The global emission of carbon dioxide (CO2) from streams and rivers is estimated to be 1.8 Â± 0.3 PgC yr-1 (Raymond et al., 2013). This is almost half of the present day increase in atmospheric COÂ¬2 concentrations. While effort has been made to refine these carbon flux estimates, the source of this riverine CO2 still remains poorly constrained. Without knowing the carbon source, it is difficult to assess how evasion of CO2 from rivers impacts the global carbon cycle. For instance, if this CO2 is ‘young’ (e.g. 10 kyrs), for instance if it has been released from old soils or sedimentary rocks, then it is a net source of CO2 to the modern atmosphere. Both pathways are poorly represented in global inventories.
The large Arctic Rivers could play an important role in the release of old carbon as CO2. First, they drain vast areas of peatland with high soil carbon stocks, where the organic matter is several thousand years old (Schuur et al., 2015). Second, they carry aged carbon associated with soil organic matter in their solid load (Hilton et al., 2015). Finally, much of this aged organic matter is found in permafrost zones, which are subject to ongoing warming, which may release carbon upon thaw and contribute to the ‘permafrost carbon feedback’ (Schuur et al., 2015). Despite this recognition, CO2 fluxes and source remain poorly constrained in many of these river systems (Striegl et al., 2012).
The Mackenzie River, Canada (Figure 1), is the focus of this investigation because it allows us to quantify CO2 evasion fluxes and the age of CO2 in a large Arctic River basin where old sources of organic carbon (Hilton et al., 2015) and inorganic carbon (Calmels et al., 2007) are known to be important. The principle aim will be to quantify the CO2 evasion over monthly timescales at the reach scale in rivers. Gas flux measurements will be combined with geochemical fingerprinting of CO2 using isotopes, alongside measurements of other major carbon pools in the river system. These measurements will inform numerical modelling experiments to provide new insight on CO2 evasion over longer time periods, with an aim to make robust predictions about future change associated with Arctic warming.
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Measuring CO2 emissions on the Mackenzie River delta
The student will use a combination of cutting-edge techniques to achieve the research objectives. First, new measurements of CO2 evasion will be made at key locations on the large rivers of the Mackenzie Basin, Northwest Territories, Canada (Figure 1) where team members have been working since 2009. These will be made via an Infra-Red Gas Analyser (IRGA) and linked to the CASE partner PP Systems who are recognised as a world leader in field-based CO2 instrumentation.
The evaded river CO2 will be trapped for isotope analysis using techniques pioneered by the supervisory team (Garnett et al., 2012). Radiocarbon will be used to examine the inputs of CO2 from different sources at ultra-high resolution, and stable isotopes used to examine source and evasion versus exchange. These CO2 measurements will be made alongside geochemical measurements to constrain the major water chemistry and the carbon content and source of the river sediments.
In addition to geochemical measurements in the field, detailed hydrometric measurements of water discharge and water flow will be made using Acoustic Doppler Current Profiler (ADCP) (Figure 2). This will also provide detailed information on the channel form and river bed topography.
Numerical experiments will be run at time scales which match the field observations (over days to weeks) of flow, using established Computational Flow Dynamic Models (Sandbach et al., 2012). In addition, lower complexity models will be used to examine longer-time scales of gas exchange.
Literature review and compilation of published datasets; receive training on novel sample collection methods and CO2 flux measurements; visit to NERC Radiocarbon Facility; planning, organisation and undertaking a field season in May-July Year 1, including close interaction with CASE Partner PP Systems; write/ defend Research Proposal;
Sample and data processing, receive training on geochemistry methods; receive training on numerical model methods; CASE Partner PP systems visit; write and submit NERC Radiocarbon Facility grant; second field trip in mid-Year 2; further develop writing skills and manuscript preparation for publication.
Synthesise field and modelling datasets; attend international conferences; publication and thesis writing
Complete and submit thesis; finalise manuscripts for publication.
Specialist training will cover novel field sampling methods. Firstly, the measurement of CO2 fluxes by IRGAs and gas chambers. This will be closely linked to case Partner PP Systems, who are considered a world leader in the design and manufacture of scientific instruments for CO2 measurement. The student will visit the CASE Partner PP Systems (based in MA, USA) for a research placement and play a central role in the development of novel applications of their instruments. In addition, in the field the student will receive training in the use of ADCP methods to quantify river flow (Figure 2).
It will also cover methods that the supervistory team have developed, including trapping of CO2 for isotope measurements (stable carbon isotopes and radiocarbon).
S/he will be trained on cutting-edge geochemical methods, including: geochemical preparation for elemental and isotopic analyses of organic matter by EA-IRMS; measurement of CO2 isotopic composition; radiocarbon measurement by AMS, with the potential for an extended visit to the NERC Radiocarbon Facility for more in depth training.
The numerical modelling component will make use of existing approaches, and the student will receive training in how to set these experiments up, run them, and process/interpret the data.
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 and the NERC Radiocarbon Facility. In addition to receiving regular supervisory meetings and support at Durham (in partnership with SUERC), 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
Garnett MH, et al. (2012) Annual variability in the radiocarbon age and source of dissolved CO2 in a peatland stream. Science of the Total Environment 427, 277-285.
Sandbach SD, Lane SN, Hardy RJ, et al. (2012) Application of a roughness-length representation to parameterize energy loss in 3-D numerical simulations of large rivers. Water Resources Research, 48, W12501.
Hilton RG, et al. (2015) Erosion of organic carbon in the Arctic as a geological carbon dioxide sink, Nature, 524, 84-87.
Raymond PA, et al. (2013) Global carbon dioxide emissions from inland waters. Nature, 503, 355-359.
Striegl RG, et al. (2012) Carbon dioxide and methane emissions from the Yukon River system. Global Biogeochem Cycles, 26, GB0E05.
Prof. Robert Hilton, Durham University, 0044191 33 41970, firstname.lastname@example.org