Deep hillslope scale inventory of mineral surface area and mineralogy in Christina River Basin Critical Zone Observatory

Abstract:

We quantify the vertical and lateral generation and distribution of mineral specific surface area (SSA) on a hillslope scale. SSA is a critical interface for biogeochemical interactions that span every Critical Zone discipline with significant feedbacks between SSA and landscape evolution. We strategically sampled two 21-meter deep drill cores of greenschist bedrock in the Christina River Basin Critical Zone Observatory, located in the Piedmont physiographic province. Drill sites were positioned at the ridge top and an interfluve with 11 meters of elevation difference to assess hillslope dynamics among the processes responsible for SSA generation. Our SSA data reveal that the inventory of SSA reaches an asymptotic relationship with depth, with a distinct change in rock physical properties presenting at 3 meters below the ground surface at both hillslope positions, identified by a distinct inflection point in the SSA inventory. Below this 3 meter deep interface SSA changes very little, which we understand to represent a very gradual transition from weathered to fresh bedrock. The mineralogical changes that influence the generation of SSA are driven by near-surface pedogenic and biotic process, including organic-matter occlusion of surface area, yet the 3-meter deep inflection in SSA inventory reveals that these processes are not the sole driving mechanism for SSA generation. We identify that the primary controls on mineral SSA production occur in near-surface environments where multiple biogeochemical processes combine to physically disrupt and chemically weather minerals.