibilities can be envisaged: sulfate from sedimentary gypsum, atmospheric pollution and oxidative weathering of sulfide. As shown by previous studies, S and O isotopes of the sulfate molecule can allow deciphering between the different sources. In the aim of constraining the origin of sulfate delivered to the ocean by rivers and to refine CO$_{2}$ consumption budgets for chemical weathering reactions, we have started to measure S and O isotopes in the largest river systems. Among them, the Mackenzie River basin is an ideal case, because it has been recognized by geologists to contain both gypsum and reduced sediments, mainly black-shales, rich in pyrite. The O and S isotopes of the sulfate molecule do show large discrepancies between the two main geomorphic units of the Mackenzie River basin: the Rocky-Mackenzie Mountains to the West and the interior platform to the East. For example, river samples from the lowlands are characterized by values of $\delta^{34}$S ranging from $\ -3.25\permil$ to $\ -18.47\permil$ and those from the mountains varying between 2.06\permil and 9.87\permil. We interpret these values and the relationships between isotopic composition of sulfate and major elements as showing the dominant contribution of sulfide oxidation in the lowlands and gypsum dissolution in the mountains. The details of our mixing model, e.g. end-member choices, will be discussed in detail; but based on our data we calculate that 54 to 96% and 18 to 40% of dissolved sulfate come from sulfide oxidation in lowland rivers and mountain rivers, respectively. The mean value obtained for the Mackenzie River Basin is 32%. Assuming that protons added in water by pyrite oxidation react preferentially with carbonate rocks, we calculate that 30% of the protons reacting with carbonate minerals do not originate from the dissolution of atmospheric CO$_{2}$ in soil water. For a long-term perspective and at a global scale, the oxidative weathering of sulfide coupled to rock weathering can lead to a release in the atmosphere of CO$_{2}$ originating from the carbonate reservoir. This process may be significant in mountainous regions, such as Himalayas, where surface rocks have been recently exhumed and where high physical weathering rates sustain a continuous contact between fresh rocks and water.
DE: 4806 Carbon cycling
DE: 1806 Chemistry of fresh water
DE: 1886 Weathering (1625)
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting