Oligocene to early Miocene silicoflagellates from the Ivorian Basin, eastern equatorial Atlantic, Site 959

Abstract

Silicoflagellate biostratigraphy and relative abundance were determined for Site 959 in the eastern equatorial Atlantic. Five silicoflagellate zones, Corbisema apiculata, Naviculopsis biapiculata, Naviculopsis lata, Naviculopsis ponticula, and Corbisema triacantha Zones, were used with the Distephanus speculum hemisphaericus Subzone occurring in the N. biapiculata Zone. The Naviculopsis quadrata Zone is missing between the N. ponticula and N. lata Zones, indicating a hiatus in the lower Miocene cores that corresponds to hiatus NH1. Silicoflagellate relative abundance was graphed as a measure of paleoproductivity for Hole 959A. One large pulse of productivity occurred in the latest early Miocene, right before silica deposition came to an abrupt halt in the earliest middle Miocene. INTRODUCTIONLeg 159 has been one of the few legs of the Ocean Drilling Program to drill the eastern equatorial Atlantic. The objectives of the cruise were to study the evolutionary stages of continental transform margins and investigate deep and intermediate water influences in the eastern equatorial Atlantic throughout the development of the basin. From the Oligocene to the present, Site 959 has been at or near the equator, and therefore, the cores recovered offer an excellent opportunity to study the paleoceanographic history of the eastern equatorial Atlantic and to determine the characteristics of the water masses that were present. In this study, silicoflagellate relative abundance data were collected to define the biostratigraphy for the upper Oligocene to the lower Miocene and investigate the paleoproductivity of the site.Site 959 is located at 3°37.7′N, 2°44.1′W on the Ivory CoastGhana Marginal Ridge south of the Ivorian Basin (Fig. 1). Cores from Site 959 provide a sediment record from the mid-Cretaceous to the present, with silicoflagellates occurring in the Oligocene to lower Miocene. Silicoflagellate preservation and abundance are excellent in the upper Oligocene to uppermost lower Miocene sediment, but downcore in the lower Oligocene, silicoflagellate preservation diminishes. Hole 959A consists of calcareous ooze in the upper 205 m (Sections 159-959A-1H-CC through 22X-CC) underlain by 237 m (Sample 159-959A-23X-1, 39−41 cm, to Sample 159-959A-46X-CC) of interbedded nannofossil ooze and biosiliceous ooze. Twelve samples from the upper portion of Hole 959D (Samples 159-959D-1R-CC through 7R-CC) contain poorly preserved, biogenic siliceous components with sparse nannofossils. Core recovery for this interval of Hole 959D was 16%, which made age designations and identification difficult. Samples downcore from 159-959D-7R-CC consist of diagenetically altered porcellanite and chert and were not examined for silicoflagellates. METHODS AND SAMPLE PREPARATIONSamples were prepared as smear slides on 22 mm × 40 mm cover slips. The number of silicoflagellates was tabulated in 10 transects across the slide, which is equal to an area of 4.92 cm 2 , more than half of the slide area (Tables 1−4). The specimens were counted at 200× m...

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“…spinosa; according to Barron (1985), this diatom assemblage indicates an age of 18.8 to 17.9 Ma. N. ponticula is otherwise reported from the eastern Equatorial Pacific (Engel & McCartney 1990), the eastern Equatorial Atlantic (Schellpeper & Watkins 1998), and outcrops of the temperate North Atlantic (Wetmore & Andrews 1991).…”