Soil residence time: A window into landscape morphologic steady state
Abstract
For a landscape in true morphologic steady state the erosion rate and the average residence time of the debris mantle regolith (including the soils) are everywhere equal. Where other factors influencing soil properties such as climate, organisms and parent material are relatively invariant the degree of weathering and extent of pedological development in the debris mantle regolith should be spatially invariant. The corollary to this argument, commonly exploited in soil-geomorphic analysis, is that variation in debris mantle regolith development in a landscape reflects inheritance of older geomorphic surfaces and hence departure from steady state, at least over some time and space scale. The Oregon Coast Range (OCR) experiences a constant rate of rock uplift and has escaped the effects of Pleistocene glacial and periglacial processes. Furthermore, rock uplift and denudation rates have been shown to be approximately in balance, and consequently the OCR is promoted as being a good candidate for a (flux) steady state landscape. This is, however, not a sufficient condition for morphologic steady state, which is often assumed in numerical landscape simulations. The rock underlying the OCR is relatively homogeneous turbidites of the Tyee formation, and climatic and vegetation factors are relatively uniform over large areas. The degree of weathering and pedological development of the regolith on hillslopes should therefore dominantly reflect variation in regolith residence time, such that significant variation implies non-morphologic-steady state conditions. Indeed, spatial variation in soil/regolith age indicates the extent of departure from morphologic steady state. We have observed ubiquitous but localised deep, highly weathered regoliths and soils on ridge tops in the OCR. The extent, depth, geometry and elevational distribution of these deep regolith patches combined with relative measures of their age derived from total element and meteoric 10Be inventory will enable us to determine the spatial and temporal scale over which morphologic steady state can be considered to apply.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.H31A1287A
- Keywords:
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- 1815 Erosion;
- 1826 Geomorphology: hillslope (1625)