CRITICAL ZONE STRUCTURE IN THE GLACIATED INTERIOR LOWLANDS, USA: A CONCEPTUAL MODEL FROM THE INTENSIVELY MANAGED LANDSCAPE CRITICAL ZONE OBSERVATORY

The concept of a critical zone (CZ) supporting terrestrial life has fostered groundbreaking interdisciplinary science addressing the coupled and complex interactions among water, soil, rock, air and life near Earth’s surface. Pioneering work has focused on the CZ in areas characterized by residual soils and steady-state or erosional topography. A new Intensively Managed Landscapes Critical Zone Observatory (IML-CZO) was established in the Interior Lowlands of North America devoted to the study of CZ processes in a region characterized by thick surficial deposits resulting from multiple continental glaciations, with bedrock at depths of up to 150 m. Here the physical (glacial ice, loess, developing soil profiles) and biological (subglacial microbes, tundra, forest prairie) components of the CZ vary significantly in time. Moreover, the spatial relationships between mineral components of the CZ record a history of glacial-interglacial cycles and landscape evolution. We assert that the physical CZ structure influences the present occurrence and rates of processes in the IML-CZO and the responses to ongoing change. We present an idealized model of CZ evolution through a glacial cycle and exhibit cross-sections typical of substrate variability in the region to provide specific examples of heterogeneity and how the mineral components of the CZ record environmental change. Due to net sediment accumulation during the Quaternary Period, the processes dominant in the IML-CZO are not well described by the flow-through reactor model developed for residual soils derived from local bedrock. We identify two main characteristics of CZ structure which should be included in a conceptual model of CZ development in the IML-CZO: (1) mineral components of the CZ have diverse origins and transport trajectories including alteration in past CZs and during transport, and, (2) variability in climate, ecosystems, and hydrology during glacial-interglacial cycles profoundly influence the composition of the CZ, creating a legacy retained in its structure.