Diagenesis and bioavailability of mercury in the contaminated sediments of Ulhas Estuary, India

Sequential extraction of Hg was performed in a core collected from intertidal area in Ulhas Estuary in order to characterize the downward distribution and diagenetic behavior of Hg in a polluted estuary. Concentration of total Hg ranged between 0.46 and 6.40 μg g⁻¹ with significant decrease in surficial sediment as a result of closing of two Hg-cell based chlor-alkali plants. Results of sequential extraction showed that >65% Hg was strongly bound to organo-sulphur and inorganic sulphide species that are not bioavailable. Flux of only 18.8% of Hg was found compared to its value deposited on sediment–water interface. Hence, it is concluded that there is no significant diagenetic remobilization of Hg in Ulhas Estuary. This core also has been analyzed for ²¹⁰Pb geochronology. The estimated sedimentation rate is 0.31 cm y⁻¹ in the present sampling region.     

Sediment and trace element depositional history from the Ajkwa River estuarine mangroves of Irian Jaya (West Papua), Indonesia

Radiotracer ²¹⁰Pb and contaminant copper were used to estimate sediment accumulation rates in 4 cores from the Ajkwa River estuary and mangrove tidal channels in western Irian Jaya. Mass accumulation rates (4.5–13 kg dry wt m⁻² yr⁻¹) were within the envelope of expectations for a region of high rainfall, great river catchment relief, and rapid tectonic uplift of mountains. Copper accumulation rates were enhanced 40 fold in surface sediments, compared to pre-1950 sections of the sediment cores. These recent sediments with enhanced copper concentrations come from Freeport Indonesia mine tailings over the last 27 years. Variations in sediment core profiles of Al, Fe, and organic carbon were small, indicating no great change in bulk sediment composition. Sulfur concentrations decline toward the sediment surface, suggesting a decline in rates of microbial sulfate reduction. Enhanced sediment copper concentrations will be a useful tracer of sediment dispersal from the Ajkwa River estuary along this coast.     

Metal concentrations, fluxes, inventories and chronologies in sediments from Sepetiba and Ribeira Bays: A comparative study

Three sediment cores were sampled at Sepetiba Bay and four cores at Ribeira Bay, Rio de Janeiro State, Brazil. Sediment accumulation rates and chronologies were obtained from ²¹⁰Pb activity-depth profiles. Sediment accumulation rates in Ribera Bay ranged from 1.2 mm y⁻¹ in the inner bay to 2.6 mm y⁻¹ close to its entrance. In Sepetiba Bay two sediment accumulation rates were observed: a lower rate of 0.35 cm y⁻¹ before the 1960s and 0.76 cm y⁻¹ since then. The cause of this change is due to the construction of the Santa Cecília impoundment (1955) that brings water from the Paraíba do Sul Basin into the Guandu River, which increased its flow from the original 20 m³ s⁻¹ to 160 m³ s⁻¹. Concentration of 44 elements was obtained by ICP-MS after total dissolution of samples from two selected cores. The relative differences between the concentrations of crustal elements, such as Al, Fe and Ti are only about 20% (p < 0.05). Cd and Zn are 15 and four times larger in Sepetiba Bay than in Ribeira Bay, respectively. Other major and minor elements show lower statistically significant differences. The enrichment factors and metal inventories show that Sepetiba Bay can be considered polluted with Bi, Cd, Cr, Cu, Sb and Zn. Particularly, Cd and Zn present concentrations three and four times higher than the Brazilian existing limits.     

Efficient trapping of organic carbon in sediments on the continental margin with high fluvial sediment input off southwestern Taiwan

Accumulation rates of marine and terrigenous organic carbon in the continental margin sediments off southwestern Taiwan were estimated from the measured concentrations and isotopic compositions of total organic carbon (TOC) and previously reported sedimentation rates. Surficial sediments were collected from the study area spanning from the narrow shelf near the Kaoping River mouth to the deep slope with depths reaching almost 3000 m. The average sediment loading of Kaoping River is 17 Mt/yr, which yields high sediment accumulation rates ranging from 0.08 to 1.44 g cm⁻² yr⁻¹ in the continental margin. About half of the discharged sediments were deposited on the margin within 120 km of the river mouth. Carbon isotopic compositions of terrestrial and marine end-members of organic matter were determined, respectively, based on suspended particulate matter (SPM) collected from three major rivers in the southwestern Taiwan and from an offshore station. All samples were analyzed for the TOC content and its isotopic composition (δ¹³C_org). The SPM samples were also analyzed for the total nitrogen (TN) content. TOC content in marine sediments ranges from 0.45% to 1.35% with the highest values on the upper slope near the Kaoping River mouth. The TOC/TN ratio of the SPM samples from the offshore station is 6.8±0.6, almost identical to the Redfield ratio, indicating their predominantly marine origin; their δ¹³C_org values are also typically marine with a mean of −21.5±0.3‰. The riverine SPM samples exhibit typical terrestrial δ¹³C_org values around −25‰. The δ¹³C_org values of surficial sediments range from −24.8‰ to −21.2‰, showing a distribution pattern influenced by inputs from the Kaoping River. The relative contributions from marine and terrestrial sources to sedimentary organic carbon were determined by the isotope mixing model with end-member compositions derived from the riverine and marine SPM. High fluvial sediment inputs lead to efficient trapping of organic carbon over a wide range of water depth in this continental margin. The marine organic accumulation rate ranges from 1.6 to 70 g C m⁻² yr⁻¹ with an area weighted mean of 4.2 g C m⁻² yr⁻¹, which is on a par with the mean terrestrial contribution and accounts for 2.3% of mean primary production. The depth-dependent accumulation rate of marine organic carbon can be simulated with a function involving primary productivity and mineral accumulation rate, which may be applicable to other continental margins with high sedimentation rates. Away from the nearshore area, the content of terrigenous organic carbon in surficial sediments decreases with distance from the river mouth, indicating its degradation in marine environments.     

Mercury contamination history of an estuarine floodplain reconstructed from a Pb-dated sediment core (Berg River, South Africa)

Mercury deposition histories have been scarcely documented in the southern hemisphere. A sediment core was collected from the ecologically important estuarine floodplain of the Berg River (South Africa). We establish the concentration of Hg in this ²¹⁰Pb-dated sediment core at <50 ng g⁻¹ Hg_T throughout the core, but with 1.3 ng g⁻¹ methylmercury in surface sediments. The ²¹⁰Pb dating of the core provides a first record of mercury deposition to the site and reveals the onset of enhanced mercury deposition in 1970. The ratio of methylmercury to total mercury is relatively high in these sediments when compared to other wetlands.     

Chemical composition of short sediment cores from Thermaikos Gulf (Eastern Mediterranean): Sediment accumulation rates, trawling and winnowing effects

Four cores recovered within the framework of the INTERPOL Project have been analysed for their grain size and geochemistry; sediment accumulation rates (SARs) were also determined from ²¹⁰Pb and ¹³⁷Cs profiles. Two cores are representative of the Axios and Aliakmon Rivers depositional environment, whilst the third core represents the Pinios River province; the fourth core represents an environment of outer shelf relict sands. Apparent SARs ranged between 0.667 g cm⁻² yr⁻¹ (Axios and Aliakmon Rivers) and 0.414 g cm⁻² yr⁻¹ (Pinios River). Trawling activities and biomixing are critical processes that may be responsible for the mixing of the surface sediments, as observed from the excess ²¹⁰Pb profiles. The thickness of the surface mixed layer was 4.5 cm in the vicinity of Axios and Aliakmon Rivers and in the area of Pinios River, 3.75 cm on the outer shelf and 1 cm in the area where no trawling was observed. Sediment accumulation appeared to be regulated by variations in the riverine discharge, shelf transport pathways and winnowing processes. Major element variations, such as Si, Al, Ti, V and Ni, were dominated by terrigenous supply as aluminosilicate minerals and quartz, whereas most Ca and Sr were biogenic. Si/Al and Ca/Al ratios have been used to express changes in sediment accumulation and winnowing. Redox processes were depicted by Mn, which showed an increase in the depth of its redoxcline, from 1 cm in inshore stations to 2 cm on the outer shelf. Si/Al ratios follow the Ca/Al ratios and can be used to assess percentage winnowing in the sediment. Increases in these ratios indicate a decrease in sediment input rates and are seen in the upper parts of most of the cores. Anthropogenic or ‘excess’ metal contents have been calculated from Zn/V and Pb/V ratios. Their distributions in the cores showed that by far the highest contamination is associated with the Axios River output, whilst sediments influenced by the Pinios River were relatively uncontaminated.     

Sedimentation rates in the central lake constance determined with210Pb and137Cs

Sediment cores from central Lake Constance were dated with²¹⁰Pb and¹³⁷Cs. A sedimentation rate of (0.11±0.02) g·cm⁻²·y⁻¹ was determined with the²¹⁰Pb method.¹³⁷Cs measurements revealed sedimentation rates of (0.11±0.01) g·cm⁻²·y⁻¹ and (0.08±0.01) g·cm⁻²·y⁻¹ respectively for two different cores sampled at the same location. The lower Cs-dated value indicates incomplete core recovery and demonstrates the sensitivity of this simple dating method to small losses of material at the water/sediment interface. An unambiguous application of the¹³⁷Cs method is, therefore, only possible if complete core recovery is ensured. Sedimentation rates based on particulate matter, collected in sediment traps at various water depths, agree with the results of the radioisotope methods. Estimates of 30–125 days residence times for suspended particulate matter were calculated from⁷Be measurements.     

Nature of sediment dispersal off the Sepik River, Papua New Guinea: preliminary sediment budget and implications for margin processes

A sediment budget is constructed for the slope and narrow continental shelf off the Sepik River in order to estimate the relative importance of turbid plumes versus bottom gravity transport through a near-shore submarine canyon in the dispersal of sediment across this collision margin. ²¹⁰Pb geochronology and inventories of Kasten cores are consistent with the northwestward dispersal of sediment from the river mouth via hypopycnal and possible isopycnal plumes. Sediment accumulation rates are 5 cm yr⁻¹ on the upper slope just off of the Sepik mouth, decreasing gradually to 1 cm yr⁻¹ toward the northwest, and decreasing abruptly offshore (<0.2 cm yr⁻¹ at 1200 m water depth). A sediment budget indicates that only about 7–15% of the Sepik River sediment discharge accumulates on the adjacent open shelf and slope. The remainder presumably escapes offshore via gravity flows through a submarine canyon, the head of which extends into the river mouth. The divergent sediment pathways observed off the Sepik River (i.e., surface and subsurface plumes versus sediment gravity flows through a canyon) may be common along high-yield collision margins of the Indo–Pacific archipelago, and perhaps are analogous to most margins during Late Quaternary low sea-level conditions.     

Dating of sediments from Lake Zurich (Switzerland) with210Pb and137Cs

²¹⁰Pb- and¹³⁷Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with²¹⁰Pb/²¹⁰Po and revealed sediment accumulation rates of (0.055±0.015) g·cm^?2·y^?1 and (0.09±0.03) g·cm^?2·y^?1, respectively. A comparable rate of (0.07±0.01) g·cm^?2·y^?1 has been obtained from¹³⁷Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm^?2·y^?1. Constant²¹⁰Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct¹³⁷Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of⁷Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant²¹⁰Pb activities. Remobilization may also be the reason for an incomplete²¹⁰Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the²¹⁰Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.     

Perchlorate accumulation and migration in the deep vadose zone in a semiarid region

For 25 years, a plant in Israel manufacturing ammonium perchlorate disposed of untreated wastewater in four unlined ponds. This study explores the transport mechanisms of perchlorate infiltrated from 1965 to 1990 from one of these active storage ponds into a deep (40 m) layered vadose zone and the underlying Israeli coastal aquifer. Perchlorate migration from 1990, when wastewater disposal ceased, until today, with infiltration due only to natural rain (500 mm y⁻¹), was also studied. Several indirect methods were used, including: mass balance in the unsaturated zone profile, δ¹⁸O and δ²H profiles below the pond, and a comparison of the same sediment profiles in 2005 and 2007. The isotopic composition of the pore water could be divided into two separate groups: lighter (depleted) and heavier (enriched) samples. All samples in the lighter group were from the shallow vadose zone, above two clayey layers, and represent natural infiltration of rainwater. The enriched samples were from the deeper section of the unsaturated zone (20–40 m) and represent water used for perchlorate manufacturing 14 years prior to drilling. Consequently, the overall maximum infiltration rate was estimated to be 1.4 m y⁻¹. Below the clayey layer almost identical perchlorate concentrations were found along the sediment profile in 2005 and 2007 (two boreholes, 3 m apart). Very different perchlorate profiles were observed above the clayey layers. This suggests that perchlorate below the clay layers (20–40 m) is practically stagnant under the current natural conditions. The reduction in perchlorate concentration in groundwater below the ponds vs. its increased concentration further downgradient supports the contention that the current migration of perchlorate from the vadose zone to the groundwater is very small. We estimate that perchlorate concentration in the groundwater under the infiltration pond, which was 187 mg l⁻¹ in 2004, will reach 10 μg l⁻¹ within about 14 years. The existence of a clayey layer crossing the thick vadose zone was thus found to significantly change the infiltration rate when ponded conditions were replaced with natural precipitation.     

Nature of decadal-scale sediment accumulation on the western shelf of the Mississippi River delta

Sediment delivered to coastal systems by rivers (15×10⁹ tons) plays a key role in the global carbon and nutrient cycles, as deltas and continental shelves are considered to be the main repositories of organic matter in marine sediments. The Mississippi River, delivering more than 60% of the total dissolved and suspended materials from the conterminous US, dominates coastal and margin processes in the northern Gulf of Mexico. Draining approximately 41% of the conterminous US, the Mississippi and Atchafalaya river system deliver approximately 2×10⁸ tons of suspended matter to the northern Gulf shelf each year. Unlike previous work, this study provides a comprehensive evaluation of sediment accumulation covering majority of the shelf (<150 m water depth) west of the Mississippi Delta from 92 cores collected throughout the last 15 years. This provides a unique and invaluable data set of the spatial and modern temporal variations of the sediment accumulation in this dynamic coastal environment.Three types of ²¹⁰Pb profiles were observed from short cores (15–45 cm) collected on the shelf. Proximal to Southwest Pass in 30–100 m water depths, non-steady-state profiles were observed indicating rapid accumulation. Sediment accumulation rates in this area are typically >2.5 cm yr⁻¹ (>1.8 g cm⁻² yr⁻¹). Kasten cores (∼200 cm in length) collected near Southwest Pass also indicate rapid deposition (>4 cm yr⁻¹; >3 g cm⁻² yr⁻¹) on a longer timescale than that captured in the box cores. Near shore (<20 m), profiles are dominated by sediments reworked by waves and currents with no accumulation (the exception is an area just south of Barataria Bay where accumulation occurs). The remainder of the shelf (distal of Southwest Pass) is dominated by steady-state accumulation beneath a ∼10-cm thick mixed layer. Sediment accumulation rates for the distal shelf are typically <0.7 cm yr⁻¹ (<0.5 g cm⁻² yr⁻¹). A preliminary sediment budget based on the distribution of ²¹⁰Pb accumulation rates indicates that 40–50% of the sediment delivered by the river is transported out of the study region. Sediment is moved to distal regions of the shelf/slope through two different mechanisms. Along-isobath sediment movement occurs by normal resuspension processes west of the delta, whereas delivery of sediments south and southwest of the delta may be also be influenced by mass movement events on varying timescales.     

Recent aquatic ecosystem response to environmental events revealed from Pb sediment profiles

The ²¹⁰Pb dating method was first introduced by Goldberg (1963), and since then has been applied to study sediment from lakes, estuaries and coastal marine environments. Hundreds of studies around the world have used ²¹⁰Pb as a geochronological tool in aquatic ecosystems. However little attention has been paid to the potential of this naturally occurring isotope as an environmental tracer of ecological events. Here we report three instances in which ²¹⁰Pb profiles measured on undisturbed sediment cores from lakes, rivers and fjords show us the potential of ²¹⁰Pb profile as a tracer of natural and anthropogenic processes. The methodology used here is a suite of techniques combining biogeochemistry (micro-electrodes), paleomagnetism (susceptibility), sediment characteristics (LOI) and visualization (SPI and X-ray) applied to the interpretation of ²¹⁰Pb profiles. We measured ²¹⁰Pb profiles on sediments from a river, Cruces River (Chile), which recorded a clear shift in the water chemistry caused by a pulp mill effluent to the river. Here metal mobilization and remobilization of the tracer may be the cause of the observed profile. We also measured ²¹⁰Pb profiles in sediment from two fjords of Southern Chile (Pillan and Reñihue), the sudden deposition change of fresh ²¹⁰Pb with depth observed could very well be the result of bioturbation but it occurred in a seafloor area deprived of bioturbators. In this case, ²¹⁰Pb recorded the onset of aquaculture activities (fish farming) that took place two decades ago. Finally, ²¹⁰Pb profiles measured in two lakes in the “pampa Argentina”: Epecuen and Venado showed a particular shape with depth. These profiles apparently registered a sudden depositional event with recent ²¹⁰Pb material, probably related to strong shifts in precipitation and drought cycles in that part of the world. These three examples show that ²¹⁰Pb profiles provide valuable information not only on geochronology, but also related to natural and anthropogenic short term processes, as shown here, but these are not always reported and well understood.     

Composition of back-arc basin volcanics, Valu Fa Ridge, Lau Basin: Evidence for a slab-derived component in their mantle source

Samples dredged from 2 localities near the crest of the Valu Fa ridge, an active back-arc basin spreading centre in the Lau Basin, consist of highly vesicular lava fragments of andesitic composition. The samples are characterized by rare, euhedral An₈₅ plagioclase phenocrysts in a hypocrystalline groundmass of An₆₀ plagioclase laths, brown glass and rare subhedral clinopyroxene. Samples from within and, to a lesser extent, between the dredge hauls show remarkable isotopic and chemical homogeneity, with: ⁸⁷Sr/⁸⁶Sr − 0.70330 ± 2; ¹⁴³Nd/¹⁴⁴Nd − 0.51303 ± 2; ²⁰⁶Pb/²⁰⁴Pb − 18.65 ± 2; ²⁰⁷Pb/²⁰⁴Pb − 15.55 ± 1; ²⁰⁸Pb/²⁰⁴Pb − 38.34 ± 4; Sr − 165 ppm; Rb − 7 ppm; Cs − 0.17 ppm; K − 3300 to 4200 ppm; Ba − 96 ppm; and REE — LREE depleted with 12–18 × chondritic abundances. On Sr-Nd, Pb-Pb and Sr-Pb plots the volcanics lie just within or on the edge of the MORB fields, overlapping with island-arc volcanics from the Marianas and Tonga. Compared with MORB and ocean-island basalts, the samples show alkali-element enrichment relative to REE and higher Cs relative to Rb. The isotopic and geochemical characteristics of the Valu Fa Ridge volcanics clearly indicate a minor, but significant, slab-derived component in the back-arc basin mantle source.     

Transport ofBe andBe in the ocean

Beryllium isotopes (¹⁰Be and⁹Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of¹⁰Be and⁹Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 10⁹ and 6 × 10¹⁶ atoms g⁻¹ of¹⁰Be and⁹Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The¹⁰Be/⁹Be ratio in trap samples varies from 3 to 20 × 10⁻⁸. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of¹⁰Be into the traps at about 1500 m are estimated as 9 × 10⁵ and 4 × 10⁵ atoms cm⁻² a⁻¹ at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 10⁵ atoms cm⁻² a⁻¹ at both locations. Good correlations exist between¹⁰Be,⁹Be and²⁷Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.     

Particulate matter fluxes in the southern and central Kara Sea compared to sediments: Bulk fluxes, amino acids, stable carbon and nitrogen isotopes, sterols and fatty acids

The Kara Sea is one of the arctic marginal seas strongly influenced by fresh water and river suspension. The highly seasonal discharge by the two major rivers Yenisei and Ob induces seasonal changes in hydrography, sea surface temperature, ice cover, primary production and sedimentation. In order to obtain a seasonal pattern of sedimentation in the Kara Sea, sediment traps were deployed near the river mouth of the Yenisei (Yen) as well as in the central Kara Sea (Kara) within the framework of the German–Russian project “Siberian River run-off; SIRRO”. Two and a half years of time-series flux data were obtained between September 2000 and April 2003 and were analyzed for bulk components, amino acids, stable carbon and nitrogen isotopes as well as sterols and fatty acids.Sediment trap data show that much of the annual deposition occurred under ice cover, possibly enhanced by zooplanktonic activity and sediment resuspension. An early bloom of ice-associated algae in April/May occurred in the polynya area and may have been very important to sustain the life cycles of higher organisms after the light limitation of the winter months due to no/low insolation and ice cover. The strong river input dominated the months June–August in the southern part of the Kara Sea. The central Kara Sea had a much shorter productive period starting in August and was less affected by the river plumes. Despite different time-scales of sampling and trapping biases, total annual fluxes from traps were in the same order of magnitude as accumulation rates in surface sediments. Terrestrial organic carbon accumulation decreased from 10.7 to 0.3 g C m⁻² a⁻¹ from the riverine source to the central Kara Sea. Parallel to this, preservation of marine organic matter decreased from 10% to 2% of primary productivity which was probably related to decreasing rates of sedimentation.     

The global-average production rate ofBe

Precipitation collected in continuously open containers for about a year at seven sites around the United States was analyzed for¹⁰Be,⁹⁰Sr,²¹⁰Pb and²³⁸U. Based on these data and long-term precipitation,⁹⁰Sr and²¹⁰Pb delivery patterns, the stratospheric, tropospheric and recycled¹⁰Be components in the collections were estimated and the global¹⁰Be production rate was assessed. Single station production rate estimates range from 0.52 × 10⁶ atoms cm⁻² yr⁻¹ to 2.64 × 10⁶ atoms cm⁻² yr⁻¹. The mean value is 1.21 × 10⁶ atoms cm⁻² yr⁻¹ with a standard error of 0.26 × 10⁶ atoms cm⁻² yr⁻¹.     

Sediment transport in distributary channels and its export to the pro-deltaic environment in a tidally dominated delta: Fly River, Papua New Guinea

Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb- and flood-dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10 g l⁻¹ from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100 cm above the bed) drop off from around 100 cm s⁻¹ within the distributary channels to <50 cm s⁻¹ on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35 h to traverse the 20-km-wide, low-gradient (2×10⁻³ degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates.     

Temporal and spatial complexity in post-glacial sedimentation on the tectonically active,Poverty Bay continental margin of New Zealand

On the eastern Raukumara Ranges of the New Zealand East Coast, active tectonics, vigorous weather systems, and human colonisation have combined to cause widespread erosion of the mudstone- and sandstone-dominated hinterland. The Waipaoa River sedimentary dispersal system is an example that has responded to environmental change, and is now New Zealand's second largest river in terms of suspended sediment discharge. This paper presents new sediment accumulation rates for the continental shelf and slope that span century to post-glacial time scales. These data are derived from radiochemical tracer, palynological, tephrostratigraphic, and seismic methods. We hypothesise on the temporal and spatial complexity of post-glacial sedimentation across the margin and identify the broad extent of sediment dispersal from the Waipaoa system. The ∼15 km³ Poverty Bay mid-shelf basin lies adjacent to the mouth of the Waipaoa River, reaching a maximum thickness of ∼45 m. A post-glacial mud lobe of an additional ∼3 km³ extends through the Poverty Gap and out onto the uppermost slope, attaining 40 m thickness in a structurally controlled sub-basin. Here, an offset in the last-glacial erosion surface indicates that deposition was sympathetic with fault activity and the creation of accommodation space, implying that sedimentation was not supply limited. Contrary to classical shelf sedimentation models, the highest modern accumulation rate of 1 cm y⁻¹ occurs on the outer-shelf sediment lobe, approximately ∼2 times the rate recorded at the mid-shelf basin depocentre, and ∼10 times faster than the excess ²¹⁰Pb rates estimated from the slope. Pollen records from slope cores fingerprint Polynesian then European settlement, and broaden the spatial extent of post-settlement sedimentation initially documented from the Poverty Bay mid-shelf. Changes in sub-millennial sedimentation infer a 2–3-times increase in post-settlement accumulation on the shelf but a smaller 1–2 times increase on the slope. Over longer time scales, seismic evidence infers slower but steady sedimentation since the last transgression, and that significant cross-shelf sediment pathways pre-date the increase in sedimentation resulting from colonisation and deforestation. From a summation of coastal bedload, shelf and slope sediment mass accumulation, the total sediment budget for the Holocene is ∼1 Mt y⁻¹. Under modern conditions a larger proportion of the Waipaoa sediment dispersal system likely extends onto the slope and beyond.     

Be in a deep-sea core: implications regardingBe production changes over the past 420 ka

The influx of¹⁰Be into a globigerinid ooze core (CH72-02) from the eastern North Atlantic has been studied. This core contains a depositional record of the first 11 δ¹⁸O stages covering the last 423 ka. It is shown that the marine deposition of¹⁰Be is strongly influenced by the sedimentation of clays. Clay particles appear 10 times more efficient than the carbonate component as a carrier in bringing¹⁰Be to the bottom sediments. In core CH72-02, the deposition rates of¹⁰Be averaged over each oxygen-isotope stage for the past 11 stages show a scatter of ±40% about the mean value of 6.6 × 10⁸ atoms cm⁻² ka⁻¹. However, after correction for changes in lithology, the data show that the production rate of¹⁰Be over the same period has varied no more than ±25%, and the variations are not systematic in that high or low¹⁰Be production appear to be associated with either cold or warm climates. On the time scale of this investigation (intervals of ca. 50 ka over the last 420 ka, with resolutions as fine as 10 ka for portions of the record), it is unlikely that the shielding effect of the solar wind has deviated by more than ±25% or the geomagnetic field intensity has deviated by more than a factor of 1.6 from their long-term averages.     

Oxygen diffusion rates in quartz exchanged with CO2

Oxygen self diffusion rates were determined in quartz samples exchanged with¹⁸O-enriched CO₂ between 745 and 900°C and various pressures, and the diffusion profiles were measured using an ion microprobe. The activation energy (Q) and preexponential factor (D₀) at P(CO₂) = P(tot) = 100 bar, for diffusion parallel to the c-axis are 159 ( ± 13) kJ/g atom and 2.10 (+0.75/ −0.55) × 10⁻⁸ cm²/s. This rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1-bar quartz-O₂ exchange experiment. The oxygen diffusion rate measured at 0.6 bar, 888°C, and at 900°C in vacuum is in agreement with the previous 1-bar exchange experiments with¹⁸O₂. The effect of higher CO₂ pressures is small. At 900°C, the diffusion rate exchanged with CO₂ is = 2.35 × 10⁻¹⁵ cm²/s at 100 bar, 2.24 × 10⁻¹⁵ cm²/s at 3.45 kbar and 8.13 × 10⁻¹⁵ cm²/s at 7.2 kbar.There is probably a diffusing species, other than oxygen, that enhances the oxygen diffusion rate in these quartz-CO₂ systems, relative to that occurring at very low pressures or in a vacuum. The effect of this diffusing species, however, is not as strong as that associated with H₂O. Preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled, high-grade metamorphic terrane will vary depending upon whether a water-rich phase was present or not. Closure temperatures will be approximately 100°C higher in rocks where no water-rich phase was present during cooling. The measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression.