SIGNIFICANCE OF ARAGONITE CEMENTS AROUND CRETACEOUS MARINE METHANE SEEPS

Detailed petrography and geochemistry of carbonate precipitates in Cre taceous cold seep mounds from the Canadian Arctic show spectacular ear ly diagenetic products: some still-preserved splays and isopachous lay ers of fine, acicular aragonite, and large botryoids and crusts of low -magnesium calcite showing unusual entanglement of former fibrous calc ite and aragonite, The latter mineralogy is suggested by clear, flat-t erminated cathodoluminescence patterns interpreted as ancient crystal growth steps, and the former by rhombohedral termi nations, The early cement phases very likely precipitated in cold Arctic water dominated hy bicarbonates derived from bacterially oxidized methane: these cemen ts have delta(13)C values around -44.0 parts per thousand and delta(18 )O values of 1.8 to 0.1 parts per thousand PDB. Coexistence of calcite and aragonite early cements in the Cretaceous seep mounds is unusual, because precipitation occurred in high-latitude, cold-water settings, and during a so-called calcite sea mode, As in modern marine hydrocar bon seeps, the chemistry of the Cretaceous system was apparently contr olled by chemosynthetic bacterial activity, resulting in high a(HCO3-) that promoted precipitation of carbonates, We suggest that, locally, fluctuations in a(HCO3-)/a(SO42-) resulted in oscillating aragonite or calcite supersaturation, and hence, controlled the mineralogy of the early precipitates.