Calcium (Ca) is the fifth most abundant element in the Earth’s crust and forms a key constituent of many rock-forming minerals. In addition, it is an essential nutrient for all living organisms, except perhaps fungi. Furthermore, the geochemical cycles of calcium and carbon are intimately linked (for example, through the formation of CaCO3 in the ocean). The analysis of calcium isotopes provides a powerful tool to trace and quantify fluxes within the calcium biogeochemical cycle.
Radiogenic isotopes trace sources and the values measured reflect mixing between different sources. Stable isotopes also reflect mixing between sources but the values can also be changed by fractionation processes, for example, by biological uptake.
What does this figure tell us?
This figure shows river water samples collected from the outlet of Leverett Glacier, Greenland and data from the local bedrock. It was expected that the river water samples would plot in the grey box defined by the isotopic compositions of the main Ca-bearing minerals as these constitute the main source of Ca to the river and there is no vegetation to fractionate Ca. However, the river points have clearly been fractionated and we think that this is due to fractionation during adsorption of Ca onto suspended sediments. If this is a common occurrence in sediment-laden rivers (not just glacial ones) then it will need to be taken into account when determining riverine stable isotope fluxes to the ocean (a key parameter in global biogeochemical cycle models).