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

Martinez, I.; Shilobreeva, S.; Alt, J.; Polyakov, V.; Agrinier, P.

Авторы: Martinez Isabelle, Shilobreeva Svetlana, Alt Jeffrey, Polyakov Veniamin, Agrinier Pierre
Журнал: Comptes Rendus - Geoscience
Том: 353
Номер: 1
Год издания: 2021
Издательство: Elsevier BV
Местоположение издательства: Netherlands
Первая страница: 319
Последняя страница: 336
DOI: 10.5802/crgeos.61
Аннотация: 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 DrillingProgram Hole 801C in the western Pacific Ocean. Reduced C contents of 0.12–0.29 wt% CO2 and δ13C values of −22.6 to −27.8h 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 δ13C of inorganic C in the UVS (−0.4 to +1.5h) indicates precipitation of seawater dissolved inorganic carbon (DIC) through the intensive circulation of seawater. δD values of −59.8 to −17.6h 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 δ13C values for total C (−18.7 to −23.5h). We propose that a proportion of this reduced C could have formed through abiotic reduction of magmatic CO2 at the ridge axis. The contents andδ13C values of inorganic carbon in the LVS (0.05–0.09 wt% CO2 and −10.7 to −9.5h, respectively) fall in the range characteristic of C in mid-ocean ridge basalt glasses, also suggesting a magmatic origin. δD values in the LVS (weighted average = −69.3h) are consistent with magmatic water. Reduced C in the basalts may also have formed through microbial activity at low temperatures, as indicated byprevious work showing negative δ34S 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 DICas 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 1.5×1012–2×1012 molC·y−1 is similar to the outgassing CO2 flux [∼1.32±0.8–2.0 × 1012 molC·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 lithospherebut remains stored as reduced and inorganic carbon.
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