T19 Tropical Small Mountainous River Biogeochemistry: Terrestrial Losses, Internal Processing, Coastal Inputs, and Marine Burial

Sunday, 9 October 2011
8:00 AM - 12:00 PM
Room 102BC

DRY SEASON FLUVIAL CARBON DYNAMICS RELATED TO LANDSLIDES IN THE SIERRA DE LAS MINAS, GUATEMALA

Paper number: T19-12
Program: Technical Sessions
Time: 10:55 AM – 11:10 AM

Authors

Abstract

Landslides mobilize and bury material, including organic carbon, along the fluvial-hillslope interface. Here, we seek to understand the role fluvial processes play in the fate of this buried material. Stream water and bed sediment samples were collected during the dry season from the south side of the Sierra de las Minas, Guatemala, where landslides triggered by Hurricane Mitch (1998) are common. The Sierra de las Minas are also bordered by two active fault systems that have triggered landslides: the Polochíc fault system to the north, and the Motagua fault system to the south. Dissolved organic carbon concentrations (DOC) range from 1.13 mg/L to 1.61 mg/L with a mean concentration of 1.34 mg/L (n=11). Streambed particulate organic carbon (SPOC) concentrations range from 0.10 weight% to 3.13 weight% with a mean concentration of 0.80 weight% (n=11). Stream dissolved inorganic carbon (DIC) concentrations range from 3.12 mg/L to 29.46 mg/L with a mean concentration of 10.02 mg/L (n=11). No measurable suspended solids were observed in any of the streams sampled, thus no measurable particulate organic carbon transport was observed.

We found no correlation between stream DOC and SPOC concentrations, suggesting little, if any, in-stream erosion of SPOC. With the exception of one sample—collected from a stream draining an active limestone quarry—stream DOC concentrations are negatively correlated both with stream DIC and with total cation concentrations (r2 = 0.64, p = 0.006 and r2 = 0.74, p = 0.002, respectively). Two samples from one stream and three samples from another stream illustrate a downstream loss of DOC and a gain of DIC, but the molar ratio between DOC lost and DIC gained is not 1:1, nor does DOC loss and DIC gain correspond with any decrease in stream dissolved oxygen concentrations. Total cation concentrations, primarily Ca and Mg, also increase downstream as DOC concentrations decrease. The trends observed among stream DOC, DIC, and total cation concentrations suggest a groundwater dilution of DOC rather than in-stream DOC mineralization. We speculate that organic carbon buried along the fluvial-hillslope interface is poorly mobilized by dry-season hydrological processes on the southern slopes of the Sierra de las Minas, Guatemala.