『Abstract
Analysis of river water, rock, travertine and soil from the high
altitude, negligible vegetation setting of the Southern Tibetan
Plateau demonstrates that Ca and Mg isotope ratios are fractionated
during weathering. Dissolved Ca in the two rivers studied is derived
primarily by limestone dissolution. δ44/42Ca in the
rivers averages 0.43‰ and is statistically distinct from limestones
at 0.31‰. The range in δ44/42Ca in these small rivers
is 0.43‰, equivalent to the entire range in δ44/42Ca
recorded in marine carbonate over the last 80 Ma. Precipitation
of isotopically light travertine with a δ44/42Ca of
0.21‰ enriches solute Ca in heavy isotopes. The Mg isotope composition
of the rivers is intermediate between limestone and silicate rock
averaging -1.5‰. Silicate soil has a δ26Mg of -0.03‰,
heavier than silicate rock by 0.5‰. These fractionations in the
soil create a companion groundwater reservoir of heavy Ca and
light Mg. Seasonal variations in Ca and Mg isotope ratios in the
dissolved load are small, but define an array which can be modelled
as a mixture between a fractionated groundwater reservoir and
surface runoff (reflecting the isotopic composition of the lithology).
Fractionation of Ca during the weathering of the continents is
of importance to the global cycle of Ca. The riverine input of
Ca to the oceans (dominated by carbonate weathering) is controlled
not only by the composition of the primary continental crust but
also by the size and composition of a fractionated reservoir on
the continents. The impact on the oceanic cycle of Ca depends
on the relative residence times of dissolved Ca in the ocean and
the storage time of fractionated Ca.
Keywords: magnesium (Mg); calcium (Ca); isotopes; weathering;
oceans; cycle』
1. Introduction
2. Materials and methods
2.1. Sample collection
2.2. Sample preparation
2.3. Mass spectrometry
3. Results
3.1. Bedrock and bedload variations in Ca and Mg isotope
ratios
3.2. Secondary material
3.3. Dissolved Mg and Ca isotope ratios
4. Discussion and interpretation
4.1. Lithological control on riverine Ca and Mg isotope ratios?
4.2. Secondary carbonate as a control on Ca isotope ratios
4.3. Controls on Mg isotope fractionation
4.4. A model for Ca and Mg isotope compositions in river waters
5. Implications
6. Conclusions
Acknowledgements
References