Master of Science
Earth and Environmental Sciences
Gray E. Bebout
Community interest in deep-Earth carbon (C) cycling has focused attention on extents of C release from subducting oceanic lithosphere and sediment and the fate of this released C. Many have suggested that, based on isotopic and other arguments, ~20% of the C subducted into the deeper mantle is in reduced form (organic); however, individual subduction zone margins show large variation in carbonate to organic C ratios. Despite the size of the potentially deeply subducted organic C reservoir, its fate in subducting sections remains largely unexplored, with most attention paid to release of carbonate C.
To characterize the forearc behavior of organic C, metamorphosed to pressures and temperatures as high as 3.0 GPa and 600˚C, reduced C concentrations and isotope compositions were obtained for mixed clastic-carbonate sedimentary rocks underthrust to varying depths in the forearc of a paleo-subduction zone represented by the Schistes Lustrés/Cignana (SLC) suite (Italian Alps) resembling sediment entering the East Sunda trench near Indonesia. In general, it appears that more Al-rich samples (shaley) have higher concentrations of reduced C, which in low-grade units preserves the C isotope composition of its organic protoliths on the paleo-seafloor. Carbonate-poor rocks in the SLC suite, and at Ocean Drilling Program Site 765 (near Indonesia), show correlated major element (Al, Mg, Mn, Ti, and P) and reduced C contents (up to 1.2 wt. %) reflecting sandstone-shale mixture. High-grade (ultrahigh-pressure; UHP) metasedimentary rocks at Lago di Cignana show lower reduced C wt. % normalized to Al2O3 concentrations, perhaps in part reflecting the very different sedimentological setting in which their protoliths were deposited (relative to the Schistes Lustrés). Clastic metasedimentary rocks in circum-Pacific high-P/T metasedimentary suites (Catalina Schist, Franciscan Complex, Western Baja Terrane) contained very little carbonate during initial subduction (only as minor diagenetic cement) and this carbonate was removed from these rocks by devolatilization at relatively shallow levels (likely <15 km) of those paleo-accretionary complexes.
Processes that could alter the concentrations and isotopic compositions of reduced C in sediment include devolatilization, closed-system exchange with carbonate, redox reactions, and isotopic exchange with C in externally-derived fluids. The extents of these effects will vary greatly as related to differences in sedimentary lithologies subducting at individual margins and degrees of open-and closed-system isotopic behavior as related to differential infiltration by externally-derived fluids. It appears that, on modern Earth, 40±20% of initially subducted C (globally, including reduced and oxidized C) is returned to the atmosphere in arcs. The results of this M.S. thesis research indicate efficient delivery of initially subducted reduced C to depths approaching those beneath volcanic fronts (to at least 75 km), where some fraction could be released during devolatilization and melting and contribute to the C output flux in volcanic gases.
Change in the isotopic compositions of the C, observed in this study, would affect the ability to model proportions of carbonate and organic C proportions in volcanic gases based on their 13C.
Kraft, Kaylee, "Fate of Subducting Organic Carbon: Evidence from Western Alps HP/UHP Metasedimentary Suites" (2018). Theses and Dissertations. 5674.