Bacterially Mediated Formation of Carbonate Concretions in the Oligocene Boom Clay of Northern Belgium

ABSTRACT Carbonate concretions in the Oligocene Boom Clay in Northern Belgium grew as a result of bacterially mediated geochemical processes. Concretion distribution patterns are controlled by glacio-eustatic controls on water depth and related sediment detrital carbonate contents. Bacterial remains are present throughout these early diagenetic concretions and can be observed by SEM analysis of etched carbonates. The oxygen isotopic composition (18O 0o/oo) of the calcite in the concretions indicates early diagenetic growth close to the sediment-water interface. Negative carbon-isotope values (13C -17 to -22o/oo) from concretion carbonates, as well as pyrite 34S values of -15 to -45o/oo, suggest precipitation resulting from bacterial sulfate reduction during shallow burial diagenesis. Additionally, organic components in the concretions are characteristic of the remains of sulfate-reducing bacteria.