Taxonomy and Bathymetric Distribution of Holocene Deep-Sea Benthic Foraminifera in the Indian Ocean and the Red Sea

Abstract

This study examines and illustrates deep-sea benthic foraminifera from 21 Holocene samples from the Indian Ocean and the Red Sea Deep Sea Drilling Project sites. The samples represent a wide range of water depths (ca 800-5600m). A total of 226 taxa were recorded and their bathymetric distribution was examined. Seventy six important taxa are illustrated using scanning electron micrography. About 76% of the total species recorded belong to the suborder Rotaliina, 13% to Textulariina, and 11% to Miliolina. Identification of 18 species was equivocal and, consequently, they are described with 'cf.' Thirty three taxa are identified up to generic level. Calcareous forms are dominant above 5000m whereas agglutinated forms are dominant below 5000m water depths. This supports the theory that the Calcite Compensation Depth (CCD) in the Indian Ocean lies at about 5000m as reported earlier. Characteristic assemblages are found to be associated with distinct deep-water masses. INTRODUCTION Taxonomy plays a significant role in the understanding of ecologic and biostratigraphic importance of benthic foraminifera. Until the 1970's deep sea benthic foraminifera were largely used for taxonomic purposes. Later, studies by Streeter (1973) and Schnitker (1974) mainly focused on relationships between modem deep-sea benthic foraminifera and deep-water masses which allowed micropaleontologists to decipher environmental changes occurring through the Neogene. These studies on faunal-water mass associations had a great impact on earlier interpretations and eclipsed advances on taxonomic reports of benthic foraminifera owing to the incorrect belief that the deep-sea fauna are cosmopolitan. Ambiguous taxonomic identifications in many studies have resulted in inconsistent ecologic interpretations while using the same taxon or taxa in different regions. Thus a stable taxonomy assists in comparing paleontologic data and offers a more reliable understanding of the paleoecological history of different regions. The first comprehensive taxonomic study on Recent deep-sea benthic foraminifera was that of Brady (1884), based on samples dredged from the world ocean during the Challenger expedition (1873-1876). Brady's study was fundamental to the development of the taxonomy of deep-sea benthic foraminifera and provided the basis for later work. Deep-sea benthic foraminifera from the Neogene sequences from the Indian Ocean Deep Sea Drilling Project (DSDP) holes were first illustrated by Boltovskoy (1978) whereas Recent deep-sea benthic foraminifera from the other southeast Indian Ocean deep-sea cores were first figured by Corliss (1979). More recently, several studies have presented illustrations and taxonomic notes of Neogene deep-sea benthic foraminifera from the Indian Ocean region (Srinivasan and Sharma 1980; Gupta 1987; Boersma 1990; Hermelin 1991; Mackensen 1992). The present study will improve our understanding of the Indian Ocean benthic foraminifera, their bathymetry, and water mass relationships, and will assist in comparing paleontologic data for understanding paleoecologic history of different regions. MATERIALS AND METHODS The present study is based on benthic faunal data from 21 DSDP holes from the Indian Ocean (16 holes) and the Red Sea (5 holes) drilled during legs 22-26 of D/V Glomar Challenger. The holes cover a wide area and a wide range of water depths (table 1, text-fig. 1). The physiography and water mass conditions in the study area have been described in detail by Gupta (1994). TEXT-FIGURE 1 Location of DSDP sites studied. Shaded area represents high productivity equatorial belt. micropaleontology, vol. 40, no. 4, pp. 351-367, text-figure 1, plates 1-6, tables 1-3, appendix 1, 1994 351 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 06:35:24 UTC All use subject to http://about.jstor.org/terms Anil K. Gupta: Taxonomy and bathymetric distribution of Holocene deep-sea benthicforaminifera in the Indian Ocean and the Red Sea TABLE 1 Details of sample/site locations. All samples are from Core Section 1-1. Site 211 213