CARBON AND NUTRIENT EXPORT DYNAMICS IN SMALL MOUNTAINOUS WATERSHEDS PRONE TO LANDSLIDING

Small mountainous rivers (SMRs) are recognized as an important component in the global carbon cycle as they transport a disproportionately large fraction of sediment and organic carbon to the world ocean. To distinguish between different sources of the particulate organic carbon (POC) as well as between fossil versus modern POC is important to predict the likelihood of oxidation of organic carbon within the fluvial system, and for burial and sequestration. Carbon characterization also provides a way to link sources to delivery mechanisms such as landsliding, sheet wash, and gully erosion. While tectonic and precipitation-triggered landsliding is important to carbon delivery in many SMRs, little is yet known about how landsliding affects carbon in watersheds altered by human activities (e.g. deforestation). This study examines the impact of landsliding on carbon and nutrient export dynamics.

The Sierra de las Minas, located at the North American-Caribbean plate boundary in central Guatemala, provides an ideal site for landslide research. In June–July 2009, water, sediment and bedrock samples were collected from 34 sites within three adjacent watersheds with various degrees of landsliding within and among the watersheds. In addition, water and sediment samples were collected before and after a rain event on July 27, 2009. The pH, temperature, dissolved oxygen and conductivity were measured at the time of sampling. Total suspended sediment (TSS) was determined gravimetrically for each sampling event. TSS in landslide dominated watersheds had a mean TSS concentration of 7.36 ± 1.47 mg/L (± 1 SE) compared to 2.22 ± 0.153 mg/L for areas with minimal landsliding. Water chemistry in areas with landsliding compared to those with minimal landsliding showed a difference in watershed response before and after the storm event. Areas with landsliding yielded a 2.5 fold increase in total loading of major cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) while areas with minimal landsliding yielded increase up to 68% or in some areas decreased. Future analysis of water, soil, suspended sediment, and rock samples for total organic carbon will be conducted in order to determine the source and routing of organic carbon.