A multi-basin redox reconstruction for the Miocene Monterey Formation, California, USA

Abstract

Deposition of the Miocene Monterey Formation into numerous basins provides an ideal opportunity to study how preservation of geochemical signatures varies within and among basins as a function of local and global climatic and oceanic conditions. This study presents commonly employed redox indicators, including iron (Fe) speciation and redox-sensitive trace metal concentrations (Mo, U, V), to constrain paleoredox conditions from three Monterey locations: the Santa Barbara Basin, the Santa Maria Basin, and the San Joaquin Basin. The Fe speciation geochemistry of the Santa Barbara and Santa Maria Basin sediments are consistent with a wide range of redox conditions from suboxic to euxinic. These results, in combination with associated phosphorites and trace metal enrichment patterns, indicate oscillatory redox conditions, perhaps with a chemocline that fluctuated above the sediment-water interface on seasonal or multi-year timescales. Overall, these sections were deposited in environments similar to modern phosphorite-forming and upwelling-dominated oxygen minimum zone settings like the Peru Margin. Geochemical evidence indicates that the San Joaquin Basin was generally more restricted and more stably anoxic compared to the other two basins, perhaps more analogous to the modern Cariaco Basin, at least for the studied interval. Iron speciation in the San Joaquin Basin reveals anoxia and euxinia over the entirety of the studied interval, although trace metal concentrations are generally at the low end of those typical of anoxic/euxinic conditions—consistent with a restricted connection with the open ocean. The one exception is an interval c. ~11.6 million years ago (Ma) with elevated concentrations of redox-sensitive elements that may represent a decrease in basin restriction or an increase in water column sulfide concentrations. The present-day California Margin does not achieve euxinic conditions in similar basinal settings comparable with these Monterey sections, suggesting fundamental differences for the Miocene California margin that may carry implications for global-scale oxygen deficiency in Miocene oxygen minimum zones.