IAP-24-020

Investigating multiple causes of saltmarsh compaction – how important is biodegradation?

Saltmarshes provide precise and near-continuous reconstructions of Common Era relative sea level (RSL) at sites globally, but particularly along the East coast of North America. However, saltmarsh sediments are prone to compaction processes that cause post-depositional distortion of the stratigraphic column used to reconstruct RSL. In turn, this causes post-deposition lowering (PDL) of sea-level index points and distorts the resultant RSL record. Without correction, this can result in erroneous interpretation of rates and drivers of sea-level change. PDL caused by mechanical compression is well understood and so geotechnical modelling can be used to return sea-level index points to the altitudes at which they were deposited, limiting the effects of sediment compaction on our interpretation of reconstructed sea level (Brain, 2015). However, use of geotechnical models assumes that the compressive strength of saltmarsh sediment remains unchanged once deposited and buried. Saltmarsh sediments are, however, highly organic and, hence, are prone to biodegradation processes that may reduce the strength of the sediment; as such, geotechnical models may not be sufficiently accurate in accounting for the effects of sediment compaction in saltmarsh records of RSL. Recent studies suggest that saltmarsh-sediment compressibility may have increased during past warmer and drier conditions (e.g., during the Medieval Climate Anomaly ~800-1300 CE) in saltmarshes in Connecticut, USA (Brain et al., 2017; Kemp et al., 2015). Despite this, the underpinning mechanisms are poorly constrained, partly due to a limited assessment of the geochemical characteristics of saltmarsh sediments throughout burial, and how this relates to variable sediment compressibility. The tentative link between increased compressibility and a changing climate is a research priority, potentially affecting the ability of saltmarshes to keep pace with and to buffer the effects of future RSL rise. To address this gap in our understanding, this project will investigate the controls on sediment compaction in organic saltmarsh stratigraphies to permit development of new geotechnical models that explicitly incorporate biodegradation processes. This will be made possible by increasing understanding of the geotechnical properties and geochemical signature of modern saltmarsh sediments that span a range of climate zones and incorporate variability of dominant plant types; and by considering the relationship between biodegradation and compressibility in saltmarsh stratigraphies. These results will be applicable to saltmarshes on a global scale.

The project will involve fieldwork to ‘end-member’ saltmarshes in the UK and along the Atlantic coast of North America to collect modern saltmarsh samples for geotechnical and geochemical analysis. In the laboratory the student will set up and run geotechnical tests in the bespoke laboratory at Durham University. In addition, the student will work in collaboration with Dr Chris Vane from the British Geological Survey to identify the geochemical properties of different saltmarsh sediments using cutting-edge Rock-Eval methods (e.g. Garcin at al., 2022) to develop key metrics of the biodegradation state of saltmarsh sediments.

Year 1

Review of existing literature relating to saltmarsh sediments, geotechnical and geochemical properties. Laboratory and field training to sample saltmarsh sediments for geotechnical and geochemical analysis. Fieldwork to saltmarshes in the UK and/or the Atlantic coast of North America. Presentation of research plans at UK-based workshop.

Year 2

Preparation and analysis of geotechnical, geochemical and physical/property data. Further fieldwork to saltmarshes in the UK and/or the Atlantic coast of North America. Data analysis and interpretation of datasets. Presentation of the results at an international meeting.

Year 3

Continued data analysis and interpretation. Development of new geotechnical model which incorporates additional biodegradation processes to incorporate newly quantified causes of compressibility of saltmarsh sediments. Presentation at a large international meeting (e.g. EGU/AGU). Production of thesis.

Year 3.5

Continued interpretation of datasets and model development and testing. Continued thesis production.

This project will provide the student with a wide range of skills in geotechnics and geochemistry alongside fieldwork and general laboratory skills. The student will be trained in a range of laboratory techniques and geotechnical model development. The student will also benefit from the research environment at the Department of Geography in Durham by becoming part of the Sea Level, Ice and Climate and Hazards and Surface Change Research Clusters.