| 12512 |
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. |
| 12512 |
label |
Pogge von Strandmann, Philip ; Burton, Kevin ; James, Rachael ; van Calsteren,
Peter ; Gislason, Sigurður R. and Sigfússon, Bergur (2008). The influence of weathering
processes on riverine magnesium isotopes in a basaltic terrain. Earth and Planetary
Science Letters, 276(1-2) pp. 187–197. |
| 12512 |
label |
Pogge von Strandmann, Philip ; Burton, Kevin ; James, Rachael ; van Calsteren, Peter
; Gislason, Sigurður R. and Sigfússon, Bergur (2008). The influence of weathering
processes on riverine magnesium isotopes in a basaltic terrain. Earth and Planetary
Science Letters, 276(1-2) pp. 187–197. |