Effects of biogeochemical processes on magnesium isotope variations in a forested catchment in the Vosges Mountains (France)

• Published in: Geochimica et cosmochimica acta 2012-06, Vol.87, p.341-355
• Main Authors: Bolou-Bi, Emile B., Vigier, Nathalie, Poszwa, Anne, Boudot, Jean-Pierre, Dambrine, Etienne
• Format: Article
• Language: English
• Subjects: aerial parts; Bedrock; C horizons; Dissolution; forested watersheds; Grasses; Horizon; illite; Isotopes; Life Sciences; losses from soil; Magnesium; mountains; Needles; Picea abies; rain; roots; saprolite; Soil (material); soil solution; spring; streams; summer; tracer techniques; trees; winter
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2012.04.005

This study investigates the potential of Mg isotopes as tracers of biogeochemical processes in a small-forested catchment located on sandstones extremely poor in Mg-bearing minerals. The average δ26Mg is −0.63±0.12‰ and 0±0.14‰ for local rainwater and bedrock, respectively. From the C horizon to the upper eluvial (E) horizon, soil δ26Mg (from 0.0±0.14‰ to 0.25±0.14‰) is close to the bedrock value, while more than 70% of Mg is lost, suggesting a small isotopic shift during illite dissolution. The surface soil horizon (Ah) δ26Mg is close to plant δ26Mg, and especially to the grass δ26Mg value (−0.49‰). The bulk δ26Mg of trees and grass (−0.32‰ and −0.41‰, respectively) are higher than the average δ26Mg values of the soil exchangeable fraction (−0.92‰ to −0.42‰), and of rainwater (−0.65‰). Within plants, roots are enriched in heavy isotopes, whereas light isotopes are preferentially translocated and stored in the above ground parts. In Norway spruce, the older needles, forming the annual litterfall, are isotopically lighter and strongly depleted in Mg compared to more recent needles. Soil solution δ26Mg shifts seasonally, from low values, lower than rainwater and close to litterfall during a high rainfall period in spring, to higher values, close to soil δ26Mg in dryer periods of winter or summer. At the watershed scale, streamwater δ26Mg varies between −0.85±0.14‰ and −0.08±0.14‰ and δ26Mg values decrease linearly with discharge. The high streamwater δ26Mg at low flow, close to bedrock δ26Mg, most likely reflects dissolution processes in the deep saprolite in relation to the very long water residence time. Conversely, we suggest that low stream level δ26Mg values are at least partly related to the contribution of surface flows from wet areas. Using a simple mass and isotopic balance approach, we compute that mineral dissolution rates in the soil (0.35kgMgha−1year−1) presently compensate for Mg losses from the soil.