A geochemical model of the Peru Basin deep-sea floor—and the response of the system to technical impacts

A geochemical model of the Peru Basin deep-sea floor, based on an extensive set of field data as well as on numerical simulations, is presented. The model takes into account the vertical oscillations of the redox zonation that occur in response to both long-term (glacial/interglacial) and short-term (El Niño Southern Oscillation (ENSO) time scale) variations in the depositional flux of organic matter. Field evidence of reaction between the pore water NO3− and an oxidizable fraction of the structural Fe(II) in the clay mineral content of the deep-sea sediments is provided. The conditions of formation and destruction of reactive clay Fe(II) layers in the sea floor are defined, whereby a new paleo-redox proxy is established. Transitional NO3− profile shapes are explained by periodic contractions and expansions of the oxic zone (ocean bottom respiration) on the ENSO time scale. The near-surface oscillations of the oxic–suboxic boundary constitute a redox pump mechanism of major importance with respect to diagenetic trace metal enrichments and manganese nodule formation, which may account for the particularly high nodule growth rates in this ocean basin. These conditions are due to the similar depth ranges of both the O2 penetration in the sea floor and the bioturbated high reactivity surface layer (HRSL), all against the background of ENSO-related large variations in depositional Corg flux. Removal of the HRSL in the course of deep-sea mining would result in a massive expansion of the oxic surface layer and, thus, the shut down of the near-surface redox pump for centuries, which is demonstrated by numerical modeling.