THE ORIGIN OF STREAMBED SEDIMENTS IN A LANDSLIDE DOMINATED MOUNTAIN RANGE

Landslides are a dominant geomorphic process around the world and especially in areas that receive large amounts of rainfall or are tectonically active. Landslide activity influences biogeochemical cycles by mobilizing plant biomass, soil, and regolith along the fluvial-hillslope interface. The north slope of the Sierra de las Minas in Guatemala is a tectonically active region that receives 2500–5000 mm of rainfall annually. As a result, landslide activity is frequent and ubiquitous throughout the mountain range. Here, we focus on the fate of landslide-mobilized organic material by exploring chemical similarities between bedrock shale and streambed sediment samples.

Ratios of carbon to nitrogen (C/N) and δ¹³C of organic carbon (C_org) in bedrock shale samples are similar to those in streambed sediment samples. Plots of δ¹³C vs. C/N show most streambed sediments fall in the same range as the shale samples (δ¹³C = -22.5 to -27.2‰, C/N = 2.04 to 8.45). A plot of δ¹³C vs. 1/C_org reveals that all but three streambed sediment samples have less organic carbon than the shale bedrock samples. This apparent dilution of C_org from shale to streambed sediments supports a mixing of the shale and granite lithologies that make up the bedrock of the mountains. The three streambed sediment samples with higher C_org concentrations than the others are also more depleted in δ¹³C and have higher C/N ratios than the shale samples (δ¹³C = -29.0 to -29.6‰, C/N = 13.0 to 18.9). These chemical differences suggest the possibility of landslide delivery and persistence of a soil source to and within the streambeds of these three low-order streams.

If soil is not a major contributor to the streambed sediments in a region as prone to landslides as the north slope of the Sierra de las Minas, then it is likely that any soil mobilized during landslide events is quickly exported by the fluvial network. Soil exported by the fluvial network draining the north slope of the Sierra de las Minas would likely be delivered to and buried in Lago Izabal. This burial of organic matter combined with eventual soil redevelopment and plant regrowth on landslide scars can be a sink of atmospheric carbon. The timescale for this sink is dependent on both the rate of landslide activity and the rate of regrowth on landslide scars.