Settling velocity,effective density,and mass composition of suspended sediment in a coastal bottom boundary layer,Gulf of Lions,France

Particle size distribution and size-specific settling velocity are critical parameters for understanding the transport of fine sediment on continental margins. In this study, observed floc size versus settling velocity, volume distributions of particles 2 μm–1 cm in diameter, and calculated effective densities for all particle sizes provided estimates of the mass distribution in suspension, which is used to apportion mass among component particles, microflocs, and macroflocs. Measurements were made during relatively quiescent environmental conditions. Observations of size distributions based on mass demonstrate an increase in the component particle fraction through time. The increase in the percentage of component particles in suspension had implications on water column properties, as small changes in the component particle fraction affected water column optical transmission in a way that was not as easily detected by changes in the volume concentration distribution or total mass concentration. Flocs larger than 133 μm in diameter only comprised one quarter to one third of the mass in suspension. This finding may explain why suspension bulk clearance rates are often an order of magnitude lower than those predicted by other methods.     

Currents and sediment transport in the Mississippi Canyon and effects of Hurricane Georges

The temporal variability in currents, temperature, and particulate matter concentration were measured in the Mississippi Canyon axis where the thalweg was 300 m deep from May–July and August–November 1998 using current meters, thermographs, a light-scattering sensor, and sediment traps. Canyon sediments were sampled by coring and observed using an ROV video camera. Currents in the upper Mississippi Canyon generally oscillated up/down canyon with diurnal periodicity and were bottom-intensified. Mean current speed at 3.5 mab was approximately 8 cm s^?1 during both deployments, reaching maximum speeds of over 50 cm s^?1 under normal conditions. Based on current velocities, critical bed shear stress for resuspension of canyon-floor sediments was exceeded about 30% of the time during both deployments. In late September, Hurricane Georges passed 150 km NE of the study site, significantly intensifying current velocities, bed shear stress, resuspension, trap fluxes and temperature fluctuations. As the hurricane passed, maximum current speed reached 68 cm^?s and temperature decreased ～7 °C in less than two hours. Critical bed shear stress for sediment resuspension was exceeded approximately 50% of the time during the five days of hurricane influence. Further evidence for sediment resuspension was the five-fold (and perhaps 70–130 fold) increase in trap fluxes and compositional similarities between canyon surface sediment and material collected by traps.     

A cross-shore suspended sediment transport shape function parameterisation for natural beaches

A new field-based parameterisation (‘shape function’) describing the distribution of cross-shore suspended sediment transport across a beach profile is presented. Time-averaged and depth-integrated suspended sediment fluxes were measured over 39 tides at Sennen Cove, Cornwall, UK, for a range of wave conditions (offshore significant wave heights 0.1–2.5 m). The suspended sediment flux data were heuristically separated into four transport components: (1) mean flux in the surf/shoaling zone; (2) oscillatory flux in the surf/shoaling zone; (3) onshore flux in the swash/inner surf zone and (4) offshore flux in the swash/inner surf zone. Each of these transport components was related to the local water depth (h) normalised by the breakpoint depth (h_b) and the four resulting suspended transport shape functions were combined to form a total suspended load shape function. Each shape function component is scaled independently by the wave energy level through h_b. The total suspended load shape function predicts onshore sediment transport under low-energy conditions, with peaks at the breakpoint and in the swash zone, in agreement with the field observations. Under high-energy conditions the total suspended load shape function predicts onshore transport in the shoaling zone, offshore transport in the surf zone and onshore transport in the inner swash zone.     

Contemporary morphodynamics of a high-energy headland-embayment shoreface

A 2-year investigation into shoreface morphodynamic behaviour off a high-energy headland-embayment coast in Northern Ireland reveals important process–response mechanisms that cannot be explained solely by existing conceptual models. Fourteen sequential bathymetric surveys, conducted every 1–2 months, show that morphologic (seabed) change is not directly related to oceanographic forcing—extensive nearshore and shoreface accretion and erosion occurs under fair-weather, modal and high-energy conditions. The main factors which seem to cause significant change are long-duration (swell) events coupled with onshore winds, availability of (recently) introduced sediment, surges and elapsed time between storms and the next scheduled survey. Several high-energy events over a short time period (<30 days) did not result in extensive seafloor changes, contrary to expectations. Net seabed change over 2 years shows an average to 0.6 m m^?2 of shoreface accretion from the nearshore to 24 m depth. Net erosion was not observed anywhere, including the subaerial beach. Geologic evidence strongly suggests that the source of the significant volume (7.7×105 m³) of sediment introduced into the study area must have been derived from the lower shoreface and/or inner shelf, beyond 24 m depth.     

The effect of Hurricane Lili on the distribution of organic matter along the inner Louisiana shelf (Gulf of Mexico,USA)

On October 3, 2002 Hurricane Lili made landfall on a previously studied region of the inner Louisiana shelf as a Category 2 storm with winds over 160 km/h. A week after the hurricane, major impacts of the storm were not evident in the water column except for the lower than expected inshore salinities (∼12 psu) for this time of year, which was characterized by low river discharge. Turbidity profiles were typical of those measured during previous investigations with suspended sediment concentrations >75 mg/L at inshore stations and <50 mg/L in surface waters and offshore. The implication is that the sediments resuspended during the hurricane settled soon after the storm passage. Water column particulate organic carbon (POC) concentrations ranged from 0.1 to over 2.0 mg/L, with the highest concentrations measured near the seabed and in the inshore portions of the study area. Suspended particles were characterized by low organic matter content (%POC of 0.5–2 wt%), low chlorophyll:POC ratios (Chl:POC<4 mg/g) and moderately elevated POC:particulate nitrogen ratios (POC:PN of 10–14 mol/mol), all suggesting their source was locally resuspended seabed sediment rather than from algal biomass or land-derived vascular plant detritus.Post hurricane sediment deposition throughout the study area resulted in a storm layer that ranged from <0.5 to 20 cm in thickness. In most locations sediment accumulation ranged from 3 to 10 cm. The storm deposits were generally composed of silty clays with a coarser, somewhat sandy 1–2 cm basal layer. Surface sediments from the storm layer were characterized by relatively high mineral surface areas (SA of 30–50 m²/g) and elevated OC contents (%OC of 1.0–2.0%). The dispersal of fine sediments following the hurricane resulted in marked changes in the SA and %OC values of surface sediments from offshore locations, which prior to the storm contained coarser, organic-poor particles (SA of 5–15 m²/g and %OC of 0.2–0.6%). The OC:SA and OC:N ratios of storm layer sediments ranged from 0.4 to 0.6 mg OC/m² and from 10 to 12 mol/mol, respectively, and were comparable to those measured in surface sediments prior to the hurricane. Such similarities in the composition of the organic matter reinforce the idea that the source of the storm deposits was the finer fraction of resuspended seabed sediments, with little evidence for inputs from local land-derived sources or autochthonous algal production. Overall, the magnitude of sediment and organic matter deposition on the seabed after the storm greatly exceeded the annual inputs from the Atchafalaya River and coastal primary production. The combined effects of hurricane-driven erosion and post-storm deposition represent a major perturbation to the benthic community of the region, which is already subject to these types of disturbances due to the combined effects of peaks in river discharge and the passage of storm fronts.     

Modern sediment accumulation on the Po shelf,Adriatic Sea

Sediment accumulation over the past century on the continental shelf near the Po delta varies with distance from the most active distributary channels. Near the Pila and Goro distributaries, sediment accumulation is rapid (1–4 cm yr⁻¹) and occurs in pulses. In these areas, the seabed is dominated by physical sedimentary structures that can be related to flood sedimentation. Between the two distributaries and in the southern portion of the dispersal system, sediment accumulation is slower (rates reach a minimum of 0.23 cm yr⁻¹ at ∼50 km from the Pila mouth) and steady-state, reflecting more continuous dispersal of sediment during non-flood periods. Sedimentary strata in these locations are composed of finer (clayey silt), mottled sediment. The similarity in the spatial distribution of long-term (100-yr) sediment accumulation to deposition resulting from the 2000 flood event suggests that the Po shelf is flood-dominated.About half of the sediment delivered by the Po River on a 100-yr time scale can be accounted for in the seabed deposit within ∼50 km of the Pila mouth. The remaining sediment is likely transported southward by the prevailing circulation, and this sediment coalesces with inputs from the Apennine Rivers.     

Winter variability of physical processes and sediment-transport events on the Eel River shelf,northern California

A 5-yr data set of near-bed current and suspended-sediment concentration measured within 2 m of the seabed in 60-m water depth has been analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River continental shelf, northern California. This data set encompasses a wide range of shelf conditions with winter events characterized as: Major Flood (1996/97), strong El Niño (1997/98), strong La Niña (1998/99), and Major Storm (1999/00). Data were collected at a site located 25 km north of the Eel River mouth, on the landward edge of the mid-shelf mud deposit. During the winter months sediment resuspension is forced primarily by near-bed oscillatory flows, and sediment transport occurs both as suspended load and as gravity-driven (fluid-mud) flows. Winter conditions that caused periods of increased sediment transport existed on average for 142 d yr⁻¹ over the total record, ranging between 89 d in the Major Flood year (1996/97) and 171 d in the La Niña year (1998/99). Hourly averaged values of significant wave height varied between 0.5 and 10.7 m and hourly averaged values of near-bed orbital velocities ranged between 0 and 125 cm s⁻¹. During the five winters, sediment threshold conditions were exceeded an average of 35% of the time, ranging from 19% in the Major Flood year (1996/97) to 52% in the La Niña year (1998/99). Mean concentration of suspended sediment, measured at 30 cmab, ranged from values close to 0–8 g l⁻¹. Among winters, major sediment flux events exhibited different patterns due to varying combinations of physical processes including river floods, waves, and shelf circulation. Within winters, the major period of sediment flux varied from a 3-d fluid mud event (Major Flood winter) to a 50-d period of persistent southerlies (El Niño winter) and a winter of continuous storm cycles (La Niña winter). Winter-averaged suspended-sediment concentration appeared to vary in response to river discharge, while total sediment flux responded to storm intensity. The net sediment flux appeared to depend on timing of river discharge and shelf conditions. On the Eel River shelf, the mid-shelf mud deposit apparently is not emplaced by deposition from the river plume, but by secondary processes from the inner shelf including off-shelf transport of sediment suspensions and gravity-driven fluid-mud flows. Thus, these inner-shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that forms and nourishes the mid-shelf deposit. Large-scale shelf circulation patterns and interannual variability of the physical forcing are also important in determining the locus of the mid-shelf deposit, and both are influenced by climate variations. Post-depositional alteration of the deposit also depends on the subsequent shelf conditions following major floods.     

Glaciation,erosion, and landscape evolution of Iceland

Stratigraphic and sedimentological studies indicate that Iceland has experienced over 20 glaciations during the last 4–5 Myr, in reasonable agreement with the number of glaciations reconstructed from the ∂¹⁸O record in deep-sea sediment. The pattern of glacial erosion was to a large part controlled by constructive volcanic processes resulting in increased topographic relief after 2.5 Myr. Between 2.5 and 0.5 Ma valleys up to 400 m deep were excavated into the Tertiary basalts of eastern and south Iceland with an average erosion rate of 10–20 cm ka⁻¹. During the last 0.5 million years rates of erosion increased to 50–175 cm ka⁻¹, with an additional 200 to over 1000 m of valley excavation. Previous estimates of the rate of landscape erosion during the Holocene vary widely, from 5 to 70,000 cm ka⁻¹. We present new studies that define the rates of landscape denudation during the major part of the Holocene (the last 10,200 years): one based on the Iceland shelf sediment record, the other from the sediment record in the glacier-fed lake, Hvítárvatn. Both studies indicate average Holocene erosion rates of about 5 cm ka⁻¹ similar to our erosion rate estimate for 4–5 Ma old strata that has not been subjected to regional glaciation.     

Glacial–interglacial sediment transport to the Meiji Drift,northwest Pacific Ocean: Evidence for timing of Beringian outwashing

A large sediment deposit known as the Meiji Drift, located in the northwestern Pacific Ocean, is thought to have formed from deep water exiting the Bering Sea, although no notable deep water forms there presently. We determine the terrigenous sources since 140 ka to the drift using bulk sediment ⁴⁰Ar–³⁹Ar and Nd isotopic analyses on the silt-sized (20–63 μm) terrigenous fraction from Ocean Drilling Program (ODP) Site 884 to reconstruct paleo-circulation patterns. There are large changes in both isotopic tracers, varying on glacial–interglacial cycles. During glacial intervals, bulk sediment ⁴⁰Ar–³⁹Ar ages range between 40 and 80 Ma, while Nd isotopic values range from ε_Nd = ? 1 to + 2. During interglacial intervals, sediments become much younger and more radiogenic, with bulk sediment ages falling to 2–15 Ma and Nd isotopic values ranging between ε_Nd = + 5 and + 9. These data and quantitative comparison to potential source rocks indicate that the young Kamchatkan and Aleutian Arcs, lying NW and NE of the Meiji Drift, contribute the majority of sediment during interglacials. Conversely, older source rocks, such as those drained by the Yukon River and northeast Russia are the dominant origin of sediments during glacials. Mixing model calculations suggest that as much as 35–45% of the sediment deposited in the Meiji Drift during glacials is from the Bering Sea. It remains unclear whether thermohaline-type circulation or focussing of Bering Sea flow lead to the glacial–interglacial sediment source changes observed here.     

Tidally-induced sediment transport patterns in the upper Bay of Fundy: A numerical study

The Minas Basin, the eastern end of the Bay of Fundy, is well known for its high tide ranges and strong tidal currents, which can be exploited to extract electricity power. The properties of the tidally-induced sediment transport in the Minas Basin, where significant changes in tidal processes may occur due to a recently proposed tidal power project, have been studied with a three-dimensional hydrodynamic model, an empirical bed load sediment transport model and surface sediment concentrations derived from the remotely-sensed images. The hydrodynamic model was evaluated against independent observational data, which include tidal elevation, tidal current (in the full water column and bottom layer), residual current profile and tidal asymmetry indicators. The evaluation shows that the model is in good agreement with the observations.The sediment transport includes two components, bed load and suspended particulate load. The bed load is calculated using the modelled bottom shear stress and the observed grain size data. The estimated features of bed load transport roughly agree with the observed patterns of the erosion and deposition in the Minas Basin and Cobequid Bay. The transport of the suspended load is estimated using the modelled velocity fields and the surface sediment concentration derived from remote-sensing images. The comparisons between the modelled results and the limited observations illustrate that the observed directions of suspended sediment transport are basically reproduced by the model. The modelled net suspended sediment input into the Minas Basin through Minas Passage is 2.4×10⁶ m³ yr^?1, which is comparable to the observed value of 1.6×10⁶ m³ yr^?1.The variations of the bed load and the suspended load in space and time are also presented. The total net transport, defined as the mean value of the sum of bed and suspended load transports during the tidal cycle, shows strong spatial variability. The magnitude of the transport flux ranges from 0.1 to 0.2 kg m^?1 s^?1 in Minas Channel and Minas Passage, 0.1 kg m^?1 s^?1 in Cobequid Bay, to 0.01 kg m^?1 s^?1 in the central Minas Basin and Southern Bight. In Minas Channel, the sediment transport follows the structure of the tidal residual circulation, which features a large anticlockwise gyre. The sediment in Minas Passage moves eastward and deposits into the central Minas Basin. However, the sediment from the eastern part of the Basin moves westward and deposits in the central Minas Basin as well. In the Cobequid Bay, sediment moves eastward and deposits in the upper bay.     

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.     

A quantitative examination of modern sedimentary lithofacies formation on the glacially influenced Gulf of Alaska continental shelf

Continental-shelf lithofacies are described from a series of cores collected in the northern Gulf of Alaska, a high-energy paraglacial shelf experiencing rapid rates of sediment accumulation. Short-lived tracers (²³⁴Th and chlorophyll-a) indicate that during the annual peak in fluvial sediment input (summer), biologic sediment mixing coefficients in the surficial seabed are generally lower than other coastal environments (<20 cm² yr⁻¹) and mixing extends downward <10 cm.²¹⁰Pb geochronology indicates that sediment accumulation rates (time scales of 10–100 yr) are 0.1–3 cm yr⁻¹. The measured bioturbation and accumulation rates lead to predictions of moderate to bioturbated lithofacies, as observed. Primary depositional fabric is preferentially preserved where sediment accumulation rates >2 cm yr⁻¹ and non-steady sediment deposition occurs. Depositional fabric is also observed in strata at 50–100 m water depths and is similar in appearance to beds that may form through deposition of wave-induced fluid-mud flows, which have been observed forming on other shelves with moderate to high wave energy. Five general lithofacies can be identified for the study area: inner-shelf sand facies, interbedded sandy mud facies, moderate-to-well-bioturbated mud facies, gravelly mud facies, and Tertiary bedrock facies. The moderate-to-well-bioturbated mud facies is areally dominant, representing over 50% of the shelf area, although roughly equal volumes (∼0.4 km³) of strata with some preservation of primary fabric are annually accumulating. Lithofacies on this paraglacial shelf generally resemble mid- and low-latitude allochthonous shelf strata to a much greater degree than Holocene glacimarine strata formed on shelves dominated by icebergs and floating ice shelves. Paraglacial strata may be differentiated from non-glacial shelf strata by lower organic carbon concentrations, a relatively lower degree of bioturbation, and increased preservation of primary depositional fabric.     

Particle fluxes and recent sediment accumulation on the Aquitanian margin of Bay of Biscay

As a part of the ANR-Forclim experiment, particle mass fluxes and sedimentation processes were investigated on the slope of Aquitanian margin of the Bay of Biscay, between the canyons of Cap-Breton and Cap-Ferret. Interface sediments were collected along a depth transect from 145 to 2000 m; simultaneously a mooring line was deployed at the deepest station (WH, 2000 m) with two traps (800 and 1700 m) for a 16-month period (June 2006–November 2007). ²¹⁰Pb activities of settling particles and of interface sediments were determined to study transport processes of particles. Sediment and mass accumulation rates, calculated from excess ²¹⁰Pb profiles in the sediment column, show the expected decreasing trend with depth, as usually observed on margins. Mean particulate mass fluxes at 800 and 1700-m depth at site WH are, respectively, 27 and 70 g m^?2 a^?1.The ²¹⁰Pb budget points out events of temporary high lateral input of particles. The comparison of mass and ²¹⁰Pb fluxes between the water column and the seabed indicates that lateral transport plays an important role in particle accumulation on the Aquitanian margin. Regarding the objectives of the ANR-Forclim program, which aims to improve significantly the interpretation of fossil foraminifera signals, as a proxy for hydrological changes in the North Atlantic ocean, these results highlight advection processes must be considered when interpreting fluxes of foraminifers on the Aquitanian margin.     

Flocculation,heavy metals (Cu,Pb, Zn) and the sand–mud transition on the Adriatic continental shelf,Italy

Across a limited depth range (5–10 m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand–mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63 μm) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20 m water depth while the SMT associated with the Pescara River varies between 15 and 25 m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT.     

Polycyclic aromatic hydrocarbons in the coastal water,surface sediment and mullet Liza klunzingeri from northern part of Hormuz strait (Persian Gulf)

The concentrations of 16 polycyclic aromatic hydrocarbons (PAHs) were determined in mullet (Liza klunzingeri), water and sediment from northern part of Hormuz strait (Persian Gulf). The concentration levels of total PAHs in L. klunzingeri, water and sediment were 133.99–268.57 ng g⁻¹ dry weight, 3.12–5.88 ng l⁻¹ and 42.29–228.9 ng g⁻¹ dry weight, respectively. Based on isomer ratios, analysis of the PAHs source in the sediment demonstrated that the PAHs come from pyrogenic and petrogenic origin. Risk assessment showed PAHs threshold concentrations to occasionally be exceeded in the study area.     

Astrochronology of the late Jurassic Kimmeridge Clay (Dorset,England) and implications for Earth system processes

The Late Jurassic Kimmeridge Clay Formation (KCF) is an economically important, organic-rich source rock of Kimmeridgian–Early Tithonian age. The main rock types of the KCF in Dorset, UK, include grey to black laminated shale, marl, coccolithic limestone, and dolostone, which occur with an obvious cyclicity at astronomical timescales. In this study, we examine two high-resolution borehole records (Swanworth Quarry 1 and Metherhills 1) obtained as part of a Rapid Global Geological Events (RGGE) sediment drilling project. Datasets examined were total organic carbon (TOC), and borehole wall microconductivity by Formation Microscanner (FMS). Our intent is to assess the rhythmicity of the KCF with respect to the astronomical timescale, and to discuss the results with respect to other key Late Jurassic geological processes. Power spectra of the untuned data reveal a hierarchy of cycles throughout the KCF with ～ 167 m, ～ 40 m, 9.1 m, 3.8 m and 1.6 m wavelengths. Tuning the ～ 40 m cycles to the 405-kyr eccentricity cycle shows the presence of all the astronomical parameters: eccentricity, obliquity, and precession index. In particular, ～ 100-kyr and 405-kyr eccentricity cycles are strongly expressed in both records. The 405-kyr eccentricity cycle corresponds to relative sea-level changes inferred from sequence stratigraphy. Intervals with elevated TOC are associated with strong obliquity forcing. The 405-kyr-tuned duration of the lower KCF (Kimmeridgian Stage) is 3.47 Myr, and the upper KCF (early part of the Tithonian Stage, elegans to fittoni ammonite zones) is 3.32 Myr. Two other chronologies test the consistency of this age model by tuning ～ 8–10 m cycles to 100-kyr (short eccentricity), and ～ 3–5 m cycles to 36-kyr (Jurassic obliquity). The ‘obliquity-tuned’ chronology resolves an accumulation history for the KCF with a variation that strongly resembles that of Earth's orbital eccentricity predicted for 147.2 Ma to 153.8 Ma. There is evidence for significant non-deposition (up to 1 million years) in the lowermost KCF (baylei–mutabilis zones), which would indicate a Kimmeridgian/Oxfordian boundary age of 154.8 Ma. This absolute calibration allows assignment of precise numerical ages to zonal boundaries, sequence surfaces, and polarity chrons of the lower M-sequence.     

Modeled tephra ages from lake sediments,base of Redoubt Volcano,Alaska

A 5.6-m-long lake sediment core from Bear Lake, Alaska, located 22 km southeast of Redoubt Volcano, contains 67 tephra layers deposited over the last 8750 cal yr, comprising 15% of the total thickness of recovered sediment. Using 12 AMS ¹⁴C ages, along with the ¹³⁷Cs and ²¹⁰Pb activities of recent sediment, we evaluated different models to determine the age–depth relation of the core, and to determine the age of each tephra deposit. The selected age model is based on a mixed-effect regression that was passed through the adjusted tephra-free depth of each dated layer. The estimated age uncertainty of the 67 tephras averages ±105 yr (95% confidence intervals). Tephra-fall frequency at Bear Lake was among the highest during the past 500 yr, with eight tephras deposited compared to an average of 3.7/500 yr over the last 8500 yr. Other periods of increased tephra fall occurred 2500–3500, 4500–5000, and 7000–7500 cal yr. Our record suggests that Bear Lake experienced extended periods (1000–2000 yr) of increased tephra fall separated by shorter periods (500–1000 yr) of apparent quiescence. The Bear Lake sediment core affords the most comprehensive tephrochronology from the base of the Redoubt Volcano to date, with an average tephra-fall frequency of one every 130 yr.     

Luminescence dating of the top of a deep water core from the NESTOR site near the Hellenic Trench,east Mediterranean Sea

In the present study, the uppermost 3 cm of muddy, seabed sediment was collected from a deep-sea sediment core, drilled from a water depth of 4 km, near the deepest site of the Mediterranean Sea, outside Pylos, Greece. The core was divided into 7 layer samples, each 3–4 mm thick, in order to get an independent age assessment for each one using luminescence dating; from polymineral coarse grains in the range 30–60 μm. Between 11 and 22 aliquots were measured for each sample using the optically stimulated luminescence, single-aliquot regenerative-dose analysis. The estimated mean equivalent dose values had an uncertainty less than 3% and resulted in optical ages ranging from 3.5 to approximately 5 ka, with uncertainties lying between 5.5 and 7.4%. Multiple-aliquot, thermoluminescence-based, additive-dose, total bleaching approach provided equivalent doses with typical errors of 10–15% and ages in the range of 3.6 and 9.4 ka, with uncertainties up to 17%. Single-aliquot optical ages are shown to be relatively successful, due to their consistency with AMS ¹⁴C radiocarbon ages, obtained from Planktonic foraminifera from the same core. Luminescence dates for the topmost 1.5 cm indicate a substantial mix and post burial disturbance of the surface sediment. Below the topmost 1.5 cm, both luminescent approaches indicate ages which increase smoothly with depth. The concordant OSL and TL age estimates for the sample O5, in conjunction with specific luminescence properties and its major-element geochemical chemistry content, suggest that it was heated during the Santorini volcanic eruption. Deposition rates of 8.6–18.9 cm over 10 ka below the sample O5, provided by optical ages, exhibit an excellent level of agreement with the accumulation rate of 7–18 cm over 10 ka at the sea bottom, already reported for the site under study.     

Field observations of instantaneous water slopes and horizontal pressure gradients in the swash-zone

Field observations of instantaneous water surface slopes in the swash zone are presented. For free-surface flows with a hydrostatic pressure distribution the surface slope is equivalent to the horizontal pressure gradient. Observations were made using a novel technique which in its simplest form consists of a horizontal stringline extending seaward from the beach face. Visual observation, still photography or video photography is then sufficient to determine the surface slope where the free-surface cuts the line or between reference points in the image. The method resolves the mean surface gradient over a cross-shore distance of 5 m or more to within ±0.001, or 1/20th −1/100th of typical beach gradients. In addition, at selected points and at any instant in time during the swash cycle, the water surface slope can be determined exactly to be dipping either seaward or landward. Close to the location of bore collapse landward dipping water surface slopes of order 0.05–0.1 occur over a very small region (order 0.5 m) at the blunt or convex leading edge of the swash. In the middle and upper swash the water surface slope at this leading edge is usually very close to horizontal or slightly seaward. Behind the leading edge, the water surface slope was observed to be very close to horizontal or dipping seaward at all times throughout the swash uprush. During the backwash the water surface slope was observed to be always dipping seaward, approaching the beach slope, and remained seaward until a new uprush edge or incident bore passed any particular cross-shore location of interest. The observations strongly suggest that the swash boundary layer is subject to an adverse pressure gradient during uprush and a favourable pressure gradient during the backwash. Furthermore, assuming Euler's equations are a good approximation in the swash, the observations also show that the total fluid acceleration is negative (offshore) for almost the whole of the uprush and for the entire backwash. The observations are contrary to recent work suggesting significant shoreward directed accelerations and pressure gradients occur in the swash (i.e., ∂u/∂t>0∼∂p/∂x<0), but consistent with analytical and numerical solutions for swash uprush and backwash. The results have important implications for sediment transport modelling in the swash zone.     

Understanding impacts of tropical storms and hurricanes on submerged bank reefs and coral communities in the northwestern Gulf of Mexico

A 100-year climatology of tropical storms and hurricanes within a 200-km buffer was developed to study their impacts on coral reefs of the Flower Garden Banks (FGB) and neighboring banks of the northwestern Gulf of Mexico. The FGB are most commonly affected by tropical storms from May through November, peaking in August–September. Storms approach from all directions; however, the majority of them approach from the southeast and southwest, which suggests a correlation with storm origin in the Atlantic and Gulf of Mexico. A storm activity cycle lasting 30–40 years was identified similar to that known in the Atlantic basin, and is similar to the recovery time for impacted reefs. On average there is 52% chance of a storm approaching within 200 km of the FGB every year, but only 17% chance of a direct hit every year. Storm-generated waves 5–25 m in height and periods of 11–15 s induce particle speeds of 1–4 m s^?1 near these reefs. The wave–current flow is capable of transporting large (～3 cm) sediment particles, uplifting the near-bottom nepheloid layer to the banks tops, but not enough to break coral skeletons. The resulting storm-driven turbulence induces cooling by heat extraction, mixing, and upwelling, which reduces coral bleaching potential and deepens the mixed layer by about 20 m. Tropical storms also aid larvae dispersal from and onto the FGB. Low storm activity in 1994–2004 contributed to an 18% coral cover increase, but Hurricane Rita in 2005 reduced it by 11% and brought coral cover to nearly pre-1994 levels. These results suggest that the FGB reefs and neighboring reef banks act as coral refugia because of their offshore location and deep position in the water column, which shields them from deleterious effects of all but the strongest hurricanes.