High Sediment Accumulation Rates Research Articles

Logjam controls on channel:floodplain interactions in wooded catchments and their role in the formation of multi-channel patterns

The role of wood in the formation and maintenance of complex dynamic floodplain surfaces is important and to date has received relatively limited attention compared to in-channel habitat processes. This paper explores the role of logjams as important agents of channel:floodplain interaction. We draw on a specific case study as well as examples from the literature to show that although the processes of interaction differ, the resulting dynamic floodplain patchwork is a common feature of rivers with logjams. In addition, we contend that the presence of logjams is an important factor in the evolution and maintenance of multiple channel patterns in both montane and lowland river environments. These observations have important implications for the definition of reference targets for river restoration. The specific results of this research show: 1) The presence of a range of types of multiple channel network dissecting the floodplains of low order channels that are strongly associated with the presence of logjams. 2) The relatively rapid formation of floodplain channels following logjam formation. 3) The dynamic nature of logjams within headwater channels on both seasonal and annual timescales that lead to a highly dynamic habitat mosaic on the floodplain surface. 4) An increased frequency of overbank flooding and high rates of floodplain sediment accumulation upstream of logjams and along floodplain channel networks that create the complex topography observed in the case study forested floodplains.

Sediment dispersal and quantitative stratigraphic architecture across an ancient shelf

Two large (200 to 300 km), near-continuous outcrop transects and extensive well-log data (ca 2800 wells) allow analysis of sedimentological characteristics and stratigraphic architecture across a large area (ca 60 000 km2) of the latest Santonian to middle Campanian shelf along the western margin of the Western Interior Seaway in eastern Utah and western Colorado, USA. Genetically linked depositional systems are mapped at high chronostratigraphic resolution (ca 0·1 to 0·5 Ma) within their sequence stratigraphic context. In the lower part of the studied interval, sediment was dispersed via wave-dominated deltaic systems with a ‘compound clinoform’ geomorphology in which an inner, wave-dominated shoreface clinoform was separated by a muddy subaqueous topset from an outer clinoform containing sand-poor, gravity-flow deposits. These strata are characterized by relatively steep, net-regressive shoreline trajectories (>0·1°) with concave-landward geometries, narrow nearshore belts of storm-reworked sandstones (2 to 22 km), wide offshore mudstone belts (>250 km) and relatively high sediment accumulation rates (ca 0·27 mm year−1). The middle and upper parts of the studied interval also contain wave-dominated shorefaces, but coeval offshore mudstones enclose abundant ‘isolated’ tide-influenced sandstones that were transported sub-parallel to the regional palaeoshoreline by basinal hydrodynamic (tidal?) circulation. These strata are characterized by relatively shallow, net-regressive shoreline trajectories ( 190 km) and relatively low sediment accumulation rates (ca ≤0·11 mm year−1). The change in shelfal sediment dispersal and stratigraphic architecture, from: (i) ‘compound clinoform’ deltas characterized by across-shelf sediment transport; to (ii) wave-dominated shorelines with ‘isolated’ tide-influenced sandbodies characterized by along-shelf sediment transport, is interpreted as reflecting increased interaction with the hydrodynamic regime in the seaway as successive shelfal depositional systems advanced out of a sheltered embayment (‘Utah Bight’). This advance was driven by a decreasing tectonic subsidence rate, which also suppressed autogenic controls on stratigraphic architecture.

Climate-related cyclic deposition of carbonate and organic matter in Holocene lacustrine sediment, Lower Michigan, USA

Records from lake sediment cores are critical for assessing the relative stability of climate and ecosystems over the Holocene. Duck Lake in south-central Lower Michigan, USA, was the focus of a study that identified how changes in the geochemical variables in lake sediments relate to variations in regional climate and local land use during the Holocene. More than 8.5 m of lacustrine sediment were recovered using Livingston and freeze corers and analyzed for organic carbon, inorganic (carbonate) carbon, total nitrogen, and trace metals. Repeating packages of sediment (1–10 cm thick) that grade from light (inorganic carbon-rich) to dark (organic carbon-rich) were found from the surface to a depth of about 8 m. Variations in the high-resolution gray scale data from core X-radiographs are highly correlated to the relative amount of inorganic carbon. Geochemical analyses of the upper 8.5 m of sediment revealed a wide range of values: 0.05–10.6% for inorganic carbon (i.e. 0.5–89% calcium carbonate) and 1.1–28% for organic carbon (i.e. 2.7–70% organic matter). Organic carbon to nitrogen ratios indicate that most of the sediment organic matter is produced within the lake. A core chronology based on eight AMS radiocarbon dates shows low sediment accumulation rates (0.05 cm/year) from 10,000 to 3,800 cal year BP and higher sediment accumulation rates (0.1–0.3 cm/year) from 3,800 cal year BP to present. We suggest that carbonate accumulates during relatively dry times, whereas organic matter accumulation dominates when nutrient input to the lake is enhanced by wetter climate. The Duck Lake core records a distinct low point in inorganic carbon deposition that may be related to the 8.2 ka cooling event now documented from several sites in North America. Spectral analysis of gray scale values shows significant ~200-year periodicities over the past 8,000 years, hypothesized to result from climate changes induced by solar forcing. Concentrations of trace metals (e.g. lead, iron, copper, zinc) indicate the onset of regional anthropogenic influence about 150 cal year BP.

Frequent failure of the continental slope: The Gela Basin (Sicily Channel)

[1] Evaluating the recurrence of sediment failure on continental margins is important to better understand the evolution of margins and to assess the geologic risk of slope failure and, possibly, tsunami hazard. This paper proposes an integrated morphological and stratigraphic reconstruction of slope failures to evaluate their timing, frequency, and responsible mechanisms. Data from seismic stratigraphy, sediment cores, and seafloor geomorphology document multiple slide scars as well as buried and exposed mass transport deposits that originated during the Quaternary period on the continental slope of the Gela Basin in the Sicily Channel (central Mediterranean). The very high resolution data provide clues to recognize (1) successive phases of failure that have impacted on the same location, (2) main geological factors conducive to recurrent sediment failure in the area, including, in particular, specific stratigraphic surfaces acting as glide planes and high pore pressure gradients likely generated by high sediment accumulation rates atop mass transport complexes, and (3) a set of failed sediments that have been deposited since the Last Glacial Maximum defining a return interval on the order of 3–4 ka.

Deep subsidence of the early catagenesis zones of organic matter in the sedimentary rocks of Northeastern Sakhalin shelf at high thermal regime of the basin

Heat flux measurements in the shelf zone of Northern Sakhalin reveal strong heating of the sedimentary complexes with a temperature gradient of the order of 36 ° C/km along the entire length of the Okhotsk Sea margin of the North Sakhalin basin [2]. However, despite the high thermal regime of the lithosphere in the region, the fact that initial stages of catagenesis gradations PC3‐MC1 ( R o = 0.50‐0.64%) are confined to great depths (3800‐5000 m) is a characteristic property of the deep distribution of the organic matter catagenesis of sedimentary rocks in the boreholes of Northern Sakhalin. It is likely that this fact is related to a high rate of sediment accumulation in the Neogene-Quaternary time. In order to check this supposition and estimate the rate of variations in the degree of transformation of organic matter (OM) in the deeper layers of the basin, we applied the modeling system of GALO basins and reconstructed numerically the history of subsidence and temperature evolution of the rocks of the sedimentary cover in the northeastern part of the Sakhalin shelf in Kaigano-Vasyukanskii area of Sakhalin-5 region (Fig. 1). In addition to the traditionally considered Cenozoic complex, our model of the sedimentary section of the basin includes Upper Cretaceous formations, whose thicknesses were estimated from the seismic data (table). The peculiarity of the shelf region considered here was in the fact that it was located on the one hand within the continental margin of the Okhotsk Sea Plate and on the other hand near the complex of the accretion prism of Eastern Sakhalin [1]. Although the close location of the accretion cline (and consequently the underthrust zone) predetermines the domination of compression stresses over the territory of the basin and consequently the nonisostatic response of the basin lithosphere to the load of water and sediments, there is sufficient evidence of active fracture tectonics in the northeastern shelf of Sakhalin with notable weakening of the lithosphere in the Late Cretaceous and Cenozoic (see below). Therefore, we considered two limiting versions of the basin development. In the first of them, we assumed that numerous fractures and displacements along the fractures significantly weakened the lithosphere strength in the region studied. Under these conditions, the local isostatic response of the lithosphere to the load of water and sediments existed during the entire history of the basin, which we consider started in the Late Cretaceous (and possibly excluded a few relatively short periods). In the second alternative version of the basin formation, we assumed that the local isostatic response of the basin lithosphere to the load of water and sediments existed only during the last 34 Ma starting from the time of the Kuril arc formation, which absorbed the main part of the regional compression stresses. The initial heat flux in the basin in the second version was assumed notably lower than in the first one. Of course, in both versions of the basin development, modern temperature distributions and OM maturity with depth (the values of the reflecting property of vitrinite, R o , %) calculated in the model were close to the values measured in the regions of the basin considered here and in the neighboring regions (Fig. 2). The main modeling results of the thermal history of the basin and variation in the OM maturity are shown in Fig. 3 for the first version of the basin development, which assumed an isostatic response of the lithosphere to the load of water and sediments and consequently high heat flux (approximately 160 mW/m 2 ) in the beginning of the Late Cretaceous. In such a model, intense subsidence of isotherms in the Pre-Cenozioc period of the basin development (Fig. 3) is determined

Phosphorus speciation and availability in intertidal sediments of the Yangtze Estuary, China

In order to better understand P cycling and bioavailability in the intertidal system of the Yangtze Estuary, both surface (0–5 cm) and core (30 cm long) sediments were collected and sequentially extracted to analyze the solid-phase reservoirs of sedimentary P: loosely sorbed P; Fe-bound P; authigenic P; detrital P; and organic P. The total sedimentary P in surface and core sediments ranged from 14.58–36.81 μmol g −1 and 17.11–24.55 μmol g −1 , respectively, and was dominated by inorganic P. The average percentage of each fraction of P in surface sediments followed the sequence: detrital P (54.9%) > Fe-bound P (23.7%) > organic P (14.3%) > authigenic P (6.3%) > loosely sorbed P (0.8%), whereas in core sediments it followed the sequence: detrital P (61.7%) > Fe-bound P (17.0%) > authigenic P (13.1%) > organic P (7.5%) > loosely sorbed P (0.7%). Post-depositional reorganization of P was observed in both surface and core sediments, converting organic P and Fe-bound P to authigenic P. The accumulation rates and burial efficiencies of the total P in the intertidal area ranged from 118.70–904.98 μmol cm −2 a −1 and 80.29–88.11%, respectively. High burial efficiency of the total P is likely related to the high percentage of detrital P and the high sediment accumulation rate. In addition, the bioavailable P represented a significant proportion of the sedimentary P pool, which on average accounted for 37.4% and 25.1% of the total P in surface and core sediments, respectively. This result indicates that the tidal sediment is a potential internal source of P for this P-limiting estuarine ecosystem.

The biogenic gas potential of the submarine canyon systems of Plio-Peistocene foreland Basin, southwestern Taiwan

Biogenic gas was accidentally discovered and produced from the Plio/Pleistocene formation of the Hsinying gas field in southwestern Taiwan in 1989. A stratigraphic trapping mechanism related to the evolution of submarine canyon systems in the Plio-Peistocene foreland basin has been proposed in a previous study which explained underestimated recoverable gas reserve before drilling. To verify this shallow gas exploration hypothesis and to systematically evaluate the biogenic gas generation and entrapment potential of the submarine canyon systems, seismic interpretation, high-resolution sequence stratigraphic interpretation, seismic attribute analysis and geochemical analysis were performed and integrated in this study. Twenty-nine submarine canyons mapped mainly trend in a NE direction, except the NW trending Eurchungchi submarine canyons located near the Chiali paleo-high. Bright seismic amplitudes were often observed at the incised valley heads of the canyon systems. The shales located near the incised valley heads and deposited during flooding stage possess the highest biogenic gas generation potential, as canyon fill reveals the second highest. Due to the high sediment accumulation rate in the foreland basin, organic matter in such a depositional environment tends to become diluted. A Class III AVO anomaly, inverted impedance lower than 4.7 e + 6 kg/M 3*M/S and A/ B (the ratio between the target horizon amplitude and the RMS amplitude from the background strata) greater than 1.78 were identified as valid direct gas indicators as sand is buried shallower than 1000 m. Class IV AVO anomaly and A/ B greater than 1.4 were concluded to be the indicators of gas sand in the case that sand is buried deeper than 1600 m. Based on the results of sequence stratigraphic interpretation and the consistency between spatial geometries of seismic attributes and those of the submarine canyons, a stratigraphic trap associated with the incised valley heads was concluded to be the original gas entrapment style of the Hsinying and the Kuantian gas fields. Biogenic gas migrated after being trapped startigraphically, hence contributing to the present-day gas field structure. Due to the prevalent erosional features of the submarine canyons on the time structural maps, different types of stratigraphic traps formed in combination with faults and submarine canyons can be recognized easily.

Recent sediment accumulation rates for the Western margin of the Barents Sea

Abstract We investigate patterns, processes, and rates of sediment accumulation within the marginal ice zone of the western Barents Sea. The Barents Sea is among the most productive of the Arctic marginal seas, with approximately half of the sediment burial flux derived from marine rather than terrestrial sources. Sediment accumulation rates were quantified by 210 Pb geochronology at 14 stations, ranging in water depths from 173 to 503 m, along a south–north latitudinal gradient of 6° (75–81°N). The average sediment accumulation rate for all stations is 0.7±0.4 mm/yr. In general, lower sediment accumulation rates are associated with coarser sediment fractions at shallower water depths (∼200 m) where currents remove fine-grained sediments and transport material to the deeper regions. Higher sediment accumulation rates are detected in deeper water stations and are largely associated with specific sedimentation events and/or features of the Barents seafloor. Although the C/N ratio of surface sediments (0–2 cm) from all stations indicates a predominance of sediments of marine origin (C/N=9.0±1.0), there is no distinct depositional pattern, indicating preferential burial in areas more heavily influenced by the marginal ice zone. We conclude that a combination of mixed sediment sources, large shifts in the location of the marginal ice zone over time, and the benthic boundary layer processes obscure the productivity signal preserved in these seafloor sediment deposits of the western Barents Sea.

A Late Holocene record of landscape degradation from Heygsvatn, the Faroe Islands

Diatom, magnetic susceptibility and loss-on-ignition (LOI) analyses of a 9-metre sediment core from Heygsvatn (Suðuroy, the Faroe Islands) were used to investigate the response of lake and catchment to mid-Late Holocene environmental change. A 30-lake training set from Faroese lakes was also used to explore the relationships between diatoms and environmental variables and assist in the interpretation of the diatom stratigraphy. Canonical Correspondence Analysis showed that lake depth, light penetration, total phosphorus (TP), pH and dissolved organic carbon (DOC) were the most significant variables affecting diatom distribution in Faroese lakes ( p < 0.06). Nine 14C AMS dates indicated that the sediment record covered the last ~ 5700 years. Exceptionally high rates of sediment accumulation, an increase in LOI and magnetic susceptibility and inversion of radiocarbon dates suggested that a period of elevated soil erosion occurred at Heygsvatn ca. 1700–1200cal. yr. BP. Because this erosion pre-dates the arrival of humans the most likely cause is climate deterioration (cooler, wetter conditions). The diatom stratigraphy supports this hypothesis by showing that during the period of erosion the lake was deeper or/or more depauperate in TP, consistent with the climate becoming wetter and leading to increased lake flushing. Subsequently, with sustained sediment erosion, the lake became more shallow and pH increased as the lake infilled. In combination, the proxies support the idea that the climate became wetter ca. 1700–1200cal. yr. BP in the Faroe Islands, but that the response of the landscape was partly dependent on the increased development of peat initiated during the onset of Neoglacial cooling after 5000cal. years BP.

Excess 210Pb inventories and fluxes along the continental slope and basins of the Gulf of Papua

Sediment samples were collected from continental slopes and marginal basins in the Gulf of Papua and analyzed for excess 210Pb to elucidate transport processes of fine‐ grained particles to this region. Estimated excess 210Pb fluxes of 1.0–12.8 dpm cm−2 a−1 were derived from measured seabed inventories. Highest sediment accumulation rates (0.28–0.35 cm a−1) were measured along the northeastern shelf edge, and they decrease in seaward directions and along isobaths to the southwest. The excess 210Pb flux could result from either focused deposition of high‐210Pb activity sediments from the continental shelf and upper slope or scavenging of 210Pb brought landward from deep‐sea waters. This sediment flux is concentrated in the northeastern Gulf of Papua, where the shelf is narrow and calcium carbonate contents are lowest. Analysis of sedimentary fabric and 210Pb distributions in cores suggests sediment delivery to the slope occurs on a 100‐year timescale as both diffuse hemipelagic deposition as well as turbidity flows. The flux of sediment in turbidity flows is not well constrained but may be producing additional deep‐sea accumulation in the Moresby Trough, as well as export from the study area.

Evidence for multiple, episodic, mid-Holocene Hypsithermal recorded in two soil profiles along an alluvial floodplain catena, southeastern Tennessee, USA

Floodplain soil–paleosol successions are valuable archives for reconstructing Pleistocene–Holocene climate changes but have been relatively unstudied in the southern Appalachian region. Two soil profiles on a small floodplain in southeastern Tennessee, USA were described and sampled in detail using both pedological and geological approaches, including stable carbon isotope analysis of soil organic matter (SOM). Correlation between the 2 profiles was constrained by uncalibrated AMS 14C ages of bulk humates, and using SOM δ 13C values, both mobile and immobile elements. Four distinct 2.5–4‰ shifts towards less negative δ 13C values for SOM suggest ∼ 300-yr cyclicity and transient warmer and drier climate events, with either water-stressed C 3 vegetation or as much as 35% C 4 plants present during the mid-Holocene. These postulated multi-episodic drier climate conditions have never before been documented in the southern Appalachian region and are tentatively correlated with mid-Holocene warming and drying in the eastern US, the nearly time-equivalent mid-Holocene events documented in Texas, the US High Plains and in the Gulf of Mexico. High rates of floodplain sediment accumulation (0.5–3 mm/yr), high clay content and maintenance of poorly drained soil conditions favor preservation of high-resolution climate archives in floodplain deposits by inhibiting oxidation and translocation of organic C.

Environmental precursors to rapid light carbon injection at the Palaeocene/Eocene boundary

The start of the Palaeocene/Eocene thermal maximum--a period of exceptional global warming about 55 million years ago--is marked by a prominent negative carbon isotope excursion that reflects a massive input of 13C-depleted ('light') carbon to the ocean-atmosphere system. It is often assumed that this carbon injection initiated the rapid increase in global surface temperatures and environmental change that characterize the climate perturbation, but the exact sequence of events remains uncertain. Here we present chemical and biotic records of environmental change across the Palaeocene/Eocene boundary from two sediment sections in New Jersey that have high sediment accumulation rates. We show that the onsets of environmental change (as recorded by the abundant occurrence ('acme') of the dinoflagellate cyst Apectodinium) and of surface-ocean warming (as evidenced by the palaeothermometer TEX86) preceded the light carbon injection by several thousand years. The onset of the Apectodinium acme also precedes the carbon isotope excursion in sections from the southwest Pacific Ocean and the North Sea, indicating that the early onset of environmental change was not confined to the New Jersey shelf. The lag of approximately 3,000 years between the onset of warming in New Jersey shelf waters and the carbon isotope excursion is consistent with the hypothesis that bottom water warming caused the injection of 13C-depleted carbon by triggering the dissociation of submarine methane hydrates, but the cause of the early warming remains uncertain.

Cenomanian Turonian Coastal Record in SW Utah, U.S.A.: Orbital-Scale Transgressive Regressive Events During Oceanic Anoxic Event II

Abstract The Cenomanian–Turonian interval of the Sevier foredeep, western U.S.A., is examined in order to (1) establish a high-resolution stratigraphic framework for marginal-marine strata of this interval and (2) test for the existence of high-frequency (tens of kyr-scale) cycles of continental runoff or sea-level change predicted by the hemipelagic record and climate models. High rates of sediment accumulation in marginal-marine environments of southwestern Utah (up to 210 m/Myr, compacted) and a northward translation of the major Sevier thrusting made possible the preservation of a highly detailed record of shoreline movements. The coeval Bridge Creek Limestone, linked with the study interval using biostratigraphic and bentonite-stratigraphic data of previous authors, provides an unprecedented, high-resolution orbital time scale. Three orders of transgressive–regressive cycles defined as genetic sequences are identified in the upper Cenomanian (S. gracile and N. juddii Zones) through lower Turonian (W. devonense through M. nodosoides Zones). The longest sequence (S. gracile Zone through V. birchbyi Zone) spans approximately 800 kyr and is penecontemporaneous with the δ13Corg positive excursion that defines Oceanic Anoxic Event II (OAE II). Medium-term and short-term sequences show durations of c. 65–160 kyr and c. 20–40 kyr, respectively. Features suggesting regression due to relative sea-level fall are described from some of the 20–40 kyr cycles in the lowermost S. gracile Zone (possibly including the uppermost M. mosbyense Zone). The data provide the first physical evidence globally of Cenomanian–Turonian changes in shoreline position and relative sea level, whose recurrence interval was as short as a few tens of kyr. These processes provide a viable depositional link between the rhythmic deposition of the Bridge Creek Limestone and the primary orbital forcing of insolation and climate. Although the possible tectonic influence is difficult to unravel, the study area represents an important reference point for climate and oceanographic modeling of the Cenomanian–Turonian greenhouse and OAE II.

Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia

AbstractAlthough facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for &gt;100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (&gt;10 000 km2) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (&gt;0·45 mm year−1 for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow.Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (&gt;200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to &gt;200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits.The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.

Architecture and sediment dynamics of the Mauritania Slide Complex

Large-scale mass wasting is an important sedimentary process along the northwest African margin, and is related to high sediment accumulation rates under an ocean margin upwelling regime. Although the margin is generally arid with limited fluvial input, additional sediment supply comes from wind-borne Saharan dust. Recent mapping of the margin off Mauritania has revealed a major sediment slide, here called the Mauritania Slide Complex, as it comprises elements of true sliding as well as more mobile distal debris flow. Seismic data image stacked slide deposits separated by undisturbed stratified sediments indicating that undisturbed sediment accumulation was interrupted by several phases of slope failure. A series of stepped headwalls, 25–100 m high, represents the source area of the youngest slide event, which most likely occurred as retrogressive type of failure. The area of seafloor affected by this mass movement is ∼30,000 km 2, while the deposit volume is ∼600 km 3. The uppermost debrite unit, which has been 14C dated at 10.5–10.9 ka, forms a broad tongue extending down to the lower slope. This debrite comprises a vertical succession of three different layers of matrix types, with a predominantly outer shelf source at the base and pelagite-dominated composition at the top. The complete sequence of three layers was deposited at a mid slope position, whereas only the upper layers reached the lower slope. A thick pile of sediments with outer shelf/upper slope derived biogenic and terrigenous debris-rich sediments at the base and hemipelagic sediments on top failed at an upper to mid slope location and disintegrated into a layered debris flow on its down-slope journey.

Recent sediment transport and deposition in the Nazaré Canyon, Portuguese continental margin

Processes, pathways and fluxes of sediment transport and deposition in the Nazaré submarine canyon, Portuguese continental margin, were investigated by water column profiling of suspended particulate matter, recording of near-bottom currents and suspended particulate matter fluxes with benthic landers, and by analysis of surface sediment cores. The results show that fine-grained suspended sediment from the adjacent shelf is captured in the upper canyon and actively transported through the canyon to the abyssal plain under influence of tidal currents and intermittent sediment gravity flows. Tidal currents are actively resuspending and transporting sediment in the upper to middle canyon, as reflected by high concentration of suspended particles in bottom waters, high horizontal and vertical sediment fluxes in the bottom water layer and high sediment accumulation rate on the seabed. Diminishing strength of tidal currents leads to focussed sediment deposition in the middle canyon. Further transport of sediments toward the lower canyon takes place predominantly by sediment gravity flows occurring on yearly or longer timescale. Turbidity currents strong enough to transport sand-sized material to the lower canyon occur only on centennial or longer timescales.

Sediment tracing and environmental history for two small catchments, Karoo Uplands, South Africa

As part of an investigation of land degradation in the South African Karoo, three types of fine sediment signatures (radionuclides, mineral magnetism and sediment geochemistry) were used to trace the origins of fine particulate sediment accumulating in two reservoirs that were constructed around the mid 1930s. Both reservoirs are fed by gullies that are known to have been in existence before their construction. Robust chronologies were derived from the 137Cs profiles in the reservoir sediments, from aerial photographic records, and from an analysis of extreme storm events that have left stratigraphic markers in the reservoir sediments. Extreme daily rainfalls have become more frequent in the last 30 years but sediment accumulation rates in the two reservoirs differ significantly. One reservoir has high sediment accumulation rates between 1958 and 1964 that correlates with an expansion in the area of cereal cultivation. The second catchment has no cultivated land, but sedimentation rates in the reservoir declined after 1941 when livestock grazing densities decreased by a factor of around three. Despite the increase in the frequency of high magnitude daily rainfalls after 1970, sediment availability, as controlled by land use, appears to be the most important limit on changes in sedimentation rates. Differences in lithology and land use provided an opportunity to evaluate the number of sources that could successfully be discriminated in each catchment at different timescales. 137Cs and unsupported 210Pb can be used to trace sources of actively transported or very recently deposited sediment and to discriminate sources of reservoir sediments between topsoils (cultivated and uncultivated) and subsoils or gully sidewalls. Lack of current 137Cs inputs means that it will become less useful as a future discriminator of topsoil/subsoil sources because it only has a half-life of ca. 30 years and will eventually be undetectable. The potential for unsupported 210Pb to provide an alternative method for discriminating these sources is demonstrated. Using Principal Component and Discriminant Function Analysis, and the three groups of sediment fingerprint signatures, it was possible to discriminate doleritic soils from other potential sources in the catchments, including valley fill and soils underlying vegetation that had been recently burned as part of the rangeland management system. Results suggest that fire was an important contributor to destabilising soil, which was subsequently delivered to the reservoir during high magnitude rainfalls. It is concluded that: (1) changes in the extent of cereal cultivation and livestock grazing density are most closely related to temporal changes in sedimentation rates in each of the two reservoirs; (2) temporal changes in the magnitude and frequency of extreme daily rainfalls do not correlate with reconstructed patterns of sediment accumulation through time; (3) the gully systems provide connectivity between the dominant hillslope sources and the reservoirs but have not contributed significantly to the sediment deposited in either reservoir; and (4) there is no evidence to suggest that sources of sediment accumulating in the reservoirs have changed significantly over the last ca. 70 years.

The Western Adriatic shelf clinoform: energy-limited bottomset

Clinoforms on modern shelves and slopes, as well as in ancient rock records, are widely recognized as a fundamental building element of continental margin growth. Regardless of their dominant lithology, clinoforms are composed of three geometric elements: topset, foreset and bottomset. Traditionally, much emphasis in the study of clinoforms was put on the geometry of the topset, viewed as the most energetic portion of a clinoform and studied to discern if aggradation was active rather than erosional truncation, and on the foreset, the area with the highest sediment accumulation rates. Here we focus on the factors forcing clinoforms to taper out and on the inferred mechanisms for bottomset creation. We base our analysis on muddy shelf clinoforms, a particular class of clinoforms that is typical of low-gradient settings and is characterized by a substantial component of shore-parallel sediment transport. This paper is based on a large dataset of CHIRP-sonar profiles, bathymetric and isopach maps of the Late Holocene clinoform on the Adriatic shelf, where integrated stratigraphic studies from sediment cores allow a very high (in some cases century-scale) chronological resolution. Knowledge on the dominant oceanographic regime affecting sediment dispersal and, ultimately, clinoform development, has been recently refined during the EuroSTRATAFORM collaboration. Muddy clinoforms are markedly three-dimensional features that cannot be entirely understood solely on two-dimensional shore normal sections. We suggest that this class of clinoform is advection dominated and that an increase in the energy regime at the toe of the foreset may prevent sediment from reaching beyond the transition to the bottomset region.

How depositional conditions control input, composition, and degradation of organic matter in sediments from the Chilean coastal upwelling region

In a comprehensive study, we compared depositional conditions, organic matter (OM) composition, and organic carbon turnover in sediments from two different depositional systems along the Chilean continental margin: at ∼23° S off Antofagasta and at ∼36° S off Concepción. Both sites lie within the Chilean coastal upwelling system and have an extended oxygen minimum zone in the water column. However, the northern site (23° S) borders the Atacama Desert, while the southern site (36° S) has a humid hinterland. Eight surface sediment cores (up to 30 cm long) from water depths of 126–1350 m were investigated for excess 210Pb ( 210Pb xs) activity, total organic and total inorganic carbon concentrations (TOC and TIC, respectively), C/N-ratios, organic carbon isotopic compositions (δ 13C), chlorin concentrations, Chlorin Indices (CI), and sulfate reduction rates (SRR). Sediment accumulation rates obtained from 210Pb-analysis were similar in both regions (0.04–0.15 cm yr −1 at 23° S, 0.10–0.19 cm yr −1 at 36° S), although total 210Pb xs fluxes indicated that the vertical particle flux was higher at 36° S than at 23° S. We propose that sediment focusing in isolated deposition centers led to high sediment accumulation rates at 23° S. Furthermore, there were no indications for sediment mixing at 23° S, while bioturbation was intense at 36° S. δ 13C-values (−24.5‰ to −20.1‰ vs. VPDB) and C/N-ratios (molar, 8.6–12.8) were characteristic of a predominantly marine origin of the sedimentary OM in both investigated areas. The extent of OM alteration in the water column was partly reflected in the surface sediments as chlorin concentrations decreased and C/N-ratios and CI increased with increasing water depth of the sampling site. SRR were lower at 23° S (areal SRR 0.12–0.60 mmol m −2 d −1) than at 36° S (areal SRR 0.82–1.18 mmol m −2 d −1), which was partly due to the greater water depth of most of the sediments investigated in the northern region and consistent with a lower quality of the sedimentary OM at 23° S. Reaction rate constants for TOC degradation that were obtained from measured SRR ( k SRR; 0.0004–0.0022 yr −1) showed a good correspondence to k TOC that were derived from the depth profiles of TOC (0.0003–0.0014 yr −1). Both, k SRR and k TOC, reflect differences in OM composition. At 36° S they were related to the degradation state of bulk OM (represented by C/N-ratios), whereas near 23° S they were related to the freshness of a small fraction of labile OM (represented by CI). Our study shows that although rates of organic carbon accumulation were similar in both investigated sites, the extent and kinetics of organic carbon degradation were closely linked to differing depositional conditions.

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 ( δ 13C 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 δ 13C org values are also typically marine with a mean of −21.5±0.3‰. The riverine SPM samples exhibit typical terrestrial δ 13C org values around −25‰. The δ 13C 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.