The role of amorphous P-bearing precursors on barite formation

Abstract

Past climate variations on Earth are recorded in sedimentary rocks by chemical and biological indicators. In this sense, investigations of sedimentary marine barites (BaSO4) have been fundamental to reconstruct geochemical evolution of palaeoenvironments. Despite the significant advances achieved during the past decade, the mechanism of barite formation in the ocean water column in undersaturated conditions and the role of microorganisms remain controversial. Phosphorus-rich, amorphous precursor phases have been suggested to play a key role, although the exact mechanism of marine barite formation from this precursor is not well constrained. In this paper, we evidence that barite can precipitate by replacing amorphous Ba-P precursor phases formed in the absence of living organisms. This occurs by a mineral replacement reaction, yielding an abiotic scenario for barite particles with morphologies, mesostructure and composition resembling barite formed in the marine water column. This demonstrates the need to investigate how this formation process, via the replacement of an amorphous precursor, affects the isotopic and trace element signatures of barite, and how this influences the environmental information obtained from this proxy.