Causes for slow weathering and erosion in the steep, warm monsoon-subjected highlands of Sri Lanka

Tropical soils, formed in low-erosion rate settings, are key to understanding the relationship between weathering and erosion. These soils are typically formed from saprolite. Saprolite is an in-situ, clay-rich layer, formed isovolumetrically from bedrock. In this study, we address three main questions: How and through which weathering processes is bedrock converted to saprolite? Which chemical and mineralogical processes lead to deep and highly weathered saprolite/soil profiles? Is there a stable isotope system with which we can trace the chemical processes?

The Sri Lankan highlands are a key area, as denudation rates and chemical weathering rates are surprisingly low for a landscape situated in a mountainous area exposed to wet, tropical climate with two monsoon seasons. Since this region is not tectonically active, all weathering processes are expected to be driven by climate. A 10 m weathering profile in the Sri Lankan highlands, formed over charnockite bedrock (left picture below), is also reflected on a small scale in a 19 cm profile through a corestone formed at the weathering front in the deep along rock fractures (picture to the right below). There, the main weathering reactions take place and bedrock is converted to saprolite.

 

 

At the top of the corestone, half of the total denudation is contributed by chemical processes and the other half by erosion, as is indicated by a CDF (chemical depletion fraction) of saprolite to unweathered bedrock of 0.5. In addition, calculated element mass transfer coefficients (τ) show strong depletion of several elements directly above the core stone and within the saprolite profile. Consistent with textural microscopic inspections, the following main mineral weathering sequence can be inferred (also illustrated in the BSE pictures below): pyroxene - plagioclase - biotite - k-feldspars

We aim at identifying the processes that convert rock to saprolite at the weathering front (formation of secondary porosity, oxidation of Fe-bearing minerals, rock-eating microbes and mycorrhiza). To do so we perform chemical mass balances across the weathering front. We identify the chemical reactions at the weathering front in the corestone and throughout the entire saprolite profile. Furthermore, detailed analyses of the inter-mineral reactions and of dissolving primary minerals and precipitating secondary minerals will be performed. The second phase of the project will be focused on the determination of a stable isotope system (e. g. Mg, Si, Li) that can serve as a fingerprint for weathering conditions in this kind of geological settings.

The project is in cooperation with the DFG Graduate School GRK 1364 of the University of Potsdam (http://www.geo.uni-potsdam.de/graduiertenkolleg.html) and
Dr. Tilak Hewawasam of the University of Peradeniya, Sri Lanka (http://www.pdn.ac.lk/arts//geography/staff/drhewa.html)

References:

Hewawasam, T., von Blanckenburg, F., Bouchez, J., Dixon, J. L., Schuessler, J. A., (2013). Chemical weathering of deep saprolite in the tropical highlands of Sri Lanka. Geochimica et Cosmochimica Acta, vol. 118, pp. 202-230.

von Blanckenburg, F., Hewawasam, T., Kubik, P., (2004). Cosmogenic nuclide evidence for low weathering and denudation in the wet, tropical highlands of Sri Lanka. J. Geophys. Res., vol. 109, pp. 1-22.

Hewawasam, T., von Blanckenburg, F., Schaller, M., Kubik, P., (2003). Increase of human over natural erosion rates in tropical highlands constrained by cosmogenic nuclides. Geology, v. 31, no. 7, pp. 597-600.

Contact

Profile photo of  Dipl.-Geow. Ricarda Behrens

Dipl.-Geow. Ricarda Behrens
Geochemistry of the Earth's surface

Telegrafenberg
Building E, room 225
14473 Potsdam
tel. +49 331 288-28645