Microscopic analysis of pyrite in the sediments of two Brazilian mangrove ecosystems

Abstract.

This study has used scanning electron microscopy coupled with X-ray micro-analysis to compare the morphology of diagenetic pyrite formed in the sediments of two contrasting Brazilian mangroves. The Sepetiba Bay site is influenced by marine conditions, and pore waters show lower dissolved iron and higher dissolved total sulfide concentrations. The Paraíba do Sul River site, on the other hand, is influenced by fluvial waters, and shows higher dissolved iron and lower dissolved sulfide concentrations in pore waters. At both sites, pore waters were oversaturated for pyrite, and pyrite morphology was similar. The finding that pyrite morphology was similar despite marked differences in pore-water chemistry can be explained by the low solubility product of pyrite and the daily exposure of surficial sediments to air, conditioned by tidal cycles. This implies that oxygen input to the sedimentary environment is a key factor in the pyrite formation mechanism.