Diatom biostratigraphy and datum events of ODP Hole 183-1140A

Ocean Drilling Program (ODP) Leg 183 Site 1140 provided a lower Oligocene to middle Miocene record of diatom assemblages from the northern Kerguelen Plateau. Samples were examined to improve the resolution of shipboard diatom biostratigraphy. The material is complementary to that recovered during ODP Legs 119 and 120, and the diatom zonation of Harwood and Maruyama could be readily applied. A standard succession of biostratigraphic zones from the middle Miocene and lower Oligocene was delineated, although some zones were unrecognizable because of poor core recovery. The detailed diatom biostratigraphy presented here agrees well with shipboard calcareous nannofossil biostratigraphy. Sediment accumulation rates based on diatom bioevents average 1.26 cm/k.y.

Diatom biostratigraphy of ODP Holes 191-1179B and 191-1179C

Although the objective of Ocean Drilling Program Leg 191 was to install a seismic monitoring station and to test a hard rock reentry drilling system, several good, near-continuous sedimentary core sequences were recovered during the cruise. Two holes, 1179B and 1179C, yielded an upper Miocene to Pleistocene diatom record through an expanded section with excellent recovery. Because diatom species included in both low-latitude and high-latitude biostratigraphies are present, zonations for the equatorial Pacific and northwest Pacific are applied to the sediments. The oldest zones from each zonation that are represented in the cores are the Nitzschia miocenica Zone and the Rouxia californica Zone, respectively. Only one zonal boundary is not observed within the diatom assemblage, that being the top of the Nitzschia jouseae Zone and base of the Rhizosolenia praebergonii Subzone A (equatorial Pacific). Preservation is good overall, and sample abundances vary from rare to abundant. This would be an excellent section for further biostratigraphic, paleoclimatic, and paleoceanographic study.

Middle Miocene stable carbon and oxygen isotope record of foraminifera of ODP Hole 120-747A

Paleoceanographic variability at southern high latitude Ocean Drilling Program (ODP) Site 747 was investigated in this study through the interval which spans the Middle Miocene Climate Transition (MMCT). Between 15.0 and 12.2 million years ago (Ma), foraminiferal d18O records derived from both benthic (Cibicidoides spp.) and planktonic taxa (Globorotalia praescitula and Globigerina bulloides) reveal a history of changes in water column thermal and salinity structure and a strong imprint of seasonality. Prior to the MMCT, in the interval between 14.35 and 13.9 Ma, G. bulloides displays relatively high d18O values similar to those of G. praescitula, interpreted to indicate weakening of the thermocline and/or increased seasonality with cooler early-spring and/or late-fall temperatures. Following this interval, G. bulloidesd18O values diverge significantly from benthic and G. praescitula values, with G. bulloides values remaining relatively low for at least 600 kyr following the benthic foraminiferal d18O shift during the MMCT at ~13.9 Ma. This divergence in d18O records occurs in direct association with the Mi3 cooling and glaciation event and may suggest: (1) a strengthening of the vertical temperature gradient, with greater cooling of deep waters than surface waters, (2) changes in the depth habitat of G. bulloides, (3) changes in the dominant season of G. bulloides calcification, (4) modification of surface-water d18O values in association with enhanced sea-ice formation, (5) increased surface-water carbonate ion concentration, and/or (6) a significant decrease in surface-water salinity across the MMCT. The first of these possible scenarios is not likely, particularly in light of recent Mg/Ca evidence for significant surface-water cooling in the Southern Ocean associated with the MMCT. Of the remaining possibilities, we favor a change in surface salinity to explain the observed trends in d18O values and hypothesize that surface salinity may have decreased by up to 2 salinity units at ~13.9 Ma. In this scenario, the development of a lower-salinity Antarctic surface layer coincided with regional cooling of both surface and deep waters of the Southern Ocean during the Mi3 glaciation of East Antarctica, and contributed into the dominance of Neogloboquadrina spp. between 13.8 and 13.2 Ma. Additionally, the distinct patterns observed in planktonic foraminiferal d18O records spanning the MMCT correspond with changes in the vertical d13C gradient between planktonic and benthic foraminiferal records and major changes in planktonic foraminiferal assemblages at Site 747, providing further evidence of the environmental significance of this climatic transition.

Faunal turnovers in the central Pacific benthic foraminifera during the Paleocene-Eocene thermal maximum

Although it is well known that the Paleocene/Eocene thermal maximum (PETM) coincided with a major benthic foraminiferal extinction event, the detailed pattern of the faunal turnover has not yet been clarified. Our high-resolution benthic foraminiferal and carbon isotope analyses at the low latitude Pacific Ocean Shatsky Rise have revealed the following record of major faunal transitions: (1) An initial turnover which involved the benthic foraminiferal extinction event (BFE). The BFE, marked by a sharp transition from Pre-extinction fauna to Disaster fauna represented by small-sized Bolivina gracilis, expresses the onset of the PETM and the abrupt extinction of about 30% of taxa. This faunal transition lasted about 45-74 kyr after the initiation of the PETM and was followed by: (2) the appearance of Opportunistic fauna represented by Quadrimorphina profunda, which existed for about 74-91 kyr after the initiation of the PETM. These two faunas, which appeared after the extinction event, are characterized by low diversity and dwarfism, possibly due to lowered oxygen condition and decreased surface productivity. The second pronounced turnover involved the gradual recovery from Opportunistic Fauna to the establishment of Recovery fauna, which coincided with the recovery about 83-91 kyr after its initiation.

Paleocene and Eocene nannofossil datums from Shatsky Rise and Walvis Ridge

The precision of late Paleocene to middle Eocene nannofossil datums is investigated by means of quantitative methods and correlated to the magnetic polarity stratigraphy, using sequences from the Northwest Pacific, Southeast Atlantic and Italy. It is the rule rather than the exception to find tails of very reduced abundances prior to, or after, a range of consistent and higher abundances. The absolutely first or final occurrence of a species, therefore, seldom provides a synchronous datum when material from different geographic areas are compared. On the other band, synchroneity is often confirmed when the initial sharp rise or the final sharp decline in abundance is used as datum level. The use of datums not employed in the two principal existing nannofossil zonal schemes can substantially improve the biostratigraphic resolution. Two established zonal markers show abundance patterns making them unsuitable as datums: the first occurrences of Ellipsolithus macellus (base NP4, diachronous) and Tribrachiatus nunnii (base NP10 and Paleocene/Eocene boundary, too rare and too short range in open ocean sections). The first occurrence of either Fasciculithus spp. or Sphenolithus spp. is a better marker near the base of NP4. The first occurrence of Discoaster diastypus at 56.6 Ma represents a suitable replacement for recognition of the Paleocene/Eocene boundary.

Pliocene stable oxygen and carbon isotope record of benthic foraminifera from ODP Site 138-846 in the eastern equatorial Pacific

Oxygen and carbon isotope ratios were measured in benthic foraminifers from the entire Pliocene and latest Miocene sections of Site 846, a 180-m section, at a sampling interval of 10 cm. This provides a temporal resolution of about 2500 yr. The documented continuity of the record is excellent. Using the time scale that was developed on the basis of orbital tuning of GRAPE density records, we observed a fairly constant phase relationship between delta18O and variations in the obliquity of Earth's rotational axis. A new numbering scheme for Pliocene isotope stages is proposed. This high-resolution delta18O record clarifies several interesting aspects of late Neogene climatic evolution, including a "glacial" event that may have caused the final Messinian desiccation of the Mediterranean Sea; one or more "interglacial" events that might have caused refilling of the Mediterranean; a well-resolved couplet of glacial events at about the age of the Sidujfall Subchron; interglacial extremes in the early part of the Gauss that could have resulted from either significant deglaciation on Antarctica or from warming of deep water; and a gradual ramp of increasingly extreme "glacial" events, starting at about the Kaena Subchron and culminating with delta18O stage 100 in the earliest Matuyama.

Oligocene-Miocene calcareous nannofossils in ODP Leg 149 holes

During Ocean Drilling Program (ODP) Leg 149, five sites were drilled on the Iberia Abyssal Plain in the northeastern Atlantic Ocean. Both Mesozoic and Cenozoic sediments were recovered. Oligocene to Miocene sediments were cored at deepwater Sites 897, 898, 899, and 900. Except for a few intervals, occurrences of generally abundant and well-preserved calcareous nannofossils suggest that the deposition of the turbidite-type sediments occurred above the calcite compensation depth (CCD). One major unconformity in the middle late Miocene is present. Detailed quantitative analyses of calcareous nannofossils are used to determine the changes occurring among the nannoflora in relation to sea-level variation. A succession of 89 biohorizons from the early Oligocene to the late Miocene are defined by combining the biostratigraphic results of the four sites studied in the Iberia Abyssal Plain. One new genus and eight new species are described: Camuralithus, Camuralithus pelliculatus, Ericsonia detecta, Helicosphaera limasera, Sphenolithus akropodus, Sphenolithus aubryae, Sphenolithus cometa, Reticulofenestra circus, and Syracosphaera lamina. Two new variations and seven new combinations are also introduced.

Latest Paleocene-earliest Eocene cyclostratigraphy

I present a new method for reconnaissance cyclostratigraphic study of continuously cored boreholes: the generation of detailed sediment color logs by digitizing Ocean Drilling Program (ODP) core photographs. The reliability of the method is tested by comparison with the spectrophotometer color log for the uppermost Paleocene-lowermost Eocene section (Chron C24r) at ODP Hole 1051A. The color log generated from Hole 1051A core photographs is essentially identical to the spectrophotometer log. The method is applied to the Chron C24r section at Holes 1051A and 690B, producing the first high-resolution geophysical log for the latter section. I calculate astronomically calibrated durations between bio- and chemostratigraphic events within Chron C24r by correlating the cyclostratigraphies for Holes 1051A and 690B. These durations are significantly different from previous estimates, indicating that the chronology of events surrounding the Paleocene/Eocene boundary will have to be revised. This study demonstrates that useful geophysical logs can be generated from digitized ODP and DSDP core photographs. The method is of practical use for sections lacking high-resolution logs, as is the case for most lower Paleogene sections.

Constructing a stacked benthic d18O record

Composite stacks were constructed by superimposing 6 to 13 benthic foraminiferal d18O records covering the period 0-850 ka. An initial timescale for each core was established using radioisotopic age control points and assuming constant sedimentation rates between these points. The average of these records is our 13-core "untuned" stack. Next, we matched the 41 kyr component of each record individually to variations in Earth's obliquity. Four of the 13 records produced timescales that were inconsistent with one or more of the known radioisotopic ages. The nine remaining cores were averaged to create a "minimally tuned" stack. Six of the minimally tuned cores were assembled into a "tropical" stack. For each stack we estimated the uncertainty envelope from the standard deviation of the constituents. Spectral analysis of the three stacks indicates that benthic d18O is dominated by a 100 kyr oscillation that has a narrow spectral peak. The contribution of precession to the total variance is small when compared to prior results from planktic stacks.

Neogene calcareous nannofossil abundances and biostratigraphy of the Ontong Java Plateau

A total of 35 calcareous nannofossil datums were found in the Neogene sediments recovered at five sites (Sites 803-807) on the Ontong Java Plateau in the equatorial Pacific during Ocean Drilling Program Leg 130. Among them, 12 datums in the Pleistocene-upper Pliocene sequences were correlated with magnetostratigraphy. Pliocene and Miocene calcareous nannofossil assemblages in 289 samples obtained from Holes 804C, 805B, 805C, and 806B were studied. Reticulofenestra coccolith size distribution patterns in these Pliocene-Miocene sediments were also revealed through the present investigation.

Bio- and magnetostratigraphic events of ODP holes of the central Kerguelen Plateau

Neogene biostratigraphic and magnetostratigraphic data are compiled from Holes 747A, 748B, and 751A drilled on the Southern Kerguelen Plateau during Ocean Drilling Program Leg 120. Neogene sections have excellent to good magnetostratigraphic signatures in many intervals. This, in addition to minimal coring gaps and the occurrence of mixed assemblages of both calcareous and siliceous microfossil assemblages, makes these valuable biostratigraphic reference sections for intra- and extraregional correlations. This paper combines the sequence of biostratigraphic events reported from diatom, radiolarian, planktonic foraminifer, calcareous nannofossil, and silicoflagellate studies of Leg 120 sediments. It correlates microfossil datums with the geomagnetic polarity time scale to test existing age estimates and to refine biostratigraphic age controls for the southern high latitudes. Significant biostratigraphic datums are presented in a series of age-depth plots. Numerous hiatuses are clearly identified through this approach, and the positions of lesser disconformities are suggested. Most Neogene intervals are represented in at least one site, although "regional" unconformities occur in the upper Pliocene, uppermost Miocene/lowermost Pliocene, middle upper Miocene, middle middle Miocene, and at the lower/middle Miocene boundaries. The longest hiatus spanned 6 m.y., with most other hiatuses representing 1 m.y. or less. This paper compiles Leg 120 biostratigraphic and magnetostratigraphic data for use in future syntheses of southern high latitude biostratigraphy and presents an age model for Leg 120 Neogene sediments.

Core alignment and composite depth scale of DSDP Site 86-577

Deep Sea Drilling Project Site 577 on Shatsky Rise (North Pacific Ocean) recovered a series of cores at three holes that contain calcareous nannofossil ooze of latest Cretaceous (late Maastrichtian) through early Eocene age. Several important records have been generated using samples from these cores, but the stratigraphy has remained outdated and confusing. Here we revise the stratigraphy at Site 577. This includes refining several age datums, realigning cores in the depth domain, and placing all stratigraphic markers on a current time scale. The work provides a template for appropriately bringing latest Cretaceous and Paleogene data sets at old drill sites into current paleoceanographic literature for this time interval. While the Paleocene Eocene Thermal Maximum (PETM) lies within core gaps at Holes 577* and 577A, the sedimentary record at the site holds other important events and remains crucially relevant to understanding changes in oceanographic conditions from the latest Cretaceous through early Paleogene.

Biogenic sedimentation of DSDP Site 85-574

The equatorial Pacific is an important part of the global carbon cycle and has been affected by climate change through the Cenozoic (65 Ma to present). We present a Miocene (12-24 Ma) biogenic sediment record from Deep Sea Drilling Project (DSDP) Site 574 and show that a CaCO3 minimum at 17 Ma was caused by elevated CaCO3 dissolution. When Pacific Plate motion carried Site 574 under the equator at about 16.2 Ma, there is a minor increase in biogenic deposition associated with passing under the equatorial upwelling zone. The burial rates of the primary productivity proxies biogenic silica (bio-SiO2) and biogenic barium (bio-Ba) increase, but biogenic CaCO3 decreases. The carbonate minimum is at ~17 Ma coincident with the beginning of the Miocene climate optimum; the transient lasts from 18 to 15 Ma. Bio-SiO2 and bio-Ba are positively correlated and increase as the equator was approached. Corg is poorly preserved, and is strongly affected by changing carbonate burial. Terrestrial 232Th deposition, a proxy for aeolian dust, increases only after the Site 574 equator crossing. Since surface production of bio-SiO2, bio-Ba, and CaCO3 correlate in the modern equatorial Pacific, the decreased CaCO3 burial rate during the Site 574 equator crossing is driven by elevated CaCO3 dissolution, representing elevated ocean carbon storage and elevated atmospheric CO2. The length of the 17 Ma CaCO3 dissolution transient requires interaction with a 'slow' part of the carbon cycle, perhaps elevated mantle degassing associated with the early stages of Columbia River Basalt emplacement.

Pliocene and Pleistocene diatom datum levels of the Northwest Pacific

Thick sections of Pliocene and Pleistocene biosiliceous clay and ooze were recovered by the Hydraulic Piston Corer (I-IPC) at three northwest Pacific sites (DSDP Sites 578, 579, and 580). They contain a well-preserved paleomagnetic record which made it possible to evaluate diatom events used in low and high latitudes in the transitional region of the northwest Pacific. Equatorial Pacific events are usually isochronons between the equatorial and subarctic regions. However, species which have short ranges in low latitudes tend to have diachronous first and last appearances in higher latitudes. All subarctic North Pacific datum species are present in the sediments at three sites which lie north and south across the subarctic front, but their ranges become shorter in southern regions. They do not penetrate into the equatorial region. Spatial distributions of these events are influenced by the paleo-position of the subarctic front. The migration of species from their home-area outwards, in the form of the first appearance, is related to the fluctuations of the subarctic front. The last appearance of species is a response to the change of the surface water temperature that is beyond the limit of tolerance of the species, or an unstable oceanic environment due to major change of climate.

Middle Eocene to Miocene calcareous nannofossil of ODP Leg 125

During Ocean Drilling Program Leg 125, a thick sequence of middle Eocene to Pleistocene pelagic sediments, volcanogenic sediments, and predominantly extrusive volcanic rocks was recovered. Calcareous nannofossils were examined from 15 holes at nine sites, but Eocene to Miocene calcareous nannofossils were found only from Holes 782A, 784A, 786A, and 786B. In portions of Holes 786A and 786B, datable nannofossil oozes were found intercalated among volcanic flows. The nannofossil biostratigraphy of these holes indicates the presence of three well-defined hiatuses: within the lower Oligocene, between the upper Oligocene and middle Miocene, and between the middle and upper Miocene. An attempt was made to correlate the magnetochronological data with the first or last occurrences of the following species: Sphenolithus distentus, Reticulofenestra bisecta, Reticulofenestra reticulata, and Cyclicargolithus floridanus abisectus n. comb. The results indicate that the FO of Sphenolithus distentus can extend down to Zone CP16 (34.7 Ma), the LO of Reticulofenestra bisecta best defines the boundary between CP19a and CP19b (23.5 Ma), and the LO of Cyclicargolithus f. abisectus n. comb, can extend up to Subzone CN5a (12.5 Ma). No latest Oligocene Cyclicargolithus f. abisectus n. comb, acme was observed. Cyclicargolithus abisectus is considered a subspecies or variant of Cyclicargolithus floridanus because their LOs coincide. As a consequence of these observations, we have modified the definitions of Bukry's Subzones CP14a, CP14b, and CNla. Analyses of sediment-accumulation rates indicate that the rates increased gradually from the Eocene to Miocene. This is especially evident since the late Miocene in Hole 782A. In different parts of the Izu-Bonin forearc basin, however, the rate is not everywhere the same and appears to vary according to the import of volcanogenic materials.