Late Quaternary carbonate and isotope stratigraphy of DSDP Site 90-594 east of South Island, New Zealand

The late Quaternary sequence off eastern South Island, New Zealand, consists of ~100 m of alternating bluish gray pelagic oozes and greenish gray hemipelagic oozes that extend uninterruptedly back to the Brunhes/Matuyama boundary (0.73 m.y.). A very high resolution (~2400 yr.) record of sediment texture, calcium carbonate content, and planktonic and benthic foraminiferal oxygen and carbon isotope composition demonstrates an in-phase cyclical fluctuation between the sedimentary parameters that closely correspond to the pelagic-hemipelagic sedimentation cycles and the isotope composition. Pelagic oozes, formed during interglacial periods of high eustatic sea level, are characterized by calcareous microfossils, relative enrichment in sand and clay sizes, high carbonate contents, reduced delta18O values, and increased delta13C values. Hemipelagic oozes, associated with glacial episodes and lowered eustatic sea level, include common terrigenous material and siliceous microfossils, are enriched in silt sizes, have low carbonate contents, high delta18O values, and low delta13C values. The history of alpine glaciations and associated erosion of the South Island of New Zealand, as expressed by the appearance of hemipelagic oozes, can be correlated directly with the major fluctuations of Northern Hemisphere ice sheets as expressed by the influence of eustatic sea-level changes on the oxygen isotope composition of both planktonic and benthic foraminifers. This high-accumulation-rate record contains conspicuous intervals of highfrequency, high-amplitude isotope variability including the presence of multiple glacial/interglacial intervals within single isotope stages, and offers one of the best sections cored to date for detailed study of the evolution and history of climate change over the last 0.75 m.y.

Investigations on seven sediment cores from the New Ireland Basin, east of Papua New Guinea

Sediment cores were recovered from the New Ireland Basin, east of Papua New Guinea, in order to investigate the late Quaternary eruptive history of the Tabar-Lihir-Tanga-Feni (TLTF) volcanic chain. Foraminifera d18O profiles were matched to the low-latitude oxygen isotope record to date the cores, which extend back to the early part of d18O Stage 9 (333 ka). Sedimentation rates decrease from >10 cm/1000 yr in cores near New Ireland to ~2 cm/1000 yr further offshore. The cores contain 36 discrete ash beds, mostly 1-8 cm thick and interpreted as either fallout or distal turbidite deposits. Most beds have compositionally homogeneous glass shard populations, indicating that they represent single volcanic events. Shards from all ash beds have the subduction-related pattern of strong enrichment in the large-ion lithophile elements relative to MORB, but three distinct compositional groups are apparent: Group A beds are shoshonitic and characterised by >1300 ppm Sr, high Ce/Yb and high Nb/Yb relative to MORB, Group B beds form a high-K series with MORB-like Nb/Yb but high Ce/Yb and well-developed negative Eu anomalies, whereas Group C beds are transitional between the low-K and medium-K series and characterised by flat chondrite-normalised REE patterns with low Nb/Yb relative to MORB. A comparison with published data from the TLTF chain, the New Britain volcanic arc and backarc including Rabaul, and Bagana on Bougainville demonstrates that only Group A beds share the distinctive phenocryst assemblage and shoshonitic geochemistry of the TLTF lavas. The crystal- and lithic-rich character of the Group A beds point to a nearby source, and their high Sr, Ce/Yb and Nb/Yb match those of Tanga and Feni lavas. A youthful stratocone on the eastern side of Babase Island in the Feni group is the most probable source. Group A beds younger than 20 ka are more fractionated than the older Group A beds, and record the progressive development of a shallow level magma chamber beneath the cone. In contrast, Group B beds represent glass-rich fallout from voluminous eruptions at Rabaul, whereas Group C beds represent distal glass-rich fallout from elsewhere along the volcanic front of the New Britain arc.