Isotopic evidence for complex microbial ecosystems in the phosphate-rich interval of the Miocene Monterey Formation

The middle Miocene Monterey Formation has long been debated as a crucial global sink for organic carbon that led to global cooling. We evaluate proxies for the microbial ecosystem to investigate organic carbon burial within the phosphate-rich interval of the Monterey Formation at Naples Beach, California by combining mineralogical evidence with δ34S analyses of carbonate associated sulfate (CAS). All δ34S are below Miocene seawater values (~22‰, VCDT) and range from +12.2‰ to +18.5‰. δ34SCAS < δ34Sseawater sulfate is typical of microbial environments at or near the interface between oxic and suboxic waters. Low pyrite concentrations characteristic of the Monterey Formation indicate that the system is iron-limited; iron reducing bacteria consume all ferric iron, producing a small amount of pyrite. Sulfate reducing bacteria then consume the excess, residual sulfate, generating free H2S in the absence of available iron. H2S diffuses upward towards the sediment-water interface (an oxic-suboxic mixing zone) where H2S is oxidized to 34S-depleted sulfate either aerobically or coupled to nitrate reduction, and lowers seawater pH. The high phosphate content and low carbonate content of this interval of the Monterey Formation supports a model of precipitation in lower pH waters. Assuming a -40‰ fractionation of δ34S due to microbial sulfate reduction, we estimate at least a 10%-20% contribution of sulfate from sulfide oxidation to marine porewater sulfate. These results suggest that the phosphate-rich interval of the Monterey Formation housed a complex suite of iron and sulfate reducing bacteria as well as sulfide oxidizing bacteria, suggesting that significant organic carbon was consumed during early diagenesis and may account for low organic carbon content described in previous studies.