Catena Position Regulates Rock Porosity in a Forested Headwater Catchment
Abstract
Weathering reactions in soil support ecosystem nutrition and carbon sequestration. To date, most weathering research in soils has focused on the fine earth (<2 mm) fraction, while emphasizing a vertical paradigm of soil development. However, in postglacial soils, the mineral landscape is spatially heterogeneous, spanning bare bedrock to deep till, and coarse fragments are often abundant. The structure of the Critical Zone in these areas creates distinct, lateral patterns in soil weathering that vary along a catena. We predicted that the magnitude of porosity in rock fragments and bedrock would be directly related to the intensity of chemical weathering in the surrounding soils and mediated by rock properties such as mineralogy and particle size. The goal of this work was to investigate the relationship between soil weathering and the porosity of rock fragments and bedrock. Rock fragments, bedrock cores, soil samples, and soil clods were collected from locations at four hillslope positions along three transects in a postglacial, forested catchment. To measure porosity and identify minerals, all samples were scanned by electron microscopy with energy dispersive spectroscopy (SEM-EDS) after inspection by optical petrography. Subsets of rock fragments and rock cores were scanned with electron microprobe and µCT to quantify mineral content and measure porosity in 3D. Porosity in the outer rind of rock fragments showed a significant relationship to elemental depletion in soils (adj. r2 = 0.39), which was in turn strongly related to catena position. Surprisingly, porosity was greater in the outer rind of weathered rock fragments than in the C horizon (parent material) of soils. In shallow bedrock soils where the C horizon is absent, the weathering front for certain minerals extends to the interior of rock fragments and bedrock, despite the absence of saprolite. We found that changes in soil properties along the catena were linked to significant differences in weathering of rock fragments and bedrock. This work increases our understanding of the dynamic relationship between pedogenesis and weathering in the Critical Zone.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.H55F..05B