Biostratigraphy And Development Conditions Of Nannoplankton In The East Pacific Equatorial Area
Abstract
Calcareous nannoplankton from the East Pacific Rise bottom sediments between Clarion and Clipperton fracture zones was researched. The sediments were taken in the 41-st cruise of R/V "Dmitry Mendeleev" by five piston cores (PC) and eleven bottom scribe apparatuses (BSA). This area is a zone of strong near-equatorial biogenic sedimentation that developed in altering environments. The deep tectonic reorganization finished up to the Oligocene. It resulted in the climatic cooling, falling of the sea level, and intensification of carbonate dissolution, deepening of carbonate compensation depth, narrowing of the carbonate sedimentation areas and developing of mostly siliceous biogenic sedimentations. Calcareous nannoplankton was studied from the sediments derived at three polygons: polygon 1 with stations 3830 and 3835; polygon 2 with stations 3833 and 3911; polygon G with stations 3875, 3884, 3886 and sublatitude transect with the stations 3834, 3847, and 3849. The sediments consist of the changed biogenic carbonate oozes (with coccoliths and plankton foraminifers) and of the clays of different compositions (with diatomes and radiolaries). The 1st and 2d polygons are situated in the Miocene and Late Oligocene folding thickness area that was overlapped by Quaternary sediments. This paper uses two nannoplankton zone schemes of the biostratigraphic division — by D. Bukry (Okada, Bukry, 1980) and S. Gartner (1977). The oldest sediments (BSA probes 3833-8) included abundant and good preserved nannofossis of Sphenolithus predistentus zone, the Lower Oligocene, Sphenolithus ciperoensis zone, the Upper Oligocene (BSA probes 3911-85, 91) and Triquetrirhabdulus carinatus zone with subzones of the Upper Oligocene and Lower Miocene (BSA probes 3833, 3833-10, 30, PC probes 3833-22, 3834). In the nannofossil associations few reworked species were determined. The preservation of coccolith material fixed the processes of bottom dissolution. The Early Pleistocene species occured in the upper part of PC 3834. They identify the Calcidiscus macintyrei and Helicosphaera sellii zones. The Quaternary sediments were also taken at sublalitude transect stations. The most complete section of BSA 3875 includes the zones Emiliania huxleyi Acme, Emiliania huxleyi, Gephyrocapsa oceanica and Pseudoemiliania lacunosa. There are lots of the Early Miocene species recorded among the Early Quaternary nannofossils. The species of the Middle, Late Miocene, and Pliocene are absent. The sediments of these ages were not revealed both in the researched sequences and in the deep sea sites sections of this area. The prolonged lack of sedimentation or partial-only sedimentation with complete dissolution of carbonate materials was fixed in sections of 3834 and 3833-22 PC from the Middle Miocene to Early Pleistocene, from about 18 ma to Recent. This "dumb thickness" was formed in the bottom environments that were useless for preservation of the carbonate micropaleontology rests. The nannofossil associations gave an opportunity to determine some climatic changes in this area during Neogene and Quaternery periods: It was a transition from Oligocene tropical environments to Miocene-Quaternary subtropical ones; the weakening of cool water flow happened together with lowering of the bottom dissolution processes, and with weakening of reworked allochton coccolth material implication.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1525D