The Earth Surface Science Institute (ESSI) PhD Projects
Unusual patterns of diagenesis in offshore sediments of the Yellow River Delta, China
Supervisors: Professor Simon Bottrell, Dr Robert Mortimer and Dr Yanqing Sheng (Yantai Institute of Coastal Zone Research (YIC), Chinese Academy of Sciences (CAS)).
The Yellow River (or 'Huang He') is the largest single source of sediment to the world's oceans and that sediment has a distinctive composition, being largely derived from the major wind blown ('loess') deposits of the North China Plain. Preliminary geochemical data from cores taken from the offshore delta show an unusual composition, with carbon:sulphur ratios (C/S) indicative of deposition in a terrestrial environment, despite being deposited in fully marine conditions. This apparent contradiction implies that processes operating during diagenesis of the Yellow River delta sediment differ from those considered typical of marine sediments (e.g. Froelich et al. 1979). The extremely large volumes of sediment involved mean that this could have a significant impact on the major biogeochemical element cycles that control ocean chemistry.
- Aerial View of the Yellow River Delta (source: www.chinadaily.com.cn/m/shandong/images/attachment/jpg/site1/20090728/0013729e44e10bd8fa3flc.jpg)
This project will involve the student taking sediment cores in the coastal Yellow Sea, China with colleagues from the Yantai Institute of Coastal Zone Research (YIC). These cores will then be used for an integrated study of the geochemistry of carbon, iron and sulfur in the Yellow River delta sediments. These three elements undergo intense biologically mediated cycling between different redox states and dissolved and mineral forms in marine sediments. The cycles of carbon, iron and sulfur are intimately interlinked and control both the evolution of the diagenetic mineralogy and porosity of the sediment (e.g. Mortimer et al. 2011) and exchange of solutes with the overlying sea-water.
A wide variety of approaches are available at Leeds to study sediment diagenetic processes, including: SEM and electron microprobe analysis of solids; traditional pore-water analysis; high resolution gel-probe pore-water analysis; and S, O and C isotope analysis. The student will use these to constrain the behaviour and fate of the different components and thus build an understanding of the diagenetic processes leading to the unusual C/S geochemistry observed.
A second phase of this study will compare findings from the Yellow River delta with published work on the offshore parts of other major river deltas, for instance the Amazon where sediment reworking in highly mobile sediment banks has a major influence on biogeochemical processes; Aller et al., 1996; 1998; 2010). This will enable reassessment of the global importance of these regions in defining regional and global sedimentary geochemistry.
Finally, this research also has significant practical implications. In the absence of biostratigraphical control, sediment C/S is commonly used in well correlations by the oil industry, so understanding causes of variation is key to its proper application. Also, conditions of iron diagenesis are thought to influence diagenetic porosity occlusion in siliciclastic marine sediments and thus the ability of these sediments to act as oil reservoirs in the subsurface (e.g. Aller et al, 1986). Understanding the controls on diagenesis of the Yellow River delta sediments will provide a useful "end-member" case study that sheds light on the interplay between biogeochemical and mineral precipitation processes taking place.
This project will be undertaken in collaboration with the Yantai Yantai Institute of Coastal Zone Research (YIC) , Chinese Academy of Sciences (CAS)) who will provide logistical support for fieldwork, laboratory facilities for initial core processing and co-supervision with Dr. Y. Sheng.
Training
This project offers the opportunity for the acquisition of a broad range of skills and will provide excellent training for a post in either industry or academia. The successful candidate will be involved in overseas field sampling, water and sediment analyses, using both novel and established techniques. The interpretation of the data will involve interdisciplinary understanding of biogeochemical processes during sediment diagenesis and is directly relevant to the petroleum industry.
Aller, R.C., Macking, J.E., and Cox, R.T. (1986). Diagenesis of Fe and S in Amazon inner shelf muds - apparent dominance of Fe reduction and implications for the genesis of ironstones. Continental Shelf Research 6, 263-289.
Aller R.C. (1998). Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors. Marine Chemistry 61, 143-155.
Aller R.C.; Blair NE; Xia Q; et al. (1996) Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments. Continental Shelf Research 16, 753-786.
Aller, R.C. Madrid, V., Chistoserdov, A., et al. (2010). Unsteady diagenetic processes and sulfur biogeochemistry in tropical deltaic muds: Implications for oceanic isotope cycles and the sedimentary record. Geochimica et cosmochimica Acta 74, 4671-4692
Froelich, P.N., Klinkhammer, G.P., Bender, et al. (1979) Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: sub-oxic diagenesis. Geochimica et Cosmochimica Acta,43, 1075-1090.
Mortimer, R.J. G., Galsworthy, A.M.J., Bottrell, S.H., Wilmot, L.E., Newton, R.J. (2011). Experimental evidence for rapid biotic and abiotic reduction of Fe(III) at low temperatures in salt marsh sediments: a possible mechanism for formation of modern sedimentary siderite concretions. Sedimentology 58, 1514-1529.