Benthic Redox Conditions and Oceanographic Variability in the Upper Central Peruvian Margin since the Nineteenth Century Depicted from Benthic Foraminifera

Abstract:

Benthic foraminifera compose the dominant biota in the central Peruvian margin, which is impinged by an intense Oxygen Minimum Zone (OMZ). Here subsurface and benthic biogeochemical conditions are characterized by oxygen deficiency in the bottom waters, strong fluxes of settling organic matter and reducing conditions in the surface sediments. These processes interact among each other and are amplified or relaxed according to the variability of oceanic conditions at different spatial and temporal scales. Recent studies on living foraminiferal assemblages determined the association of species to a geochemical condition (in terms of redox and organic matter (OM)). Species such as Bolivina costata, Nonionella auris and Virgulinella fragilis were characteristic of sulphidic/labile OM-rich sediments, whereas Bolivina pacifica headed the assemblage representative of postoxia (no oxygen, non sulphidic)/preserved OM.

The present study aims to reconstruct the decadal to multidecadal variation of benthic paleo-redox conditions for the last two centuries, based on laminated sedimentary records of benthic foraminiferal assemblages and redox-sensitive metals (Mo, Re, etc.) in the upper margin off Callao (12°S). Three major multidecadal periods were determined for the record: i) the mid to late nineteenth century, characterized by the occurrence of massive diatom-rich sedimentation events, development of bottom anoxia, and higher abundance of Bolivina costata and Nonionella auris; ii) the late nineteenth century to mid-twentieth century, featuring interdecadal variations of redox conditions; and iii) the late twentieth century until the early 2000’s, in which redox-sensitive metal records exhibit a trend towards less reducing conditions, accompanied by higher relative abundances of postoxia foraminiferal species (e.g. Bolivina pacifica or Bolivina plicata). Remarkably, the first multidecadal period is associated with a higher ENSO activity, whereas the latest one is parallel to the period of coastal cooling and increasing fluxes of productivity proxies. Our findings suggest that besides export productivity, the upper margin benthic redox states are modulated by vertical mixing and/or subsurface ventilation.