Plio-Pleistocene benthic foraminifera of the Tyrrhenian Sea

Benthic foraminifers from Site 652, Site 653 (Hole 653A), and Site 654 of Leg 107 (Tyrrhenian Sea, Western Mediterranean), which penetrated with more or less good recovery the Plio-Pleistocene stratigraphic interval, were studied in a total of 699 close-spaced samples. A total number of 269 species have been classified and their quantitative distribution in each sample is reported. The benthic foraminifers assemblage is more diversified in Site 654, less diversified in Site 652. Less than a half of the benthic foraminifers species listed from Plio-Pleistocene Italian land sections are present in the coeval deep-sea Tyrrhenian record, in which shallow water species are missing and Nodosarids are poorly represented. A very few species have comparable stratigraphic distribution in the three deep-sea sequences and in Italian land sections when compared against calcareous plankton biostratigraphy. In the same three sites, the first appearance levels of several species are younger and younger, and last appearance levels are earlier and earlier from Site 654 to Site 653 and Site 652. Five biostratigraphic events, biochronologically evaluated and occurring at the same level in the deepsea Tyrrhenian record and in several land sections, have been selected as zonal boundaries of the proposed benthic foraminifers biostratigraphic scheme. The Plio-Pleistocene interval has been subdivided into four biozones and one subzone, recognizable both in the deep-sea and land-based sequences. The Cibicidoides (?) italicus assemblage zone stretches from the base of the Pliocene to the extinction level of the zonal marker, biochronologically evaluated at 2.9 Ma. The Cibicidoides robertsonianus interval zone stretches from the Cibicidoides (?) italicus extinction level to the Pliocene Mediterranean FO of Gyroidinoides altiformis, evaluated at 2.4 Ma. The Gyroidinoides altiformis interval zone stretches from the Mediterranean Pliocene FO of the zonal marker to the appearance level of Articulina tubulosa, evaluated at 1.62 Ma. The Articulina tubulosa assemblage zone stretches from the appearance level of the zonal marker to the Recent. In the Articulina tubulosa biozone, the Hyalinea baltica subzone is proposed. The appearance level of Hyalinea baltica is evaluated at 1.35 Ma, well above the Plio-Pleistocene boundary as defined in the Vrica stratotype section.

Neogene planktonic foraminifera of ODP Leg 101 holes

Leg 101 of the Ocean Drilling Program recovered a large volume of Neogene sediments from sites in the Straits of Florida, Little Bahama Bank, and Exuma Sound. In varying amounts, shallow-water, platform-derived carbonate debris is nearly ubiquitous. Reworked planktonic foraminifers are common, especially in the Pliocene-Pleistocene. At Site 626 in the Straits of Florida, a sequence of Holocene to upper Oligocene sediments was recovered. The greatest Neogene hiatus at this site spans the latest Miocene through Pliocene. Below this, several minor hiatuses are present in a generally conformable sequence. From the Little Bahama Bank transect (Sites 627, 628, and 630), a nearly complete composite Neogene section was sampled. At Site 627, a major unconformity separates lowermost Miocene sediments from middle to upper Eocene sediments. A second major unconformity occurs at Site 628. Here, middle Miocene sediments lie above uppermost Oligocene deposits. Sites 632, 633, and 631 in Exuma Sound all bottomed in a thick, lower Pliocene section. The mid-Pliocene is very thin at Sites 633 and 631, while it is better represented at Site 632. Major unconformities at Sites 627 and 628 appear to correlate with periods of elevated sea level, which suggests that carbonate platform shedding may be greatest during this part of the sea-level cycles. One of the salient features of the Bahamas is the lack of any systematic temporal distribution of hiatuses. Only a brief hiatus in the late Pliocene may be regional. It appears that local platform-shedding events were of equal or greater importance in developing the stratigraphy of the Bahamas than regional or eustatic events.

Eocene-Oligocene diatoms in the western Indian Ocean

The occurrence of diatom species in the Eocene-Oligocene sections of Ocean Drilling Program (ODP) Leg 115 sites and Deep Sea Drilling Project (DSDP) Sites 219 and 236 in the low-latitude Indian Ocean are investigated. Diatoms are generally rare and poorly preserved in the Paleogene sequences we studied. The best-preserved assemblages are found close to ash layers in early Oligocene sediments. The low-latitude diatom zonation established for the Atlantic region by Fenner in 1984 is fully applicable to the Paleogene sequences of the western Indian Ocean. Correlation of the diatom zones to the calcareous nannofossil stratigraphy of the sites places the Coscinodiscus excavatus Zone of Fenner within calcareous nannofossil Subzone CP16b. For the Mascarene Plateau and the Chagos Ridge, the times when the sites studied, together with the areas upslope from them, subsided to below the euphotic zone are deduced from changes in the relative abundance between the group of benthic, shallow-water species and Grammatophora spp. vs. the group of fully planktonic diatom species. The Eocene section of Site 707, on the Mascarene Plateau, is characterized by the occurrence of benthic diatoms (approximately 10% of the diatom assemblage). These allochthonous diatoms must have originated from shallow-water environments around volcanic islands that existed upslope from ODP Site 707 in Eocene times. In Oligocene and younger sediments of Sites 707 and 706, occurrences of benthic diatoms are rare and sporadic and interpreted as reworked from older sediments. This indicates that the area upslope from these two Mascarene Plateau sites had subsided below the euphotic zone by the early Oligocene. Only Grammatophora spp., for which a neritic but not benthic habitat is assumed, continues to be abundant throughout the Oligocene sequences. The area of the Madingley Rise sites (Sites 709-710) and nearby shallower areas subsided below the euphotic zone already in middle Eocene times, as benthic diatoms are almost absent from these Eocene sections. Only sites located on abyssal plains, and which intermittently received turbidite sediments (e.g., Sites 708 and 711), contain occasionally single, benthic diatoms of Oligocene age. The occurrence of the freshwater diatom Aulacosira granulata in a few samples of late early Oligocene and late Oligocene age at Sites 707, 709, and 714 is interpreted as windblown. Their presence indicates at least seasonally arid conditions for these periods in the source areas of eastern Africa and India. Three new species and two new combinations are defined: Chaetoceros asymmetricus Fenner sp. nov.; Hemiaulus gracilis Fenner, sp. nov.; Kozloviella meniscosa Fenner, sp. nov.; Cestodiscus demergitus (Fenner) Fenner comb, nov.; and Rocella princeps (Jouse) Fenner comb. nov.