Carbon geochemistry of the Atlantis Massif (IODP Expedition 357):
Implications for carbonation of mantle peridotites
Abstract
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.