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Using the ⁸⁷Sr/⁸⁶Sr of modern and paleoseep carbonates from northern Cascadia to link modern fluid flow to the past

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  • Most authigenic carbonates previously recovered from the Cascadia slope have Sr-⁸⁷/Sr-⁸⁶ signatures that reflect shallow precipitation in equilibrium with coeval seawater. There is also evidence for carbonate formation supported by fluids that have been modified by reactions with the incoming Juan de Fuca plate (Sr-⁸⁷/⁸⁶(Sr) = 0.7071; Teichert et al., 2005) or with terrigenous turbidites (Sr-⁸⁷/Sr-⁸⁶ = 0.70975 to 0.71279; Sample et al., 1993). We report on the strontium isotopic composition of carbonates and fluids from IODP Site U1329 and nearby Barkley Canyon (offshore Vancouver Island), which have strontium isotope ratios as low as 0.70539. Whereas the strontium and oxygen isotopic compositions of carbonates from paleoseeps in the uplifted Coast Range forearc indicate formation in ambient bottom seawater, several samples from the Pysht/Sooke Fm. show a Sr-⁸⁷-depleted signal (Sr-⁸⁷/Sr-⁸⁶ = 0.70494 and 0.70511) similar to that of the anomalous Site U1329 and Barkley Canyon carbonates. Our data, when analyzed in the context of published elemental and isotopic composition of these carbonates (Joseph et al., 2012), point to two formation mechanisms: 1) shallow precipitation driven by the anaerobic oxidation of methane (AOM) with δ¹C values as low as -50%[parts per thousand] and contemporaneous Sr-⁸⁷/Sr-⁸⁶ seawater ratios, and 2) carbonate precipitation driven by fluids that have circulated through the oceanic crust, which are depleted in Sr-⁸⁷. Carbonates formed from the second mechanism precipitate both at depth and at sites of deep-sourced fluid seepage on the seafloor. The Sr-⁸⁷-depleted carbonates and pore fluids found at Barkley Canyon represent migration of a deep, exotic fluid similar to that found in high permeability conglomerate layers at 188 mbsf of Site U1329, and which may have fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52-57 Ma igneous Crescent Terrane, which supplies fluids with high calcium, manganese and strontium enriched in the non-radiogenic nucleide. Tectonic compression and dehydration reactions then force these fluids updip, where they pick up the thermogenic hydrocarbons and ¹³C-enriched dissolved inorganic carbon that are manifested in fluids and carbonates sampled at Barkley Canyon and at Site U1329. The Crescent Terrane may have sourced cold seeps in this margin since at least the late Oligocene. (C) 2012 Elsevier B.V. All rights reserved.
  • Keywords: Authigenic carbonate, Paleoseep, Crescent Terrane, Barkley Canyon, Site U1 329, Strontium isotopes
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  • Joseph, C., Torres, M. E., Martin, R. A., Haley, B. A., Pohlman, J. W., Riedel, M., & Rose, K. (2012). Using the 87Sr/86Sr of modern and paleoseep carbonates from northern cascadia to link modern fluid flow to the past. Chemical Geology, 334, 122-130. doi: 10.1016/j.chemgeo.2012.10.020
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  • 334
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  • This research was funded in part by the Department of Energy (DOE) National Energy Technology Laboratory (NETL) and an NSF supported USSP grantto M.T. C.J. thankfully acknowledges support via an NETL-sponsored Oak Ridge Institute for Science and Education (ORISE) Fellowship.
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