Trace element abundance, and Sr and Nd isotope ratios of dust samples in the Pacific Ocean (Table 2)

Eolian dust preserved in deep-sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate. In order to identify the provenance of eolian dust, Nd and Sr isotopic compositions and Rb, Sr and rare earth element (REE) concentrations have been determined for the silicate fractions of deep-sea sediments from the north and central Pacific Ocean. Different regions of the Pacific Ocean are characterized by distinct air-borne inputs, producing a large range in epsolin-Nd (-10 to +1), 87Sr/86Sr (0.705-0.721), La/Yb (5-15), EuN/EuN* (0.6-1.0) and Sr/Nd (4-33). The average Nd isotopic composition of Pacific deep-sea sediments (epsilon-Nd = -6), is more radiogenic than the average from the Atlantic (epsilon-Nd = -8). In contrast, the average147Sm/144Nd ratio for Pacific sediments (0.114) is identical to that of Atlantic sediments and to that of global average riverine suspended material. The values of epsilon-Nd and147Sm/144Nd are positively correlated for the Pacific samples but negatively correlated for Atlantic samples, reflecting a fundamental difference between the dominant components in the end members with radiogenic Nd (island-arc components in the Pacific and LREE-enriched intraplate ocean island components in the Atlantic). Samples from the north central Pacific have distinctive unradiogenic epsilon-Nd values of -10, 87Sr/86Sr > 0.715, high La/Yb (> 12), and low EuN/EuN* (0.6) and Sr/Nd (3-6). These data are virtually identical to the values for loess from Asia and endorse the use of these sediments as indicators of Asian paleoclimate and paleowind directions. Island-arc contributions appear to dominate in the northwest Pacific, resulting in higher epsilon Nd (-1 to +1) and lower 87Sr/86Sr (~ 0.705) and La/Yb (~ 5). Sediments from the eastern Pacific tend to have intermediate Sr and Nd isotopic compositions but regionally variable Sr/Nd and REE patterns; they appear to be derived from the west margin of the North and South American continents, rather than from Asia. Our results confirm that dust provenance can be constrained by isotopic and geochemical analyses, which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate.

Nd isotope ratios of surface sediments from the Pacific Ocean (Table 2)

The neodymium isotopic composition of the silicate fraction of Holocene pelagic sediments from the North Pacific define two provinces: a central North Pacific province characterized by unradiogenic and remarkably homogeneous end (-10.2 +/- 0.5) and a narrow circum-Pacific marginal province characterized by more radiogenic and variable end (-4.2 +/- 3.8). The silicate fraction in the central North Pacific is exclusively eolian; based on prevailing wind patterns, meteorological data, and neodymium isotopic data, the only significant sediment source is Chinese loess. Leaching experiments on Chinese loess confirm that leachable Nd is isotopically indistinguishable from bulk and residual silicate Nd. Silicates in the circum-North Pacific marginal province comprise eolian loess, volcanic ash, and hemipelagic sediments derived from volcanic arcs. A compilation of Pacific seawater and Mn nodule epsilon-Nd data shows no clear spatial variation except for a general decrease from surface to deep waters from -3 to -4 and slightly lower epsilon-Nd in bottom waters along the western North Pacific due to the incursion of Antarctic Bottom Water. The relative homogeneity of bottom water epsilon-Nd, which contrasts sharply with the distinctive variation in sediment epsilon-Nd, plus the large difference between the average end of bottom waters and the central North Pacific eolian silicates (-4 vs. -10), suggests that any contribution of REE to seawater from eolian materials is insignificant. Furthermore, leaching of REE from eolian particles as they sink though the water column must be insignificant because Nd in shallow waters is more radiogenic than Nd in deeper waters. That there is no contrast in the Nd isotopic composition of bottom waters that overlie the central and marginal sediment provinces suggests that the ash and hemipelagic sediments derived from Pacific rim volcanic arcs also contribute minimal REE to seawater. The elimination of eolian, ash, and hemipelagic sediments leaves only near-shore riverine particulates as a possibly significant particulate source of REE to seawater.

Element concentrations of aluminosilicate components from ODP sites in the Pacific Ocean

We measured major and trace element concentrations in the operationally defined, chemically extracted, residual aluminosilicate component of sediment from Ocean Drilling Program Sites 1215 and 1256 in the central and eastern equatorial Pacific Ocean and found that this residual component contains volcanogenic and authigenic aluminosilicates in addition to inferred eolian material. While the residual component younger than 20 Ma from the central Pacific (ODP Site 1215) is similar compositionally to upper continental crust and suggests an increase in the delivery of Asian dust material since 20 Ma, the residual in sediment older than 20 Ma indicates significant amounts of volcanogenic and authigenic materials. Volcanogenic debris comprises as much as ~ 40% of the residual between 23-40 Ma, which coincides with the mid-Tertiary "ignimbrite flare-up" that occurred in much of western North America. The residual component extracted from the 50 Ma biogenic sediment reflects authigenic signatures (seawater-like negative cerium anomalies and elevated Fe/Si ratios). The previously interpreted increase in an andesitic detrital source in North Pacific locations may instead be authigenic material, presenting significant challenges for many paleoclimate proxies. Additionally, in the eastern Pacific (ODP Site 1256), the residual component contains ~70% of volcanogenic material, most likely originating from Central America, and also includes refractory barite. The ability to separately identify eolian, volcanogenic, and authigenic materials in the aluminosilicate component of pelagic sediment allows resolution, respectively, of the climatic, geologic, and chemical processes contributing to the paleoceanographic archive in this critical oceanic region.

Abyssal community analysis from replicate box cores in the central North Pacific

A 0.25 m**2 United States Naval Electronics Laboratory box corer was used to take replicate samples from an oligotrophic bottom under the North Pacific Central Water Mass (~28°N, 155°W). The bottom is a red clay with manganese nodules at a depth of 5500-5800 m. Macrofaunal density ranges from 84 to 160 individuals per m**2 and is therefore much the same as in Northwest Atlantic Gyre waters. Of the macrofaunal taxa, polychaetes dominate (55 %), followed by tanaids (18 %), bivalves (7 %), and isopods (6 %). Meiofaunal taxa were only partially retained by the 297 µm screen used in washing. Even then, they are 1.5-3.9 times as abundant as the macrofaunal taxa, with nematodes being numerically dominant by far. Foraminifera seem to comprise an important portion of the community, but could not be assessed accurately because of the inability to discriminate living and dead tests. Remains of what are probably xenophyophoridans are also very important, but offer the same problem. Faunal diversity is extremely high, with deposit feeders comprising the overwhelming majority. Most species are rare, being encountered only once. The distributions of only three species show any significant deviation from randomness. The polychaete fauna from box cores collected from 90 miles to the north was not significantly different from that of the principal study locality. Concordance appeared at several taxonomic levels, from species through macrofaunal/meiofaunal relationships. As a result, the variation in total animal abundance shows aggregation among cores. We discuss Sokolova's concept of a deep-sea oligotrophic zone dominated by suspension feeders, and reconcile it with our present findings. The high diversity of the fauna combined with the low food level contradict theories that relate diversity directly with productivity.

(Table 2) Species composition of agglutinated foraminifers from the abyssal zone of the Pacific Ocean

The quantitative study of distribution and taxonomic composition of recent living and dead (without plasma) benthic foraminifers revealed three foraminiferal assemblages in bottom sediments of the Pacific Ocean at depths of 3350 to 4981 m. The assemblage dominated by epibenthic Lagenammina difflugiformis, Reophax dentaliniformis, and Saccorhiza ramose occupies slopes of underwater hills. The assemblage with a high share of infaunal Cribrostomoides subglobosum, C. nitidum, and Ammobaculites agglutinans is registered on an abyssal plateau. The assemblage with a significant proportion of large Astrorhiza and Reophax species, which are characterized by active way of life, populates gentle slopes and narrow depressions with potentially strong bottom currents.

Chemical compositions of Fe-Mn nodules and host sediments from the Northwest Pacific

A number of regularities of ore element accumulation in iron-manganese nodules along the profile from the Tsugaru Strait to the Wake Atoll is identified in the paper. It is shown that the ore process is enhanced to the pelagic zone: in nodules content of ore material increases and content of mineral insoluble residue decreases from near-shore areas to central parts of the ocean. Diagenetic redistribution of the elements between host sediments and nodules resulting to enrichment of the latter increases from bottom sediments of the ocean periphery to fine grained pelagic muds. At absolute enrichment by Fe, Mn, Cu, Ni, Co, Mo, W, Ti, Zr, and V (as compared to host sediments) nodules are relatively enriched in Mn group elements (Cu, Ni, Co, Mo, W) and consequently depleted in Fe group elements (Ti, V, Zr) in the direction from the periphery to the center of the ocean. The ratio of reactive forms of Fe and Mn in host sediments is a factor determining the ratio of Fe group and Mn group elements in nodules.

Diversity of benthic foraminiferal faunas in Tertiary and Quaternary sediments of DSDP Hole 33-317_Site from the Manihiki Plateau, Pacific Ocean (Table 1)

A study is made of the benthic foraminifers (size fraction > 63 µm) recovered from 59 upper Eocene through Quaternary sediment samples at DSDP Site 317 (Leg 33), located at a depth of 2598 m in the central part of the Manihiki Plateau (South Pacific). The sediments cored are disturbed in only two samples. The stratigraphic assignements used are based on previous studies of planktic foraminifers and other microfossils. In total, 216 taxa are identified. A cluster analysis based on the 77 species which comprised 5% or more of the entire foraminiferal assemblage in at least one sample suggests the presence of 3 major biostratigraphic zones corresponding approximately to the following ages, zone A: middle Miocene-Quaternary; zones B-C: early Miocene-Oligocene; and zone D: Eocene. The most important faunal turnover occurred between the Eocene and the Oligocene; a less pronounced break took place between the early and the middle Miocene, and an additional minor turnover between the Oligocene and the early Miocene. Eighteen taxa are long-ranging, being recorded from the middle Eocene through the Pliocene-Quaternary. It is concluded that, in general, benthic foraminifers of the bathyal zone are poor worldwide stratigraphic guide fossils; the following taxa are conditionally considered as the most suitable in the Eocene-Quaternary sequence: Aragonia aragonensis, Quadrimorphina profunda, Nuttallides truempyi, Abyssamina poagi, Buliminella grata, Bulimina jarvisi, B. macilenta, Turrilina alsatica, Cibicides notocenicus, C. wuellerstorfi, Pyrgo murrhina. However, most of these species are relatively rare.

Phosphorus dynamics in waters of the Pacific Ocean

Tagged phosphorus was used to measure principal indices of mineral phosphorus variations in the euphotic zone of the East Pacific, i.e. total rate of uptake of phosphate phosphorus by microplankton (A_t), fraction consumed by phytoplankton (A_p/A_t), and turnover time (T). A_t reached its greatest values (150-280 ng/l/hour) in the upwelling zone of the Peru traverse, where development of phytoplankton was induced by upwelling. In other areas of this traverse values were 40-80 ng/l/hour in surface layers. In less productive waters on two other profiles (off Central America and California), values were lower, between 20 and 40 ng/l. On the vertical profile maxima of A_t were found at the upper boundary of the thermocline. Turnover time of PO4 phosphorus (T) in zones of phytoplankton abundance was very short, between 1.5 and 4 days. At most other stations it was 10-40 days, increasing to 100-200 days or longer at the lower boundary of the euphotic zone. In areas of phytoplankton abundance it accounted for 60-80% of total uptake of PO4 phosphorus. But in zones of elevated bacterial abundance, A_p/A_t fell to 20-40%. Data indicating lack of correlation between PO4 phosphorus and productivity are presented. It is emphasized that the above measures of PO4 phosphorus dynamics can be used for obtaining measures of functional condition and successional phase of marine plankton communities.

Radiogenic Hf isotopic composition of continental eolian dust in Pacific Ocean sediments

The inorganic silicate fraction extracted from bulk pelagic sediments from the North Pacific Ocean is eolian dust. It monitors the composition of continental crust exposed to erosion in Asia. 176Lu/177Hf ratios of modern dust are subchondritic between 0.011 and 0.016 but slightly elevated with respect to immature sediments. Modern dust samples display a large range in Hf isotopic composition (IC), -4.70 < epsilon-Hf < +16.45, which encompasses that observed for the time series of DSDP cores 885/886 and piston core LL44-GPC3 extending back to the late Cretaceous. Hafnium and neodymium isotopic results are consistent with a dominantly binary mixture of dust contributed from island arc volcanic material and dust from central Asia. The Hf-Nd isotopic correlation for all modern dust samples, epsilon-Hf= =0.78 epsilon-Nd = +5.66 (n =22, R**2 =0.79), is flatter than those reported so far for terrestrial reservoirs. Moreover, the variability in epsilon-Hf of Asian dust exceeds that predicted on the basis of corresponding epsilon-Nd values (34.76 epsilon-Hf < +2.5; -10.96< epsilon-Nd <-10.1). This is attributed to: (1) the fixing of an important unradiogenic fraction of Hf in zircons, balanced by radiogenic Hf that is mobile in the erosional cycle, (2) the elevated Lu/Hf ratio in chemical sediments which, given time, results in a Hf signature that is radiogenic compared with Hf expected from its corresponding Nd isotopic components, and (3) the possibility that diagenetic resetting of marine sediments may incorporate a significant radiogenic Hf component into diagenetically grown minerals such as illite. Together, these processes may explain the variability and more radiogenic character of Hf isotopes when compared to the Nd isotopic signatures of Asian dust. The Hf-Nd isotope time series of eolian dust are consistent with the results of modern dust except two samples that have extremely radiogenic Hf for their Nd (epsilon-Hf =+8.6 and +10.3, epsilon-Nd =39.5 and 39.8). These data may point to a source contribution of dust unresolved by Nd and Pb isotopes. The Hf IC of eolian dust input to the oceans may be more variable and more radiogenic than previously anticipated. The Hf signature of Pacific seawater, however, has varied little over the past 20 Myr, especially across the drastic increase of eolian dust flux from Asia around 3.5 Ma. Therefore, continental contributions to seawater Hf appear to be riverine rather than eolian. Current predictions regarding the relative proportions of source components to seawater Hf must account for the presence of a variable and radiogenic continental component. Data on the IC and flux of river-dissolved Hf to the oceans are urgently required to better estimate contributions to seawater Hf. This then would permit the use of Hf isotopes as a monitor of past changes in erosion.

Nutrients of pore water in sediments of the central equatorial Pacific

Shipboard whole-core squeezing was used to measure pore water concentration vs depth profiles of [NO3]-, O2 and SiO2 at 12 stations in the equatorial Pacific along a transect from 15°S to 11°N at 135°W. The [NO3]- and SiO2 profiles were combined with fine-scale resistivity and porosity measurements to calculate benthic fluxes. After using O2 profiles, coupled with the [NO3]- profiles, to constrain the C:N of the degrading organic matter, the [NO3]- fluxes were converted to benthic organic carbon degradation rates. The range in benthic organic carbon degradation rates is 7-30 ?mol cm**-2 y**-1, with maximum values at the equator and minimum values at the southern end of the transect. The zonal trend of benthic degradation rates, with its equatorial maximum and with elevated values skewed to the north of the equator, is similar to the pattern of primary production observed in the region. Benthic organic carbon degradation is 1-2% of primary production. The range of benthic biogenic silica dissolution rates is 6.9-20 µmol cm**-2 y**-1, representing 2.5-5% of silicon fixation in the surface ocean of the region. Its zonal pattern is distinctly different from that of organic carbon degradation: the range in the ratio of silica dissolution to carbon degradation along the transect is 0.44-1.7 mol Si mol C**-1, with maximum values occurring between 12°S and 2°S, and with fairly constant values of 0.5-0.7 north of the equator. A box model calculation of the average lifetime of the organic carbon in the upper 1 cm of the sediments, where 80 +/- 11% of benthic organic carbon degradation occurs, indicates that it is short: from 3.1 years at high flux stations to 11 years at low flux stations. The reactive component of the organic matter must have a shorter lifetime than this average value. In contrast, the average lifetime of biogenic silica in the upper centimeter of these sediments is 55 +/- 28 years, and shows no systematic variations with benthic flux.