Lead supervisor: Ed Tipper, Earth Sciences
Co-supervisor: Jotis Baronas, University of Durham
Brief summary:
Chemical weathering is thought to regulate climate on long time-scales; terrestrial sediments provide a largely untapped record of this through time.
Importance of the area of research concerned:
Chemical weathering is the process by which rocks dissolve in rainwater, which is naturally acidic. This is because atmospheric carbon dioxide dissolves in rain to form carbonic acid, and the rainwater interacts with rocks making them dissolve. The dissolved carbon dioxide becomes trapped in river and seawater, as bicarbonate, where it resides stably for thousands, or tens of thousands of years, and is then stored permanently in a mineral form as calcium carbonate. Rock dissolution or chemical weathering is a major process in the global carbon cycle and it is thought that this terrestrial chemical weathering of rocks, and subsequent burial of carbon as calcium carbonate, acts as the feedback which has controlled the carbon cycle and thus climate over Earth history. Our work on river water carbon transfer in SE Asian rivers suggests they are carbon “hot-spots”. The problem with waters is that they are “here today, gone tomorrow” and only reflect dissolution rates over very short time-scales. River sediments on the other hand are transported at much slower rates and could provide a time-integrated of chemical weathering rates at the spatial scale of a continent.
Project summary :
We have one of the largest collections of suspended sediment from large rivers in the world. The student will determine the time-integrated weathering fluxes at a millennial time-scale, by exploiting changes in the chemistry of suspended sediment during transit from mountain front to river mouth over the residence time of the sediment in major sedimentary basins. Sediment records are the only way to track weathering fluxes at millennial timescales at continental scales. The method is similar to determining the chemical index of alteration in a soil profile. However, the key advantage of suspended sediments is that they integrate the spatial area and geological complexity of entire basins, unlike soil profiles. The student will make new measurements, and will model and interpret the data.
What will the student do?:
The student will determine the mineralogy and mineral composition of the sediments using XRD, Qemscan and EPMA. The mineralogy is key to unlocking the major and trace element chemistry of suspended sediments and hence the chemical weathering signal. Isotope ratios such as Nd and Sr will be used to determine the sediment provenance. The core of the project will be to develop new modelling techniques using for example the R or Python programming languages to evaluate the change in sediment composition during transport and deposition that can be attributed to chemical weathering over the transport time-scale of the sediment. The difference in suspended sediment composition between the mountains and river mouths therefore corresponds to the degree of weathering from source to mouth integrated over the time-scale of sediment transport provided that the source of the sediment remains the same, mainly delivered by erosion in the mountainous headwaters. Subject to current logistical constraints, additional fieldwork may be possible if the student is keen. There will be opportunities to present the work at national and international conferences such as Goldschmidt and the AGU.
References - references should provide further reading about the project:
Baronas et al, 2020. Integrating Suspended Sediment Flux in Large Alluvial River Channels: Application of a Synoptic Rouse-Based Model to the Irrawaddy and Salween Rivers. Journal of Geophysical Research, vol. 125, pp e2020JF005554., doi: 10.1029/2020JF005554
Lupker et al, 2012. Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin). Geochim. et Cosmochim. Acta, vol. 84, pp 410-432 doi: 10.1016/j.gca.2012.02.001
Bouchez et al, 2012. Floodplains of large rivers: Weathering reactors or simple silos? Chemical. Geol., vol 332-333, pp166-184, https://doi.org/10.1016/j.chemgeo.2012.09.032
Applying
You can find out about applying for this project on the Department of Earth Sciences page.