Seasonal sensitivity of weathering processes: Hints from magnesium isotopes in a glacial stream

Seasonal changes in river chemistry offer the potential to assess how weathering processes respond to changing meteorological parameters and ultimately how chemical weathering might respond to climatic parameters. Systematic seasonal variations in magnesium isotope ratios (the 26Mg/24Mg ratio expres...

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Bibliographic Details
Published in:Chemical geology 2012-06, Vol.312-313, p.80-92
Main Authors: Tipper, Edward T., Lemarchand, Emmanuel, Hindshaw, Ruth S., Reynolds, Ben C., Bourdon, Bernard
Format: Article
Language:English
Subjects:
Catchments
Chemical weathering
Discharge
distillation
fractionation
Freshwater
hydrochemistry
isotopes
leaching
Magnesium
Mathematical models
melting
Mg isotopes
oxides
rain
River geochemistry
river water
Rivers
Rocks
seasonal variation
Streams
summer
watersheds
Weathering
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Summary:Seasonal changes in river chemistry offer the potential to assess how weathering processes respond to changing meteorological parameters and ultimately how chemical weathering might respond to climatic parameters. Systematic seasonal variations in magnesium isotope ratios (the 26Mg/24Mg ratio expressed as δ26Mg in per mil units) are reported in stream waters from a mono-lithological granitic, weathering-limited, first order catchment from the Swiss Alps (Damma glacier). Rain, ground, and pore-waters, in addition to plants, rocks, mineral separates and soil are also reported. The concentration response of the river waters is attenuated compared to the large changes in discharge. However, the systematic trends in the isotope data imply that either the source of the Mg changes in a systematic manner, or that the process by which Mg is released into solution changes as a function of discharge. The two first order observations in the data that need to be explained are 1) the systematic enrichment in 24Mg in the stream waters compared to the granitic rocks they drain and 2) a systematic increase in δ26Mg in the waters during the summer melt season. Both observations (which are similar to many other rivers draining silicate rock) can either be accounted for by 1) conservative mixing between at least two different sources of Mg (in addition to precipitation inputs), or 2) process related fractionation. If the stream water compositions can be rationalised by multi-component mixing, there is at least one unidentified component with a δ26Mg
ISSN:0009-2541
1872-6836
DOI:10.1016/j.chemgeo.2012.04.002