Abstract

Massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid Bathymodiolus sp. containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and U-series isotopes. Stable carbon isotopes and 87Sr/86Sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon-rich fluids. Samples from five seepage sites on the northern Gulf of Mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to >2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives.

In general, paired 14C and 230Th dates of carbonate buildups consisting of aragonite, high-Mg calcite, and dolomite mineralogies are highly discordant. The cause of the discordance, established on the basis of paired Δ14C and δ13C values (Δ14C = −898‰ to −992‰ δ13C = −9.5‰ to −53.3‰, for n = 27), lies with the impairment of the radiocarbon dates resulting from dilution of the 14C pool with fossil-hydrocarbon–derived carbon. The validity of the ionium dates based on U-rich (2.4–7.6 ppm) samples is demonstrated by the concordance between 234U/238U and 230Th/234U evolution in time, and by the (234U/238U)o activity ratios that are generally within the range of sea-water value of 1.14 ± 0.04 (2 sigma). Some dolomite-rich samples are exceptional because their (234U/238U)o ratios are significantly higher (1.22–1.38) than sea water, suggesting deposition from anoxic pore waters where the soluble U reached anomalous 234U/238U ratios. The formation of the carbonates from sea-water–derived fluids, rather than from formation fluids advecting from deep aquifers, is supported by the 87Sr/86Sr composition of the samples (mean 0.709145 ± 19 × 10−6, n = 14) that compares well with modern nonseep marine carbonates and the ambient Gulf of Mexico sea water (0.709171 ± 8 × 10−6). Calcareous shells, the δ13C values of which indicate a carbon source in sea water (δ13C = −4.3‰ to −1.1‰), yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates. Isotope migration attested by the observed U uptake in the fossil shells is likely to affect the accuracy of their ionium dates.

Radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late Pleistocene time (195–13 ka). Ages derived from nascent seep sites at mid-bathyal and abyssal depths (12.3–0.0 ka) indicate that currently vigorous seepage was initiated at the end of the last deglaciation. These radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor.

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