Weathering of Garnet and Chain-Silicates in Forest and Agricultural Soils at the Kellogg Biological Station and Coweeta Hydrologic Laboratory LTER Sites, and Other Great Lakes and Southern Blue Ridge Regional Localities

Velbel, M.A., J.T. McGuire, N.P. Mellott, and M.J. Formolo

Presented at the All Scientist Meeting (1998-07-21 to 1998-07-22 )

Scanning electron microscope (SEM) investigations of garnets and chain-silicate minerals (pyroxenes and amphiboles) were conducted to qualitatively characterize their weathering textures as a function of weathering regime, parent material, and land use. Samples were collected from soils developed on weathered glacial deposits (including an agricultural field and a deciduous forest at Kellogg Biological Station (KBS) LTER site in Battle Creek, Michigan and a deciduous forest at Sanford Natural Area in East Lansing, Michigan), and from deep saprolitic weathering profiles of the Carroll Knob Complex at the Coweeta Hydrologic Laboratory Southern Hardwood Forest LTER site (North Carolina) and the Laurel Creek Complex (Georgia). The chain-silicate minerals at all locations and under all land-use regimes exhibit the same types of weathering morphologies as observed at the other sites, and as previously reported elsewhere, despite differences in parent material, geomorphic history, presence/absence and identity of secondary-minerals (weathering products), vegetation, and land use.Garnets from all Michigan localities exhibit a wide range of features, including features apparently inherited from their source rocks, and features formed during glacial transport. Widespread preservation of inherited pre-pedogenic surface textures suggests that garnet in Michigan soils has undergone little weathering since deglaciation. Garnets from the Michigan localities appear less weathered than garnets from other weathered glacial deposits in Canada. The garnets collected from KBS (both forested and agricultural soils) differed from those collected at Sanford Natural Area in that those from KBS have well-formed etch pits on their surfaces while those from Sanford are relatively unweathered. It is unclear why KBS garnets are more etched than Sanford garnets.Garnet weathering features formed during prolonged deep saprolitic weathering at the Coweeta Hydrologic Laboratory Southern Hardwood Forest LTER site are distinctly different from those developed on post-glacial soils in Michigan, and closely resemble weathering features developed in many (oxidizing) lateritic and saprolitic regoliths. Pre-weathering (parent-material) surface textures are completely obliterated by weathering at Coweeta. Layers of gibbsite and goethite replace iron-bearing (almandine) garnet in saprolitic weathering environments; etch pits are uncommon on garnet in the saprolite. However, in many soils, almandine surfaces are directly exposed to soil solutions. These grains exhibit numerous well-formed etch pits. Garnet etch pits at Coweeta are larger, more abundant, and crystallographically better developed than at KBS.Garnets in Michigan glacial soils are minimally weathered compared with Coweeta garnets; this suggests that the relative extent of garnet grain-surface modification by weathering depends on the length of time the material has been exposed to weathering. It may be that the time required to develop diagnostic weathering textures on garnet is longer than the age of Michigan soils. However, chain-silicates in Michigan soils appear to have weathered to a degree comparable to Coweeta chain-silicates, so regolith age alone may not be sufficient to explain differences in garnet surface textures. Further work is required to determine whether differences in soil-solution chemistry among sites are sufficient to explain the preservation and apparent passivity of pre-pedogenic garnet surfaces in Michigan soils. Work to estimate the “response time” of garnet and chain-silicate surface textures to environmental change is in progress.Return to Contents

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