Middle Eocene to Oligocene Planktonic Diatom Stratigraphy from Deep Sea Drilling Sites in the South Atlantic, Equatorial Pacific, and Indian Oceans

Abstract

Indian Ocean. Biostratigraphy and paleoceanographic implications of the differences in assemblage compositions are discussed. The assemblage analysis for the middle and late Eocene at Sites 356 and 357 in the mid-latitude southwest Atlantic provides insight into the paleolatitudinal distribution of diatom species. In the middle Eocene the diatom assemblages at these sites are composed of low-latitude and cosmopolitan species, whereas during the late Oligocene high-latitude species are present, reflecting an extension toward the equator of the circum-Antarctic assemblage that can be related to climatic cooling and probably intensified meridional wind and surface water circulation. In addition, the Eocene-Oligocene planktonic diatom zonation developed in the low-latitude Atlantic and the Caribbean (Fenner, in press) is tested against the moderately to well-preserved early Oligocene diatom assemblages of the Pacific sites (except for Site 292, at which preservation of diatoms was too poor), and those of middle and late Eocene age at DSDP Site 167. The age assignments obtained by applying this stratigraphic zonation are in accordance with those obtained using other planktonic microfossil groups, thus proving the applicability of this zonation. The wide occurrence of the same species through the equatorial Atlantic and Pacific oceans and the identical sequence of first appearances have both to be seen as a result of the homogenizing effect of unobstructed circum-equatorial circulation in Tethys. In the diatomaceous Eocene-Oligocene section of Site 167 in the equatorial Pacific, the change in assemblage composition toward an assemblage dominated by Cestodiscus spp. in the early Oligocene occurs at about the Eocene/Oligocene boundary, as it also does at DSDP Site 366 in the equatorial Atlantic. The value of this change in assemblage composition as a paleoclimatic indicator and stratigraphic marker is suggested. The only Indian Ocean sites (Sites 216, 217, 220) which contain Eocene-Oligocene diatoms were located just south of the equator during the Eocene. But preservation of diatoms is too poor to allow stratigraphic assignments or to discuss assemblage compositions in relation to other known sections.

Early Oligocene glaciation and productivity in the eastern equatorial Pacific: Insights into global carbon cycling

“…Controlled partially by an increase in diatom accumulation [ Salamy and Zachos , 1999], the increase in opal suggests the initiation of significant seasonal production in the Southern Ocean facilitated by deep mixing and upwelling supplying silica to the photic zone [ Salamy and Zachos , 1999; Berger , 2007]. In this context the modest pulse in diatom abundance that occurs in the early Oligocene at Site 1218 and several other equatorial Pacific sites may take on added significance [ Fenner , 1984; Baldauf , 1992; Barron et al , 2004; J. Bauldauf, unpublished data, 2010].…”

Site 647

“…The assemblage present has some affinities to the assemblage observed in the high-latitude South Atlantic (Gombos and Ciesielski, 1983), with that ob served in the Norwegian Sea (Schrader and Fenner, 1976) but SITE 647 containing less benthic species, and with assemblages observed by Fenner (1984a) in the South Atlantic, Equatorial Pacific, and Indian oceans.…”

“…The occurrence of Coscinodiscus excavatus without C. retic ulatus in Sections 105-647A-25R, CC, through 105-647A-27R, CC, allows placement of this interval in the Coscinodiscus ex cavatus Zone of Fenner (1984b). Most of the species listed above also occur in this interval.…”

“…The Oligocene diatom assemblage present at Site 647 is com posed of high-and low-latitude species and contains a mixture of species observed from the Norwegian-Greenland Sea (Schrader and Fenner, 1976;Dzinoridze et al, 1978), the low-latitude Atlantic (Fenner, 1977(Fenner, , 1984a, the Pacific (Fenner, 1984a;Barron, 1985), and the high-latitude south Atlantic (Gombos and Ciesielski, 1983). Most of the diatoms observed in samples from Core 105-647A-14R are typical of the Eocene-Oligocene and may be re worked.…”

“…The sporadic occurrence of Sceptroneis pupa and Cymatosira compacta in Samples 105-647A-16R-2, 39-41 cm, through 105-647A-25R-2, 120-122 cm, and 105-647A-24R-4, 39-41 cm, respectively, suggests that this inter val is early Oligocene to early late Oligocene in age, according to the range of these species in the Norwegian-Greenland Sea (Schrader and Fenner, 1976). The rare occurrence of Rhizosolenia gravida in Samples 105-647A-19R-5, 124-126 cm, through 105-647A-26R, CC, suggests that this interval is early Oligocene in age, on the basis of the range of this species in the high-lati tudes of the South Atlantic (Gombos and Ciesielski, 1983;Fen ner, 1984b Section 105-647A-23R, CC, through Sample 105-647A-25R-4, 119-121 cm, is assigned to the Cestodiscus reticulatus Zone of Fenner (1984b). This zonal assignment is based on the occur rence of Cestodiscus reticulatus and C. robustus in addition to the assemblage listed above.…”