Pelletal structures in Peruvian upwelling sediments

Abstract

Late Quaternary sediments from the Peru upwelling zone have been investigated using backscattered electron imagery. The dysaerobic to anaerobic conditions which persist within the strong oxygen minimum zone intersecting the shelf and upper slope sediments, have resulted in the preservation of a wide range of microscopic fine structure relating to biological activity. Two forms of pelletal structures are present: pellets and aggregate structures. (1) Pellets are oval in cross section with a mean diameter of 610μm and sharp boundaries; three common pellet types have been identified: (a) crushed diatom and clay pellets; (b) intact diatom pellets; and (c) silt pellets. (2) Aggregate structures are smaller than pellets with a mean diameter of 360 μm. They have indistinct boundaries and may be composed either of pure diatom ooze or a mix of clay, silt and diatoms. The size, shape and mode of occurrence of aggregate structures suggest that they are the product of a meiofauna, redistributing the topmost sediment during periods of very slight increases in oxygen. Most of the crushed diatom/clay pellets and intact diatom pellets are probably also the product of a surficial deposit-feeding fauna tolerant of low dissolved oxygen levels such as polychaete worms. Silt pellets may record the chamber building activities of agglutinating foraminifera.