The biogeochemistry of a landslide-dominated catchment in the Luquillo Mountains of eastern Puerto Rico

U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303-1066, USA

Abstract

The U.S. Geological Survey Water Energy and Biogeochemical Budgets (WEBB) Project has been investigating the effects of hillslope-scale physical and biological processes on the biogeochemistry of weathering and erosion in the forested Luquillo Mountains of Eastern Puerto Rico and in adjacent agricultural catchments to the west of the Mountains. This is a landscape where mass wasting is dominated by landslides (soil avalanches and slips). These landslides remove soft soil down to a hard saprolite at a natural rate of roughly one percent of the land surface per century. This process may generate more than 90 percent of the solid load of the rivers.

A single landslide on a small tributary of the Icacos River was instrumented to monitor the biogeochemical transformations of water moving under the landslide and through the debris deposits below the slide. An adjacent tributary catchment lacking large slides, the Guabá, served as a control. The flow path was about 100 meters long starting on a ridge, where an array of zero-tension lysimeters and rainfall and throughfall collectors was installed. Approximately 40 meters along the flow path, a well was drilled into almost-fresh exfoliations within the quartz diorite bedrock under the landslide. Water eventually emerged below the slide debris, and the resulting stream was also sampled.

Rainwater chemistry was dominated by seasalt. Organic anions were important to the charge balance in the throughfall and lysimeter water. By the time the water reached the well, it contained little organic carbon and was instead, at times, a dilute nitric acid solution. None of the plants in inventories of macrophytes along the flow path were associated with nitrogen fixation, leaving cyanobacteria in lichens and ubiquitous bacterial coatings as the likely source. This water has considerably less sulfur than would be expected from a seasalt source. Upon passing through the landslide debris, the nitrate was converted to nitrogen and ammonia, presumably through action of bacteria associated with buried organic debris, and bicarbonate became the dominant anion in solution. Iron-II mobilized in the reducing environments in the debris precipitated out in the stream.

The water from the Guabá had less combined nitrogen and dissolved iron and more sulfur than waters from the landslide stream. The landslide face is an especially fertile setting, presumably due to the high concentrations of phosphate in the deeper soil horizons that had been exposed and the high nitrate concentrations in the deeper groundwater.

Landslides on igneous or metamorphic bedrock in humid climates are an especially effective way to generate newly fixed particulate organic carbon (POC) along with the sediment to transport, protect, and bury that carbon. Rates of POC fixation exceed the fixation of carbon as bicarbonate ion by chemical weathering. Globally, the landslide cycle may therefore play an important role in the carbon cycle during times in the Earth's history when humid mountainous regions are extensive.

• WEBB Project Bibliography
• WEBB Project Chief Home Page
• Caribbean Water Science Center home page