I apologize for the dearth of posts in the past few weeks. It has not been a lack of things to write about. I’ve been busy, but mostly I had no desire to sit at my computer any longer than I already am when I am working. This post is all about what I mean when I say “work”. I am going to talk about the background for research project that I’ve been working on for the past several weeks.
[This is a long post, with no pictures, about the geology of Taiwan. If that doesn’t interest you, please don’t feel the need to read this! I hope to have some other posts coming soon about other topics.]
I wrote this both for you who might be interested, and for me. As you’ll see, this project is advantageous because it builds directly from a solid basis of existing knowledge and large databases. Although this is generally a good thing, I’ve been facing the difficulties keeping everything straight when it comes to what we know already know about these data, why, when, how, and then how what I am working on is new and important. The last few days I’ve been reviewing everything that I can to try to construct a good, solid background and I hope that writing it up may help me remember things. The language may be technical, but I’ll do my best to keep things low on the jargon.
In summary, this is the story of sediment carried by the rivers of the island of Taiwan and how typhoons influence this process. Understanding sediment carried by rivers has several purposes. This material represents not only movement of land mass (ie. erosion rate vs. uplift rate) but also transfers of individual elements through the global system. Thus quantifying this sediment movement is an important part of tracking fluxes of important elements between reservoirs on Earth. Specifically, the significant movement of carbon as part of this process is occurring and what we hope to better understand to better characterize the global carbon cycle. Understanding the carbon cycle is critical for the future; we have significantly perturbed the natural processes through combustion of fossil fuels and need to understand how the system will react.
Taiwan is an interesting place geologically as it is an orogenic region (area of mountain-building) that is still undergoing rapid uplift. Specifically, the island of Taiwan can be described as Luzon Volcanic Arc collision with a package of sediments that have been pushed up out of the sea by the Philippine oceanic plate with the Eurasian continental plate convergence (Teng, 1990). To the north of Taiwan, the Philippine plate is being pushed under the Eurasian plate (subducting), whereas to the south the opposite is happening and the Eurasian plate is subducting underneath the Philippine plate. As the Philippine plate approached from the east, it plowed sediments that were deposited off the coast of the Asian continent up out of the ocean. These uplifted sediments are most Taiwan’s land mass. The Luzon Arc is/was situated on the Philippine plate, but as the two plates have moved together, islands from the Luzon arc have crashed into the built-up sediments and are now the east side of Taiwan. As a result, Taiwan is a high-relief island composed largely of sedimentary and metamorphic rocks that is still undergoing uplift.
These characteristics have placed Taiwan into an interesting geological classification known as a “high-standing island”, with the good company of Indonesia, Malaysia, Papua New Guinea, Philippines and New Zealand. The high-standing islands of the Pacific have been the subject of much research because of their disproportionately high contribution to sediment movement to the ocean (Milliman & Syvitski, 1992). Simply put, geologic processes are constantly recycling material: just as mountains are pushed up, they also erode back into the ocean. And just as the rates of uplift across the globe vary from place to place, so do the rates of erosional processes. A common method to quantify erosion (although exactly how to quantify erosion rates is not a straightforward matter – I won’t get into that) is through measurements of material carried in streams and rivers. The sediments and other materials carried by these water bodies represent movement of material from their watersheds into the ocean.
When it comes to this sediment movement from land to sea, Taiwan and the other high-standing islands in the Pacific are the world’s superstars – combined they contribute approximately ~33% of total land-to-sea sediment movement (Lyons et al., 2002; Milliman & Syvitski, 1992) . Taiwan by itself is probably putting in a formidable 1-3% (which doesn’t sound like a lot at first, but glance at a map and consider how little of the Earth’s land surface is “Taiwan” before you scoff). Taiwan’s mountains are eroding at a rate of 3-6mm/year (Dadson et al., 2003)*. Furthermore, this sediment is likely carrying significant amounts of C: the rocks of the earth’s crust contain significant stores, as well as the more recently fixed organic carbon in soils and plant matter that may also be swept along as sediments in rivers (Sundquist & Visser, 2004). When it comes to particulate organic carbon (a subset, but significant subset of the total pool of carbon), the high standing islands are possibly contributing ~35% of the global transfer from land to sea (Lyons et al., 2002).
However, these estimates still require more refinement, primarily for two reasons: 1) sediment export from Taiwan is highly episodic; 2) not all riverine POC is created equal when it comes to the goal of better understanding the influence of this sediment movement on the global carbon cycle.
Firstly, several factors contribute to Taiwan’s large yields of sediment export (that is, sediment carried per area). The island is in a subtropical climate with high rainfall. This rain provides enough water energy in the rivers to transport sediments, and therefore the erosional processes are likely to be supply-limited. Thus, the common but stochastic events of earthquakes, typhoons and their associated landslides are essential for providing material to rivers to be transported. Sediment transport rates change drastically during one of these events (Dadson et al., 2003; Kao et al., 2005). Due to the episodic nature of these events, total sediment movement for a period of time (eg, annually) is may be largely dependent on the number of these large events. Therefore, estimating sediment discharge for period of time requires data that are representative and can be generalized for other periods of time than when the sampling occurred.
Secondly, the POC carried in river sediments comes from a variety of sources, and only after properly allocating this POC to the correct source can we understand this process’s influence on the carbon cycle. An obvious source of organic carbon are the living organisms on the land of these river watersheds: pieces of plants and other organisms. Furthermore, there are organic compounds in the soils that erode into the rivers. And finally, there are rocks that can also be carried as “sediments”. While organic carbon content is not often high in rocks, it can be stored in sedimentary rocks if bits and pieces of formerly living creatures are solidified into rock. Of course, as a geological process, organic carbon storage in rock is generally considered a long-term affair, and this organic carbon is aptly called “fossil carbon”, as it is essentially very old, dead things that have become rock. Taiwan’s high mountains are sedimentary and metamorphic rocks that are relatively high in fossil carbon (Hilton et al., 2008). The plants and soil contributions to POC are “non-fossil” carbon. Non-fossil carbon will not necessarily become fossil carbon with age – only if it is buried and lithified can it enter the longer-term storage pool of fossil carbon.
Why is it important to distinguish between these two? On a geologic time scale, the plant and soil C has recently been fixed from the atmospheric pool through primary productivity whereas the fossil carbon has been stored in the rocks for millions of years. The sedimentation pattern of Taiwan likely leads to a quick route between initial physical weathering to burial in the deep sea and lithification. In the past, mountain-building events were consider a way that fossil organic C trapped in rocks is exhumed, oxidized and finally released to the atmosphere. However, it is possible that in the case of Taiwan, this fossil C doesn’t have the chance for oxidization before reburial. Thus, as Taiwan is pumping this POC to long-term storage on the bottom of the ocean, we want to know how much of it is C that has been out of the atmosphere for millennia (fossil) and how much of it may have just recently been in the atmosphere (nonfossil). Is rock storage even longer than we thought before? Is Taiwan acting as a giant carbon sink, pulling C from the atmosphere through primary productivity and then sloughing it off into the ocean?
Back to the data – how do we hope to answer these questions? The most extensive datasets about Taiwanese rivers has been collected by the Water Resources Agency and includes data for many rivers across the island going back for decades (the earliest records are in the first half of the 20th century). The length of this record will help to create an understanding of “average” which can help avoid the issues presented by the dependence on typhoons and earthquakes, which vary from year to year. Having similar data for rivers all across the island will also allow to understand Taiwan more accurately rather than having to extrapolate from a single location on the island. For these reasons, this is an excellent data set.
But unfortunately, the majority of these data is not necessarily about sediment at all, but is actually daily measurements of water discharged by the river. By using selected samples that have both suspended sediment values and river discharge measurements, we can summarize the relationship between the two and allow for extrapolation of about of sediment carried by certain water discharge rates (Kao et al., 2005). Through other experiments with limited, but more detailed sampling, we also understand the relationships between POC and discharge, and understand whether this POC is fossil or non-fossil (Hilton et al., 2008; Hilton et al., 2012; Hilton et al., 2011; Hovius et al., 2011).
However, the extreme events of the earthquakes and typhoons most likely break these relationships. Consider a typhoon in comparison with a normal storm. The typhoon will drop lots of water in a relatively short amount of time, as well as bring huge winds which may mobilize different material than a gentle rain. For example, a deep landslide may put a lot more rock (fossil C) into the river than water flowing over the land surface, which will likely be carrying soil and plant particles (Hilton et al., 2008). Thus, separate sampling to characterize typhoon water discharge and sediment content (POC, fossil and nonfossil) have been carried out (Hilton et al., 2008). However this sampling during typhoons is extremely dangerous – hello, it’s a typhoon! – and therefore we have only limited data available to understand these events. Still, these events have given us some insight into how POC content and contributions change during large storms.
And now we finally arrive at where I am considering the data. Climate projections have estimated that there will be changes in typhoon behavior in the future. How will this change how much of the sediment, POC fossil and nonfossil movement is occurring in Taiwan? Within the span of the data that we have already collected, is it possible to see any changes occurring? What will happen for the different possible changes, such as shifts in typhoon frequency, intensity or location?
Someone whom I am working with phrased it kind of like this: the carbon cycling is speeding up before we even understand it. And that may be true, but I find it more dramatic that, understand it or not, at this point, Taiwan is along for the ride no matter what. So we might as well try to figure some things out as we go.
*for context, consider these uplift rates: Andes 0.6-3mm/yr; Colorado plateau 0.8mm/yr; Sierra Nevada 1-2 mm/yr; Himalayas 10mm/yr and Taiwan 5-7mm/yr. So when it comes to a geological timescale, things are happening pretty quickly in Taiwan compared to the rest of the world.
Dadson, S. J., Hovius, N., Chen, H., Dade, W. B., Hsieh, M.-L., Willett, S. D., Hu, J.-C., et al. (2003). Links between erosion, runoff variability and seismicity in the Taiwan orogen. Nature, 426(6967), 648–51. doi:10.1038/nature02150
Hilton, R. G., Galy, A., & Hovius, N. (2008). Riverine particulate organic carbon from an active mountain belt: Importance of landslides. Global Biogeochemical Cycles, 22(1), 1–12. doi:10.1029/2006GB002905
Hilton, R. G., Galy, A., Hovius, N., Chen, M.-C., Horng, M.-J., & Chen, H. (2008). Tropical-cyclone-driven erosion of the terrestrial biosphere from mountains. Nature Geoscience, 1(11), 759–762. doi:10.1038/ngeo333
Hilton, R. G., Galy, A., Hovius, N., Horng, M.-J., & Chen, H. (2011). Efficient transport of fossil organic carbon to the ocean by steep mountain rivers: An orogenic carbon sequestration mechanism. Geology, 39(1), 71–74. doi:10.1130/G31352.1
Hilton, R. G., Galy, A., Hovius, N., Kao, S.-J., Horng, M.-J., & Chen, H. (2012). Climatic and geomorphic controls on the erosion of terrestrial biomass from subtropical mountain forest. Global Biogeochemical Cycles, 26(3), 1–12. doi:10.1029/2012GB004314
Hovius, N., Galy, A., Hilton, R. G., Sparkes, R., Smith, J., Shuh-Ji, K., Hongey, C., et al. (2011). Erosion-driven drawdown of atmospheric carbon dioxide: The organic pathway. Applied Geochemistry, 26, S285–S287. doi:10.1016/j.apgeochem.2011.03.082
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