Abstract
Recent work has suggested that weathering processes occurring in the
subsurface produce the majority of silicate weathering products discharged to the
world’s oceans, thereby exerting a primary control on global temperature via the
well-known positive feedback between silicate weathering and CO2. In addition,
chemical and physical weathering processes deep within the critical zone create
aquifers and control groundwater chemistry, watershed geometry and regolith
formation rates. Despite this, most weathering studies are restricted to the shallow
critical zone (e.g., soils, outcrops). Here we investigate the chemical weathering,
fracturing and geomorphology of the deep critical zone in the Bisley watershed in the
Luquillo Critical Zone Observatory, Puerto Rico, from two boreholes drilled to 37.2
and 27.0 m depth, from which continuous core samples were taken. Corestones
exposed aboveground were also sampled. Weathered rinds developed on exposed
corestones and along fracture surfaces on subsurface rocks slough off of exposed
corestones once rinds attain a thickness up to ~1 cm, preventing the corestones
from rounding due to diffusion limitation. Such corestones at the land surface are
assumed to be what remains after exhumation of similar, fractured bedrock pieces
that were observed in the drilled cores between thick layers of regolith. Some of
these subsurface corestones are massive and others are highly fractured, whereas
aboveground corestones are generally massive with little to no apparent fracturing.
Subsurface corestones are larger and less fractured in the borehole drilled on a road
where it crosses a ridge compared to the borehole drilled where the road crosses the
stream channel. Both borehole profiles indicate that the weathering zone extends to
well below the stream channel in this upland catchment; hence weathering depth is
not controlled by the stream level within the catchment and not all of the water in the
watershed is discharged to the stream.
