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Iron Concretions in a Typic Dystrochrept in Taiwan

Taiwan Forestry Research Inst., Taipei, Taiwan

J. K. Torrance^*

Dep. of Geography, Carleton Univ., Ottawa, Canada, K1S 5B6

L. H. Bowen

Dep. of Chemistry, North Carolina State Univ., Raleigh, NC, 27695-8204

C. Wang

Land Resource Research Centre, Agriculture Canada, Ottawa

* Corresponding author.

ABSTRACT

Prominent Fe concretions, singly and in clusters up to 30 cm across, occur in a Typic Dystrochrept pedon developed under evergreen mixed hardwood, on 60% slopes, by in situ weathering of Tertiary shales. The round to subangular polygonal structures contain a gray matrix (N 5) surrounded by two Fe-rich layers. The inner layer is dark reddish brown (5YR 3/3) and is composed of 60% Fe₂O₃, almost equally as hematite and goethite. Energy-dispersive x-ray analysis indicates higher concentrations of Fe, Mn, Ca, and some trace elements than in the soil matrix. The outer reddish-yellow (7.5YR 6/6) layer is goethite rich: only Fe and Zn are consistently in greater abundance than in the soil matrix. Thin-section studies and chemical evidence suggest that the concretions commence their formation during the weathering of saprolite, are most prominently developed between 50- and 100-cm depth, and remain mainly as fragments above 30 cm. The Fe forming these concretions appears to be cycled downward during their disintegration, to become involved again in the process. A life cycle for the concretions, based on the balance of erosion, Fe mobilization, and reprecipitation at the site, is proposed.

Contribution from the Dep. of Geography, Carleton Univ., Ottawa. LRRC Contribution no. 89-52.

Received for publication August 8, 1988.