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

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. r s w l d 0 e 9 o r fi o in e n , but nvir m lte on INTRODUCTION Leg 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). Co from Site 959 provide a sediment record from the mid-Cretaceou the present, with silicoflagellates occurring in the Oligocene to lo Miocene. Silicoflagellate preservation and abundance are exce in the upper Oligocene to uppermost lower Miocene sediment, downcore in the lower Oligocene, silicoflagellate preservation minishes. Hole 959A consists of calcareous ooze in the upper 2 (Sections 159-959A-1H-CC through 22X-CC) underlain by 237 (Sample 159-959A-23X-1, 39 −41 cm, to Sample 159-959A-46X CC) of interbedded nannofossil ooze and biosiliceous ooze. Tw samples from the upper portion of Hole 959D (Samples 159-95 1R-CC through 7R-CC) contain poorly preserved, biogenic silice components with sparse nannofossils. Core recovery for this inte of Hole 959D was 16%, which made age designations and identi tion difficult. Samples downcore from 159-959D-7R-CC consist diagenetically altered porcellanite and chert and were not exam for silicoflagellates. METHODS AND SAMPLE PREPARATION Samples were prepared as smear slides on 22 mm × 40 mm cover slips. The number of silicoflagellates was tabulated in 10 trans 1Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program). 2Geology Department, University of Nebraska, 214 Bessey Hall, Lincoln, NE 68588-0340, U.S.A. 00091355@bigred.unl.edu es to er lent but i5 m m lve Dus val caf ed cts 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 × magnification, and a magnification of 400 × was used for more precise identification. Only specimens with over half of the skeleto were counted. On a few slides, over 300 specimens were counted for most of the slides, 50 −100 specimens were found in the 10 transects. Because aberrant specimens may be an indication of e ronmental stress, they were counted on a separate tally. Silicoflagellates from Holes 959A and 959D were examined fo biostratigraphic purposes. The slides from Hole 959A, ranging fro lower Miocene to upper Oligocene, exhibit good silicoflagellate ske etal preservation. In sediments of the lower Oligocene, incomple diatom and silicoflagellate skeletons show evidence of dissoluti and diagenesis. BIOSTRATIGRAPHY The biostratigraphy used here is derived from the previously published zonations of Bukry (1981), Perch-Nielsen (1985), and Locker and Martini (1986). Silicoflagellate counts in the oldest cores of Hole 959A and all of Hole 959D are so low that it is difficult to obtain a good biostratigraphic record for the ?lower Oligocene to lowermost upper Oligocene. Corbisema triacantha Zone Definition: This zone is bracketed by the last occurrence of Naviculopsis to the last occurrence of Corbisema triacantha triacantha. Author: Martini, 1971. Common species: The common species in this zone include Dictyocha pulchella, Dictyocha fibula ausonia, Corbisema triacantha triacantha, Bachmannocena elliptica, and D. fibula ausonia (naviculopsoid). Distephanus speculum hemisphaericus also is present in the lower part of the zone, but is not as common as the other listed species. Remarks: The upper part of the C. triacantha Zone is not recognized in the sediments of Site 959, because there was a halt in siliceous preservation by the middle Miocene, before the last occurrence of C. triacantha. The last occurrence of Naviculopsis lies in Sample 159-959A-25X-CC. The minimum thickness of the zone is 27.2 m. Naviculopsis ponticula Zone Definition: This zone is the interval defined by the first occurrence of Naviculopsis ponticula to the last occurrence of Naviculopsis. Author: Bukry, 1981. Common species: The common species include C. triacantha, N. ponticula, Distephanus crux, and B. elliptica. Other species include Distephanus speculum patulus, Distephanus speculum hemisphaericus, and Distephanus speculum pentagonus. Remarks: This zone was found in Hole 959A with the first occurrence of N. ponticula in Sample 159-959A-29X-3, 38−40 cm, and the last occurrence