The behaviour of major and trace elements in complete vertical peat profiles from three Sphagnum bogs

Abstract

The distribution of major elements (Al, Ca, K, Mg, Na) in peat profiles from two contrasting ombrotrophic Sphagnum bogs is interpreted in respect to the abundance and mineralogical composition of ash, atmospheric deposition, plant bioaccumulation and leaching. High concentrations of Ca are found in the minerotrophic peats from a continental bog underlain by calcareous sediments (Luther bog, Ontario). Aluminum is more abundant in the minerotrophic peats from a coastal bog in a siliceous terrain (Barrington bog, Nova Scotia). The peats at the coastal site contain an average of 3 times as much Na and Mg as the peats from the continental bog because of sea spray. Potassium is highly enriched in the surface peats at both sites because of plant bioaccumulation. The behaviour of trace metals (Cr, Cu, Fe, Mn, Ni, Pb, U, V, Zn) in peat profiles is explained in terms of the abundance and distribution of mineral matter, plant bioaccumulation, the response of metals to acidic, anaerobic conditions, and anthropogenic atmospheric deposition. The distribution of Ni, V and Cr in the peats from a third bog (Galbraith Bog, Ontario) follows that of mineral-matter content, and the metals are highly correlated with ash. Relative to the mean composition of the underlying mineral sediments (clays) collected at 30 other sites, the peats are neither significantly (> 2 times) enriched nor depleted in Ni, V and Cr. Manganese is strongly enriched in surface and near-surface peat layers because of plant bioaccumulation. In contrast, Fe and Mn are relatively depleted in subsurface peats due to their enhanced solubilities in acidic, anaerobic waters. The opposite is true for Cu and U which accumulate in peats because of their diminished solubilities in anaerobic environments. Enrichments of Zn and Pb also occur, but these are restricted to surface peats, and are due to anthropogenic metal deposition.