Understanding fracture distribution and its relation to knickpoint evolution in the Rio Icacos watershed (Luquillo Critical Zone Observatory, Puerto Rico) using landscape‐scale hydrogeophysics

Abstract

The Rio Icacos watershed in the Luquillo Mountains (Puerto Rico) is unique due to its extremely rapid weathering rates. The watershed is incised into a quartz diorite that has developed a large knickzone defining the river profile. Regolith thickness within the watershed generally decreases from 20 to 30 m at the ridges to several meters in the quartz diorite‐dominated valley to tens of centimeters near the major river knickpoint, as determined from previous studies. Above the knickzone, we observe spheroidal corestones, but below this weathering is much less apparent. Measured erosion rates from previous studies are also high in the knickzone compared with upper elevations within the river profile. A suite of near‐surface geophysical methods (i.e. ground penetrating radar and terrain conductivity) capable of fast data acquisition in rugged landscapes, was deployed at kilometer scales to characterize critical zone structure. Concentrations of chaotic ground penetrating radar (GPR) reflections and diffraction hyperbolas with low electrical conductivity were observed in vertical zones that outcrop at the land surface as areas of intense fracturing and spheroidally weathered corestones. The width of these fractured and weathered zones showed an increase with proximity to the knickpoint, and was attributed to dilation of these sub‐vertical fractures near the knickpoint, as postulated theoretically by a stress model calculated for the topographic variability across the knickzone in the Rio Icacos, and that shows a release of compressive stress near the knickpoint. We hypothesize that erosion rates increase in the knickzone because of this inferred dilation of fractures. Specifically, opened fractures could enhance access of water and in turn promote spalling, erosion, and spheroidal weathering. This study shows that ground‐based hydrogeophysical methods used at the landscape‐scale (traditionally applied at smaller scales) can be used to explore critical zone architecture at the scales needed to explain the extreme variability in erosion rates across river profiles. © 2018 John Wiley & Sons, Ltd.

Are catchments leaky?

“…Buss et al (2013) states that, "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." In a recent study at the Luquillo CZO, but at a catchment overlaying quartz diorite bedrock, Comas, Wright, Hynek, Fletcher, and Brantley (2019), using geophysical imaging and modeling, also finds bedrock fractures at tens-of-meter below the valley floor, except at knickpoints of stream incision. Deep fracturing and weathering under a humid tropical climate may be the norm, and thus bedrock weathering, and its capacity as long-term CO 2 sink, may be best constrained by accounting for both river and groundwater export of the weathering products.…”

Subsurface particle transport shapes the deep critical zone in a granitoid watershed

“…Ground penetrating radar (GPR) surveys indicated that eroding channels in the hillside leading to the river are usually co-located with vertical zones of enhanced GPR reflection. These were interpreted as deep fracture zones filled with rindletted corestones (Comas et al, 2019). Field measurements across these fracture zones showed coincident decreases in terrain conductivity and increases in magnetic susceptibility (Comas et al, 2019).…”

“…These were interpreted as deep fracture zones filled with rindletted corestones (Comas et al, 2019). Field measurements across these fracture zones showed coincident decreases in terrain conductivity and increases in magnetic susceptibility (Comas et al, 2019). To test what might cause this, we conducted magnetic susceptibility (MS) measurements in the laboratory and observed that the MS values decreased from corestones ≈ rindlets > seep sediments > soils (Table S-1).…”

“…To explore the significance of subsurface particle transport in the Luquillo Mountains, Puerto Rico at a site where such transport has been noted (Harrison et al, 2020), we examined "seep sediments" collected from a seep along a steep slope and stream sediments from stream beds in the Río Icacos watershed. As one of the fastest weathering and eroding granitoid systems in the world (White and Blum, 1995), this site has been well studied (White et al, 1998;Buss et al, 2008;Shanley et al, 2011;Comas et al, 2019), allowing us to elucidate how sub-surface particle transport contributes to shaping the deep critical zone.…”

Subsurface Weathering Revealed in Hillslope‐Integrated Porosity Distributions

“…Therefore, they may not be spatially representative of subsurface weathering at a site. Near‐surface geophysics can augment data from cores, penetrating through the weathering profile in surveys spanning 10 1 –10 3 ‐m horizontal scales (Comas et al, 2019; Parsekian et al, 2015). For example, hillslope‐scale seismic refraction surveys can be used to compute seismic velocity tomograms, which represent horizontal and vertical variations in weathering (e.g., Befus et al, 2011).…”