Benthic foraminifera in the Nazaré Canyon, Portuguese continental margin: Sedimentary environments and disturbance

Abstract

Living (rose Bengal stained) benthic foraminifera were investigated from thirteen stations ranging from 146–4976 m water depth in the Nazaré Canyon, located on the Western Iberian continental margin. The total standing stocks (TSS), species assemblages and in-sediment distributions are compared between stations located on the highly disturbed axis of the upper (< 2700 m) and middle canyon (2700–4000 m), the adjacent low energy terraces and the lower canyon (> 4000 m). In addition, the community changes were investigated in relation to water depth and bio-available, organic carbon. Overall, low total standing stocks of foraminifera were found in the disturbed canyon axis, which experiences frequent sediment resuspension and gravity flows. At the upper canyon axis station at 1118 m a rather exceptional fauna was recovered, dominated by a species of Technitella that made up 75% of the TSS. The highest TSSs were recorded in the upper canyon terrace stations, where fine grained sediment and organic carbon are able to accumulate. Standing stocks on the terraces decreased with increasing water depths. The quiescent terrace stations recorded high abundances of infaunal species, including Melonis barleeanum, Globobulimina spp. and Chilostomella oolina. The occurrence of these species reflects the low pore water oxygen concentrations and high bio-available carbon supply. Several tubular Bathysiphon species were found in the upper canyon terrace stations. Instead, the deepest stations of the lower canyon were dominated by another large agglutinated protozoan, Saccorhiza ramosa. The change in the arborescent/tubular foraminiferal community may reflect the increase in sediment oxygenation with increasing water depth in the canyon, Bathysiphon spp. occurring in sediments with higher bio-available carbon flux and S. ramosa inhabiting more oligotrophic areas.