Rare earth elements as tracers for microbial activity and early diagenesis: A new perspective from carbonate cements of ancient methane-seep deposits

Abstract

Ancient methane-seep carbonates are geological archives of environmental conditions and record past microbial activity. To better understand the information stored in this archive, a comparison was made of phase-specific major, trace, and rare earth element (REE) patterns, mineralogies, and textures of two early diagenetic carbonate phases in five ancient methane-seep deposits including Oligocene limestones from the Satsop and Canyon rivers (Washington State), the Cretaceous Tepee Buttes (Colorado), and the Carboniferous Ganigobis limestones (Namibia). (1) Cryptocrystalline, yellow aragonite and (2) banded and botryoidal aggregates of clear, fibrous aragonite typify many Phanerozoic seep limestones, and are common features of authigenic seep carbonates forming close to the sediment-water interface today. Light REE enrichment in yellow aragonite is observed together with high organic matter contents, indicated by intense autofluorescence of yellow aragonite, and occurs in combination with low Mg/Sr ratios. Conversely, heavy REE enrichment is observed in neomorphic yellow calcite spar typified by high Mg/Sr ratios, indicating progressive diagenetic alteration during early diagenesis in marine pore waters affected by a succession of biogeochemical reactions. These observations suggest that REE contents and patterns of early diagenetic carbonate phases are a function of the quality of preservation. Transformation of primary aragonite to calcite resulting from changing pore water compositions during later-stage early diagenesis is apparently capable to significantly modify elemental composition. The new results reveal that rare earth elements do not necessarily behave conservatively if early diagenesis is controlled to a large extent by evolving pore water compositions in environments strongly affected by biogeochemical processes. For such settings a careful assessment of diagenetic alteration is consequently required before calcite resulting from aragonite transformation can be used for reliable paleoenvironment reconstruction.