『Abstract
　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 δ²⁶Mg compositions, compared to the parent basaltic glass (δ²⁶Mg=-0.29‰), ranging from -0.96 to +0.64‰, while precipitation and hydrothermal waters possess δ²⁶Mg 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 δ²⁶Mg 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 δ²⁶Mg (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 δ²⁶Mg compositions values both higher and lower than unweathered basalt. In the glacier-fed and direct-runoff rivers where the δ²⁶Mg 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.

Keywords: magnesium isotopes; Iceland; chemical weathering; physical weathering; secondary mineral formation/stability』

1. Introduction
2. Geology, climate and river setting
3. Field and analytical techniques
　3.1. Sampling
　3.2. Cations, anions and trace elements
　3.3. Mg isotopes
4. Results
　4.1. Major and trace element data
　4.2. Mg isotope and elemental data
5. Discussion
　5.1. Basalt and soil samples
　5.2. Suspended load
　5.3. External inputs
　5.4. Dissolved load
　　5.4.1. Magnesium isotope and elemental variations
　　5.4.2. Magnesium isotope behaviour
6. Conclusions
Acknowledgements
References