Carbon transfers by Large Rivers of South East Asia: CO2 source or sink?

Biogeochemical Cycles

IAP2-19-041

Overview

The maintenance of an equable climate over most of the Earth’s history implies a negative feedback which regulates atmospheric CO2, the dominant greenhouse gas. Whilst silicate weathering and carbonate formation has been cited as the major negative feedback, the burial of organic carbon may outweigh CO2 consumption through silicate weathering in many environments (Galy et al., 2007; Hilton et al., 2015). The fundamental problem is that both the magnitude and control mechanisms of these global carbon transport fluxes are poorly known.

Organic matter is eroded and transported as part of the suspended load in rivers. This transfer can result in the release or consumption of CO2, depending on the source of the river carbon and its fate during sedimentary processes. For example, 70-85% of the organic carbon buried in the Bengal Fan is from the erosion recent terrestrial biosphere (soils and vegetation), which acts to remove CO2 from the atmosphere for millions of years (Galy et al., 2007). Alternatively, erosion can supply organic carbon from sedimentary rocks (e.g. organic matter from shales). If this is weathered and oxidised during transport, it can release CO2 back to the atmosphere after millions of years of storage (Hilton et al., 2014).

Mountainous regions on the planet are thought to have the highest rates of CO2 consumption through weathering and organic carbon burial. The Irrawaddy, Salween and Mekong Rivers in SE Asia, all with their headwaters in the Himalaya-Tibetan-Plateau region, together provide one of the largest terrestrial carbon inputs to the world’s oceans (Bird et al., 2008). On our recent fieldwork, we determined the inorganic carbon flux as the equivalent amount of carbon as driving 10,000km, every second in an average family sized car. Despite these preliminary findings, the fluxes and sources of riverine organic carbon remain poorly constrained, particularly in the Irrawaddy and Salween rivers.

This PhD project will thus focus on these key questions:
– what is the organic carbon flux from the Irrawaddy-Salween-Mekong rivers, from their headwaters to the oceans?
– How does the source of organic matter vary during river transport and floodplain transit?
– What is the balance between CO2 release via the oxidation of rock bound organic carbon and CO2 consumption via the burial of modern organic carbon from the biosphere?

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Image Captions

The upper Irrawaddy River, Myanmar, viewed while sampling August 2018 (©ET Tipper)

Methodology

Over the last 4 years, members of the project team have built a large sample archive of river sediments and waters from this region. However, depending on the interests and skill set of the student, there is significant scope for the student to conduct fieldwork in Myanmar (Irrawaddy and Salween), Laos, Cambodia and Vietnam (Mekong) and China (Irrawaddy, Salween and Mekong) where we have local contacts and infrastructure that can assist with fieldwork.

In the laboratory, the student will receive training and apply a range of state-of-the-art techniques (stable and radioactive isotope geochemistry, biomarker abundance and isotope composition) to determine whether the organic matter is from the modern biosphere, or whether it is ancient organic matter from rocks, with very different climate implications. These include stable isotope geochemistry, radiocarbon and biomarker methods. There is also the potential to use organic compound specific isotopes to track the source of carbon and organic matter cycling within the river system.

The project will further determine the amount of organic carbon oxidation on route to the oceans using trace metal proxies for rock weathering. These include rhenium dissolved in river waters.

In addition to geochemical measurements, detailed hydrometric measurements of water discharge and water flow will be made using Acoustic Doppler Current Profiler (e.g. Figure 2).

Overall, the training and analyses are designed to enable the research will shed new light on the sensitivity of climate to mountain building and tectonics.

Project Timeline

Year 1

Literature review and compilation of existing samples and datasets; receive training on sample preparation for isotope and biomarker analyses; depending on the student – planning, organisation and undertaking a field season in May-July Year 1; write/ defend Research Proposal;

Year 2

Sample and data processing, receive further training on isotope geochemistry methods; help to write and submit NERC Radiocarbon Facility grant; further develop writing skills and manuscript preparation for publication.

Year 3

Synthesise field and modelling datasets; attend international conferences; publication and thesis writing;

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

Methods, including: i) geochemical preparation for elemental and isotopic analyses of organic matter by EA-IRMS; ii) radiocarbon measurement by AMS, with visit to the NERC Radiocarbon Facilty for more in depth training; iii) biomarker identification and abundance by GC-MS (and potentially isotope composition by GC-IRMS).

In addition, depending on the skills and interests of the student, there is an opportunity to receive training in novel field sample methods, which include sample collection, filtration and the use of ADCP methods to quantify river flow (Figure 2).

The supervisory team has the necessary expertise to train the student in these specialist skills, all supported by a dedicated team of technicians in Durham Geography. In addition to receiving regular supervisory meetings and support at Durham (in partnership with NERC Radiocarbon Facility, SUERC, and Glasgow University), the student will also be enrolled in a graduate training programme at Durham University and through IAPETUS-specific training, gaining a range of transferable skills relevant to completion of the PhD and developing a career path, including writing research proposals and giving oral presentations.

S/he will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment. The student will attend and contribute to the programme of regular departmental seminars and paper reading groups on a wide range of topics, to support the development of a well-rounded scientist. S/he will attend national and international conferences (e.g. AGU, EGU, Goldschmidt), networking events and outreach activities, developing networks for feedback and future employment.

References & further reading

Bird, M.I., et al., 2008, A preliminary estimate of organic carbon transport by the Ayeyarwady (Irrawaddy) and Thanlwin (Salween) Rivers of Myanmar. Quaternary International, 186, 113-122.

Galy, V., et al., 2007, Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system, Nature, 450, 407-410.

Hilton, R.G., et al., 2014, Geological respiration of mountains revealed by the trace element rhenium, Earth Planet. Sci. Lett., 403, 27-36.

Hilton, R.G., et al., 2015, Erosion of organic carbon in the Arctic as a geological carbon dioxide sink, Nature, 524, 84-87

Further Information

Prof. Bob, Durham University, 0044191 33 41970, r.g.hilton@durham.ac.uk

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