Occurrence of Neogloboquadrina pachyderma new subspecies in the shelf-slope sediments of northern Indian Ocean

Abstract

Specimens of @iNeogloboquadrina pachyderma@@ (Ehrenberg) which are considered to be typical bipolar, coolerwater species, are described in the recent shelf-slope sediments of the northern Indian Ocean as far north as 23~'N in the Arabian Sea and 18~'N in the Bay of Bengal. Studies of hydrological conditions in the Indian Ocean reveal that the Subtropical Subsurface Water Mass is traceable as far north as the Gulf of Aden, and the Indian Ocean Deep Bottom Water Mass originating in the deepest levels of the South Polar region moves northwards and passing through the equator enters the Arabian Sea and the Bay of Bengal; and also the Indian Central Water Mass and the Equatorial Water Mass having different T-S relationships are responsible for the transport of this species to the middle and low latitudes. It is proposed that the forms which occur in the low latitudes (up to 25~'N) in the northern Indian Ocean be referred to henceforth as (1) (a) @iN. pachyderma dextralis typica@@ Setty, n. subsp. (sensu stricto) if the species is right-coiled, and (b) @iN. pachyderma sinistralis typica@@ n. subsp. Setty (sensu stricto) if the species is left-coiled; (2) as @iN. pachyderma dextralis@@, Setty n. subs. (sensu stricto) for those which occur in the mid-latitudes; and (3) all those forms which occur in the high latitudes as @iN. pachyderma@@ (sensu stricto) where they are typically left coiled