Factors regulating the distribution of elements in marine sediments predicted by a simulation model

The contents of oxyanionic elements (V, Se and Mo) and cationic transition metals (Mn, Fe, Co, Ni, Cu and Zn) in sediments from near-shore to deep-sea environments were measured to clarify factors regulating the distribution of these elements in marine sediments. For cationic transition metals of which contents increase from near-shore to deep-sea environments, the chemical composition of pelagic clays is modeled by a mixture of aluminosilicates having the chemical composition of average shale and Fe–Mn oxides having the chemical composition of associated manganese nodules. The content of V is fairly constant in sediments from near-shore to deep-sea areas. The mixture model of average shale and manganese nodules holds also for V, although most of the V is located in the aluminosilicate lattices. The content of Se in the near-shore sediments is higher and that in the deep-sea sediments is lower than that in average shale. The high content in the near-shore sediments is interpreted as the addition of biogenic materials to aluminosilicates with average shale composition and the low content in deep-sea sediments is explained by oxidative release of Se from aluminosilicates. The content of Mo in sediments increases from near-shore to deep-sea environments. The general distribution of Mo in marine sediments is expressed by the mixture model. An anomalously high content of Mo in a near-shore sediment is attributed to adsorption of molybdate on manganese oxides.     

Circulation and Material Transport in Suo-Nada during Spring and Summer

An attempt was made to reproduce the circulation pattern in Suo-Nada, Japan during spring and summer season in order to elucidate the water exchange mechanism in the basin. Two hydrographic surveys at the end of each season were conducted covering the entire Suo-Nada area. A three-dimensional hydrodynamic Princeton Ocean Model (POM) was used to compute the current resulting from the observed density and wind field. During spring, a very pronounced counter clockwise gyre is situated near the opening of the basin. This is replaced by a clockwise circulation which seemed to occupy the whole domain during summer. Within each season, however, the vertical distribution of current does not show any remarkable differences, indicating the dominance of horizontal current and a very weak estuarine flow. These observational and numerical results were used to estimate the remnant function and the corresponding average residence time of permanently dissolved matter (PDM) and transformable matter (TM). The results revealed a small difference in the average residence times of materials within each season but a large seasonal variability between spring and summer. Furthermore, calculations based on climatological density fields have indicated a similar trend of variation between the seasonal values of average residence times. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterisation of Heterotrophic Microorganisms Isolated form the "Grotta Azzura" of Cape Palinuro (Salerno, Italy)

Abstract. Hydrothermal vent ecosystems have been extensively investigated during the last 20 years; shallow water hot springs (typically found below 50 m depth) have also been studied, and from these ecosystems many thermophilic and hyperthermophilic organisms were isolated and described.
The submarine caves of Cape Palinuro were discovered long ago, but only during the last decades were geological and biological investigations carried out, yielding a large number of reports on this fascinating environment. During the last two years, several samples of water, mud, and/or bacterial mat were collected within the "Grotta Azzurra", and enrichment experiments were performed using peptone, starch, glucose or pullulan as main organic nutrients. Many heterotrophic strains were isolated and most of them grew optimally between 37 and 43 ° C with the addition of 0-2 % NaCl. Growth was usually observed between 10 and 50 ° C the range of growth pH was 4 to 9, and NaCl concentrations up to 10–12 % were tolerated in most cases.
The analysis of 16S rDNA performed with the most representative isolates showed a close phylogenetic relationship with the genera Bacillus, Vibrio, Citrobacter and Escherichia. Moreover, an isolate showing morphological and physiological similarities with the genus Enterococcus was characterised and taxonomically described based on the 16S rDNA sequence.     

Temporal Variations in the Dissolved Nutrient Stocks in the Surface Water of the Western North Atlantic Ocean

Changes in patterns of undetectability and molar ratios of dissolved nutrients in the euphotic zone of the oligotrophic western North Atlantic Ocean were investigated utilizing the Bermuda Atlantic Time-series Study (BATS) data set of the US Joint Global Ocean Flux Study (JGOFS). Our aim was to examine the temporal dynamics of nutrient stocks over a decade (1989∼1998) and to gain insight into the interactions between the different biotic and abiotic factors underlying BATS. Patterns of nutrient undetectability clearly revealed the depleted nature of the nutrients in surface water at the BATS location, particularly phosphorous. The N:P ratio was consistently far above the nominal Redfield ratio (mean, 38.5) but was significantly lower during the 1993∼1994 period (22.1). Over the same period the proportion of samples depleted in N only increased while the proportion of samples depleted in P only decreased. This indicates an overall reduction of N relative to P in the surface water at BATS during the 1993∼1994 period, the reasons for this anomaly, though, are not clear. The correlation analysis between the biotic and abiotic variables at BATS has indicated some interesting relationships that can help understand some of the parameters affecting nutrient stocks in the euphotic zone and their consequent impacts on marine biota. Although nutrient stocks in the oligotrophic environment are limited, they might be subject to interannual variation that may become anomalous in some cases. These variations might underlay significant feedback mechanisms by affecting marine productivity, the prime factor controlling the sequestration of atmospheric CO₂ by the oceans. This revised version was published online in July 2006 with corrections to the Cover Date.     

N2 fixation variability in the oligotrophic Sulu Sea, western equatorial Pacific region over the past 83 kyr

N₂ fixation is an important biological process that adds new nitrogen to oceans and plays a key role in modulating the oceanic nitrate inventory. However, it is not known how, when, and where N₂ fixation rates have varied in response to past climate changes. This study presents a new record of nitrogen isotopic composition (δ¹⁵N) over the last 83 kyr from a sediment core (KH02-4 SUP8) taken in the Sulu Sea in the western equatorial Pacific region; data allow the N₂ fixation variability in the sea to be reconstructed. Sediments, sinking, and suspended particulate organic matter (POM) all have lighter isotopic values compared to the δ¹⁵N values of substrate nitrate (av. 5.8‰) in North Pacific Intermediate Water. These lighter δ¹⁵N values are regarded as reflecting N₂ fixation in the Sulu Sea surface water. A δ¹⁵N mass balance model shows that N₂ fixation rates were significantly enhanced during 54–34 kyr in MIS-3 and MIS-2. It has been speculated that higher interglacial denitrification rates in the Arabian Sea and the eastern tropical Pacific would have markedly decreased the global oceanic N inventory and contributed to the increase in N₂ fixation in oligotrophic regions, but such a model was not revealed by our study. It is possible that changes in N₂ fixation rates in the Sulu Sea were regional response, and accumulation of phosphate in the surface waters due to enhanced monsoon-driven mixing is thought to have stimulated enhancements of N₂ fixation during MIS-3 and MIS-2.     

Precipitation of authigenic uranium in suboxic continental margin sediments from the Okinawa Trough

Concentrations of U and Th isotopes in Okinawa Trough and East China Sea sediment cores were determined by isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) to investigate the behavior of redox sensitive uranium in suboxic hemipelagic sediments and determine their significance in oceanic uranium balance. ²³⁸U concentrations and ²³⁸U/²³²Th activity ratios in the East China Sea sediments showed no remarkable variation with depth. However, ²³⁸U and ²³⁸U/²³²Th ratios in the Okinawa Trough sediments were low in the surface oxidizing layer but increased where the suboxic condition was encountered. The distribution profiles of ²³⁰Th and ²³²Th concentrations were relatively constant with depth in both the Okinawa Trough and East China Sea sediment cores. These results suggested that there has been post-depositional precipitation of authigenic uranium within the suboxic Okinawa Trough sediment column. The post-depositional precipitation rates of authigenic uranium were estimated to be 47 ± 5 to >62 ± 8 ng cm⁻² yr⁻¹; these rates were comparable to those previously reported for several anoxic sediments. A mechanism controlling precipitation of uranium may be the downward diffusion of uranium U(VI), reduction to U(IV) and finally precipitation onto the solid phase. The accumulation rate of uranium for the Okinawa Trough sediments was approximately eight times higher than the world average rate reported for suboxic sediments. This removal of uranium in the oceanic budget increases the importance of the suboxic sediment sink.     

Surface current and vertical thermal structure on the continental slope in Tosa Bay

In order to specify a vertical thermal structure related to surface current variation on the continental slope in Tosa Bay, Japan, we analyzed monthly regular hydrographic measurements in the years 1991–2004. Subsurface temperature below 200 m on the slope was found to vary synchronously with the vertical displacement of the main thermocline around 200 m. It is shown that the vertical-averaged temperature below 200 m is significantly correlated with an along-isobath/southwestward surface current velocity on the slope. This correlation indicates that when a strong (weak) southwestward surface current is observed, temperature below 200 m decreases (increases) simultaneously, that is, isotherms below the 200 m are displaced upward (downward) together with the main thermocline. Moreover, when the strong southwestward flow is detected, across-isobath isotherms around 200 m slope upward toward the offshore direction. Furthermore, it is suggested that as the Kuroshio axis moves offshore south of the bay, the southwestward flow tends to be weakened by the combined effect of other Kuroshio parameters such as transport and stream width as well as the Kuroshio axis position. As a result, it is inferred that the correlation between the surface current and subsurface temperature can be interpreted in terms of the formation and decay of an anticlockwise circulation interacting with a cold eddy.     

Volume transport and distribution of deep circulation at 165°W in the North Pacific

We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O₂>4.2 ml l^?1), and lowest in silica (Si<135 μmol kg^?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=10⁶ m³ s^?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.     

Latitudinal and downcore (0–750 ka) changes in n-alkane chain lengths in the eastern equatorial Pacific

The n-alkane C₃₁/(C₂₉ + C₃₁) ratios from surface sediments in the eastern equatorial Pacific (EEP) exhibit higher values to the north and lower values to the south across the southern edge (2–4°N) of the Intertropical Convergence Zone (ITCZ). Since plants tend to synthesize longer chain length n-alkanes in response to elevated temperature and/or aridity, the higher C₃₁/(C₂₉ + C₃₁) ratios at northern sites suggest a higher contribution of vegetation under hot and/or dry conditions. This is consistent with the observation that northern sites receive higher levels of plant waxes transported by northeasterly trade winds from northern South America, where hot and dry conditions prevail. Furthermore, from a sediment core covering the past 750 ka (core HY04; 4°N, 95°W) we found that C₃₁/(C₂₉ + C₃₁) ratios exhibit a long-term decrease from MIS (marine oxygen isotope stage) 17 to 13. During this period, the zonal SST (sea-surface temperature) gradient in the equatorial Pacific increased, suggesting an increase in Walker circulation. Such intensified Walker circulation may have enhanced moisture advection from the equatorial Atlantic warm pool to the adjacent northern South America, causing arid regions in northern South America to contract, which may explain long-term decrease in n-alkane chain lengths.     

Accreting white dwarfs as supersoft X‐ray sources

I review various phenomena associated with mass‐accreting white dwarfs (WDs) in the view of supersoft X‐ray sources. When the mass‐accretion rate is low (Ṁ_acc < a few × 10^–7 M⊙yr^–1), hydrogen nuclear burning is unstable and nova outbursts occur. A nova is a transient supersoft X‐ray source (SSS) in its later phase which timescale depends strongly on the WD mass. The X‐ray turn on/off time is a good indicator of the WD mass. At an intermediate mass‐accretion rate an accreting WD becomes a persistent SSS with steady hydrogen burning. For a higher mass‐accretion rate, the WD undergoes “accretion wind evolution” in which the WD accretes matter from the equatorial plane and loses mass by optically thick winds from the other directions. Two SSS, namely RX J0513‐6951 and V Sge, are corresponding objects to this accretion wind evolution. We can specify mass increasing WDs from light‐curve analysis based on the optically thick wind theory using multiwavelength observational data including optical, IR, and supersoft X‐rays. Mass estimates of individual objects give important information for the binary evolution scenario of type Ia supernovae (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)