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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
3. (May), Pages 430-438
Significance of Aragonite Cements Around Cretaceous Marine Methane Seeps
Martine M. Savard (1), Benoit Beauchamp (2), Jan Veizer
(3)
ABSTRACT
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 d13C
values around -44.
and d18O values of 1.8 to 0.1
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 aHCO3-, that promoted precipitation of carbonates.
We suggest that, locally, fluctuations in aHCO3-/aSO42-
resulted in oscillating aragonite or calcite supersaturation, and hence,
controlled the mineralogy of the early precipitates.
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