Abstract: Autochthonous marine pelagic and allochthonous coastal and freshwater diatoms were encountered in five samples from the Ocean Drilling Program (ODP) Leg 116 Holes 717A, 717B, and 717C. Allochthonous diatoms were transported to the equatorial Indian Ocean from inland locations and coastal waters, respectively. The pelagic diatom assemblage consists mainly of warm and/or tropical diatoms known from the Neogene to Holocene. A stratigraphic marker, Rhizosolenia praebergonii var. robusta, which indicates Pliocene-Pleistocene (2.4-1.55 Ma) age of the sediments, was present in the samples from Core 116-717C-27X. Species composition of the whole assemblage is most similar to the tropical Indian Ocean Diatom Zone 6 of Schrader and supports an estimated age of 1.8-2.1 Ma for the sediments. Approximately 100 samples were investigated for the presence of diatoms in the Bengal Fan sediments, ODP site 717. Diatom remnants were present in a number of samples, but their occurrence was rare. Samples with highest frequencies of diatoms were cleaned following the standard cleaning procedure (Kaczmarska, 1976) and then fractionated in heavy liquid (Kotlarczyk and Kaczmarska, 1988). Only five samples yielded diatom frustules in quantities and qualities suitable for further analysis; two samples from the uppermost strata at Holes 717A and 717B (Samples 116-717A-1H-2, 50-52 cm, -717B-1H-CC, 0-2 cm) and three samples from Hole 717C (116-717C-27X-1, 50-54 cm, -717C-27X-2, 3-8 cm, and -717C27X-3, 3-8 cm). The samples and taxa encountered are listed in Table 1. Species composition in all these samples is very similar. Diatom assemblages comprise several floristic elements of different ecological signature. Freshwater diatoms were represented by isolated specimens of Cymbella affinis, Epithemia adnata w.proboscioidea, E. turgida, Gomphonema parvulum v. micropus, Melosira granulata, Rhopalodia gibba, and Synedra ulna. More common were benthic coastal (shelf) diatoms such as several species of the genus Diploneis, Navicula hennedyi, Rhaphoneis amphiceros v. geminifera, Trachyneis aspera, and Triceratium favus. Most diversified and numerous, however, were representatives of neritic and oceanic plankton: Actinocyclus spp., Chaetoceros spp. (mostly present as resting spores), Coscinodiscus spp., Thalassiosira spp., Thalassionema nitzschioides s.l. and Thalassiothrix spp. Several stratigraphically useful taxa present at Site 717 {Actinocyclus ellipticus, A. ellipticus f. lanceolata, Azpeitia nodulifera, Hemidiscus cuneiformis, Nitzschia marina, and Thalassiosira oestrupii) are known from sediments of late Neogene to Holocene.
Publication Year: 1990
Publication Date: 1990-01-01
Language: en
Type: article
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