IAP-24-104

Weathering vs. climate change: Quantifying recent silicate weathering using novel isotope tracers

Climate change poses a significant threat to our planet, threatening ecosystems, human health, and global economies. To prevent this, we need to develop techniques that allow us to rapidly stabilise and reduce atmospheric carbon dioxide (CO2) concentrations. Chemical weathering of silicate rocks is a natural process that consumes atmospheric CO2 and stabilises climate over geological timescales. Artificially speeding up this process by spreading crushed rocks on soils is also being investigated as a potentially promising CO2 removal technique and thus holds promise for mitigating climate change.
However, the efficiency of silicate weathering depends on climate and particularly on the rainfall and subsurface flow dynamics in soils and aquifers. Efficient weathering is thought to require deep flowpaths that reach reactive minerals but also short water transit times to maintain chemical disequilibrium. These requirements are often incompatible in natural systems and there is an urgent need to improve our understanding on how water flow through soils and aquifers controls weathering efficiency at river catchment scale. This knowledge will help both predict how natural weathering is affected by changing climate (Li et al. 2024), and to design the most effective enhanced weathering techniques.

Typically, silicate weathering is quantified at catchment scale by measuring dissolved element concentrations in streams. A major disadvantage of this approach is that concentrations do not reveal how long ago the weathering took place, i.e. “new” water (i.e. water that entered the landscape only recently) can, and often does, carry “old” solutes (i.e. those derived from weathering which took place decades or even millennia ago (Torres and Baronas 2021)). The aim of this project is to develop a novel approach that uses isotope ratios to distinguish between “new” and “old” solutes in streams, revealing how weathering efficiency varies across catchments spatially and with varying discharge. Isotopes of hydrogen and oxygen (δ2H, δ18O) in water will be used to quantify the fraction of “new” water entering streams, using well-established approaches (Knapp et al. 2019). At the same time the isotope ratios of silicon (δ30Si) and lithium (δ7Li) will be used to constrain the degree of clay formation (reflective of water-mineral equilibria), building on recent developments and increasing understanding of these tracers in weathering systems (Baronas et al. 2018; Hatton et al. 2019). Radiogenic strontium (87/86Sr) isotopes will be used to determine the contribution of soils vs. deeper groundwater flowpaths to water and weathering fluxes at catchment scale (Shand et al. 2007).

Ultimately, the new hydrochemical data generated at the study catchments in Plynlimon, Wales, as part of this project will be combined with historical multi-decadal data in a novel isotope-enabled hydrogeochemical model. Overall, the project will develop and apply methodology to assess if there are any long-term climate change-driven trends in water supply, storage, and routing, and how these changes are impacting weathering rates and rock-derived nutrient supply to ecosystems. This project will thus be utilising and developing cutting-edge scientific approaches to understand and predict carbon storage through silicate weathering.

The project will combine field work, laboratory analyses (MC-ICP-MS, TIMS, CRDS, ICP-OES) and statistical data evaluation to tackle the above problem. The supervisory team combines expertise on weathering (Baronas), travel times of water (Knapp), silicon isotopes (Hatton), and soil carbon (van Soest). The PhD student will be based at Durham University under the supervision of Baronas and Knapp, also spending time in Bangor and Plynlimon, Wales, under the supervision of Hatton and van Soest (UKCEH, Bangor), while conducting fieldwork, sample processing, and sensor data analysis. Field data will be collected during multiple trips to the field site and samples will be analysed in the world-class geochemical laboratory facilities at Durham University.

The student will:
1) constrain chemical and isotopic (δ18O, δ2H, δ7Li, δ30Si, and 87/86Sr) signatures in contrasting weathering regimes: soil waters and groundwaters in the study catchments;
2) collect stream chemical and isotopic timeseries data over a range of timescales and hydrologic regimes, coupled with stream discharge measurements;
3) combine hydrological, chemical, and isotopic data in a model, unmixing the contributions of different endmember components and relating isotopic solute signatures to water age.

The project will focus on three well-established study sub-catchments in Plynlimon, Wales, managed by UKCEH (under supervision by Hatton) and within one of three observatory catchments managed by FDRI (Floods and Droughts Research Infrastructure). These sites have been hydrochemically monitored for several decades (Neal et al. 2011). The work of the PhD project will be placed in context to the existing baseline data, which has shown that dissolved Si and ion concentrations exhibit a dynamic response to discharge, reflecting a shift in flowpaths and water transit times (Neal et al. 2011, van Soest et al. 2024). Si and Li are ideal tracers for catchment-scale silicate weathering at Plynlimon, since >90% of Si and Li exported by the Plynlimon rivers is produced within the catchments (Kirchner and Neal 2013) and these elements are therefore ideal tracers for catchment-scale silicate weathering. Previous work has also shown that radiogenic Sr isotope signatures can be used to distinguish and quantify groundwater contributions to total streamflow in Plynlimon (Shand et al. 2007).

UKCEH and FDRI are actively maintaining the research infrastructure in the Plynlimon catchments, including rainwater samplers, river weirs coupled with discharge loggers, a monthly river chemistry sampling programme, and planned groundwater well refurbishments. This project will benefit greatly from the existing infrastructure and the student will have access to additional training, expertise and funding from the wider FDRI network. All fieldwork will be fully supported by the direct participation of UKCEH and FDRI in the project.

Research and training programme

The research will build on previously published datasets, as well as recent unpublished pilot data collected at the site by Baronas, Hatton, and van Soest through 2023-2025. The existing datasets will be complemented by extensive new data acquired during project duration.

The research tasks will involve:
• Establishing a semi-automated stream monitoring and sampling strategy capturing different hydrological conditions.
• Sampling and analysing all representative end-members contributing to streams: soils and soil porewaters (new soil pits and lysimeters), groundwaters (existing wells) and rainwaters (existing rain samplers).
• Relating newly collected data with historical datasets to establish multi-decadal trends in local climate and stream chemistry.
• Combining hydrological, chemical, and isotopic data in a new coupled hydrochemical model, quantifying silicate weathering efficiency in different catchment compartments under different hydrological regimes.

Year 1

The student will conduct a literature review, compiling existing chemical and hydrological datasets. The student will learn sampling and sensor data processing techniques during initial fieldwork (months 2-4) and set up long-term semi-automated time-series sampling programme (for the next 2 years). The student will learn advanced clean laboratory and isotope mass spectrometry techniques at the Durham Geochemistry Centre.

Year 2

Regular sample and sensor data retrieval will be carried out in coordination with UKCEH (Plynlimon fieldtrips every 2-3 months). Additional extensive soil and groundwater sampling campaigns will be conducted by the full supervisory team. The student will work in the clean laboratory to regularly process and analyse the geochemical and isotopic composition of collected soil and water samples.

Year 3

The student will focus on the modelling and the interpretation of collected datasets, followed by manuscript writing and submission.

Year 3.5

The student will focus on writing and submitting the dissertation and related manuscripts.

Training programme components include:

• Fieldwork techniques, sampling soils, groundwater, rainwater, and streams. Collecting and processing water quality and flow sensor data. Collecting and processing meteorological station data.
• Sample processing, clean lab, and isotope geochemistry (H2O, Li, Si, and Sr) techniques.
• Hydrological and geochemical (thermodynamic and reactive transport) modelling.
• Transferrable skills in project management, scientific communication, peer-reviewed writing and publication, leadership & teamwork, funding acquisition, and other techniques – provided both directly by the supervisory team and the UKRI DTP and FDRI training networks.