Biogenic and Abiotic Carbonates in Altered Oceanic Crust (AOC) and Implications to Subduction Zone Carbon Cycle

Abstract

Altered oceanic crust (AOC) is an important carbon reservoir for subduction-zone carbon recycling. However, the character of the carbon reservoir (e.g., total budget and isotopic signatures) in AOC has not been well constrained. Here, based on analyses of contents and isotope compositions of 73 DSDP/ODP/IODP samples (22 pillow basalts, 7 sheeted dikes, 22 gabbros and 22 peridotites) and 7 gabbro samples from the Oman Ophiolite, we show that carbonates in gabbros and peridotites were all precipitated at near surface low temperature conditions instead of being precipitated at their corresponding depths in the deeper oceanic crust, and thus cannot represent the true values of the lower oceanic crust sections. Our modeling yielded a new value for carbon input flux in AOC of 1.3 × 1012 moles C/year, which is only 30-60% of previous estimates. We also observed that carbonate in some oceanic crust segments, such as the Cocos plate (ODP/IODP Site 1256D) and the Southwest Indian Ridge (ODP Site 735B), is predominately biogenic in origin with weighted average δ13C values of -20.4‰ to -6.3‰. If such AOC is subducted and provides a carbon source for diamond formation, the diamond can inherit the “organic”-like δ13C values but maintain normal mantle δ15N values. This may provide an alternative explanation for the decoupled δ13C and δ15N signatures in some eclogitic diamonds.