Paleocene radiolarian distribution and biostratigraphy from offshore eastern New Zealand

A 100-m-thick Paleocene sequence of mainly pelagic sediments at ODP Site 1121, on the eastern flanks of the Campbell Plateau, contains few to common radiolarians of relatively low diversity in the lower 40 m (Early to early Late Paleocene) and abundant, diverse radiolarian assemblages in the upper 60 m (mid-Late Paleocene). The 150 taxa recorded from the entire Paleocene interval are thought to under-represent the actual species diversity by at least one half as many morphotypes have not been differentiated below the level of genus. Assemblages in the lower 40 m are similar to those described from onland New Zealand and DSDP Site 208 (northern Lord Howe Rise); they are correlated with South Pacific radiolarian zones RP4 and RP5. Assemblages in the upper 60 m differ from other known Late Paleocene assemblages in the great abundance of plagiacanthids and cycladophorids. Similarities are noted with later Cenozoic cool-water assemblages. This upper interval is correlated with South Pacific zone RP6, as revised herein, based on comparison with faunas from Site 208 and Marlborough, New Zealand. The interval is also correlated with the upper part of North Atlantic zone RP6 (RP6b-c) based on the presence of Aspis velutochlamydosaurus, Plectodiscus circularis and Pterocodon poculum. Other species, such as Buryella tetradica and Buryella pentadica, are valuable for local correlation but exhibit considerable diachroneity between the Pacific, Indian and Atlantic Oceans. An age model for the Paleocene interval at Site 1121, based on well-constrained nannofossil and radiolarian datums, indicates that the rate of compacted sediment accumulation doubles from 15 to 30 mm/ka at the RP5/RP6 zonal boundary. In large part this is due to a sudden and pronounced increase in accumulation rates for all siliceous fossils; radiolarians and larger diatoms increase from <100 to >10 000 specimens/cm2/ka. This apparent increase in biosiliceous productivity is age-equivalent to a mid-Paleocene cooling event (57-59 Ma) identified from global stable isotope records that is associated with the heaviest delta13C values for the entire Cenozoic.

Geochemistry of ODP Hole 135-841B

Two igneous rock units were recovered at Site 841. More than 200 m of island-arc rhyolites, rhyolitic tuffs, lapilli tuffs, and pumice breccias, divided into five units, compose the basement at the site. These rhyolitic volcanics are late middle Eocene or older and formed part of a subaerial rhyolitic volcano. These low-K rhyolites were produced by fractional crystallization of a more mafic arc-tholeiitic lava or by dehydration melting of lower crustal arc tholeiites. The Site 841 basement rocks are similar in composition to high-SiO2 lavas in the Eocene basement on 'Eua and crystallized from depleted island-arc-tholeiitic basalts like those exposed on 'Eua. No evidence is present in the rhyolites, or in the clasts enclosed within them, for boninite series magmas at Site 841. The Site 841 rhyolitic complex bears no resemblance to Cretaceous rhyolites from the Lord Howe Rise, which are enriched in K and incompatible elements. The volcanic rocks at Site 841 are part of a widely distributed Eocene volcanic episode that marked the earliest phases of subduction in the Tonga region; they are not part of an older crustal fragment. The second igneous sequence is a series of basaltic dikes and sills that intruded Miocene sediments. These basalts have trace element abundances and ratios identical to upper Miocene lavas from the Lau Ridge. The Site 841 basalts do not have any geochemical characteristics that suggest they were generated by unusual thermal conditions in the shallow sub-forearc mantle. They are most reasonably interpreted as intrusions fed by basement dikes propagated from the associated active arc. No evidence for local serpentinite exposures, like those that are common in the Mariana forearc, was found at Site 841. The results from Site 841 provide strong support for hypotheses of forearc evolution that have been advanced for the Izu-Bonin-Mariana system.