VARIATION IN BIOGENIC AND DETRITAL FLUXES AND FORMATION OF LAMINAE IN LATE QUATERNARY SEDIMENTS FROM THE PERUVIAN COASTAL UPWELLING ZONE

Studies of laminated sediments from the Peru margin using back scattered electron imagery (BSEI) provide a high resolution record of variation in detrital and biogenic fluxes and highlight the uncertainty involved in preserving a record of surface processes in bottom sediments. Intermittently laminated sediments, which predominate in the Peru forearc basins, record episodic major diatom bloom events from which there was adequate flux to counteract water column processes of dissolution and comminution. A distinctive, sub-millimetre lamination with a limited occurrence during Isotope Stage 5, comprises a regular alternation between silt-rich and clay-rich couplets (which may represent annual deposition) with irregularly interlaminated diatom ooze containing near monospecific, bloom assemblages. The absence of diatom ooze layers between silt-clay couplets may be controlled by variation of either the intensity of upwelling and/or the nutrient content of upwelled waters. Macroscopically obvious (0.5-3 cm-thick) pelleted or homogeneous zones which occur throughout the sub-millimetre laminated sequence represent periods of increased oxygenation with a probable 8-16 year duration occurring at irregular intervals of a few years to a few hundred years. A slight but pervasive microscopic bioturbation is common within laminated intervals which would macroscopically be described as anaerobic sediments.