SIGNATURES OF SOIL CARBON IN DEEP CRITICAL ZONE STRUCTURE AND FUNCTION (Invited Presentation)

The cycling of organic carbon in the near surface occurs over a broad range of timescales, reflecting a suite of reactive and transport pathways which must be accurately described in order to forecast system behavior. At one extreme, soils may be isolated as a balance between inputs, exports and transformations within. In contrast, soils may be treated as a boundary condition allowing transfer between surface and subsurface systems. The Critical Zone merges these perspectives and thus requires new model structures that honor the complexity of reactive transport within soil and link these transformations to the larger environmental system. In the present study, we demonstrate the necessity of this revised perspective based on a novel dataset of solute and gas chemistry as a function of depth across an 18 m thick vadose zone of weathered argillite (shale) in the Eel River Critical Zone Observatory (ERCZO) using novel sub-horizontal distributed samplers (Vadose Zone Monitoring System). We illustrate a year of major and trace ion chemistry obtained from water samples collected approximately biweekly using two complementary sampling systems, one applying active pressure to extract matrix-bound pore fluid, and the other using a passive collection method to extract freely draining water, in comparison to gas phase measurements across the fractured regolith. Our observations illustrate carbon transport and transformation commonly observed within soil profiles, operating many meters below the soil – bedrock transition. Solute concentrations reflect this carbonic acid driven weathering, and, surprisingly, normalized ion ratios span the full range of values reported for the world’s largest rivers. Collectively, our results revise the spatial and temporal scale over which organic carbon cycling and storage should be considered in the achitecture and funciton of the Critical Zone.