Abstract. Foraminifer count and multispecies isotopic data from

The Neogene biostratigraphy presented here is based on the study of 230 samples through 737 m of pelagic sediment in Hole 806B. Sediment accumulation is interrupted only once in the uppermost lower Miocene (Zone N6), apparently coincident with a widespread deep-sea hiatus. Preservation of planktonic foraminifers through the section ranges from good to moderately poor. One hundred and ten species of planktonic foraminifers were identified; taxonomic notes on most species are included. All of the standard low-latitude Neogene foraminiferal zones are delineated, with the exceptions of Zones N8 and N9 because of a high first occurrence of Orbulina, and Zones N18 and N19 because of a high first occurrence of Sphaeroidinella dehiscens.Good agreement exists between the published account of the variation in planktonic foraminiferal species richness and the rates of diversification and turnover, and measurements of these evolutionary indexes in the record of Hole 806B. The global pattern of change in tropical/transitional species richness is paralleled in Hole 806B, with departures caused by either ecological conditions peculiar to the western equatorial Pacific or by inexactness in the estimation of million-year intervals in Hole 806B.Temporal changes in the relative abundance of taxa in the sediment assemblages, considered in light of their depth habitats, reveal a detailed picture of historical change in the structure of the upper water column over the Ontong Java Plateau. The dominance of surface dwellers (Paragloborotalia kugleri, P. mayeri, Dentoglobigerina altispira, Globigerinita glutinata, and Globigerinoides spp.) throughout the lower and middle Miocene is replaced by a more equitable distribution of surface (D. altispira and Globigerinoides spp.), intermediate {Globorotalia menardii plexus), and deep (Streptochilus spp.) dwellers in the late Miocene, following the closing of the Indo-Pacific Seaway and the initiation of large-scale glaciation in the Antarctic. The shoaling of the thermocline along the equator engendered by these climatic and tectonic events persisted through the Pliocene, when initial increases in the abundance of a new set of shallow, intermediate, and deep dwelling species of planktonic foraminifers coincide with the closing of the Panamanian Seaway.

Ocean Drilling Program Site 925 is situated on Ceara Rise in the western tropical Atlantic at 4û12 ′ N, 43û30 ′ W and in 3041 m water depth. The section contains well-preserved planktonic microfossils through the late Miocene, with only moderate dissolution of carbonate in the middle Miocene. No unconformities are apparent.Datums used to establish the biozonation, and to further reÞne the age model, were constrained shipboard to within 1.5 m. In general, one sample per core has been re-examined to identify all species present. Globorotalia menardii, Globorotalia tumida, Globorotalia truncatulinoides, Pulleniatina obliquiloculata, and Neogloboquadrina dutertrei vary considerably in abundance, and each is occasionally absent in the Pleistocene, which suggests signiÞcant oceanographic change through this interval. The published ages assigned to planktonic foraminifer datums are compared to those suggested by the astrochronologic time scale. Several signiÞcant discrepancies (>0.25 m.y.) are found. Some are due to taxonomic problems (e.g., Þrst occurrence [FO] of Pulleniatina Þnalis, FO and last occurrence [LO] of Hirsutella cibaoensis, LO Neogloboquadrina acostaensis ), but many may identify real diachrons (e.g., LO Hirsutella margaritae and FO Truncorotalia crassaformis ). Discrepancies that involve zonal boundary markers (FO Globoturborotalita nepenthes and LO Fohsella fohsi ) should be further studied, as revisions to the Neogene zonation may be in order.Global rates of planktonic foraminiferal taxonomic evolution are compared to values measured at Site 925. SigniÞcant discrepancies are found, which suggests that the closing of the Central American Seaway decoupled patterns of evolution in the tropical Atlantic and PaciÞc.An assessment of planktonic foraminiferal dissolution at all Þve sites in the Ceara Rise depth transect reveals three apparent ÒtransgressionsÓ of corrosive bottom water to shallower depths on the rise between 10 and 12 Ma, at 7 Ma, and at 1 Ma.

Ten ODP sites drilled in a depth transect (2164^4775 m water depth) during Leg 172 recovered high-deposition rate ( s 20 cm/kyr) sedimentary sections from sediment drifts in the western North Atlantic. For each site an age model covering the past 0.8^0.9 Ma has been developed. The time scales have a resolution of 10^20 kyr and are derived by tuning variations of estimated carbonate content to the orbital parameters precession and obliquity. Based on the similarity in the signature of proxy records and the spectral character of the time series, the sites are divided into two groups: precession cycles are better developed in carbonate records from a group of shallow sites (21642 975 m water depth, Sites 1055^1058) while the deeper sites (2995^4775 m water depth, Sites 1060^1063) are characterized by higher spectral density in the obliquity band. The resulting time scales show excellent coherence with other dated carbonate and isotope records from low latitudes. Besides the typical Milankovitch cyclicity significant variance of the resulting carbonate time series is concentrated at millennial-scale changes with periods of about 12, 6, 4, 2.5, and 1.5 kyr. Comparisons of carbonate records from the Blake Bahama Outer Ridge and the Bermuda Rise reveal a remarkable similarity in the time and frequency domain indicating a basin-wide uniform sedimentation pattern during the last 0.9 Ma. ß

The relative abundance of all planktonic foraminifer species and the d 18 O and d 13 C values of three species ( Globigerinoides sacculifer, a mixed-layer dweller; Neogloboquadrina dutertrei, a thermocline dweller; and Truncorotalia crassaformis, a deep dweller) was determined in 25 samples at ~250 k.y. intervals through the last 6 m.y. at Ocean Drilling Program Site 925. Combined, the data indicate that mixed-layer depth was a minimum at the end of the early Pliocene (4.0 Ma) and gradually increased toward the Pleistocene.The d 18 O and d 13 C gradients in the mixed layer and the thermocline were derived by subtracting isotopic values of Globigerinoides sacculifer from Neogloboquadrina dutertrei and values of N. dutertrei from Truncorotalia crassaformis. Changes in d 18 O gradients were used as a proxy record for variation in thermocline strength and position, while changes in d 13 C gradients served a like purpose for productivity variation.The early Pliocene was characterized by relatively high numbers of thermocline-dwelling taxa, relatively small mixed-layer d 18 O gradients, and relatively large d 13 C gradients. These indicators suggest that downwelling was at a minimum for the last 6 m.y. during this period at Site 925, the temperature difference between G. sacculifer and N. dutertrei habitats was small, and surface productivity was relatively great. After 2.5 Ma thermocline-dwelling speciesÕ numbers were relatively low, mixed-layer d 18 O gradients were relatively large, and mixed-layer d 13 C were relatively small. The increased pole-to-equator temperature gradient that accompanied the growth of Northern Hemisphere ice sheets caused the intertropical convergence zone to move toward the equator and exert a growing inßuence at Site 925 through the last 4 m.y.