Oceans house a vast majority of microscopic life with millions of species living at different depths from the coast to the abyssal plains. The composition of these microorganisms vary from one group to another. One of the most important microorganisms are Foraminifera with well-preserved fossil record from the Cambrian to the Recent. Foraminifera are a foremost group of calcareous microfossils, and have spectacular fossil record throughout the world ocean. Following the discovery of the microscope by two Dutch spectacle-makers – father-and-son Hans and Zacharias Janssen in 1590, van Leeuwenhoek reported the discovery of micro-organisms on 9 October 1676. Later, French naturalist Alcide d’Orbigny (1826) revolutionized the study of foraminifera, which was followed by numerous contributions to the understanding of foraminifera and their applications. Living both on the bottom (benthic) and floating (planktic) in the water column, these microorganisms inhabit intertidal zone to over 4000 m water depths from equator to poles. Majority of foraminifera are bottom dwelling forms, i.e., benthic in nature which also have a longer range since the Cambrian epoch (~ 541 million years) in contrast to planktic forms which range from the Jurassic (~ 200 million years) to the Recent.
Benthic foraminifera have good fossilization potential and because of the well-established knowledge of their ecological and water mass preferences, they have been widely used to infer Quaternary climate variability as well as ocean circulation turnovers in different ocean basins. Several workers from across the world have contributed to the understanding of deep sea benthic foraminifera and associated bottom water masses in the world ocean including the Indian ocean since the World War – I. Moreover, it has been felt that taxonomy plays a vital role in the selection of right genus and species for their use in marine geosciences and paleoceanography.
Indian Ocean Deep water masses and Benthic Foraminiferal associations
The deep water in the northern Indian Ocean at depths ~ 1200 to 3800 m is North Indian Deep Water (NIDW). The transition from calcite-supersaturated to calcite–undersaturated water lies at about 3600 m which closely coincides with the local transition between NIDW and Antarctic Bottom Water (AABW) in the Indian Ocean. The present day Calcite Compensation Depth (CCD) lies near 5000 m water depth in the Indian Ocean whereas foraminiferal lysocline lies near 3800 m in the equatorial Indian Ocean. The dissolution of calcium carbonate below the lysocline has a major impact upon the benthic foraminiferal assemblages. The effect of dissolution differs in the hyaline, porcelaneous, trochospiral taxa, etc. Therefore, understanding the preservation state of deep-sea benthic foraminifera along with the taxonomic identification is crucial for paleoceanographic studies.
During my doctoral dissertation, I analysed sediment samples from Deep Sea Drilling Project sites 214, 216, 216A and 217, located along the Ninetyeast Ridge in the northern Indian Ocean. A rich assemblage of calcareous benthic foraminifera has been recorded from sites shallower than 5000 m water depths, whereas, below 5000 m, agglutinated taxa are dominating. This corroborates that the Calcite Compensation Depth (CCD) in the Indian Ocean lies at about 5000 m depth. A detailed taxonomic description and systematics of deep-sea benthic foraminifera with scanning electron micrograph (SEM) pictures have been written and compiled in this Monograph. The species have been described with their original references with their distribution and stratigraphic ranges. A small write-up about biostratigraphic importance of benthic foraminifera has also been added.
Diverse assemblages with many rare species in the deep sea and inadequate documentation make the identification of species more difficult. Taxonomic identifications in the present study largely follow Loeblich and Tappan (1964, 1988) up to genus level, and Schwager (1866), Barker (1960), Boltovskoy (1978), and Boersma (1990) up to species level. A total of 257 benthic foraminiferal species and 103 genera belonging to 8 orders, 26 superfamilies and 45 families were recorded from DSDP sites 214, 216, 216A, and 217. The assemblages found at different sites are rather similar. The most common and consistent species which make up the faunas are almost the same at all sites examined. The most dominant orders are Buliminida (83 species) followed by Rotalida (65 species) and Lagenida (61 species). Most of the species of benthic foraminifera reported in the present study from the northern Indian Ocean are cosmopolitan with wide range of bathymetry from upper bathyal to the lower bathyal and upper abyssal zone.
The Indian Ocean has not attracted much efforts to analyze benthic foraminifera as compared to other oceans. This monograph was aimed to fill this gap and should cater to greater needs of University students and corporate professionals. The species have been displayed with different views using Scanning Electron Micrographs (SEM photographs) in 36 beautiful plates.
Barker, R.W., 1960. Taxonomic nates on the species figured by H.B. Brady in his report on the Foraminifera dredged by H.M.S. Challenger during the years 1873-1876. Special Publication, Society of Economic Paleontologists and Mineralogists, no. 9, Tulsa, Oklahoma, USA, p. 1-238, pls. 1-115.
Boersma, A. 1990. Late Oligocene to late Pliocene benthic foraminifers from depth traverses in the central Indian Ocean. In: Duncan, RA; Backmann, J; Peterson, LC; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), v. 115, p. 315-379.
Boltovskoy, E., 1978. Late Cenozoic Benthonic Foraminifera of the Ninetyeast Ridge (Indian Ocean). Marine Geology, v. 26, p. 139-175, pl. I-VIII.
Loeblich, A.R., Jr. and Tappan, H., 1964 Treatise on Invertebrate Paleontology; Part C, Protista 2, Sarcodina, chiefly “Thecamoebians and Foraminiferida”. v. 1-2, Geological Society of America and University of Kansas Press, p. Cl- C900.
Loeblich, A.R. and Tappan, H., 1987. Foraminiferal Genera and Their Classification, v. 1–2. Van Nostrand Reinhold, New York.
Orbigny, Acide Dessalines d’, 1826. Tableau methodique de la class des Céphalopodes. Annales des Sciences Naturelles Paris, ser.1, v. 7, p. 245-314, p1. 10-17.
Schwager, C., 1866. Fossile Foraminiferen von Kar Nikobar. Reise der Österreichischen Fregatte Novara um die Erde in den Jahren 1857, 1858, 1859 unter den Befehlen des Commodore B. von Wüllerstorf-Urbair, Geologischer Theil, vol. 2, no. 1 (Zweite Abtheilung, Paläontologische Mittheilungen), p. 187-268, pl. IV-VII.