IAP-24-042

Blue Carbon accumulation, transformation and storage: Quantifying biogeochemical processes in saltmarsh ecosystems.

Blue carbon habitats such as saltmarsh, seagrass and mangroves are recognised as hotspot for the burial and storage of organic carbon (OC)[1]. In the United Kingdom saltmarsh is the primary blue carbon habitat covering an area of 452 km2 and is estimated to accumulate 46,563 tonnes of OC annually[2]; with a total of 5.2 million tonnes of OC stored in the soil of these marshes[3]. The magnitude of these intertidal OC stores has attracted the attention of NGOs and national agencies to which seek to preserve, protect and expand the sedimentary C resources within these habitats and to include them in national and international climate budgets; natural capital assessments; and greenhouse gas reporting.

However, our understanding of the mechanisms that govern the accumulation, transformation and storage of organic matter (OM) in saltmarsh environments is poorly constrained. In the absence of quantitative understanding of the biogeochemical functioning of saltmarshes any management and policy interventions would be reckless and the inclusion of saltmarshes in national and international climate frameworks, natural capital accounting and GHG reporting will remain speculative at best.

In this project we will bring state-of-the-art approaches from peatland[4,5] and marine science[6,7] to quantify the biogeochemical mechanisms governing the accumulation, transformation and storage of OM in UK saltmarshes. This improved understanding of biogeochemical functioning of saltmarsh ecosystems will allow robust evidence led policy and management interventions to be developed and lay the foundation for UK saltmarshes to be included in national C budgets, natural capital accounting and GHG reporting.

The project will build upon well characterised saltmarshes that have pre-existing OC accumulation and stock data from across the UK[2,3]. These cover the full range of marsh types. These saltmarshes will be sampled with respect to both internal zonation relative to the tidal frame; management type (natural and realigned); and geographic location (e.g., north to south and east to west of the UK). The selected saltmarshes will be subject to the following analysis.

Stoichiometric and enthalpy analysis – it is possible to constrain reactions into and through an ecosystem by understanding the stoichiometry, and enthalpy of formation, of the OM reservoirs and fluxes. This is achieved through quantifying the elemental composition (CHNOS) of OM reservoirs such as, biomass and of the sediment profile, but also of organic matter entering the marsh system from the marine and terrestrial environments.

Macromolecular analysis – the reactions of organic matter into and through a saltmarsh can be constrained by knowing the changes in macromolecular composition, e.g. carbohydrates. The collected samples will be analysed using thermogravimetric analysis and differential scanning calorimetry and py-GCMS.

Thermodynamic stability – it is possible to calculate thermodynamic parameters for naturally-occurring OM (Gf) and in accumulating ecosystems, such as peatlands and saltmarshes, it is possible to measure a profile of Gf and so assess the extent, or even presence, of a reaction. Therefore, we can measure whether the OM has become stable.

Mineralogy – OC-mineral interactions exert a primary control over OM preservation in sediments[6,7] but little is known about these OC preservation mechanisms in vegetated intertidal environments. The mineral reactivity will be determined by measuring mineral surface area (MSA) following the Brunauer-Emmett-Teller (BET) methodology and the cation exchange capacity (CEC)[7].

Binding of OC to reactive iron (Fe) compounds is a known mechanism for OC preservation that persist despite diagenetic processes[6]. Iron compounds will be sequentially extracted from the sediments through the application of the citrate-dithionite Fe reduction[6] with the quantity of each Fe fraction being determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Quantification of the OC characteristics (i.e., OC content, δ13C, Δ14C) prior to and post extraction will facilitate the quantification of the type and age of the OC bound to reactive Fe.

Year 1

Commence literature review. Familiarization with the field and laboratory approaches that will be used in the project. Development of data analysis protocols. Plan and undertake initial phase of sample collection across natural and realigned UK saltmarshes. Begin laboratory analysis.

Year 2

Continue laboratory analysis. Undertake final phase of sample collection. Begin data analysis and modelling. Begin drafting thesis. Present research at a national conference (Challenger, MASTS, Saltmarsh specialist meeting).

Year 3

Complete laboratory work. Finalise data analysis and modelling. Continue to draft thesis and develop first peer reviewed manuscript. Present research at an international conference (EGU, Goldschmidt, AGU, ASLO).

Year 3.5

Complete thesis, prepare for viva defence, drafting manuscripts for publication.

Project specific training will be provided alongside the development of transferable skills. Each year a training needs analysis will be undertaken to identify the skills, both generic and subject-specific, that are required to complete the research and prepare the student for a future in academia or industry.

The project includes both laboratory and field skills. The laboratory training will allow the student to plan and undertake analysis using a wide variety of techniques including calorimetry, elemental analysis, thermogravimetric analysis and radionuclide dating. Through the planning and execution of the fieldwork the student will gain skills including experimental design, in situ measurements and sampling at the land ocean interface. All data will be processed in R and ArcGIS allowing for the opportunity to develop the in-demand skills

Transferable skills training will be provided within IAPETUS and from al Education and Lifelong Learning Institute (IELLI) at the University of St Andrews and the Durham Centre for Academic Development at Durham University.