Geochemistry and lithology of clayey nannofossil ooze turbidites and hemipelagites at ODP Holes 135-834A and 135-835A

The western Lau Basin, between the Central and Eastern Lau Spreading Centers and the Lau Ridge, contains several small, elongate, fault-bounded, partially sediment-filled sub-basins. Sites 834 and 835 were drilled in the oldest part of the Lau Basin in two of these small extensional basins close to the Lau Ridge, formed on late Miocene to early Pliocene oceanic crust. Both sites show a similar sediment sequence that consists of clayey nannofossil oozes and mixed sediments interbedded with epiclastic vitric sands and silts. The vitric sands and silts are largely restricted to the deeper part of the sediment column (early Pliocene-late Pliocene), and the upper part of the sediment column at both sites consists of a distinctive sequence of brown clayey nannofossil ooze, stained by iron and manganese oxyhydroxides (late Pliocene-Holocene). However, the clayey nannofossil ooze sequence at Site 835 is anomalously thick and contains several medium- to very thick beds of matrix-supported, mud-clast conglomerate (interpreted as muddy debris-flow deposits), together with large amounts of redeposited clayey nannofossil ooze and coherent rafted blocks of older hemipelagic material. Redeposited clayey nannofossil oozes can be distinguished from hemipelagic nannofossil oozes using several sedimentological criteria. These include variation in color hue and chroma, presence or absence of bioturbation, presence or absence of scattered foraminifers, grain-size characteristics, variability in calcium carbonate content, presence or absence of pumice clasts, and micropaleontology. Clayey nannofossil ooze turbidites and hemipelagites are also geochemically distinct, with the turbidites being commonly enriched in Mn, Ni, Pb, Zn, Cr, and P. The sediment sequence at Site 835 is dominated by allochthonous sediments, either muddy debris-flow deposits, coherent rafted blocks, or thick clayey nannofossil ooze turbidites. Since 2.9 Ma, only 25% of the 133 m of sediments deposited represents hemipelagic deposition, with an average sedimentation rate of 1.5 cm/k.y.. Allochthonous sediments were the main sediment type deposited during the Brunhes geomagnetic Epoch and make up 80% of the thickness of sediment deposited during this period. Short intervals of mainly hemipelagic deposition occurred from 0.4 to 0.9 Ma, 1.0 to 1.4 Ma, and 1.7 to 2.1 Ma. However, allochthonous sediments were again the dominant sediment type deposited between 2.1 and 2.5 Ma, with a large slide complex emplaced around 2.5 Ma. We conclude that the adjacent high ground, surrounding the basin in which Site 835 was drilled, was affected by marked instability throughout the late Pliocene and Pleistocene. In contrast, sedimentation at Site 834 during this period has been dominated by hemipelagic deposition, with redeposited sediments making up slightly less than 17% of the total thickness of sediment deposited since 2.3 Ma. However, there was a marked increase in frequency and magnitude of redeposited sediments at around 0.2 Ma at Site 834, which broadly corresponds to the onset of a major episode of turbidite and debris-flow emplacement beginning about 0.4 Ma at Site 835. This episode of instability at both sites may be the effect of the approach and passing of the Central Lau propagator at the latitude of Sites 834 and 835 at about 0.5 Ma.

Late Oligocene through early Pleistocene calcareous nannofossils of ODP Leg 115 holes

The chronostratigraphy, the calcareous nannofossil biochronology, and the biostratigraphy of the Miocene and Pliocene sediments retrieved during Leg 115 in the equatorial western Indian Ocean are presented and discussed. Most of the zonal boundaries of the standard 1971 zonation of Martini and the 1973 zonation of Bukry are easily recognized in these low-latitude sediments. We also comment on the secondary events that are proposed in the literature to improve the biostratigraphic resolution provided by the standard zonations. The study of calcareous nannofossil biostratigraphy and taphonomy of sequences from the Northern Mascarene Plateau area, which was drilled to investigate the Neogene history of carbonate flux and dissolution, indicate that the accumulation of carbonates in this area results from a complex interplay among carbonate bioproductivity, carbonate removal by chemical dissolution and mechanical erosion, and carbonate addition by mass and current transport. In spite of these drawbacks, major changes and trends in carbonate accumulation can be recognized, most of which, if not all, correlate with major steps in the evolution of the Neogene climatic system.

Sea surface productivity and temperature detected by coccolith accumulation rates from the southeastern Pacific off Chile

Recent coccoliths from 74 surface sediment samples recovered from the southeastern Pacific off Chile were examined quantitatively to investigate modern regional gradients of sea surface productivity and temperature. All findings are based on coccolith accumulation rates. Therefore an approach was designed to estimate recent sedimentation rates based on 210Pb and bulk chemistry analyses of the same set of surface samples. Highest total coccolith accumulation rates were found off north-central Chile, where seasonal upwelling takes place. Based on amultiple linear regression between calculated coccolith accumulation rates andWorld Ocean Atlas derived sea surface temperatures, a calibrationmodel to reconstruct annual average temperatures of the uppermost 75 mof thewater column is provided. Themodelwas cross-validated and the SST estimateswere compared with SST observed and SST estimates based on diatoms and planktonic foraminifera, showing a good correlation.

(Table S1) Relative abundances of coccolithophore taxa in surface sediment samples from the Equatorial and Southeastern Pacific

This study aims to contribute to a more detailed knowledge of the biogeography of coccolithophores in the Equatorial and Southeastern Pacific Ocean. Census data of fossil coccoliths are presented in a suite of core-top sediment samples from 15°N to 50.6°S and from 71°W to 93°W. Following standard preparation of smear slides, a total of 19 taxa are recognized in light microscopy and their relative abundances are determined for 134 surface sediment samples. Considering the multivariate character of oceanic conditions and their effects on phytoplankton, a Factor Analysis was performed and three factors were retained. Factor 1, dominated by Florisphaera profunda and Gephyrocapsa oceanica, includes samples located under warm water masses and indicates the occurrence of calcite dissolution in the water column in the area offshore Chile. Factor 2 is related to cold, low-salinity surface-water masses from the Chilean upwelling, and is dominated by Emiliania huxleyi, Gephyrocapsa sp. < 3 µm, Coccolithus pelagicus and Gephyrocapsa muellerae. Factor 3 is linked to more saline, coastal upwelling areas where Calcidiscus leptoporus and Helicosphaera carteri are the dominant species.