Detailed petrography and geochemistry of carbonate precipitates in Cretaceous cold seep mounds from the Canadian Arctic show spectacular early diagenetic products: some still-preserved splays and isopachous layers of fine, acicular aragonite, and large botryoids and crusts of low-magnesium calcite showing unusual entanglement of former fibrous calcite and aragonite. The latter mineralogy is suggested by clear, flat-terminated cathodoluminescence patterns interpreted as ancient crystal growth steps, and the former by rhombohedral terminations. The early cement phases very likely precipitated in cold Arctic water dominated by bicarbonates derived from bacterially oxidized methane: these cements have delta 13 C values around -44.0 per thousand and delta 18 O values of 1.8 to 0.1 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 hydrocarbon seeps, the chemistry of the Cretaceous system was apparently controlled by chemosynthetic bacterial activity, resulting in high a (sub HCO 3 ) (super -) , that promoted precipitation of carbonates. We suggest that, locally, fluctuations in a (sub HCO 3 ) (super -) /a (sub SO 4 ) (super 2-) resulted in oscillating aragonite or calcite supersaturation, and hence, controlled the mineralogy of the early precipitates.