Rare earth elements in seep carbonates as tracers of variable redox conditions at ancient hydrocarbon seeps

Marine hydrocarbon seeps are dynamic systems, which are typically reflected in complex lithologies of limestones forming at these sites. In order to test if seep limestones with their multiple generations of carbonate phases reflect variable redox conditions, the rare earth element (REE) patterns of limestones from five ancient hydrocarbon-seep localities, Hollard Mound (Middle Devonian), Iberg (Mississippian), Beauvoisin (Oxfordian), Canyon and Satsop River (Oligocene), and Marmorito (Miocene) have been investigated. The total REE contents vary widely (0.3-43.7 p.p.m.). Early microcrystalline phases tend to show higher contents than spatially associated later phases (microspar, sparite, and blocky cement). The shale-normalized REE patterns of the carbonates show both negative and positive Ce anomalies even for individual deposits, suggesting that the redox conditions varied significantly. It is apparent that in addition to anoxic conditions, oxic conditions were common in some ancient seep environments. Intermittent oxygenation is presumed to have resulted from abrupt changes in fluid flow.