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UBC Theses and Dissertations

Pedogenesis of soils derived from ultramafic rocks and tephra in southwestern British Columbia Bulmer, Charles Ernest


Ultramafic rocks and soils are of interest because (1) they provide unique plant growth environments, (2) environmental hazards such as chrysotile asbestos may be present, and (3) valuable deposits of asbestos, and minerals containing Mg, Cr, Ni, Co and Pt may be present. Information on ultramafic soils is useful for alleviating environmental problems associated with ultramafic terrain, and for exploiting the opportunities associated with ultramafic mineral deposits. This thesis presents new information on the weathering status of ultramafic soils that formed since the most recent advance of continental ice sheets in southwestern British Columbia. The effects of eolian and tephra additions on soil formation were also described, and soil processes in ultramafic- and tephra-derived soils were compared. A total of 27 soil profiles were described and sampled at three areas in southwestern British Columbia. Brunisolic soils with composite profiles of tephra overlying serpentinite covered much of the Shulaps ultramafic complex. These soils developed in a dry environment and showed weak profile development. The Tulameen area was characterized by a moderately dry climate, and Brunisolic soils had developed in serpentinized peridotite and dunite. The soils of the Coquihalla serpentine belt had Podzolic profiles which resulted from intense weathering in a moist environment. The weathering status of the soils was investigated using a variety of chemical and mineralogical techniques. Chlorite and mica weathering was observed in clay fractions for the Podzolic soils at the Coquihalla area, while profiles from the Shulaps area had similar clay mineralogy throughout the profiles, as determined by XRD. Serpentine dissolution was indicated by chemical analysis of upland soil profiles. Weathering and leaching depleted Mg from B horizons relative to parent materials. Nickel and Co were redistributed to the C horizons in all but the driest environments, while Mn was less mobile. Chromium was retained in the B horizons of all profiles. The influence of tephra on ultramafic paleosols was minimal, but the addition of colluvial and eolian ultramafic materials to the tephra-derived soils gave rise to soils with the cation exchange complex dominated by Mg, which is more typical of ultramafic soils. Lack of buried organic layers in ultramafic paleosols was interpreted as evidence that modern-day tephra soils in the Shulaps Range were more productive than the ancient ultramafic soils. Weathering experiments showed that trace metals such as Cr, Mn, Co and Ni were preferentially released from serpentine to dilute citric acid solutions compared to major elements such as Mg, Si, and Fe. The results suggest that these potentially toxic trace metals may be released in significant amounts in the early stages of pedogenesis.

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