Large earthquakes can trigger widespread landslides that act to strip particulate organic carbon (POCbiosphere) from the vegetation and soils from mountain forests. This sort of POC is of global significance, because breakdown of the associated carbon has been implicated as a source of CO2 to the atmosphere due to oxidation in the landscape. However, this is at odds with observations that mountain rivers can rapidly erode and export landslide-mobilized POC. Preservation of some of this eroded POCbiosphere in sedimentary deposits could instead store carbon. The 2008 Mw 7.9 Wenchuan (China) earthquake caused more than fifty thousand landslides covering a total area of >396 km2 which stripped an estimated ~14 Megaton of carbon from vegetation and soil, equating to ~10% of the global annual POCbiosphere discharge by rivers and ~15% of the annual CO2 release from volcanoes. Despite this recognition, the erosion of landslide-mobilized POCbiosphere following large earthquakes remains unconstrained, mainly because of lack of relevant data before and after such events.
Using a unique set of river sediment samples collected before and after the 2008 Wenchuan earthquake, the sources of POCbiosphere in rivers and its discharge were measured and the fate and flux of these POCbiosphere was then assessed following the earthquake. The daily to multi-annual impacts on river POCbiosphere discharge were addressed for the Zagunao River, a major tributary of the Min Jiang. Finally the erosion of POCbiosphere over decades was modeled, accounting for both its river export and degradation.
Fig. 1 (A) Mixture of the biospheric POC and rock-derived petrogenic POC end-members in the Zagunao River before and after the 2008 Mw 7.9 Wenchuan earthquake. (B) Discharge of POCbiosphere quantified as averaged downstream POCbiosphere gain (the ratio of downstream to upstream POCbiosphere flux from two nested gauging stations on the Zagunao River). (Image by JIN, et al.)
In a latest online scientific paper in the journal Geology, doubled riverine POCbiosphere export following the 2008 Wenchuan earthquake was demonstrated. The increase in downstream POCbiosphere input observed immediately following the earthquake is sustained over the three years which followed. The lack of a declining trend in downstream POCbiosphere gain following the earthquake suggests that export of POCbiosphere mobilized by the earthquake may have been limited by available runoff across this reach. A single-pool model calculation suggests that erosion and river transport can rapidly move carbon from landslides and export it from the mountains, before it is released as CO2.
Fig. 2 Modeled time evolution of earthquake-mobilized POCbiosphere residing in the Zagunao River catchment. (Image by JIN, et al.)
The enhanced POCbiosphere discharge following a large earthquake may thus be prone to efficient sedimentary burial. These results challenge the assumption that carbon stripped by earthquake-triggered landslides is a source of CO2. Instead, if exported carbon is buried and preserved, earthquakes may contribute to sequestration of carbon in sedimentary rocks over geologic time, together with the potential for large earthquakes to increase CO2 consumption via silicate-derived alkalinity. These processes link active tectonics to CO2 drawdown, providing further insight into the connections of mountain building, erosion, and weathering to the global carbon cycle.
This is the third Geology paper after “Wang et al. Controls on fluvial evacuation of sediment from earthquake-triggered landslides. Geology, 2015, 43(2): 115-118” and “Jin et al. Seismically enhanced solute fluxes in the Yangtze River headwaters following the A.D. 2008 Wenchuan earthquake. Geology 44(1): 47-50” by the same team. This international team is led by professor JIN Zhangdong at Institute of Earth Environment, Chinese Academy of Sciences, the team members including Dr. HILTON Robert, professor DENSMORE Alexander and GROCKE Darren at Durham University, Dr. WEST Joshua and LI Gen at University of Southern California, XU Xiaomei at University of California, Irvine, and his two PhD students. This work was funded by 973 Program, NSFC, U.S. NSF, and CAS.
Contact: JIN Zhangdong, State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China