Biogenic arsenopyrite in holocene peat sediment, India

The plant organic matter in the peat layer deposited around 6650 +/- 110 and 4800 +/- 80 years BP, during the transgressive and regressive phases of sea level changes, respectively in the dried part (playa) of the present Pulicat lagoon in Palar Basin (southeast coast of India) was studied to elucidate the biogenic pyrite generation and associated trace elements. The scanning electron microscopic (SEM) observations show strongly curved unique C-shaped bacteria of uniform coccoidal shape and size (1 mum) freely scattered on the plant epidermal microfragments. These form spheroidal microcolonies 8-15 mum in diameter attached to the epidermis in a linear fashion or haphazardly enclosed in the translucent sheath as observed in surface view. These colonies are dense with a number of toxic trace elements as analyzed under an SEM-EDX spectrometer indicating its affinity toward metal binding. On maturity the sheath ruptures disseminating tetrad form of bacteria along with some C-shaped bacteria and few rhombohedral crystals of framboidal pyrite. The pyrite crystals intact in spheroidal shape in cavities on the epidermal fragments indicate its biogenic precipitation mediated by the bacteria. The pyrite crystals are rich in sulfur, iron, and traces of arsenic when analyzed by the SEM-EDX spectrometer. It is suggested that cubic and pyritohedran forms of crystals are perhaps arsenopyrite precipitated biogenically in saturated coastal wetland ecosystem in the past mediated by tetrad strain of the observed coccoid bacteria. Thus, the strata-bound peat layer could be the potent residing place for arsenopyrite or other toxic trace elements that may contaminate the groundwater aquifer. (C) 2003 Elsevier Science (USA). All rights reserved.