Chemical weathering of silicates and carbonates in actively uplifting landscapes

Chemical weathering of silicates and carbonates in actively uplifting landscapes

Silicate weathering
The chemical dissolution (weathering) of calcium and magnesium silicate minerals with carbonic acid removes CO2 from the atmosphere, and regulates the climate on geologic timescales. Current minimum estimates of global CO2 consumption by silicate weathering are around 0.5 Gigatons (GT) of CO2 per year. This rate is equivalent to ~1.4% of the human released CO2 from fossil fuel consumption (36 GT in 2014). There are many challenges in estimating fluxes of chemicals from silicate weathering in global river waters and, in particular, in rivers from active mountain ranges. For example, we do not fully understand how the variability in precipitation and river discharge affects our estimates of chemical fluxes, how important landslides are in exposing rocks to weathering, and how we can predict the importance of changes in climate, tectonic rock-uplift, and different erosion processes for the dissolution of rocks.

Weathering fluxes during a storm event
During my postdoc at the GFZ, I study chemical fluxes from mountain ranges and how they are affected by changes in precipitation and erosion processes. During a field season in Taiwan in May 2017, we were “lucky” to experience a major rain storm during which 1.3 m of rain fell within three days. These rainfall amounts are equivalent to the rain that fell on Houston during Hurricane Harvey! We are currently generating a detailed record of changes in the river chemistry across this short, major storm event.

Landscapes dominated by soil erosion and landsliding
We also sampled river water from catchments around the southern tip of the island. In the southernmost part of the islands, the mountains are characterized by thick soils and the hillslopes are dominantly eroded by slow soil erosion processes. In contrast, further north, the hillslopes are eroded mostly by landslides. By investigating how the chemistry of rivers changes across this transition, we may be able to learn how these two different erosion processes affect the weathering of rocks. Finally, I am attempting to acquire funding for more detailed work on how landslides in active mountain ranges affect the fluxes of inorganic carbon from mountain landscapes.

Carbonate weathering and sulfide oxidation
An important point to note is that the weathering of minerals other than silicates could control the CO2 budget. For example, when organic carbon rich rocks (such as shales) weather, the organic carbon can be oxidized and produce CO2. Moreover, the dissolution of carbonates by sulfuric acid, derived from the oxidation of sulfides (e.g. pyrite), releases CO2 from the solid Earth and can lead to transient increases in atmospheric CO2 concentrations for >10 My. Together with Albert Galy (CRPG, France) and Jordon Hemingway (Harvard University, USA) we will attempt to measure the relative importance of carbonate and silicate weathering, and the contribution of sulfuric acid to the weathering budget. In turn, the mobilization of organic carbon is studied by collaborators including Robert Hilton (Durham University), Joel Scheingross (GFZ), Dirk Sachse (GFZ) and others.