Temporal and Spatial Carbon Cycle Dynamics of an Evolving Tropical Delta.

Biogeochemical Cycles



Deltas and associated wetlands are a relatively minor surface feature, comprising less than 4-6% of the terrestrial land area1,2. However, deltas have a disproportionately critical role in carbon stocks and fluxes3,4, a role that is subject to dynamic changes both natural and anthropogenic. Deltas are dynamic systems that actively migrate across the landscape. Consequently, their connections to the landscape, river network, climate and population (Fig. 1) evolve over time. Understanding this evolution from a carbon cycle perspective is critical at a time of significant climatic transition, resulting from atmospheric greenhouse gas (GHG) accumulation.

Carbon dioxide (CO2) and methane (CH4) are significant greenhouse gases adding to anthropogenic climate disruption, and the key gases degassed from delta / wetland regions. Fluxes of these GHGs vary significantly over time and space and evidence suggests deltas can act as both sinks to CO2 and significant sources of CH45,6. This balance is dependent on factors such as inundation patterns, availability of organic substrates and water chemistry / salinity. Consequently, this balance is likely to change as deltas evolve and quantifying this effect is crucial to effectively modelling these systems in a changing climate.

Deltas are crucial to understanding the role of sediment export and gas ebullition as they are carbon ‘hotspots’ in the global carbon cycle. They play a key role in modulating how much carbon is exported from what are the transitional areas between freshwater and marine systems. These systems have also been recognised as important accumulation zones for a range of riverine sediments, accumulation that can subsequently lead to organic matter processing and GHG degassing.

Thus, a framework is needed that:
i) quantifies fluxes of GHGs from modern delta ecosystems, sufficiently capturing spatial and temporal variability.
ii) assesses the degree to which hydrological connectivity in an evolving delta affects GHG fluxes and carbon storage.
iii) identifies the landscape and biogeochemical controls on the GHG efflux strengths
iv) estimates the amount of carbon stored in various forms on the delta.

Making a significant contribution to this framework is the over-arching objective of this Ph.D. but it will also work towards producing a process-based model that describes delta carbon cycling more generally and the relevant controls / drivers. This Ph.D will feed into the broader concept of sustainable delta futures, underpinned by the UKRI GCRF Living Deltas Hub, of which Bass and Henderson are Co-investigators. This PhD aims to derive a detailed understanding of delta development and its effect on regional carbon dynamics.

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To address the research aims the student will conduct their research across a transect of the Ganges-Brahmaputra-Magna (GBM) delta, primarily situated in Bangladesh. The GBM delta is currently migrating in an eastward direction, leading to westerly regions losing hydrological connectivity with the parent river, consequently significantly changing the underlying hydrology and thus landscape functioning. By evaluating carbon dynamics in and east-west transect (Fig. 2), the project will use a space-for-time approach to constrain our current understanding of delta carbon fluxes.

To fulfil the overarching aims and research questions this project will use a combination of field sensors and laboratory-based analysis techniques. The student will be trained in the measurement of geochemical parameters that provide information on the aquatic carbon cycle (e.g., dissolved in/organic carbon concentrations, CO2 / CH4 efflux, dissolved oxygen) as well as the concentrations of dissolved CO2 and CH4. From initial field-surveying, sites will be identified with sufficient CO2 and/or CH4 dynamism that mechanisms of transfer can be studied (e.g., diffusion vs. ebullition fluxes)7,8. Where appropriate, stable and radiocarbon isotopes9 will be utilised in order to identify the sources and formation processes of aquatic GHG fluxes10,11. Combined with GIS techniques the student will quantify landscape effect on reginal scale delta GHG budgets.

A number of Holocene sediment cores will be recovered from across the GBM representing the gradient of delta stability. These cores will be used to establish longer-term records of carbon accumulation on the GBM and how a deltas stability controls the potential stocks of carbon. Total Carbon/Total Inorganic Carbon/Total organic carbon will be determined using an Analytik Jena elemental analyser, which when coupled with sedimentology and sediment geochemistry (lithology, grain size, XRF) and radiocarbon geochronology will be used to establish Holocene carbon accumulation rates and environmental change on the GBM.

Project Timeline

Year 1

Training in appropriate field and laboratory techniques. Establish field site monitoring stations in selected regions, carry out first field campaign. Use preliminary field campaigns to select suitable sites for detailed process study in year 2.

Year 2

Second field season. Having identified suitable sites, utilise a range of techniques to characterise the mechanisms of GHG flux from the different landscapes. Use this understanding to produce process-based model of delta-scale GHG dynamics.

Year 3

Refine and finalise process-based model. Synthesise findings and prepare draft publications. Present findings to IAPETUS and at an international conference.

Year 3.5

Submit thesis and finalise manuscripts for publication.

& Skills

Project Support: The facilities and instrumentation available within the supervisors and collaborative institutions will provide the student with all the necessary laboratory, field and analytical equipment to carry out this project, maximising the likelihood of a successful PhD completion. This includes the ability to analyses concentrations and isotope ratios of dissolved GHGs representing a significant added value.

Scholar Support: The student will join the Life’s Interactions with Dynamic Environments research cluster in the Department of Geographical & Earth Science (GES) at Glasgow University. GES has a large, active research community that will provide peer-support throughout the Ph.D. In Glasgow the student will be part of the Carbon Landscape Research Group (www.carbonlandscapes.org). (something on Newcastle). The student will also collaborate with the Living Deltas UKRI GCRF research hub, allowing effective integration of their research into a largescale and impactful research network. The student will participate in post-graduate training courses and be involved in annual post-graduate research conferences to allow for networking and collaboration with colleagues. All project supervisors and collaborators are highly research active; the student will frequently interact with all members of the research group providing opportunities to learn about various techniques and research areas related to their core experience.

Skills Developed: The student will receive training in world-class biogeochemical and hydrological techniques in GES and at the Newcastle University, including Cavity Ring-down Spectrometry, GHG flux measurement, dissolved gas measurement, hydrological sensor technology, UV-Vis Spectrometry, infrared gas analysis, freshwater chemistry and stable isotope analysis. In addition the student will be trained in essential research skills including scientific method, experimental design, data collection, and statistical analysis. IAPETUS, Glasgow & Newcastle University each offer transferable skills programmes adding to the employability of the student after completion.

References & further reading

1. Mitsch WJ, Gosselink JG. 2000. Wetlands third edition. John Wiley & Sons, New York.
2. Kuehn KA, et al. 2004. Ecology. 85: 2504-2518.
3. Dutta MK, et al. 2017. Frontiers in Marine Science. 4: 187.
4. Miller RL. 2011. Wetlands. 31: 1055-1066.
5. Lund M, et al. 2010. Global Change Biology. 16: 2436-2448.
6. Livesley SJ, Andrusiak SM. 2012. Estuarine & Coastal Shelf Science. 97: 19-27.
7. Prairie YT, del Giorgio PA. 2013. Inland Waters 3: 311-320.
8. Bastviken D et al. 2011. Science 331: 50.
9. Garnett et al. 2016. Ecohydrology. 9: 113-121.
10. Waldron S et al. 1999. Geochim. Cosmochim. Acta, 63: 2237-2245.
11. Waldron S et al. 1999. Global Biogeochemical Cycles 13: 93-100.


Further Information

Applications: to apply for this PhD please use the url: https://www.gla.ac.uk/study/applyonline/?CAREER=PGR&PLAN_CODES=CF18-7316

Dr Adrian Bass (Adrian.bass@glasgow.ac.uk)
Dr Andrew Henderson (andrew.henderson@newcastle.ac.uk)
Dr Pauline Gulliver (Pauline.gulliver@glasgow.ac.uk)
Living Deltas – www.livingdeltas.org.

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