Benthic Foraminifera as Proxy for Oxygen-depleted Conditions Off the Central West Coast of India

Abstract In order to study the response of benthic foraminifera, especially the rectilinear bi- and tri-serial benthic foraminifera (RBF) to oxygen-depleted conditions from the Arabian Sea off central west coast of India, 103 surface sediment samples collected from a water depth of 15 m to 3300 m were studied for the benthic foraminiferal content. A total of 383 recent benthic foraminiferal species were identified. Out of the total 383 benthic foraminiferal species identified in the study area, 65 species belong to 18 rectilinear bi- and tri-serial benthic foraminifer genera. The RBF abundance remains consistently high (>40%) between 90 m to -1200 m water depth. The intermediate depth zone of increased abundance of RBFs coincides with the prominent intermediate water oxygen minimum zone of the Arabian Sea (upper boundary -100 m and lower boundary -1000 to 1250 m). The other significant abundance (>40%) of rectilinear benthic foraminiferal forms is reported from the water depth range of -50 to -60 m with the maximum abundance reaching upto -78% at the water depth of 56 m. This zone has also been reported to be affected by seasonal severe oxygen-depleted conditions. Therefore, we propose that the increased abundance of RBF may be used as an indicator of oxygen-depleted conditions in the Arabian Sea. This benthic foraminiferal response to oxygen-depleted conditions, especially from the shallow water depths is significant as it can be applied to document the geologic extent of shallow water oxygen-depleted conditions that have been suggested as a consequence of spurt in the anthropogenic activities in the coastal regions.