『Abstract
The magnesium (Mg) isotope composition of rivers is sensitive
to changes in the balance of primary mineral dissolution and secondary
mineral formation. As these processes are regulated by climate
then changes to the Earth's climate system, such as the onset
of glaciation, could potentially alter the isotopic behaviour
of Mg in rivers. To investigate how Mg isotopes behave during
glacial weathering, we have determined the Mg concentration and
Mg isotope ratio for the dissolved, suspended and bedload phases
of glacial and non-glacial rivers in west Greenland. This region
is essentially monolithological and there is little biological
activity; hence isotope variations largely result from differences
in weathering processes. Dissolved Mg in rivers is always enriched
in the light Mg isotopes relative to the bedload (δ26Mg
= -0.4‰), and the glacial rivers have lighter Mg isotope compositions
(δ26Mg -1 to -1.3‰) than the non-glacial rivers (δ26Mg
〜 -0.6‰). Enrichment of light Mg in the dissolved load is consistent
with the preferential uptake of heavy Mg from solution during
the formation of secondary silicate minerals. However, evidence
from saturation state modelling suggests that little secondary
mineral formation is likely to have occurred in the glacial rivers
because the concentration of dissolved solids is very low. Moreover,
the glacial derived suspended sediment has identical Mg isotope
ratios to the bedload. These observations suggest that the formation
of secondary weathering minerals is not controlling the behaviour
of Mg isotopes in these rivers. Rather, variations in the δ26Mg
value of the dissolved load are attributed to incongruent weathering
of the solid phase: preferential dissolution of carbonate minerals,
which have low δ26Mg (-1.1 to -5.2‰), imparts a more
negative δ26Mg signal to the glacial river waters.
Although our study indicates that the Mg isotopic composition
of the dissolved load is sensitive to changes in weathering congruence,
it is unlikely that glaciation will result in major changes in
the δ26Mg composition of continental runoff.
Keywords: Magnesium isotopes; Glacial weathering; Greenland; Weathering
fluxes; isotope fractionation』
1. Introduction
2. Geology, climate and river settings
3. Methods
3.1. Sample collection and preparation
3.2. Cation analyses
3.3. Mg isotope analyses
4. Results
4.1. Physicochemical properties of river waters
4.2. Major element concentrations
4.2.1. Solid phases
4.2.2. Dissolved load
4.2.3. Colloids
4.3. Magnesium isotopes
5. Discussion
5.1. Physicochemical properties
5.2. Major element behaviour
5.2.1. Solid phases
5.2.2. Dissolved load
5.3. Magnesium isotope behaviour
5.3.1. Secondary mineral formation
5.3.2. Biological controls
5.3.3. Source controls
5.4. Implications for Mg cycling
6. Conclusions
Acknowledgements
References