Fig. 1.

(Upper) Roadcut exposure of the Rio Blanco Quartz Diorite, used for illustrative purposes to conceptualize the subsurface weathering system at Guaba Ridge within the Rio Icacos watershed of the Luquillo Critical Zone Observatory in Puerto Rico. The rindlet zone, approximately delineated between the solid line (bedrock–rindlet interface) and the dashed line (rindlet–saprolite interface), overlies the corestones of bedrock and is the zone of active weathering targeted in this study. (Scale bar, 10 cm.) Upper, Inset shows a plan view of the rindlet zone exposed elsewhere. (Lower) Total 0.5 M HCl extractable Fe(II) (red circles) and ATP content (blue squares) of the actual subsurface regolith obtained by hand auger atop Guaba Ridge (core A), including soil, saprolite, and the outer rindlet zone, which was partially penetrated with auger refusal occurring prior to reaching the bedrock–rindlet interface (note that the subsurface rindlet zone is substantially thicker than that revealed by the roadcut). Data points and error bars denote the mean and range of triplicate measurements.