Magnesium and calcium isotope systematics in a headwater catchment underlain by amphibolite: Constraints on Mg–Ca biogeochemistry in an atmospherically polluted but well-buffered spruce ecosystem (Czech Republic, Central Europe)

Abstract

Lithogenic and biologically-fractionated magnesium (Mg) and calcium (Ca) are viewed as alternative controls of the δ26Mg and δ44Ca isotope signatures of streams and rivers. Understanding of Mg–Ca isotope systematics and of runoff generation in headwater catchments is crucial for upscaling riverine variability in δ26Mg and δ44Ca values to Mg–Ca isotope inputs into the oceans, and for global climate change considerations. Here we report Mg–Ca–Sr isotope systematics in a small, spruce-forested, high-elevation catchment in western Czech Republic, which has received high Mg and Ca inputs from dissolution of base-rich bedrock amphibolite and from industrial emissions. Our study site NAZ (Slavkov Forest) experienced a relatively long period of acid rain (ca. 1950–1995), accompanied by deposition of partly soluble dust from coal-burning power plants. Bedrock contained as much as 11 and 10 wt% of MgO and CaO, respectively. Input–output monitoring revealed a large net export of Mg, Ca, but also S, from the catchment via surface runoff. The δ26Mg values increased in the order: open-area precipitation (−1.44‰) < soil water (−1.10‰) < spruce canopy throughfall (−1.00‰) < runoff (−0.96‰) < spruce bark (−0.59‰) < spruce needles (−0.46‰) < spruce xylem (−0.40‰) < bedrock (−0.32‰) < soil (−0.30‰) < spruce fine roots (−0.07‰). The δ44Ca values increased in the order: spruce bark (−1.91‰) < spruce xylem (−0.86‰) < spruce fine roots (−0.83‰) < spruce needles (−0.43‰) < spruce canopy throughfall (0.31‰) < soil water (0.44‰) < soil (0.46‰) < open-area precipitation (0.69‰) < plagioclase (0.83‰) < bulk bedrock (0.86‰) < runoff (0.92‰) < amphibole (1.10‰). Our data indicated a relatively small Mg isotope fractionation toward higher δ26Mg values in organic matter, and a large Ca isotope fractionation toward lower δ44Ca values in all studied organic compartments. In contrast to Mg, most Ca isotope variability at NAZ was driven by biological fractionations. While excess Mg and Ca in NAZ export fluxes via catchment runoff are most easily explained by lithological control of runoff Mg–Ca, the Mg–Ca–Sr isotope systematics were also consistent with measurable atmospheric contribution to Mg and Ca in runoff.