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Carbon geochemistry of the Atlantis Massif (IODP Expedition 357): Implications for carbonation of mantle peridotites
  • Lotta Ternieten,
  • Gretchen L. Früh-Green,
  • Stefano M. Bernasconi
Lotta Ternieten
ETH Zurich

Corresponding Author:[email protected]

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Gretchen L. Früh-Green
ETH Zurich
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Stefano M. Bernasconi
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The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered during IODP Expedition 357 on the Atlantis Massif (AM) was examined to characterize carbon sources and the fate of dissolved organic (DOC) and inorganic carbon (DIC) in seawater during long-lived hydrothermal circulation and serpentinization. Carbon isotopes reveal three stages of carbonate formation, starting at least 38,000 yr ago: (1) Early dispersed carbonate precipitation, with low water/rock ratios and high temperatures (50 to 190°C); (2) carbonate vein formation related to high and focused fluid fluxes still at higher temperatures (30 to 190°C); and (3) seawater circulation leading to cold carbonate precipitation controlled by late, brittle fractures during uplift and unroofing of the oceanic core complex. Our study reveals three main DIC sources in the system: (1) DIC from abiotic hydrothermal degradation of dissolved organic matter; (2) DIC from seawater; and (3) DIC from mantle-derived volatiles. Basement rocks containing dispersed carbonates are characterized by high concentrations (~800 ppm) of total organic carbon (TOC) and 13C-depleted carbonates. We propose that high seawater fluxes in the southern part of the AM likely favour the transport and incorporation of marine dissolved organic carbon in serpentinites and that carbonates record isotopic signals of organic matter decay. Our study indicates that organic carbon accounts for a significant proportion of the total carbon stored in the Atlantis Massif and suggests that serpentinites may be an important sink of DOC from seawater.