Accumulation rates, grain-size fractions and calcium carbonate at DSDP Sites 89-586 and 90-591

Carbonate oozes recovered by hydraulic piston coring at DSDP Site 586 on Ontong-Java Plateau and Site 591 on Lord Howe Rise have carbonate contents that are consistently higher than 90% with only minor variations. Consequently, paleoceanographic signals were not recorded in detail in the carbonate contents. However, mass accumulation rates of carbonate increased in the late Miocene to mid-Pliocene, reflecting an increase in productivity, then abruptly decreased from mid-Pliocene to the present. Variations in relative abundances of coarse material (foraminifers) and fine material (mostly calcareous nannofossils) do reflect histories of current winnowing and biogenic productivity at the two sites. The late Miocene from 10.5 to 6.5 m.y. ago was a time of relatively constant, quiet, pelagic sedimentation with typical southwest Pacific sedimentation rates of 20-25 m/m.y. The average coarse-fraction abundances are always higher at Site 586 than at Site 591, which reflects winnowing at Site 586. These conditions were interrupted between 6.5 to 4.0 m.y. ago when increased upwelling at the Subtropical Divergence and the Equatorial Divergence produced greater productivity of calcareous planktonic organisms. The increased productivity is suggested by large increases in both fineand coarse-fraction material and constant ratios of foraminifers to nannofossils. The maximum of productivity was about 4.0 m.y. ago. This period of increased upwelling is coincident with the inferred development of the West Antarctic ice sheet. The high productivity was followed by an abrupt increase in winnowing about 2.5 m.y. ago at Site 591, but not until about 2.0 m.y. ago at Site 586. By 2.0 m.y. ago in the late Pliocene, quiet, pelagic sedimentation conditions prevailed, similar to those of the late Miocene. The last 0.7 m.y. has been a period of relatively intense winnowing on Lord Howe Rise but not on Ontong-Java Plateau. The coarse-fraction data have both long- and short-period fluctuations. Long-period fluctuations at Site 591 average about 850 *10**3 yr./cycle and those at Site 586 average 430*10**3 yr./cycle. The highest amplitudes are found in the Pliocene and Quaternary sections. The short-period fluctuations range from 100 to 48*10**3 yr./cycle at Site 586 and from 250 to 33 *10**3 yr./cycle at Site 591. The effects of local fluctuations of productivity and winnowing have modified the primary orbital forcing signals at these two sites to yield complex paleoceanographic records.

Nature, chemistry, and origin of late Cenozoic megascopic tephras in Legs 89 and 90 Cores from the Southwest Pacific

Several thin (1-10 cm) megascopic vitric tephras occur in the late Cenozoic calcareous oozes on Lord Howe Rise in the Tasman Sea and off eastern South Island, New Zealand. Of the 18 tephras analyzed 15 are silicic (75-78% SiO2) with abundant clear glass shards and a biotite ± hypersthene ± green hornblende ferromagnesian mineralogy. The Neogene silicic tephras were derived from the now-extinct Coromandel volcanic area in New Zealand, and the Quaternary ones from the presently active Central Volcanic Region of New Zealand. On the basis of glass chemistry and age, several of the Quaternary tephras are probably correlatives, and at least two can be matched to the major on-land Mt. Curl tephra (-0.25 m.y.). The occurrence of correlative silicic tephras both northwest and southeast of New Zealand may result from particularly violent eruptions, the ash below and above an altitude of -20 km being dispersed in opposite directions toward the Pacific Ocean and Tasman Sea, respectively. Ash drifting eastward into the southeasterly trade wind belt off northeastern New Zealand could also be carried into the central and northern Tasman Sea. Three megascopic tephras consist of altered basic shards and common labradorite crystals. They record Neogene explosive basaltic to andesitic activity from nearby ocean island or ridge sources in the Ontong-Java Plateau and Vanuatu regions. The megascopic tephras are a very incomplete and biased record of late Cenozoic explosive volcanism in the southwest Pacific because the innumerable, thin, green argillaceous layers in the cores (Gardner et al., this volume) probably represent devitrified intermediate to basic tephras derived mainly from oceanic arc volcanism along the Pacific/Australia plate boundary. In contrast to the New Zealand-derived silicic glass shards, the preservation potential of these more basic shards in Leg 90 calcareous sediments was low.