Silicate particles are transported through the deep critical zone at Rio Icacos, Puerto Rico

Abstract:

Geophysical surveys and observations of corestones and outcrops reveal that the fractures in rock at meters to tens of meters under the Río Icacos watershed in the Luquillo Critical Zone Observatory, Puerto Rico are complex and laterally discontinuous. Weathering weakens the minerals along fracture surfaces until particles detach and erode. Such subsurface physical erosion is known as seepage erosion. However, neither the geomorphological nor geochemical significance of seepage erosion has not been well documented. To elucidate seepage erosion at Río Icacos, we sampled sediments from a seep (seep sediments) emitting between corestones at ~10 m depth at Río Icacos during Aug-Nov, 2016.

The seep sediments are 100s of microns to mm-sized. The yield of these seep sediments is proportional to runoff/precipitation. Geochemical and microscopic characteristics of seep sediments are close to the materials on the edge of the fractures that subtend the rindlets around corestones. Rindlets contain primary silicates such as plagioclase, biotite, and hornblende but are partially weathered. These cm-sized, onionskin-like rindlets are generated through spheroidal weathering, which is in turn initiated by biotite oxidation that may drive fracturing. In the overlying saprolite and soil, the primary silicates are almost completely weathered. The elemental compositions of most stream sediments lie between that of seep sediments and soils/saprolites, which indicates that the seepage erosion may contribute significantly to total denudation. We infer that seepage erosion could be a key process that sustains the high weathering flux at Río Icacos by exposing the surface of fresh minerals.