Strontium as a tracer of weathering processes in a silicate catchment polluted by acid atmospheric inputs, Strengbach, France

Abstract

This paper determines the weathering and atmospheric contributions of Ca in surface water from a small spruce forested Ž . silicate catchment N-E France receiving acid atmospheric inputs. The bedrock is a granite with K-feldspar and albite as dominant phases. The calcium content in plagioclase is low and the CarNa ratio in surface water is high, reflecting other sources of calcium from those expected from the weathering of major mineral phases. The biotite content is low. Only traces of apatite were detected while no calcite was found in spite of a major hydrothermal event having affected the granite. The strontium isotopic ratio 87 Srr 86 Sr and Sr content was used as a tracer of weathering and was determined in minerals and bulk bedrock, open field precipitation, throughfall, soil solution, spring and stream water. The Sr isotopic ratio of the reacting weathering end-member was predicted by simulating the alteration of the granite minerals by incorporating strontium into the water-rock interaction kinetic code KINDIS. In the early stages of water-rock interaction, K-feldspar and biotite strongly influence the isotopic composition of the weathering solution whereas, the Na-rich plagioclase appears to be the main long-term reactive weathering end-member. Approximately 50% of dissolved Sr in streamwater are atmospherically derived. The 87 Srr 86 Sr ratios of exchangeable Sr in the fine fraction at 1-m depth from a soil profile indicate that the amount of exchangeable Sr seems essentially controlled by atmospheric inputs. The exception is the deep saprolite where weathering Ž . processes could supply the Sr and Ca . Na-Plagioclase weathering obviously control the chemistry and the isotopic composition of surface waters. The weathering of trace mineral plays a secondary role, the exception is for apatite when plagioclase is absent. Our hydrochemical, mineralogical and isotopic investigations show that a major part of the strong Ca losses detected in catchment hydrochemical budgets that result from the neutralization of acid precipitation has an ) Corresponding author. atmospheric origin. Consequently, in the long term, in such areas, the availability of such an exchangeable base cation might be strongly limited and surface waters consequently acidified.

Heavy metal distribution in some French forest soils: evidence for atmospheric contamination

“…More recently Probst et al (2000) and Aubert et al (2001) have clearly detected using Nd and Sr as tracers, the influence of atmospheric pollutant inputs on forest soils from this region.…”

“…Sr and Pb isotopes have been demonstrated to be powerful tools to determine the respective contribution of lithogenic and anthropogenic sources in soils (Steinmann and Stille, 1997;Probst et al, 2000;Aubert et al, 2001;Semlali et al, 2001;Teutsch et al, 2001). …”

Variability of87Sr/86Sr in water draining granite revealed after a double correction for atmospheric and anthropogenic inputs

“…In order to characterize the theoretical Sr isotopic signature of a water interacting with a granite, a dissolution model was applied based on the hypothesis that most of the Sr comes from the dissolution of plagioclase, potassium feldspar and biotite (Fritz et al, 1992;Zuddas et al, 1995, Bullen et al, 1997Probst et al, 2000). Moreover, as the newly formed phases are in equilibrium with their parent solution, only the Sr content of fluid is modified; the This model was computed on the Chirac granite where the mineralogical composition of the parent rock is known (30% plagioclase, 10% biotite, 20% potassium feldspar).…”

“…The weathered zone is commonly exposed to pollution (Négrel, 1999;Probst et al, 2000;Griffioen, 2001), whereas the deeper aquifer in the fractured bedrock is better preserved and better reflects the water-rock interaction (Franklyn et al, 1991;Négrel, 1999;Pauwels et al, 2000).…”

Variation des rapports 87Sr/86Sr dans les sols et les plantes d’un inselberg tropical

“…In granite, a fraction of the Sr may be located in Ca-rich minerals such as plagioclase and apatite, with another fraction in K-rich minerals with a higher 87 Sr/ 86 Sr ratio like microcline and muscovite. Compared to feldspar, the Sr isotopic ratio of mica is very high, but its content in Sr is very low Clow et al, 1997;Probst et al, 2001), and especially that of muscovite. Muscovite is weathered in the deep horizon at TR6 and is absent from the other profiles.…”