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.