Quantifying cation exchange: Re-assessing the weathering signature of continental waters

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences

Abstract

Chemical weathering mediates Earth's carbon cycle and hence global climate over geological time-scales. Ca and Mg from silicate minerals are released to the solute phase during dissolution with carbonic acid. This solute Ca and Mg gets subsequently buried as Ca and Mg carbonates in ocean basins transferring carbon from the atmosphere to the carbonate rock reservoir. This simple reaction has provided the climatic feedback that has maintained Earth's climate equable and inhabitable over the entire history of the Earth. To understand how Earth's climate functions, it is therefore vital to understand silicate weathering and to estimate the flux carbon dioxide associated with modern chemical weathering. Modern day silicate weathering fluxes are estimated from the chemistry of rivers or natural waters. Natural waters contain positively charged elements or cations such as Ca, Mg, Na and K, and it has been understood for decades that the relative and absolute concentrations of these elements depend of the type of rocks that are drained. For example, Ca is mainly derived from the weathering of limestones, whereas Na and K are mainly derived from the weathering of silicate minerals such as feldspar. This distinction is important because only the Ca derived from silicate weathering is important for carbon dioxide consumption. Therefore, the Ca flux from silicate weathering is usually estimated based on Na, which has been thought to a more reliable estimate of silicate weathering. However, chemical weathering is more complex than simple mineral dissolution and a series of other chemical reactions also occur such as cation exchange. This is a process whereby the positively charged cations in solution are attracted to negatively charged mineral surfaces on clays, a process known to buffer groundwaters. One of the key chemical exchanges is Ca for Na, meaning that Na may not provide a true estimate of silicate weathering at all. Recent isotopic data suggests that cation exchange might be more significant that previously thought, which until now has been very hard to fingerprint. One method is to use naturally occurring tracers or isotopes, that allow chemical reactions to be tracked. In this work, it is proposed to examine the naturally occurring isotopes of the elements Li and Mg to examine the role of cation exchange in global budgets. However, to be able to do this successfully, a series of experimental work is proposed to examine the behaviour of the isotopes of Mg and Li during cation exchange. Once we understand how our tracers work we can use them to re-evaluate our understanding of natural waters, and better estimate fluxes of carbon dioxide associated with chemical weathering, with the ultimate aim of better understanding Earth's climate.

Planned Impact

There will be numerous beneficiaries from this research including scientific, social, economic and environmental beneficiaries:

Scientific Impact
The scientific impact is outlined in more detail in the section "academic beneficiaries" but in summary, the scientific impact will be broad across the spectrum of Earth system and environmental science, including:
1. Geochemists,
2. Soil scientists,
3. Contaminant transport,
4. Hydrologists

Research led undergraduate teaching
There is a strong reputation of research led teaching at St Andrews, and Tipper currently lectures environmental geoscience to third year undergraduates, with a focus on chemical weathering and the chemistry of natural waters. This grant will provide the opportunity to engage undergraduate students (the majority of which gain in employment in industry rather than academia) in the problems of natural water chemistry including the key results of the research project about cation-exchange. This grant will also provide the perfect opportunity to engage undergraduates with the use of isotope tracers, including training in the analytical protocols, adding significant value to their BSc. They will even be given the opportunity to interpret the project data in their practical labs, and potentially be encouraged to work on similar topics for their dissertations or the new masters degree beginning at St Andrews.

Environmental impact
Understanding the fundamentals of how contaminants are transported in the environment is intimately linked to cation exchange and is of obvious societal and economic interest. Media interest in this topic in the past twelve months is highlighted by the Fukushima nuclear disaster in Japan, where transport of leaked radioactive material by groundwaters has been significant. Significant research is in progress, in particular in the US, concerning the mitigation of radio-Cs. The main way of doing this is via cation exchange with clay, the direct content of the present proposal.
One of the fundamental controls on groundwater composition is accepted to be cation exchange. Therefore developing novel isotopic tracers of cation exchange is of fundamental importance to both hydrologists and for water quality. In the UK this research will be of interest to the Environment Agency, over a 5 year time-scale.

Commercial Impact
The commercial impact of understanding and constraining contaminant transport is significant. This is of interest to the nuclear industry as mentioned above because of the direct link with Cs to the scientific content of the proposal, but the the research is applicable to any heavy metal with significant sorption characteristics. Indeed the experimental approach can be extended to almost any element and will be of significance for the mitigation of contaminated waters.

Impact to the public and future generations of scientists
The next generation of scientists is of fundamental social and economic interest to the UK. Tipper is committed to communicating the research and its founding principles to the general public and school children. Two ways of doing this have already been identified (GeoBus and Our Dynamic Earth) described in detail in the pathways to impact section. GeoBus is a mobile outreach forum from the Dept. Earth Sciences at the University of St Andrews, that provides workshops on geoscience for school pupils. Our Dynamic Earth is a public forum based in Edinburgh and Tipper will help coordinate workshops on the carbon cycle to the general public.

Publications

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Related Projects

Project Reference Relationship Related To Start End Award Value
NE/K000705/1 01/05/2013 31/08/2014 £60,692
NE/K000705/2 Transfer NE/K000705/1 26/01/2015 25/09/2015 £25,055
 
Description Chemical weathering mediates Earth's carbon cycle and hence global climate over geological time-scales. Ca and Mg from silicate minerals are released to the solute phase during dissolution with carbonic acid and subsequenty buried as marine carbonate. This mechanism has provided the climatic feedback that has maintained Earth's climate equable over geological history. Quantitative models of contemporary silicate weathering processes coupled to estimates of modern day carbon fluxes associated with silicate weathering are therefore fundamental to understanding Earth's carbon cycle, and the feedbacks between the carbon cycle, climate and chemical weathering.
However, it has been proposed that Na-Ca exchange reactions with clay on mineral surfaces could account for 80% of the Na in rivers waters. At present, none of the methods to estimate silicate weathering fluxes and associated CO2 consumption account for cation-exchange reactions largely because physical and chemical weathering were assumed to be steady state processes implying that cation exchange has no net influence on weathering fluxes.
In our research we have begun to quantify the fractionation of Li and Mg stable isotope ratios by cation exchange processes. we have achieved this by 1) analysis of a series of natural samples (suspended sediments) having developed a sequential extraction process 2) the initiation of a series of experiments to test the fractionation. The latter work is still in progress.
Exploitation Route The findings, once complete will be of importance beyond their blue sky impact. For example:
1) The exchangeable cation reservoir in soils is the major nutrient pool where the biosphere is able to source nutrients for growth.
2) There are implications for water quality
3) There are implications for remediation of contaminated land
Sectors Environment

 
Description NERC Urgency grant: Himalayan chemical weathering and carbon transfer triggered by the April 2015 Nepal Earthquake
Amount £51,818 (GBP)
Funding ID NE/N007441/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 07/2015 
End 07/2015
 
Description NERC standard grant: The key to quantifying chemical weathering intensity: clay stable isotope fractionation factors
Amount £472,431 (GBP)
Funding ID NE/M001865/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 12/2014 
End 12/2017
 
Description Collaboration with Dr Nick Tosca, University of Oxford 
Organisation University of Oxford
Department Department of Earth Sciences
Country United Kingdom 
Sector Academic/University 
PI Contribution We are working to determine the isotopic fractionation of stable isotopes Mg, Si and Li between clay and aqueous solutions. The clay synthesis is being conducted in collaboration with Dr Tosca, and the isotopic analysis is being conducted in my research group. The isotopic work involves us setting up new methods, with most of the development work relating to isotopic spikes being very novel.
Collaborator Contribution Dr Tosca is conducting the clay synthesis experiments, a highly specialist task that is essential to the work.
Impact Data was presented at the Goldschmidt conference 2015 in Prague.
Start Year 2014