Admissions > PhD by research > Research Projects > Probing the Depths of the Critical Zone: Weathering of Deep Fractured Bedrock in a Tropical Environment

Probing the Depths of the Critical Zone: Weathering of

Deep Fractured Bedrock in a Tropical Environment

Supervisor: Dr Heather L. Buss



Ca and Mg are released from silicate minerals during chemical weathering, react with atmospheric CO2, and are deposited as carbonate in the oceans, regulating global climate on geological time scales [1]. Thus to estimate and predict CO2 consumption rates, it is important to identify and quantify the sources, processes, and fluxes of Ca and Mg that are discharged to the world’s oceans via rivers. Tropical watersheds are of particular interest because mineral weathering in these environments are the source for most of the material discharged to the oceans globally. In fact, although the tropics only cover about 25% of the land surface, they contribute 50% of the water, 38% of the dissolved ions, and 65% of the dissolved Si to the world’s oceans [2-3] and thus are disproportionately important to global weathering fluxes.

Fig 1: Exposed weathering profile in the
Luquillo Mountains of Puerto Rico

Thick soils in tropical forests often isolate bedrock weathering processes from the surface environment [4]. However, estimations of bedrock weathering rates are frequently made based on chemical measurements in soils. These rates are then used to interpret and predict effects of environmental changes on nutrient cycles, weathering fluxes, and global climate. Weathering reactions and rates can be completely different in soil and in fractured bedrock and in many environments, particularly tropical environments, weathering along fractured bedrock may be more important in terms of Ca and Mg fluxes.

In other cases, solute fluxes in rivers and streams are used to calculate weathering rates. However, such indirect methods only reflect modern rates and provide no information about specific mineral sources, weathering mechanisms, or the location of weathering reactions in the watershed, limiting their usefulness for predictive or paleo models. This project will directly examine weathering reactions in fractured bedrock and groundwater zones for comparison to solute fluxes in tropical watersheds of differing lithology under different flow conditions [5].



The student will investigate weathering reactions along bedrock fractures and materials of various stages of weathering including drilled bedrock cores, landslide-exposed rock, saprock, and waters (surface, ground, and precipitation). Data will be collected utilizing a suite of analytical techniques including optical microscopy, scanning electron microscopy, electron microprobe, ICP-MS, ion chromatography, and UV/VIS spectrophotometry, among others.

In cooperation with the US National Science Foundation’s Luquillo Critical Zone Observatory, this studentship will include field work on the Caribbean island of Puerto Rico [e.g., 4-7] and significant interaction with Critical Zone scientists from the US and UK. The student will benefit from the excellent facilities and analytical expertise at the University of Bristol and will learn a variety of concepts and skills drawing from geology, geochemistry, hydrology, and geomorphology in both the field and the laboratory. This project will provide the student with an excellent foundation for a variety of careers in the Earth and environmental sciences.

References

  1. Berner R.A., Lasaga A.C. and Garrels R.M., 1983. The carbonate-silicate geochemical cycle and its effect on the atmospheric carbon dioxide over the past 100 million years. American Journal of Science, 283: 641-683.
  2. Stallard R.F. and Edmond J.M., 1983. Geochemistry of the Amazon: 2. The influence of the geology and weathering environment on the load. Journal of Geophysical Research, 88: 9671-9688.
  3. Meybeck M., 1987. Global chemical weathering of surficial rocks estimated from dissolved river loads. American Journal of Science, 287(5): 401-428.
  4. Buss H.L., Mathur R., White A.F. and Brantley S.L., 2010. Phosphorus and iron cycling in deep saprolite, Luquillo Mountains, Puerto Rico. Chemical Geology, 269: 52-61.
  5. Kurtz A.C., Lugolobi F., and Salvucci G. 2011. Germanium-silicon as a flow path tracer: Application to the Rio Icacos watershed. Water Resources Research, 47: W06516.
  6. Buss H.L, Bruns M.A., Schultz M.J., Moore J., Mathur C.F., and Brantley S.L. 2005. The coupling of biological iron cycling and mineral weathering during saprolite formation, Luquillo Mountains, Puerto Rico. Geobiology, 3(4): 247-260.
  7. Buss H.L., Sak P.B., Webb S.M. and Brantley S.L., 2008. Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing. Geochim. Cosmoch. Acta, 72: 4488-4507.

 

Last updated: 24/11/11