The Architecture of the Deep Critical Zone: The Role of Lithology and Geologic Texture in Regolith Formation, Hydrologic Flowpath Development, and Weathering Dynamics

Abstract

The critical zone (CZ) represents the living skin of the Earth’s surface, which extends from the bottom of freely circulating groundwater to the top of the vegetative canopy. CZ biogeochemistry includes abiotic and biotic weathering processes that occur within pores and fractures, which aggregate (or average) along the myriad of hydrologic flowpaths that make up the shallow and deep CZ. Landscapes evolve as a function of the both bottom-up controls, dependent on the initial geologic template and past weathering/geologic forcing (e.g. tectonics, sedimentary reworking, etc.), and top-down forcing driven by climate. As such, the dynamic interaction between top-down forcing and bottom-up controls is evident in the architectural framework of the CZ, exhibited by weathering profiles in the subsurface, and contemporary biogeochemical processes visible as exported solutes in surface, soil, and groundwater. Within this framework, point measurements collected along flowpaths, coupled with bulk solid sampling are traditionally used to identify weathering reactions occurring in the subsurface. Much of the foundational CZ science has been conducted in relatively simple, monolithic geologic settings, whereby spatial variability in mineral and geochemical compositions are explained by ongoing weathering front propagation and contemporary fluid/rock interactions. However, within the CZ literature, little attention has been paid to complex geologic settings, where past weathering processes may impart significant mineral and geochemical overprints not related to ongoing, contemporary CZ processes. Yet, CZ processes in these settings are dependent on past geologic processes as they alter both geologic texture (e.g. pore size and distribution) and mineral composition, especially at the fluid/surface interface, compared to fresh, unaltered protoliths. This framework provides the underpinning for the present study, where resolving weathering profiles in the complex geologic setting of the Jemez River Basin Critical Zone Observatory, located in the Valles Caldera National Preserve, northern New Mexico, required characterization of the CZ architecture as a function of both geologic legacy and contemporary CZ processes. The CZ architecture (to 50 m) in a small zero-order basin (ZOB, area approximately 16 ha) located in the JRB-CZO was characterized by an array of complementary approaches, analogous to needing as many equations as there are unknowns. In the summer of 2015, surface geophysical surveys were conducted in the ZOB, which included surface seismic and electrical resistivity surveys along two transects (one roughly north-south, the other roughly east-west). These surveys identified a slow p-wave velocity zone that extended 4 to 6 m deep that mantled the ZOB, coinciding with a highly weathered surface. Weathering decreased with depth (as shown with increasing p-wave velocities), with competent bedrock (p-wave velocities approximately 4,000 m s-1) present at a depth of approximately 50 m. Electrical resistivity profiles showed that the eastern portion of the ZOB had less water and/or clay than the western portion of the ZOB. Using these geophysical data, the depth of the weathering profile was estimated to extend to 50 m in depth. Informed by the geophysics, a drilling campaign was undertaken in the summer of 2016, excavating continuous cores down to approximately 46 m (eastern and western slopes) and 20 m (catchment convergent zone) at three locations in the ZOB, which reflected contrasting geologies and landscape positions (i.e. eastern drill location: Bandelier Tuff; western drill location: volcanic breccia; central drill location: convergent zone with mixed lithology). Core sample analysis, collected as subsamples approximately every 40 to 60 cm, revealed that the upper 15 to 18 m of the western and eastern borehole locations consisted of matrix dominated morphology, transitioning to fracture dominated morphology below ca. 15 m. The eastern borehole was composed of welded to sub-welded tuff with a mineral composition that included quartz, alkali and plagioclase feldspar, smectite, zeolites, and amorphous minerals in the upper 15 m, showing evidence of prior interaction with alkaline hydrothermal fluids. Below 15 m, the fractured welded tuff was comprised mostly of primary minerals (quartz, alkali and plagioclase feldspar, cristobalite, and volcanic glass), exhibiting relatively little weathering within the bulk. However, fracture surfaces were coated with oxidized Fe and Mn. The western borehole was composed of rounded to subrounded quartz, frayed biotite and calcite nodules in the upper 15 m, with a significant amount of illite and smectite present. At 14 m, the lithology changed to vesicular tuff, with significant zeolitization in the first 10 m coinciding with stratified ash deposits. The convergent zone was a mixture of smectite and oxidized Fe and Mn with weathering feldspar. Traditional, sigmoidal weathering front propagation was not visible in geochemical mass transfer coefficients (i.e. tau), likely as a result of incomplete weathering of rock fragments in the upper profile. However, trends in mass transfer calculations coincided with changes in mineral composition and geophysical data (e.g. magnetic susceptibility, seismic velocities, and electrical conductivity) as a function of depth. Multivariate statistical analysis of the complementary data sets, using linear discriminant analysis (LDA), identified zones within the weathering profiles, which was used to develop a conceptual framework of the deep CZ architecture and weathering profiles in the ZOB. A key finding from the deep CZ characterization study was the depth distribution and character of hydrothermally altered secondary mineral assemblages and overall secondary mineral phases in the contrasting geologies in the ZOB. It was hypothesized that these secondary mineral coatings likely played a significant role in aqueous geochemistry evolution along flowpaths due to fluid/surface interactions. Surface coatings included smectite and zeolites in the hydrothermally altered tuff on the eastern portion of the watershed down to 15 m, which transitioned to fracture surfaces coated with oxidized Fe and Mn. At all portions of the borehole at this location, larger primary mineral grains, such as alkali and plagioclase feldspars, were largely unweathered with some surface weathering occurring where smectite and zeolite precipitates on the surfaces. It was hypothesized that zeolites were precipitated as a result of glass weathering, and the smectites likely precipitated as a result of both zeolite and feldspar weathering. Enrichment of Sr and Ba, compared to Ca was observed in zeolitic zones. Enrichment of rare earth elements and Y (REY) was observed at depths where smectite content increased, indicating sorption of REY on clay surfaces. In addition, oxidized Fe and Mn on fracture surfaces appeared to shield bulk rocks from weathering. Reactions at the fluid/surface interface, whose surface composition and characteristics appear to be a result of both past and contemporary weathering processes, was hypothesized to control aqueous solution chemistry in the subsurface. Experimental weathering reactions of core materials were performed at increasing time steps, up to 1 year, to pinpoint geochemical reactions and mineral weathering rates occurring in the subsurface. Experimental weathering showed that colloidal dispersion released zeolites to solution, which were subsequently weathered to smectite. Calcite dissolution/precipitation reactions appeared to regulate solution chemistry in the volcanic breccia. Deep core samples, representing the fractured portion of the deep CZ, had experimental solution chemistry that did not match the groundwater signatures collected from deep monitoring wells in the ZOB. However, experimental solution chemistry from altered tuff samples appeared to closely match deep groundwater chemistry collected from monitoring wells, indicating deep groundwater chemistry in the ZOB derived most of its solution chemistry from reactions in the shallow CZ (e.g. <14 m) rather than at depth. The findings of this study suggest that contemporary CZ processes greatly rely on the geologic architecture and past geologic legacy in addition to climatic forcing. This is particularly true in complex volcaniclastic terrains, such as the Valles caldera. In addition, complementary analysis, including geophysics, drilling, mineralogy, geochemistry, and experimentation provide an in-depth view into spatial as well as temporal controls on deep CZ processes.