5:15 PM - 7:15 PM
[BBG01-P01] Incorporation of over 80% of O2-derived oxygen during the abiotic oxidation of hydrogen sulfide to sulfate: Implication for the source of sulfate in Marinoan cap carbonates
Keywords:Marinoan glaciation, sulfate with negative D17O anomaly, H2S oxidation, Oxygen isotope fractionation, Oxygenase
The Marinoan glaciation (635 Ma termination) is a global glaciation in Cryogenian period, which drastically impacted the earth surface environment and the evolution of organisms. The non-mass-dependent 17O anomaly or Δ'17O value (defined as Δ'17O = ln(1+δ17O)-0.5305ln(1+δ18O)) of barite crystal fans in Marionoan cap carbonates display distinct negative spike as low as -1.25‰. This anomaly is attributed to the accumulation of atmospheric CO2 at hundreds of times of the present atmospheric level (PAL) at the termination of the Marinoan glaciation. Since only tropospheric O2 has a negative Δ'17O value, its incorporation is necessary to impart the 17O anomaly to barite. However, barite with the most negative Δ'17O value have a high δ18O value (ca. +17‰). The δ18O value are too high to be accounted for by the riverine pyrite oxidation pathway because oxygen atom in sulfate mainly derived from water during pyrite oxidation and δ18O value of riverine and ground water is low which ranges from -25‰ to -5‰. As a results, Bao et al (2024) has recently proposed that H2S oxidation in shallow shelf as the source of the sulfate. To test if H2S oxidation can account for the high δ18O value for the product sulfate, we conducted microbial H2S oxidation experiments with Cupriavidus pinatubonensis JMP134, which utilizes persulfide dioxygenase (PDO) during H2S oxidation. We also conducted abiotic H2S oxidation experiments to elucidate the impact of microbial activity for the sulfate production in seawater.
Contrary to our expectations, the oxidation rate of H2S to sulfate in both biotic and abiotic experiments was similar under our experimental conditions (approximately 10% in 2 days). The similarity indicates that sulfate formation proceeded chiefly by abiotic reaction with a trivial impact of microbial activity and suggests that abiotic H2S oxidation is the dominant process in sulfate production in shallow shelf environments. Thus, we conducted oxygen isotope measurements of the product sulfate and water only for the samples of the abiotic experiments. The oxygen isotope measurements revealed that 88.6 ± 1.1% of oxygen in the sulfate was derived from dissolved O2 during aerobic abiotic H2S oxidation. This fraction exceeds both the 25% predicted for the PDO pathway by JMP134 and the less-than-75% predicted by the abiotic H2S oxidation model of Zhang and Millero (1993). According to Zhang and Millero (1993), HSO2- is produced via the reaction of H2S (or HS-) with O2, and its conversion to HSO3- incorporates oxygen from H2O, limiting the O2-derived fraction in sulfate to less than 75%. Thus, our data suggest that the dominant portion of the sulfate must have been formed via a pathway that does not involve the intermediate sulfite or bisulfite, as originally proposed by Hoffmann and Lim (1979), (HSO2- + O2 → SO42- + H+).
In addition to determining the fraction of O2-derived oxygen in the product sulfate, we determined the apparent kinetic isotope effect (AKIE) during O2 incorporation into sulfate as 18εSO4-O2 = -12.2 ± 0.7‰ based on the following equation:
δ18OSO4 = X (δ18OH2O + 18εSO4-H2O) + (1 - X) (δ18OO2 + 18εSO4-O2)
where X denotes fraction of H2O-derived oxygen in sulfate. The AKIE of H2O incorporation into sulfate was assumed as 18εSO4-H2O = +3.6 ± 1.53‰ (Balci et al., 2007). Since 88.6% of oxygen in the product sulfate was O2-derived, its δ18O value primarily depends on the δ18O value of O2 and the AKIE of -12.2‰. Assuming barite sulfate originated solely from the abiotic H2S oxidation, we estimate that the δ18O value of atmospheric O2 at the time of Marinoan cap carbonate was ~+29.2‰, which is 5‰ higher than the modern value (+24.05‰). These results support the hypothesis that seawater sulfate in the Marinoan cap carbonate was produced by H2S oxidation and subsequently trapped as barite without isotopic alteration during the upwelling of sulfidic, Ba-rich deep water into oxic shallow seawater.
Contrary to our expectations, the oxidation rate of H2S to sulfate in both biotic and abiotic experiments was similar under our experimental conditions (approximately 10% in 2 days). The similarity indicates that sulfate formation proceeded chiefly by abiotic reaction with a trivial impact of microbial activity and suggests that abiotic H2S oxidation is the dominant process in sulfate production in shallow shelf environments. Thus, we conducted oxygen isotope measurements of the product sulfate and water only for the samples of the abiotic experiments. The oxygen isotope measurements revealed that 88.6 ± 1.1% of oxygen in the sulfate was derived from dissolved O2 during aerobic abiotic H2S oxidation. This fraction exceeds both the 25% predicted for the PDO pathway by JMP134 and the less-than-75% predicted by the abiotic H2S oxidation model of Zhang and Millero (1993). According to Zhang and Millero (1993), HSO2- is produced via the reaction of H2S (or HS-) with O2, and its conversion to HSO3- incorporates oxygen from H2O, limiting the O2-derived fraction in sulfate to less than 75%. Thus, our data suggest that the dominant portion of the sulfate must have been formed via a pathway that does not involve the intermediate sulfite or bisulfite, as originally proposed by Hoffmann and Lim (1979), (HSO2- + O2 → SO42- + H+).
In addition to determining the fraction of O2-derived oxygen in the product sulfate, we determined the apparent kinetic isotope effect (AKIE) during O2 incorporation into sulfate as 18εSO4-O2 = -12.2 ± 0.7‰ based on the following equation:
δ18OSO4 = X (δ18OH2O + 18εSO4-H2O) + (1 - X) (δ18OO2 + 18εSO4-O2)
where X denotes fraction of H2O-derived oxygen in sulfate. The AKIE of H2O incorporation into sulfate was assumed as 18εSO4-H2O = +3.6 ± 1.53‰ (Balci et al., 2007). Since 88.6% of oxygen in the product sulfate was O2-derived, its δ18O value primarily depends on the δ18O value of O2 and the AKIE of -12.2‰. Assuming barite sulfate originated solely from the abiotic H2S oxidation, we estimate that the δ18O value of atmospheric O2 at the time of Marinoan cap carbonate was ~+29.2‰, which is 5‰ higher than the modern value (+24.05‰). These results support the hypothesis that seawater sulfate in the Marinoan cap carbonate was produced by H2S oxidation and subsequently trapped as barite without isotopic alteration during the upwelling of sulfidic, Ba-rich deep water into oxic shallow seawater.