Exploring critical zone architecture and its influence on Earth surface processes using drilling, sampling, and geophysical imaging

Abstract:

The critical zone (CZ) is Earth’s breathing skin, consisting of soil, saprolite, and fractured rock stacked together in a heterogeneous layer that can be tens to hundreds of meters thick depending on factors such as climate, lithology, and topography. Quantifying CZ properties and how they vary across landscapes is important across a wide range of problems in Earth surface processes: Though the deep CZ may seem far removed from the surface, it is vital to shaping it, preparing rock-derived nutrients for infusion into ecosystems, creating subsurface pathways for through-flowing fluids, and reducing the rock itself into particles that can be swept from landscapes by erosion. Despite their broad significance, the factors that regulate CZ architecture remain poorly understood, in part because the CZ is difficult to directly observe over relevant scales. Digging up and thus directly observing the full depth of the CZ on a single ridge would require an excavation on the scale of a modern skyscraper’s foundation — an impossible task in mountain landscapes where understanding the deep CZ is vital to a complete understanding of surface processes. Here we show how this limitation can be overcome using the lens of geophysical imaging to quantify CZ architecture at scales ranging from landscapes to boreholes. We use this lens to explore a series of exciting new hypotheses about factors that drive CZ weathering and thus influence the thickness and properties of the CZ. These hypotheses have emerged from recently developed process-based models of: fracturing in subsurface stress fields; weathering due to drainage of bedrock under hydraulic head gradients; rock damage due to frost cracking in temperature gradients; mineral reactions with fluids in subsurface chemical gradients; and topographically driven groundwater flow of reactive fluids. The models predict distinct patterns of subsurface weathering that can be compared with observations from drilling, sampling, and geophysical imaging. We present an integrated conceptual model of fracturing and weathering for Earth materials as they are exhumed to across gradients in stress, hydraulic head, temperature, and chemistry. We discuss how predictions of the model can be tested using complementary observations from drilling and geophysical imaging.