The influence of weathering processes on riverine magnesium isotopes in a basaltic terrain Philip A.E. Pogge von Strandmann a,b, ⁎, Kevin W. Burton a,c, 1 , Rachael H. James a,2 , Peter van Calsteren a , Sigurður R. Gislason d , Bergur Sigfússon d a Department of Earth Sciences, CEPSAR, The Open University, Milton Keynes, MK7 6AA, UK b Bristol Isotope Group, Department of Earth Sciences, Bristol University, Bristol, UK c LMTG — UMR 5563 UR 154 CNRS Université Paul-Sabatier, Observatoire Midi-Pyrénées,14, avenue Edouard Belin, 31400 Toulouse, France d Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland abstract article info Article history: Received 7 March 2008 Received in revised form 10 September 2008 Accepted 20 September 2008 Available online 26 October 2008 Editor: M.L. Delaney Keywords: magnesium isotopes Iceland chemical weathering physical weathering secondary mineral formation/stability This study presents major-, trace-element and Mg isotope data for the dissolved load and suspended particulates of Icelandic rivers draining dominantly basaltic catchments, including both glacier-fed and direct-runoff rivers. These samples provide the opportunity to understand the behaviour of Mg isotopes during chemical weathering, where variations due to lithology are not extant. Given the significant role of Mg in the carbon cycle, such variations may provide important information on the regulation of Earth's climate. Hydrothermal waters, groundwater, precipitation (glacial ice), basalt glass, olivine and representative soils have also been analysed. The dissolved load shows a wide range of δ 26 Mg compositions, compared to the parent basaltic glass (δ 26 Mg= - 0.29‰), ranging from - 0.96 to + 0.64‰, while precipitation and hydrothermal waters possess δ 26 Mg values of - 0.83‰ and +0.85‰, respectively, with lower Mg concentrations than the dissolved load. Biomass activity in vegetation and organic material in soils and rivers (colloids) appear to have little effect on the Mg isotope compositions. Rather, the data suggest that Mg elemental and isotopic variations are largely controlled by the formation and stability of secondary phases in response to differing hydrological conditions. In some samples seawater, in the form of direct precipitation or glacial runoff, also appears to be an important source of Mg. Glacier-fed rivers, groundwaters, and some direct-runoff rivers, with a high pH, have higher δ 26 Mg than basalt, which is most likely due to the incorporation of light Mg isotopes in secondary minerals. In contrast, those direct-runoff rivers which have a relatively low pH, have low δ 26 Mg (relative to basalt), consistent with preferential incorporation of heavy Mg isotopes into secondary phases, although it is not possible to rule out some contribution from precipitation. Riverine suspended particulates are depleted in mobile elements, and have δ 26 Mg compositions values both higher and lower than unweathered basalt. In the glacier-fed and direct-runoff rivers where the δ 26 Mg of the dissolved phase is heavy, due to the formation of secondary phases, the suspended load is light, because it contains more of those phases. The opposite is true for the remainder of the direct-runoff rivers which have low pH. This could be due to dissolution of secondary minerals, enriched in light Mg, which are unstable at low pH, or the formation of new secondary phases. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The transfer of magnesium (Mg) from the continents to the oceans, and its removal through hydrothermal exchange at mid-ocean ridges, plays a major role in the global carbon cycle, and temporal variations in the Mg concentration of seawater have been invoked as a con- trolling factor on the carbonate mineralogy of the oceans (Berner et al., 1983; Albarede and Michard, 1986; Wilkinson and Algeo,1989; Tipper et al., 2006b). The major source of Mg to the oceans is from the weathering of carbonate and silicate rocks of the continental crust, principally delivered by rivers and groundwaters (Berner and Berner, 1996; Burnett et al., 2001; Holland, 2003). This suggests that Mg isotopes have the potential to serve as a powerful tracer of chemical weathering, and ultimately variations in global climate, because they have been shown to be fractionated by weathering processes (Tipper et al., 2006a,b). Magnesium is removed from the ocean through exchange with Ca during hydrothermal reaction at mid-ocean ridges, the formation of dolomite, and ion exchange reactions with clays (Elderfield and Schultz, Earth and Planetary Science Letters 276 (2008) 187–197 ⁎ Corresponding author. Bristol Isotope Group, Department of Earth Sciences, Bristol University, Bristol, UK. Fax: +44117 9253385. E-mail address: P.PoggevonStrandmann@bristol.ac.uk (P.A.E. Pogge von Strandmann). 1 Now at: Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, UK. 2 Now at: National Oceanography Centre Southampton, Southampton, SO14 3ZH, UK. 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.09.020 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl