The origin and fate of C during alteration of the oceanic crust

The contents and isotope compositions of water and carbon, including total, reduced, and inorganic (carbonate) C, were studied in 170 My altered oceanic basalts from Ocean Drilling Program Hole 801C in the western Pacific Ocean. Reduced C contents of 0.12–0.29 wt% CO2 and $\delta ^{13}$C values of $-22.6$ to $-27.8‰$ occur throughout the basement section. High total C concentrations in the upper volcanic section (UVS), above 300 m sub-basement, are dominated by inorganic C, and concentrations of both decrease with depth, from 1.92 to 0.57 wt% CO2 and 1.76 wt% CO2 to 0.66 wt% CO2, respectively. The $\delta ^{13}$C of inorganic C in the UVS ($-$0.4 to ${+}1.5{‰}$) indicates precipitation of seawater dissolved inorganic carbon (DIC) through the intensive circulation of seawater. $\delta $D values of $-$59.8 to ${-}17.6{‰}$ in the UVS also result from seawater interaction. In contrast, total C contents in the lower volcanic section (LVS) are low (0.22–0.39 wt% CO2) and dominated by reduced C, resulting in negative $\delta ^{13}$C values for total C ($-$18.7 to ${-}23.5{‰}$). We propose that a proportion of this reduced C could have formed through abiotic reduction of magmatic CO2 at the ridge axis. The contents and $\delta ^{13}$C values of inorganic carbon in the LVS (0.05–0.09 wt% CO2 and $-$10.7 to ${-}9.5{‰}$, respectively) fall in the range characteristic of C in mid-ocean ridge basalt glasses, also suggesting a magmatic origin. $\delta $D values in the LVS (weighted average $= {-}69.3{‰}$) are consistent with magmatic water. Reduced C in the basalts may also have formed through microbial activity at low temperatures, as indicated by previous work showing negative $\delta ^{34}$S values in the basalts.Our results show: (1) that magmatic C can be stored in altered oceanic basalts both as reduced and inorganic C resulting from high-temperature processes at mid-ocean ridges; (2) that microbial activity may add reduced C to the basalts during low-temperature alteration on ridge flanks; and (3) that circulation of cold seawater in the uppermost few hundred meters of basement adds seawater DIC as carbonate to the basalts and filling fractures in the basement. We estimate the content of magmatic C stored in the altered basaltic crust to be 0.126 wt% CO2. Compared with previous estimates, this concentration probably represents an upper limit for magmatic C. This resultant magmatic C flux into the crust, ranging from $1.5\times 10^{12}$–$2\times 10^{12}$ molC${\cdot }$y$^{-1}$ is similar to the outgassing CO2 flux [${\sim }1.32\pm 0.8$–$2.0 \times 10^{12}$ molC${\cdot }$y$^{-1}$, Le Voyer et al., 2019 and Cartigny et al., 2018, respectively]. Further data are needed to better constrain the fraction of magmatic CO2 that does not escape the oceanic lithosphere but remains stored as reduced and inorganic carbon.