MORPHOLOGICAL AND COMPOSITIONAL EVIDENCE FOR BIOTIC PRECIPITATION OF MARINE BARITE

Barite formation in the surface oceans is generally assumed to be domi nated by abiotic precipitation. Acceptance of this pathway is largely the result of the absence of a pelagic marine organism known to precip itate the ovoid to rounded-rectangular barite crystals typically obser ved in marine waters and sediments. Barite crystals observed in net-to w particles and on substrates retrieved from the seafloor (both in the central North Pacific) were examined by scanning electron microscopy and energy dispersive X-ray spectrometry. Three distinct crystalline f orms of barite were observed: ovoid and hexagonal crystals several mic rons in diameter, and aggregates of submicron-sized crystals. Ovoid an d hexagonal-type crystals contained between 0 and 26 mole percent SrSO 4. The microcrystalline barite contained no detectable Sr (<0.05 perce nt). Hexagonal-type crystals were precipitated by an unusual benthic f oraminifera. Comparison of the morphology and composition of the barit e crystals observed in this study to crystals precipitated by a variet y of biotic and abiotic processes suggests a biotic origin for the ovo id barite crystals, the most common form of barite observed in this re gion.