Calcareous nannofossils across the Eocene-Oligocene boundary

This work presents the stratigraphic distribution of several species of calcareous nannofossil in the middle Eocene early-Oligocene from four Ocean Drilling Program (ODP) sites located between 60° and 65°S paleolatitude in the Southern Atlantic and Indian Oceans. Useful nannofossil datums that should facilitate construction of age-models and contribute to an integrated chronology for the upper Paleogene Southern Ocean sediments from ~42 to 33 Ma are summarized. The distribution patterns of calcareous nannofossils, studied by means of quantitative and semiquantitative methods, provide an improvement of the classical Southern Ocean biozonations, introducing new biostratigraphically useful biohorizons, and testing their reproducibility within and outside the region.

(Table 2) Correlation of Mi zones with hiatuses in ODP Site 181-1124

Results from Ocean Drilling Program sites 1121-1124 show the Eastern New Zealand Oceanic Sedimentary System (ENZOSS) evolved in response to: (1) the inception of the circum-Antarctic circulation, (2) orbital and nonorbital regulation of the global thermohaline flow, and (3) development of the New Zealand plate boundary. ENZOSS began in the early Oligocene following opening of the Tasmanian gateway and inception of the ancestral Antarctic Circumpolar Current (ACC) and SW Pacific Deep Western Boundary Current (DWBC). Widespread erosion, marked by the Marshall Paraconformity, was followed by extensive drift formation in the late Oligocene- early Miocene. Alternating nannofossil chalk and nannofossil-rich mud deposited in response to 41-kyr orbital regulation of the abyssal circulation, with the mudstones representing times of increased inflow of corrosive southernsource waters. Drift deposition at the deepest sites was interrupted by bouts of erosion coincident with Mi 1-5 isotopic events signifying expansions of the East Antarctic Ice Sheet and enhanced bottom water formation. By late Miocene times, the basic ENZOSS was established. South of Bounty Trough, the energetic ACC instigated an erosional/low depositional regime. To the north, where the DWBC prevailed, orbitally regulated drift deposition continued. Increased convergence at the New Zealand plate boundary enhanced the terrigenous supply, but little of this sediment reached the deep ENZOSS as the three main sediment conduits - Solander, Bounty and Hikurangi channels - had not fully developed. The Plio-Pleistocene heralded a change from a carbonate- to terrigenous-dominant supply caused by interception of the DWBC by the three channels (~1.6 Ma for Bounty and Hikurangi, time of Solander interception unknown). The Solander and Bounty fans, and Hikurangi Fan-drift systems formed, and drifts downstream of those systems, received terrigenous detritus. Supply increased with accelerating uplift along the plate boundary, but delivery to the DWBC was regulated by eustatic fluctuations of sea level. Times of maximum supply to all three channels was during glacial lowstands whereas the supply either ceased (Bounty, Solander), or reduced (Hikurangi) in highstands. In glacial times, sediment was entrained by a DWBC invigorated by an increased input of Antarctic bottom water. The ACC also accelerated under strengthened glacial winds. Thus, glacials were times of optimum sediment supply to ENZOSS depocentres where depositional rates were 2-3 times more than interglacial rates.

(Table 1) Benthic foraminifera at DSDP Site 72-516

DSDP Site 516 contains a complete middle Eocene to lower Miocene interval with a well-developed Oligocene sequence that is more than 300 m thick. In this paper, the most important and characteristic benthic foraminiferal species from this interval are described and illustrated, and their quantitative and biostratigraphic distribution is given. Middle Eocene benthic assemblages, derived from pelagic intercalations in a partly turbiditic sequence, are low in diversity. Benthic assemblages of fairly high diversity occur in limestones, chalks, and oozes of the upper Eocene to lower Miocene. The consistently high rate of new species appearances at Site 516 during late Eocene and Oligocene contrasted greatly with the very slow rate of change in abyssal faunas at that time; there were no significant faunal changes at the Eocene/Oligocene boundary. The assemblages are dominated by Cibicidoides (mostly C. ungerianus or C. kullenbergi) and Lenticulina. Buliminids were also important during the Eocene and early Oligocene. Faunal comparison with other Atlantic DSDP sites and drill holes in the Gulf of Mexico suggest an approximately mid-bathyal (500-1500 m) depth of deposition during late Eocene and Oligocene.

Biostratigraphy and magnetostratigraphy of ODP Leg 125 holes

Samples from 15 holes at nine sites in the Izu-Bonin-Mariana region were examined for calcareous nannofossils, foraminifers, diatoms, and radiolarians. The ages of the containing sediments range from middle Eocene to Holocene. Biostratigraphic indicators date the sediments flanking Conical Seamount in the Mariana forearc as Pleistocene, whereas sediments flanking a seamount at Site 784 in the Izu-Bonin forearc were dated as middle Miocene. Sediments in the Izu-Bonin forearc are as old as the middle Eocene. Useful magnetostratigraphic results range from Holocene to mid-Miocene. Nannofossils provided the most useful biostratigraphic framework, but were supplemented with satisfactory agreement by data from foraminifers, radiolarians, and diatoms. Evidence from the biostratigraphic framework shows the likely presence of a sedimentary hiatus in the early Miocene. The presence of a single short hiatus in the early Oligocene and two in the late Miocene and early Pliocene is suggested, but supporting evidence other than nannofossil data is sparse. Evidence from approximate age-depth plots shows that sediment accumulation varies from hole to hole. The fastest rates of sediment accumulation were found to be in the late Miocene to Holocene whereas the slowest rates are present in the middle Eocene to Oligocene. The increased sedimentation rates in the late Miocene to Holocene resulted from an increase in volcanogenic sediment content from an uncertain source.

Age profiles and isotopes d18O and d13C in Globigerina angiporoides and Oridorsalis umbonatus at DSDP Site 90-593 and Holes 12-111, 29-277 and 90-592

The late Eocene through earliest Miocene stable-isotope composition of southwest Pacific microfossils has been examined in a traverse of high-quality sedimentary sequences ranging from subantarctic (DSDP Site 277) through temperate regions (DSDP Sites 592 and 593). Changes in oxygen-isotope values, measured in benthic and planktonic foraminifers, document the Oligocene development and strengthening of latitudinal thermal zonation from water masses with broad temperature gradients during the Eocene to the steeper gradients and more distinct latitudinally distributed surface water-mass belts of the Neogene. The oxygen-isotope records can be divided into three intervals: late Eocene, early Oligocene, and middle to late Oligocene. Each interval represents a successive stage in the evolution of latitudinal thermal gradients between subantarctic and temperate regions in the Southern Hemisphere. During the late Eocene, oxygen-isotope values at subantarctic Site 277 were similar to those at temperate Sites 592 and 593. The isotope values suggest that, although the inferred paleotemperatures at Site 277 are slightly cooler on average than those at the temperate sites, there is no evidence for a major thermal boundary between the regions at this time. All three sites record the well-known oxygen-isotope enrichment of about 1 per mil in both planktonic and benthic foraminifers in close association with the Eocene/Oligocene boundary. In contrast to the earliest Oligocene enrichments in the planktonic and benthic oxygen-isotope composition at Site 277, more northern Sites 592 and 593 exhibit a depletion through the early-middle Oligocene. This documents the beginning of thermal segregation as subantarctic waters cooled relative to those at temperate latitudes. During the Oligocene, this surface-water differentiation continued, as measured by planktonic d18O values. The oxygen-isotope records of the benthic foraminifers also began to diverge in the earliest Oligocene. The most enriched oxygen-isotope values in all records cluster in the middle Oligocene, marked by oscillating episodes of enrichments >0.5 per mil occurring most prominently in the subantarctic record of Site 277. These values can be interpreted as recording either the coldest oceanic temperatures of the Paleogene and/or accumulations of Antarctic ice. After this interval, latitudinal thermal differentiation developed rapidly during the middle Oligocene, especially in the surface waters which actually warmed in temperate areas. If the enriched Oligocene oxygen-isotope values indicate that ice had accumulated, this ice must have disappeared by the early Miocene, when depleted oxygen-isotope values suggest very warm conditions. The data presented in this chapter document the progressive increase of latitudinal temperature gradients from the late Eocene through the late Oligocene. This pattern of increasing isotopic offset between latitudinally distributed southwest Pacific sites is linked to the establishment and strengthening of the Circum-Antarctic Current, previously considered to have developed during the middle to late Oligocene. The intensification of this current system progressively decoupled the warm subtropical gyres from cool polar circulation, in turn leading to increased Antarctic glaciation.

(Table 2) Taxonomic composition of diatoms in dredged samples from the island-side slope of the Kuril-Kamchatka Trench

Studies of diatoms from dredge samples collected on the island slope of the Kuril-Kamchatka Trench have allowed to recognize well-preserved marine diatom assemblages corresponding to assemblages of the followed Oligocene zones: Rhizosolenia oligocaenica (subzone ''a'', 33.6-31 Ma), Cavitatus rectus (29.6-28.2 Ma), and Rocella gelida (28.2-24.0 Ma) as identified in the North Pacific zonal scale. Description of these assemblages and their complete taxonomic composition are presented. Diversity of species together with abundance and degree of preservation of diatoms and accompanying siliceous microorganisms suggests their autochtonous origin and favorable conditions of their development. Assemblages of the Early Oligocene zones Rhizosolenia oligocaenica and Cavitatus rectus recognized in sediments of the outer zone of the Lesser Kuril Ridge (submarine slope of the Shikotan Island) and on the Vityaz' submarine ridge were most probably formed under conditions of a vast shelf, while assemblage of the Late Oligocene zone Rocella gelida encountered only in the region of the Lesser Kuril Ridge formed under more deep-water conditions, presumably, over an island slope. Deepening of the basin in the region of the outer zone of the Lesser Kuril Ridge in Late Oligocene probably reflects one of stages of evolution of the Kuril-Kamchatka Trench.

Early Albian palynoflora of ODP Hole 120-750B

A well-preserved, diverse sporomorph flora of over 60 species has been found in Cores 120-750B-12W through -14R from the Southern Kerguelen Plateau. Analysis of the flora indicates that the terrestrial sediments overlaying the basaltic basement are late Early Cretaceous in age. Ranges of the sporomorphs in other parts of Gondwana and the morphology and paucity of angiosperm pollen grains confine the age of this section to the early to possibly early middle Albian. The Albian palynomorph assemblages in Hole 750B are composed primarily of fern spores and podocarpaceous pollen, and show most similarity to those from southern Australia. Changes in the flora through time reflect the successional vegetation changes on barren volcanic islands, beginning with high percentages of colonizing ferns and maturing into conifer (podocarp) forests. The flora shows some signs of endemism, which may be a result of the isolated position of the Kerguelen Islands during the Early Cretaceous. This endemism is expressed by high percentages of a distinctive monosulcate pollen species Ashmoripollis woodywisei n.sp. of pteridosperm or cycadophytean origin, and by a thick-walled, monosulcate angiosperm pollen species of the genus Clavatipollenites. The climatic conditions were probably cool to temperate (mean annual temperature approximately 7°-12°C) and humid (annual rainfall >1000 mm), analogous to modern Podocarpus-dominated forests in New Zealand and in South American mountain regions.

Bolboforma in Paleogene sediments of the Subantarctic

Sediments from ODP Holes 699A, 700B, 702B, 703A, and 704B were studied in order to determine and to understand the distribution of Bolboforma. Ten Bolboforma taxa were detected in the 294 samples analyzed. Based on Bolboforma species, a zonation correlated to the paleomagnetic record is proposed for the late middle Eocene to early Oligocene. Four biozones can be defined: the Bolboforma indistincta Zone for the lower part of the upper middle Eocene sequence, the Bolboforma eocena Zone for the upper part of the upper middle Eocene to upper Eocene, the Bolboforma geomaris Zone for the upper Eocene, and the Bolboforma latdorfensis Zone for the lower Oligocene sequence.

Seafloor hydrothermal activity in sediment core MV0502-15JC

A large diameter piston core containing 8.35 m of metalliferous sediment has been recovered from a small abyssal valley in the remote Southwest Pacific Basin (31°42.194'S, 143°30.331'W; 5082 m water depth), providing unique insight into hydrothermal activity and eolian sedimentation there since the early Oligocene. A combination of fish-teeth Sr-isotope stratigraphy and INAA geochemical data reveals an exponentially decreasing hydrothermal flux 31 Ma to the present. Although hydrothermal sedimentation related to seafloor spreading explains this trend, a complex history of late Eocene/early Oligocene ridge jumps, propagating rifts and plate tectonic reorganization of South Pacific seafloor could have also played a role. A possible hiatus in deposition, as recorded by changes in core composition just below 2 m depth, is beyond the resolution of the fish teeth Sr isotope dating method employed here; however, the timing of this interval may be coincident with extinction of the Pacific-Farallon Ridge at ~20 Ma. A low flux eolian component accumulating at this site shows an increase relative to the hydrothermal component above 2 m depth, consistent with dust-generating continental sources far to the west (Australia/New Zealand). This is the first long-term paleoceanographic record obtained from within the South Pacific "bare zone" (Rea et al., 2006), an anomalous region where Pacific seafloor has largely escaped sediment accumulation since the Late Cretaceous.