Using mineralogy and geochemistry to constrain formation of carbonates at NanTroSEIZE reference sites C0011, C0012

Two reference sites were drilled seaward of the Nankai Trough subduction system to characterize properties of materials before entering the zone of deformation. Secondary carbonates can increase sediment strength and record variations in pore water characteristics over time. For this study, we analyzed up to 86 carbonates for mineralogy, major element geochemistry, and stable isotope geochemistry. Carbonate cement abundances range from 2 % to 79 % (in vein samples). In order from most to least abundant, the carbonate mineralogy is Mn-calcite, ankerites, magnesium calcite, and dolomite. Total carbonate abundances show no trend with depth, and ankerites with abundances of 2-5%, are common. High-Mg calcite and dolomite are scarce and are most abundant in the upper 500 m at site C0011. General textural relationships obtained from scanning electron microscopy display primarily Mn-calcite and ankerite pore-filling cements with less abundant secondary smectites.;Carbon isotope delta values (vs. VPDB) range from -23.36 / to 0.26 / at Site C0011 and from -22.48 / to 0.09 / at Site C0012. The values are consistent with mixtures of seawater dissolved inorganic carbon and contributions from methane oxidized from organic sources. 13C values become depleted starting at ∼400 mbsf at site C0011 and ∼180 mbsf at site C0012.;Oxygen isotope values (vs. VPDB) range from -11.88 / to 1.65 / at Site C0011 and from -15.96 / to 5.70 / at Site C0012. The strongest 18 O depletions occur below a depth of 420 mbsf at C0011 and 330 mbsf at C0012. Carbonate clumped isotope thermometry of samples from C0012 indicate that four carbonates have formed at temperatures in equilibrium with current downhole temperatures. These carbonates may have formed very recently, and their 18O depletions can be explained strictly by temperature. Low-temperature water-rock interactions likely also contribute to 18O depletion in some of the carbonate cements.;Carbonate cements are present throughout the sediments, although carbonate cementation is not the dominant process in altering sediment properties at Sites C0011 and C0012. Formation of carbonates at shallow depths as well as deeper depths demonstrate that carbonate formation occurs both early and late in the burial history.