The influence of bacterial activity on phosphorite formation in the Miocene Monterey Formation, California

Authigenic phosphorites from the Miocene Monterey Formation (California) including an autochthonous phosphatic laminite were analyzed for molecular biomarkers, element content, and sulfur isotopic composition of associated pyrite and sulfate to evaluate the role of bacterial activity in the precipitation of phosphate minerals. The phosphorites formed in a depositional environment typified by upwelling with dynamic bottom currents and hardground formation. Pyrite enclosed in the phosphorites shows δ ³⁴S values as low as -36.5‰ VCDT, which is consistent with bacterial sulfate reduction. In a three-step extraction-phosphorite dissolution-extraction procedure, molecular fossils of sulfate-reducing bacteria (di-O-alkyl glycerol ethers and short-chain branched fatty acids i- and ai-C _15:0, i- and ai-C _17:0, and 10MeC _16:0) were preferentially released from the mineral lattice. This suggests that the molecular fossils were tightly bound to carbonate fluorapatite, indicating that sulfate-reducing bacteria were involved in mineral formation. A close association of sulfate-reducing bacteria with large sulfide-oxidizing bacteria, which was previously suggested to favor carbonate fluorapatite precipitation, could neither be confirmed nor excluded for the Miocene Monterey Formation phosphorites.