Small Amounts Of Sediment Research Articles

Aquatic Oligochaete (Annelida: Clitellata) Fauna from the Jungnang Stream in Seoul, Korea, with Eight New Korean Records

Abstract Aquatic oligochaetes were investigated from a small amount of sediment sampled from the Jungnang Stream, an urban stream in Seoul, Korea, in November 2012. Sorted oligochaetes were observed under a light microscope in a drop of carbonated water, and then preserved in 70% ethanol. Some specimens were slide mounted using glycerin and Eupharal. As a result, the following 12 species were identified including 8 new Korean records, Bothrioneurum vejdoskyanum Stolc, Branchiodrilus hortensis (Stephenson), Chaetogaster cristalinus Vejdosky, Nais communis Piguet, Nais pardalis Piguet, Dero obtusa d`Udekem, Piguetiella denticulata Liang, and Haemonais waldvogeli Brescher and 4 additional known species, Limnodrilus hoffmeisteri Cleparede, Branchiura sowerbyi Beddard, Chaetogaster diastrophus (Gruithuisen), and Stylaria fossularis Leidy. This study shows how freshwater oligochaetes are diverse even in urban streams in Korea, if a proper sampling and examining method is employed.

Today's sediment budget of the Rhine River channel, focusing on the Upper Rhine Graben and Rhenish Massif

The river bed of the Rhine River is subject to severe erosion and sedimentation. Such high geomorphological process rates are unwanted for economical, ecological, and safety reasons. The objectives of this study were (1) to quantify the geomorphological development of the Rhine River between 1985 and 2006; (2) to investigate the bed erosion process; and (3) to distinguish between tectonic, hydrological, and human controls. We used a unique data set with thousands of bedload and suspended-load measurements and quantified the fluxes of gravel, sand, silt, and clay through the northern Upper Rhine Graben and the Rhenish Massif. Furthermore, we calculated bed level changes and evaluated the sediment budget of the channel. Sediment transport rates were found to change in the downstream direction: silt and clay loads increase because of tributary supply; sand loads increase because of erosion of sand from the bed; and gravel loads decrease because of reduced sediment mobility caused by the base-level control exerted by the uplifting Rhenish Massif. This base-level control shows tectonic setting, in addition to hydrology and human interventions, to represent a major control on morphodynamics in the Rhine. The Rhine bed appears to be in a state of disequilibrium, with an average net bed degradation of 3mm/a. Sand being eroded from the bed is primarily washed away in suspension, indicating a rapid supply of sand to the Rhine delta. The degradation is the result of an increased sediment transport capacity caused by nineteenth and twentieth century's river training works. In order to reduce degradation, huge amounts of sediment are fed into the river by river managers. Bed degradation and artificial sediment feeding represent the major sources of sand and gravel to the study area; only small amounts of sediment are supplied naturally from upstream or by tributaries. Sediment sinks include dredging, abrasion, and the sediment output to the downstream area. Large uncertainties exist about the amounts of sediment deposited on floodplains and in groyne fields. Compared to the natural situation during the middle Holocene, the present-day gravel and sand loads seem to be lower, whereas the silt and clay loads seem to be higher. This is probably caused by the present-day absence of meander migration, the deforestation, and the reduced sediment trapping efficiency of the floodplains. Even under natural conditions no equilibrium bed level existed.

Hybrid submarine flows comprising turbidity current and cohesive debris flow: Deposits, theoretical and experimental analyses, and generalized models

Hybrid flows comprising both turbidity current and submarine debris flow are a significant departure from many previous influential models for submarine sediment density flows. Hybrid beds containing cohesive debrite and turbidite are common in distal depositional environments, as shown by detailed observations from more than 20 modern and ancient systems worldwide. Hybrid flows, and cohesive debris flows more generally, are best classified in terms of a continuum of decreasing cohesive debris flow strength. High-strength cohesive debris flows tend to be clast rich and relatively thick, and their deposit extends back to near the site of original slope failure. They are typically confined to higher gradient continental slopes, but may occasionally form megabeds on basin plains, in both cases overlain by a thin turbidite. Intermediate-strength cohesive debris flows typically contain clasts, but their deposits may be <1 or 2 m thick on low-gradient fan fringes, and are encased in turbidite sand and mud. Clasts may be far-traveled, and meter-sized clasts can be rafted long distances across very low gradients if they are less dense than surrounding flow. Low-strength cohesive debris flows generally lack mud clasts, and as cohesive strength decreases further there is a transition into fluid mud layers that do not support sand. Intermediate- and low-strength cohesive debrites are consistently absent in more proximal parts of submarine systems, where faster moving sediment-charged flows are more likely to be turbulent. Intermediate-strength debris flows can run out for long distances on low gradients without hydroplaning. Very low strength cohesive debris flows most likely form through late-stage transformations near the site of debrite deposition, and emplaced gently to avoid mixing with surrounding seawater. The location and geometry of cohesive debrites in hybrid beds are controlled strongly by seafloor morphology and small changes in gradient. Debrites occur as fringes around raised channel-levee ridges, or in the central and lowest parts of basin plains lacking such ridges. Small variations in mud fraction produce profound changes in cohesive strength, flow viscosity, permeability, and the time taken for excess pore pressures to dissipate that span multiple orders of magnitude. Reduction in flow speed can also cause substantial increases in viscosity and yield strength in shear thinning muddy fluids. Small amounts of sediment can dampen or extinguish turbulence, especially as flow decelerates, affecting how sediment is supported or deposited. This ensures that cohesive debris flows and hybrid flows have a rich variety of behaviors.

Tsunami-induced coastal change: scenario studies for Painan, West Sumatra, Indonesia

There exists a high probability of a great earthquake rupture along the subduction megathrust under the Mentawai Islands of West Sumatra in the near future. Six rupture models were used to assess the tsunami inundation and the accompanying sediment movement in Painan, West Sumatra, Indonesia. According to a worst scenario, the potential tsunami might hit the coast of Painan about 26 minutes after the rupture and the entire city could be inundated with a maximum inundation depth of about 7 m. Severe erosion may also occur in the near-shore region. Two scenarios, one scenario with a positive leading wave and the other with a negative leading wave, were selected to simulate the tsunami-induced morphological changes. A positive leading wave would cause severe erosion in the shoreline area and a large sandbar in the offshore area adjacent to the shoreline; a small amount of sediment could be deposited in the city area; a negative leading wave could cause moderate erosion in the further offshore area due to the strong retreating wave front, an offshore sandbar could form in the bay area, while no noticeable large area of sand deposit could be found in the city area. The difference in the erosion and deposition patterns between these two scenarios provides very helpful information in the investigation of historical tsunamis through tsunami deposits.

A method for dry extracting large volumes of fine particulate matter from bulk soil samples

A significant part of ambient dust consists of fine particles derived from natural soils. Because many toxic constituents are concentrated in the smallest particle fractions, and because these small particulates are the most easily inhaled, the fine-particle fraction of soil (PM10 and smaller) is what presents the greatest health concern. Assessing the potential risk of a soil requires examining its chemical content, its biological and mineralogical composition, its sedimentological characteristics, detailed data on grain size distribution, and other information. This analysis requires a relatively large mass of appropriate size fraction(s), obtained by dry extraction. Most methods for extracting fine particulates from soil collect only small amounts of sediment, or use wet extraction. This paper describes the Soil Fine Particle Extractor, a setup for dry extracting large volumes (dozens of grams and more) of fine particulate matter (PM10 and smaller) from bulk soil samples. Separation takes place in two steps. Primary separation in a cylindrical separator 70 cm high and 40 cm in diameter removes all coarse particles from the sample. The remaining sediment then flows through a 200 cm long and 7 cm diameter inclined elutriator connected to a three-engine vacuum cleaner. The setup allows extraction of different size fractions depending on the adjustment of the control parameters (suction rate, length, diameter and inclination of the elutriator, thickness of the sediment layer drawn in by the system). The article presents examples of extractions from soil collected in loess and desert environments, ranging from PM2.5 to PM10. It also evaluates the efficiency of the technique by investigating the extraction efficiency for a large number of particle sizes, varying from 0.1 to 70 μm. Extraction occurs most efficiently for particles between 1 and 10 μm, with an optimum around 5 μm.

A short life‐cycle test with the epibenthic copepod Nitocra spinipes for sediment toxicity assessment

A new short life-cycle test methodology is presented to evaluate sublethal effects of contaminated sediments to the harpacticoid copepod species Nitocra spinipes. The method combines a 4-d survival-gravidity test with a 7-d development test. For water-only Cu exposures, the sensitivity of the development test endpoints were compared with tests using nonexposed gravid females to initiate the 7-d development test phase, and also with a multiple generation test comprising three successive phases: 9-d development; 21-d survival-gravidity; second-generation 9-d development. The results indicated that the development endpoints were the most sensitive, with median effective concentration (EC50) values of 95 µg Cu/L for nauplii/gravid female and 101 µg Cu/L copepodites/gravid female endpoints, followed by the gravidity (144 µg Cu/L) and survival (347 µg Cu/L). The sensitivity of the short life-cycle test endpoints was similar to the multiple-generation test endpoints, and the shorter test had less variability in controls. The multiple-generation test showed a large amount of stimulation of reproduction and development at Cu concentrations of 50 to 100 µg/L. The suitability of the short life-cycle test for assessing sediment toxicity was demonstrated using Cu-spiked and naturally contaminated whole sediments. Although the small nauplii were more difficult to isolate from sediments, the small amounts of sediments used for the tests and the large effects of the contaminated sediments on nauplii and copepodite numbers resulted in significant differences to controls. For sediment exposures, the sensitivity of the endpoints was in the order development > gravidity > survival. The short life-cycle test was demonstrated to detect, within 11 d of exposure, similar levels of effects on reproduction and development to those detected using a 39-d multiple-generation exposure.

Spatial and temporal variation of in‐reach suspended sediment dynamics along the mainstem of Changjiang (Yangtze River), China

Four major reaches of Chiangjang (Yangtze River) were analyzed for the period 1956–2007 in terms of long‐term trends in suspended sediment gain and loss in each reach, the relation of input and output to within‐reach sediment fluxes, and the impacts of dam construction on reach sediment dynamics. The temporal and spatial patterns of within‐reach sediment dynamics correspond closely with river morphology and also reflect basin conditions over the past few decades; thus they are conditioned by coeval trends in climate, hydrology, and land use. The study period encompasses both unusually wet years (e.g., 1998) and unusually dry periods (e.g., 2006). There are suggestive coincidences between changes in the suspended sediment regime and innovations in land use, sediment mining, and dams. The relatively strong statistical relation between sediment input and output for each of the four study reaches, and weak relations between storage and output, imply that most sediment supplied to each reach is delivered from upstream and that relatively small amounts of sediment are recruited within each reach. Cumulative departure plots reveal epicycles of sediment exchange varying from 15 to 40 years, implying a characteristic fine sediment storage time on the order of decades. Shorter epicycles (5–8 years) are commonly related to sediment influx associated with major floods.

Oxygen isotope constraints on the structure and evolution of the Hawaiian Plume

The oxygen isotope stratigraphy of Ko‘olau volcano, Hawaii, is constructed by analyzing olivine phenocrysts from the KSDP drill core and submarine land-slide deposits. Along with those of subaerial (Makapu‘u) Ko‘olau olivines (Eiler and others, 1996a), they span the full range of the δ^(18)OVSMOW variation previously observed in Loa-trend Hawaiian volcanoes (Lō‘ihi, Mauna Loa, Hualalai, and Ko‘olau), vary systematically with the stratigraphic position, and correlate with other geochemical properties of their host lavas (Tanaka and others, 2002; Haskins and Garcia, 2004; Huang and Frey, 2005; Salters and others, 2006; Fekiacova and others, 2007). These observations can be explained if the Loa-trend volcanoes (including Ko‘olau) are constructed of magmas made by mixing peridotite melt with variable proportions of eclogite melt derived from a mafic constituent of the Hawaiian plume having a composition resembling recent mid-ocean-ridge basalts. We present a model of this magma mixing process that simultaneously explains the correlations among oxygen isotopes, major elements, trace elements and radiogenic isotopes. Although a number of models of this kind, differing in several parameters, describe the data equally well, all statistically acceptable models require an component with a MORB-like HREE pattern and enriched oxygen isotope composition (δ^(18)OVSMOW = 7.8-9.7‰), consistent with this component being an upper crustal (layer 1 or 2) basalt or gabbro with a low-temperature alteration history, possibly containing a small amount of sediment. Abundances of some minor elements—Ni and Ti—are not well described by this model; we show that these shortcomings are derived from the compositional assumption and operational difficulties, that is, TiO_2 content is too high in our assumed eclogite end-member, and the inversion of NiO content in the melt by the olivine addition calculation is imprecise due to the sensitivity of D_(NiO)^(olivine/melt) to the melt composition and to crystallization process. Previous studies have advocated that Hawaiian lavas were derived from partial melts of an olivine-free pyroxenite formed by reaction of eclogite-derived melt with peridotite (for example, Sobolev and others, 2005). Our study shows that the peridotite-derived and eclogite-derived melt-mixing model can explain the geochemistry of Hawaiian lavas as well, including high-Ni Ko‘olau olivines. We find that an olivine-free mantle source for Hawaiian lavas is unnecessary, although melt-rock interaction could be important in modifying melt composition. Inverting for mixing proportions and degree of melting, we estimate that the amount of recycled crust in the Hawaiian plume is <24 weight percent. Comparison of the late shield-stage Loa-trend (particularly Ko‘olau lavas) and Kea-trend (particularly Mauna Kea lavas) suggests that the geochemical diversity of Hawaiian lavas is produced by a thermally and chemically-zoned plume.

Feeding Habits of the Mangrove Sesarmid Crab &lt;i&gt;Neosarmatium trispinosum&lt;/i&gt; (Crustacea) in the Ryukyu Islands, Japan

Sesarmid crabs are reported to dominate the Indo West-Pacific mangroves and consume huge amounts of mangrove leaves and detritus. Neosarmatium trispinosum Davie (1994) is common in the landward mangroves of the Ryukyu Islands in southern Japan. Stomach contents of the crabs collected from Okinawa, Miyako and Iriomote Islands showed that their diet consisted of mainly mangrove leaves with small amounts of sediments indicating that N. trispinosum is primarily detritivorous. The feeding preference of N. trispinosum was investigated by offering three types (green, yellow and brown) of Bruguiera gymnorrhiza leaves. Crabs were found to prefer brown leaves than the green or yellow ones. Consumption rate of brown leaves was significantly higher (p&lt;0.0001) in cases where green, yellow and brown leaves were provided together, than when provided separately. The C/N ratio shows that the burrow sediments had C/N ratios 2/3 times lower than that of leaves, indicating that mangrove sediments could have higher nutritional value than mangrove leaves.

Subrecent sediment dynamics and sediment budget of the braided sandur system at Sandane, Erdalen (Nordfjord, Western Norway)

A study was carried out of the subrecent sediment budget of a braided sandur system in a U-shaped valley linked to the Jostedalsbreen ice field in Nordfjord, in Western Norway. Special focus was on (i) the detection of different zones with negative, positive or balanced subrecent (following the Little Ice Age advance) sediment budget within the braided sandur system, (ii) the identification of sediment sources upstream of Sandane and from the slope systems to both sides of the braided sandur system, and (iii) the analysis of the subrecent sediment budget of the entire Sandane system. A combination of methods was applied in the analysis. The upstream part of Sandane was found to be characterised by a negative subrecent sediment balance, with erosion of coarse sediments from the Little Ice Age advance. In comparison, the downstream parts of Sandane have a balanced to slightly positive subrecent sediment budget, with formation of younger flood sediments and more stable channels. Thus, the subrecent sediment budget of Sandane appears to be slightly negative. Present-day coupling of slope and fluvial systems is limited and only a rather small amount of sediments is directly transported from the slopes into the braided sandur. In sum, fluvial sediment transport following the Little Ice Age period appears to be supply-limited.

On the geometry of core-catcher holders for hot-water based ice coring of sediment-laden ice

Analysis of sediment-laden ice accreted at the base of ice sheets provides useful information on subglacial conditions and processes that otherwise cannot be observed or are very difficult to observe. While sediment-laden accreted basal ice can be accessed directly at the terminus of alpine glaciers, the basal zone of ice sheets is only accessible by drilling. Small amounts of sediment in the ice have provided few or no problems for conventional ice-core drilling (Gow and others, 1979; Weis and others, 1997); however, the higher-sediment-content accreted ice beneath West Antarctic ice streams (Vogel and others, 2005) has not been recovered using the California Institute of Technology (Caltech) hot-water ice corer (Engelhardt and others, 2000).

Paleoparasitological analysis applied to museum-curated sacra from Meridional Patagonian collections

We analyzed residue of sediment recovered from sacral foramina for evidence of parasites. The sacra had been excavated from Meridional Patagonian inhumations and had been cleaned as part of the museum curating process. Using small brushes, sediments were collected from the foramina. The sediments were rehydrated in a trisodium phosphate solution and analyzed using paleoparasitological techniques. Nematode eggs identified as Capillaria spp. and Ascaris lumbricoides were found. These results showed that small amounts of sediment can be recovered from museum collections, even after bones have been cleaned. The results showed the importance of preserving sediments, as well as other materials obtained after cleaning archaeological remains, to perform parasite and food residue analyses, and the importance of studying the museum collections as sources for analysis of ancient parasitism and diet.

Holocene and modern sediment storage in the subtropical macrotidal Fitzroy River estuary, Southeast Queensland, Australia

The Fitzroy River estuary is a macrotidal, tide-dominated estuary located in the dry tropics of central Queensland, and represents the major source of terrestrial sediment to the southern Great Barrier Reef (GBR) lagoon. The estuary currently receives most of its sediment during large episodic floods that are typically associated with cyclones. Mean annual sediment budgets for such systems are difficult to estimate due to the sporadic nature of flood discharge events, which are highly seasonal and vary greatly in magnitude between years. We have estimated the quantity and long-term rate of accumulation of catchment-derived sediment in the estuarine floodplain using the Holocene stratigraphic sequence determined from a series of sediment cores, dated with radiocarbon and optically stimulated luminescence (OSL) methods. Approximately 13,760 million tonnes (Mt) of fluvial sediment has accumulated in the Fitzroy estuary during the past 8000 years, which equates to an average of 1720 kt yr − 1 . Over the past 100 years, sediment accumulation has been focused around mangrove and tidal creek environments, which cover an area of 130 km 2. Cores from the tidal creeks, dated using 137Cs, 210Pb, and OSL, provide sedimentation rates of approximately 15 mm yr − 1 for the past 45–120 years, or sediment mass accumulation of 1700 kt yr − 1 , which includes a component that is reworked into the estuary by tidal currents. Combined with the small amount of sediment that accumulates on the floodplain during floods (∼ 1 mm yr − 1 , 640 kt yr − 1 ), we estimate that approximately 2350 kt yr − 1 of sediment is trapped in the modern lower floodplain and estuary. This estimate of sediment storage suggests that greater than 50% of the modern mean annual sediment discharge of the Fitzroy River, 4162 kt yr − 1 , may be retained in the lower floodplain and estuary. These results provide useful insights into the spatial pattern of sedimentation, long-term rates of accumulation and estimates of sediment trapping in a tropical, macrotidal sedimentary system.

Large volume TENAX® extraction of the bioaccessible fraction of sediment-associated organic compounds for a subsequent effect-directed analysis

Background, Aims and Scope Effect-directed analysis (EDA) is a powerful tool for the identification of key toxicants in complex environmental samples. In most cases, EDA is based on total extraction of organic contaminants, which may lead to an erroneous prioritisation with regard to hazard and risk. Bioaccessibility-directed extraction aims to discriminate between contaminants that take part in partitioning between sediment and biota in a relevant time frame and those that are enclosed in structures that do not allow rapid desorption. Standard protocols of targeted extraction of the rapidly desorbing, and thus bioaccessible, fraction using TENAX® are based only on small amounts of sediment. In order to obtain sufficient extract for subsequent biotesting, fractionation and structure elucidation, a large volume extraction technique needs to be developed applying one selected extraction time and excluding toxic procedural blanks.

Petrology of an apatite-rich layer in the Finero phlogopite–peridotite, Italian Western Alps; implications for evolution of a metasomatising agent

We discovered an apatite-rich layer in the Finero phlogopite–peridotite body, Western Alps. Fine-grained apatite grains are concentrated, up to 10 modal percent locally, to form a lens within or adjacent to a thin layer, <1 cm in thickness, consisting of very fine-grained minerals of olivine, orthopyroxene, spinel, amphibole, phlogopite, sulfide minerals and apatite with small amount of carbonate and clinopyroxene. In the thin layer, apatite-rich part is richer in amphibole and carbonate minerals than apatite-poor part, suggesting that an alkali- and phosphate-rich carbonatitic metasomatising agent locally existed in the thin layer. On the other hand, film-shaped orthopyroxenes occur at grain boundaries of olivine in the thin layer. The film-shaped orthopyroxenes are distinctively low in Al 2O 3, Cr 2O 3 and CaO contents compared with those of the host peridotite, suggesting the consumption of a SiO 2-rich metasomatising agent as the thin layer formed. These two distinctive metasomatising agents would be caused by the immiscibility of a homogeneous metasomatising agent which was evolved to be a high CO 2/H 2O with the result that hydrous minerals and other metasomatised minerals were formed by interaction between a parent metasomatising agent and host peridotite in the late stage of exhumation of the Finero complex. The parent metasomatising agent is probably derived from a subducted slab, possibly containing small amount of sediments.

A rapid screening method for acid‐volatile sulfide in sediments

A new, direct method for the determination of acid-volatile sulfide (AVS) in sediments is presented. The method utilizes the direct reaction of Clines reagent with small amounts of sediment followed by colorimetric determination of AVS and offers fast analysis times without the need for specialized glassware or equipment. A comparison between AVS measured by this method and that measured by a purge-and-trap AVS method shows a linear relationship (r = 0.986, p < 0.001), although the method underestimates the AVS concentration measured by the purge-and-trap method. Sample throughput by the direct AVS method is more than 10 times that of purge-and-trap methods. The applicability of the direct AVS method to the rapid screening of AVS concentrations in sediments and for evaluating sediment quality with respect to metal contamination is demonstrated.

Carboniferous sand provenance in the Pennine Basin, UK: constraints from heavy mineral and detrital zircon age data

The integration of heavy mineral analysis and detrital zircon age dating has enabled high-resolution differentiation and characterisation of Carboniferous sandstone provenance in the Pennine Basin of the UK. Heavy mineral data have identified a number of distinct mineralogical groups with different provenance histories and source-area compositions. Single-grain zircon dating on each mineralogical type has placed constraints on the geochronology of the various source terrains. This combination of mineralogical and isotopic data has led to the identification of four distinct source terrains and sediment transport pathways. During the Namurian, the majority of sediment was supplied from the north via the ‘Pennine delta’. The source region comprised a high-grade metasedimentary terrain with granitic intrusions. Zircon age data indicate that this lay within the part of Laurentia–Baltica affected by the Caledonian orogeny. Small amounts of sediment were shed northwards from the Wales-Brabant High, on the southern margin of the basin. Most of this was recycled from the Old Red Sandstone but some of it came directly from late Proterozoic igneous basement. Supply via the Pennine delta declined markedly in the Westphalian, with most of the Westphalian A and B being fed from the west. The western source mainly comprised pre-existing sediment, with variable contributions from ultramafic rocks. The precise location of this source remains conjectural: it is unlikely to be within the British Isles given the size and scale of the Westphalian fluvial systems, but the zircon age spectrum cannot be reconciled with derivation from the Appalachians–Newfoundland–Labrador area. Supply from the uplifting Variscan massif to the south became important in late Westphalian B times and continued into Westphalian D. Zircon age data indicate sourcing from Late Carboniferous granites and Cadomian and Icartian basement.

SEDIMENT TRANSPORT IN A SHALLOW COASTAL ESTUARY DURING THE WINTER SEASON

ABSTRACT: This project analyzes suspended sediment flux through the upper Barataria basin in Louisiana during the winter season defined from November through April. The Barataria is a shallow coastal estuary located in southeastern Louisiana. The controls exerted by environmental parameters (such as wind or atmospheric pressure) in wetlands-shallow bay ecosystems on transport of water and sediment were examined. Water samples were taken at a bayou (which serve as the inlet for flow to the estuary) on a regular basis. These samples were analyzed for total suspended solids and volatile suspended solids. Velocity, depth, temperature, salinity, conductivity, and meteorological measurements were all recorded at the time of each sampling. A multi-parameter field probe was employed to continually monitor turbidity, water level, conductivity, and temperature during frontal events. These data were used in a regression analysis to examine the factors that drive carbon flux in the region. Investigations have determined that synoptic climate and prevailing weather conditions explain much of the variations in water levels, flow circulation patterns, salinity, and suspended sediment. Relatively small amounts of sediment appear to leave the estuary during normal tidal activity, but winter storm fronts result in significant fluxes of sediment in both up-basin and down-basin directions.

Slope basins, headless canyons, and submarine palaeoseismology of the Cascadia accretionary complex

A combination of geomorphological, seismic reflection and geotechnical data constrains this study of sediment erosion and deposition at the toe of the Cascadia accretionary prism. We conducted a series of ALVIN dives in a region south of Astoria Canyon to examine the interrelationship of fluid flow and slope failure in a series of headless submarine canyons. Elevated head gradients at the inflection point of canyons have been inferred to assist in localized failures that feed sediment into a closed slope basin. Measured head gradients are an order of magnitude too low to cause seepage‐induced slope failure alone; we therefore propose transient slope failure mechanisms. Intercanyon slopes are uniformly unscarred and smooth, although consolidation tests indicate that up to several metres of material may have been removed. A sheet‐like failure would remove sediment uniformly, preserving the observed smooth intercanyon slope. Earthquake‐induced liquefaction is a likely trigger for this type of sheet failure as the slope is too steep and short for sediment flow to organize itself into channels. Bathymetric and seismic reflection data suggest sediment in a trench slope basin between the second and third ridges from the prism’s deformation is derived locally. A comparison of the amounts of material removed from the slopes and that in the basin shows that the amount of material removed from the slopes may slightly exceed the amount of material in the basin, implying that a small amount of sediment has escaped the basin, perhaps when the second ridge was too low to form a sufficient dam, or through a gap in the second ridge to the south. Regardless, almost 80% of the material shed off the slopes around the basin is deposited locally, whereas the remaining 20% is redeposited on the incoming section and will be re‐accreted.

Revised Technique for Calculation of Calcareous Nannofossil Accumulation Rates

A rapid and easy settling technique designed to estimate quantitative abundances of calcareous nannofossils is described. Compared with other methods, the time spent in weighing and other time-consuming steps are compensated by the possibility of standardizing and producing large series of samples at the same time. Formulas for the calculation of absolute abundances and accumulation rates are presented. A test in which the examination of several samples where the amount of sediment as well as other parameters have been changed demonstrates a random distribution of nannoliths across the slide. Variations in the calculated accumulation rates were minimal, suggesting high reliability. INTRODUCTION Coccolithophores, and probably most of the calcareous nannoplankton, are primary producers whose distribution in the oceans is controlled by several parameters. Sea surface temperature (SST), related to climatic zoning, is one of the most important limiting parameters e.g.: different species in varying abundances characterize equatorial to subpolar regions (McIntyre et al. 1970; Haq 1980; Winter et al 1994). At both local and global level, biolimiting nutrient distribution is another important factor related to coccolithophore abundance (e.g. Molfino and McIntyre 1990; Young 1994). Changes in SST or in nutrient concentrations in the oceans are related to paleoceanographic changes and knowledge of coccolithophore (nannofossil) abundance in sediments could therefore help to understand the evolution of the world's oceans. The aim of the present study was to test an easy and rapid technique for estimating nannolith abundance per gram of sediment. This technique allows one to convert the amount of nannofossils into nannofossil fluxes when precise age models and sediment densities are available. Several techniques have been proposed to estimate the absolute abundances of nannofossils (Wei 1988; Beaufort 1991; Okada 1992; Henriksson 1995). Some of these techniques are very time-consuming, or involve instrumental problems in the preparation of well spread surfaces for nannofossil counting, often meaning than many visual fields (nannofossils) must be counted. In other cases, the proposed techniques allow only afford an estimation of the true abundance, as for example in Backman and Shackleton (1983). PROCEDURE The aim of the technique proposed here is to obtain slides on which the nannoliths are distributed homogeneously, and to achieve the possibility of standardizing the procedure in order to save time. Our experience is that with this procedure it is possible to prepare sets of up to one hundred samples at one time; i.e., a rate of around 50 samples per day. This technique was tested using both deep-sea sediments, where nannofossils were the most important component, and sediments, in which the calcareous nannofossil component was diluted by organic or inorganic material. The steps below describe the procedure, and should be followed carefully. Step 1. Sample drying. A small amount of sediment is dried in a heater at a temperature of about 50?C. Step 2. Weighing of an amount of dried sediment. This amount depends on the richness of the samples in nannofossils, as well as on the parameters discussed below. For routine analysis we used mass s between 0.1 to 0.3 g. Step 3. The weighed sediment is placed in a bottle, and a varying volume of distilled water is added (we used 10 to 30 ml). The mixture is shaken and, if necessary, placed in an ultrasound apparatus for a few seconds. For samples rich in organic matter, the distilled water can be substituted with H202. It is advisable to leave the mixture for a few hours under conditions of regular