Sedimentation across the central California oxygen minimum zone: an alternative coastal upwelling sequence

Abstract

Distribution, abundance, and diversity of terrigenous, authigenous, and biogenous material provide evidence of the effect of bottom currents and oxygen minimum zone (OMZ) on continental slope sedimentation offshore central California. Box core samples and bottom photographs collected across the oxygen minimum zone of the coastal upwelling system were analyzed for lithologic and biologic trends. Three major OMZ facies are identified, along the upper and lower edges of the OMZ and one at its core. The upper boundary (525 m) of the central California OMZ (< 0.5 ml/liter O 2 ) is characterized by glauconitic gravelly sands relatively depleted in detrital silicates and enriched in carbonate and total organic carbon (TOC). These upper-boundary sands also have a maximum macrofauna density and a high abundance of foraminiferal fragments (> 80% of total forams) with Cassidulina the dominant benthic foraminifer. With increasing water depth, decreasing current speed, and dissolved oxygen content toward the core of the OMZ (750 m), grain size, glauconite, carbonate, TOC, and foraminiferal fragments all decrease as the abundance of mica and detrital silicates increase. The greatest number of genera of benthic foraminifera occurs at the core of the OMZ where their relative concentration is at a minimum and Bolivina is the dominant genus. No heavily calcified macrofauna were found at the core of the OMZ; however, near-surface sediments are highly bioturbated. As the lower boundary (1,025 m) of the OMZ is approached, grain size and abundance of detrital silicates continue to decrease as glauconite disappears. Concomitantly, the relative abundance of mica, carbonate, TOC, siliceous microfauna, benthic foraminifers, and foram fragments increase. The benthic foraminifer population consists largely (> 40%) of Globobulimina and Praeglobobulimina . The lower edge of the OMZ is highlighted by a distinct peak of fecal pellets (25% of sand-size material). Lithofacies and biofacies associated with the OMZ are controlled by an interplay of oceanic circulation, dissolved oxygen concentrations, and possibly by increased biological activity within the OMZ edge environment and slight changes in the slope gradient. Recognition of key lithofacies and biofacies trends may prove to be powerful paleoenvironmental/paleoceanographic indicators and may be useful in identifying ancient zones of upwelling analogous to the modern central California system where phosphorites, organic-rich shales, and/or chert are absent.