Lithology and approximate sub-bottom depths and ages of the transition of soft to firm ooze and of firm ooze to chalk at DSDP Leg 89 and 90 Sites

Leg 90 recovered approximately 3705 m of core at eight sites lying at middle bathyal depths (1000-2200 m) (Sites 587 to 594) in a traverse from subtropical to subantarctic latitudes in the southwest Pacific region, chiefly on Lord Howe Rise in the Tasman Sea. This chapter summarizes some preliminary lithostratigraphic results of the leg and includes data from Site 586, drilled during DSDP Leg 89 on the Ontong-Java Plateau that forms the northern equatorial point of the latitudinal traverse. The lithofacies consist almost exclusively of continuous sections of very pure (>95% CaCO3) pelagic calcareous sediment, typically foraminifer-bearing nannofossil ooze (or chalk) and nannofossil ooze (or chalk), which is mainly of Neogene age but extends back into the Eocene at Sites 588, 592, and 593. Only at Site 594 off southeastern New Zealand is there local development of hemipelagic sediments and several late Neogene unconformities. Increased contents of foraminifers in Leg 90 sediments, notably in the Quaternary interval, correspond to periods of enhanced winnowing by bottom currents. Significant changes in the rates of sediment accumulation and in the character and intensity of sediment bioturbation within and between sites probably reflect changes in calcareous biogenic productivity as a result of fundamental paleoceanographic events in the region during the Neogene. Burial lithification is expressed by a decrease in sediment porosity from about 70 to 45% with depth. Concomitantly, microfossil preservation slowly deteriorates as a result of selective dissolution or recrystallization of some skeletons and the progressive appearance of secondary calcite overgrowths, first about discoasters and sphenoliths, and ultimately on portions of coccoliths. The ooze/chalk transition occurs at about 270 m sub-bottom depth at each of the northern sites (Sites 586 to 592) but is delayed until about twice this depth at the two southern sites (Sites 593 and 594). A possible explanation for this difference between geographic areas is the paucity of discoasters and sphenoliths at the southern sites; these nannofossil elements provide ideal nucleation sites for calcite overgrowths. Toward the bottom of some holes, dissolution seams and flasers appear in recrystallized chalks. The very minor terrigenous fraction of the sediment consists of silt- through clay-sized quartz, feldspar, mica, and clay minerals (smectite, illite, kaolinite, and chlorite), supplied as eolian dust from the Australian continent and by wind and ocean currents from erosion on South Island, New Zealand. Changes in the mass accumulation rates of terrigenous sediment and in clay mineral assemblages through time are related to various external controls, such as the continued northward drift of the Indo-Australian Plate, the development of Antarctic ice sheets, the increased desertification of the Australian continent after 14 m.y. ago, and the progressive increase in tectonic relief of New Zealand through the late Cenozoic. Disseminated glass shards and (altered) tephra layers occur in Leg 90 cores. They were derived from major silicic eruptions in North Island, New Zealand, and from basic to intermediate explosive volcanism along the Melanesian island chains. The tephrostratigraphic record suggests episodes of increased volcanicity in the southwest Pacific centered near 17, 13, 10, 5 and 1 m.y. ago, especially in the middle and early late Miocene. In addition, submarine basaltic volcanism was widespread in the southeast Tasman Sea around the Eocene/Oligocene boundary, possibly related to the propagation of the Southeast Indian Ridge through western New Zealand as a continental rift system.

Distribution of planktonic foraminifers, d18O and d13C in Globigerinoides sacculifer as well as Cibicidoides kullenbergi at DSDP Holes 90-590A and 90-590B

High biogenic sedimentation rates in the late Neogene at DSDP Site 590 (1293 m) provide an exceptional opportunity to evaluate late Neogene (late Miocene to latest Pliocene) paleoceanography in waters transitional between temperate and warm-subtropical water masses. Oxygen and carbon isotope analyses and quantitative planktonic foraminiferal data have been used to interpret the late Neogene paleoceanographic evolution of this site. Faunal and isotopic data from Site 590 show a progression of paleoceanographic events between 6.7 and 4.3 Ma, during the latest Miocene and early Pliocene. First, a permanent depletion in both planktonic and benthic foraminiferal d13C, between 6.7 and 6.2 Ma, can be correlated to the globally recognized late Miocene carbon isotope shift. Second, a 0.5 per mil enrichment in benthic foraminiferal d18O between 5.6 and 4.7 Ma in the latest Miocene to early Pliocene corresponds to the latest Miocene oxygen isotopic enrichment at Site 284, located in temperate waters south of Site 590. This enrichment in d18O coincides with a time of cool surface waters, as is suggested by high frequencies of Neogloboquadrina pachyderma and low frequencies of the warmer-water planktonic foraminifers, as well as by an enrichment in planktonic foraminiferal d18O relative to the earlier Miocene. By 4.6 Ma, benthic foraminiferal d18O values become depleted and remain fairly stable until about 3.8 Ma. The early Pliocene (~4.3 to 3.2 Ma) is marked by a significant increase in biogenic sedimentation rates (37.7 to 83.3 m/m.y.). During this time, heaviest values in planktonic foraminiferal d18O are associated with a decrease in the gradient between surface and intermediate-water d13C and d18O, a 1.0 per mil depletion in the d13C of two species of planktonic foraminifers, and a mixture of warm and cool planktonic foraminiferal elements. These data suggest that localized upwelling at the Subtropical Divergence produced an increase in surface-water productivity during the early Pliocene. A two-step enrichment in benthic foraminiferal d18O occurs in the late Pliocene sequence at Site 590. A 0.3 per mil average enrichment at about 3.6 Ma is followed by a 0.5 per mil enrichment at 2.7 Ma. These two events can be correlated with the two-step isotopic enrichment associated with late Pliocene climatic instability and the initiation of Northern Hemisphere glaciation.

Stable carbon isotope record and geochemistry of late Miocene-early Pliocene sediments from DSDP Site 90-590 in the Tasman Sea

Biogenic components of sediment accumulated at high rates beneath frontal zones of the Indian and Pacific oceans during the late Miocene and early Pliocene. The delta13C of bulk and foraminiferal carbonate also decreased during this time interval. Although the two observations may be causally linked, and signify a major perturbation in global biogeochemical cycling, no site beneath a frontal zone has independent records of export production and delta13C on multiple carbonate phases across the critical interval of interest. Deep Sea Drilling Project (DSDP) site 590 lies beneath the Tasman Front (TF), an eddy-generating jetstream in the southwest Pacific Ocean. To complement previous delta13C records of planktic and benthic foraminifera at this location, late Neogene records of CaCO3 mass accumulation rate (MAR), Ca/Ti, Ba/Ti, Al/Ti, and of bulk carbonate and foraminiferal delta13C were constructed at site 590. The delta13C records include bulk sediment, bulk sediment fractions (<63 µm and 5-25 µm), and the planktic foraminifera Globigerina bulloides, Globigerinoides sacculifer (with and without sac), and Orbulina universa. Using current time scales, CaCO3 MARs, Ca/Ti, Al/Ti and Ba/Ti ratios are two to three times higher in upper Miocene and lower Pliocene sediment relative to overlying and underlying units. A significant decrease also occurs in all delta13C records. All evidence indicates that enhanced export production - the 'biogenic bloom' - extended to the southwest Pacific Ocean between ca. 9 and 3.8 Ma, and this phenomenon is coupled with changes in delta13C - the 'Chron C3AR carbon shift'. However, CaCO3 MARs peak ca. 5 Ma whereas elemental ratios are highest ca. 6.5 Ma; foraminiferal delta13C starts to decrease ca. 8 Ma whereas bulk carbonate delta13C begins to drop ca. 5.6 Ma. Temporal discrepancies between the records can be explained by changes in the upwelling regime at the TF, perhaps signifying a link between changes in ocean-atmosphere circulation change and widespread primary productivity.

Isotope age and relative abundance of planktonic foraminifera in Core Vulk-34-90, Sea of Okhotsk

Based on analyses of taxonomic composition of planktonic foraminifera and oxygen isotopic composition of their shells in the sediment core V34-90 from the eastern slope of the USSR Academy of Sciences Rise in the southeast Sea of Okhotsk the main climatic and hydrological variations over the past 20 Ka have been traced and dated by the radiocarbon method. The followed periods have been revealed: glacial (20-12.5 Ka), deglaciation (12.5-8.0 Ka) with a pronounced cooling event (Late Drias about 10.8 Ka) and post deglaciation (last 8.0 Ka).