July 17, 2017

The science before (and during) the storm – Part 1

Posted by larryohanlon

Setting the Scene

By Robert Emberson

Recently I was fortunate to participate in a fieldtrip to one of most interesting places I’ve ever worked – the central mountains of Taiwan. We were looking to sample the products of chemical weathering (the dissolution of rocks by fluid) within landslides, as well as their impact on the chemistry of the highly dynamic rivers draining these tropical mountains. We ended up getting caught in a huge rainstorm while we were there which dramatically changed our plans; in this first post, I’ll explain what we are researching, and why Taiwan is so fascinating, and in the follow-up post I’ll relate how that affected our sampling and the lessons we took from it.

You may be wondering how landslides can be important to chemical weathering. One way of thinking about them is as analogous to a coffee grinder. Chemical dissolution of rock relies on a number of factors, but much like trying to make coffee with whole beans, solid chunks of bedrock don’t dissolve very fast. Large landslides, like the one shown in the video below, smash and break large amounts of fresh bedrock, and much like ground coffee beans the resulting concentration of dissolved elements (or coffee!) in any water that percolates through will be much higher.

As such, we’re investigating the extent to which the physical process of landsliding affects chemical weathering in fast-eroding mountains. Chemical weathering is a key part of the cycles of elements into the ocean, and can play an important long-term role in regulating atmospheric carbon dioxide.

Taiwan is a great place to study the links between physical and chemical processes; because of strong seismic activity and regular, intense, tropical cyclones, there are regularly big triggers for landslides. The landscape in the mountains is shaped to a great extent by the largest of these events, which generate such extensive landsliding that the rivers fill with sediment for many years:

River valley in South West Taiwan, filled with sediment derived from landslides.

The presence of so many landslides gives us the opportunity to study them both as individual pieces in the landscape as well as how they influence the chemical load of large rivers.

One of the largest recent storms in Taiwan was typhoon Morakot, in the summer of 2009. In some places as much as 2.5 metres of rain fell in only 3 days, which led to widespread landsliding and collapse of hillslopes into river valleys. Some of these landslides have been discussed on AGU blogs before (e.g. http://blogs.agu.org/landslideblog/2017/05/11/taiwan-landslide-hotspots/) but it is striking to see the effects close up, particularly where they have impacted settlements:

While it was fascinating (and somewhat sobering) to see the effect of this giant storm on the landscape and the people living there, we hadn’t planned on encountering anything like it while we were in the field. As it turned out, the best-made plans don’t always pan out as one might expect. In the next post, I’ll share what happened when we were caught in one such huge flood.

Robert Emberson was working with Dr Aaron Bufe in Taiwan on research funded by the GFZ in Potsdam, Germany, where Dr Bufe currently works. Robert completed his PhD work also at the GFZ in December 2016, and is currently working as a freelance science writer in British Columbia, Canada. Questions or comments can be directed to @RobertEmberson or via www.robertemberson.com