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Suspended particulate organic carbon (POC) and sedimentary total organic carbon (TOC) in coastal areas play critical roles in the global carbon cycle, yet sources and dynamics of coastal POC and TOC have been affected by various anthropogenic activities such as aquaculture, sewage discharge, dam construction and land reclamation. To better understand the anthropogenic impacts on coastal organic carbon, this study was carried out in a representative semi-enclosed bay, Dongshan Bay, Southeast China. Through analyses of stable isotopic compositions of both POC (δ13CPOC and δ15NPN) and TOC (δ13CTOC and δ15NTN), the ratio of total organic carbon vs. total nitrogen (C/N), grain size, Chl-a concentrations and hydrological parameters, our study led to the following main findings: 1) During flood season, the distribution of δ13CPOC, δ13CTOC, δ15NPN and δ15NTN values within the bay did not follow the conventional land-sea transition pattern. This distribution pattern indicated more terrestrial organic matter input seaward, which contrasts with the conventional organic matter distribution along the estuarine gradient. 2) Using the organic δ13C, δ15N and C/N signatures of different endmembers, we found that the sources of organic matter deposited in the bay were strongly related to anthropogenic activities, including municipal wastewater discharge, aquaculture, land reclamation and sluice-dyke construction. Furthermore, 3) by applying the Grain Size Trend Analysis Model and the previously-estimated residual current directions, we suggested that human activities have not only altered the sources of organic matter to the semi-enclosed bays, but also significantly modified their transportation and deposition patterns, and might influence the ultimate fate of organic matter into and out of Dongshan Bay. The conclusions of this study should be applicable to similar coastal bays around the world.

Sediment transfer from land to ocean begins in coastal settings and, for large rivers such as the Amazon, has dramatic impacts over thousands of kilometers covering diverse environmental conditions. In the relatively natural Amazon tidal river, combinations of fluvial and marine processes transition toward the ocean, affecting the transport and accumulation of sediment in floodplains and tributary mouths. The enormous discharge of Amazon fresh water causes estuarine processes to occur on the continental shelf, where much sediment accumulation creates a large clinoform structure and where additional sediment accumulates along its shoreward boundary in tidal flats and mangrove forests. Some remaining Amazon sediment is transported beyond the region near the river mouth, and fluvial forces on it diminish. Numerous perturbations to Amazon sediment transport and accumulation occur naturally, but human actions will likely dominate future change, and now is the time to document, understand, and mitigate their impacts.

Located along the Aleutian Megathrust Boundary, South-Central Alaska is one of the most seismically active areas in the world. Central to this region, Prince William Sound is a glacially carved basin that receives abundant sediment from multiple sources. Primary inputs include the Columbia Glacier and the Copper River, which have diagnostic signatures of Sr/Pb, Cu/Pb, K/Ca, Rb/Sr, and Rb/Ca. As a result, earthquake induced sediment gravity flows originating from different locations deposit event layers that have distinct provenance signatures. A previous study (Kuehl et al., 2017) identified five such event layers preserved in two cores from the southern part of the deep central channel of Prince William Sound that were attributed to large historical earthquakes. To better understand the spatial continuity of these event layers along the entire length of the central channel, seven new gravity cores were collected in a north-south transect. Sedimentation rates were determined by 210Pb, 137Cs, and 239,240Pu geochronology, and cores were assessed for variations in elemental content, grain size patterns, bulk organic and stable isotopic signatures (C/N, δ13C, δ15N). Based on a spatial analysis of these cores, local earthquakes of Mw 7.0 (including an event layer from the 1964 great Alaskan earthquake) have mixed signatures resulting from the widespread generation of numerous flows throughout Prince William Sound. Complete gravity flow records are captured within deep ponded sediment basins and the area spanning the southern end of the central channel. Considering the thick (>100 m) late-Holocene sequence, the approach utilized in this study has the potential to provide a rich record of earthquake recurrence intervals to 4 ka.

How do humans affect the ways that Asia’s mega rivers deliver sediment and dissolved matter to farms, river deltas, and, eventually, the sea? A proposed study would construct an integrated picture.

Over the past three decades, particle-reactive cosmogenic 7Be (t1/2 = 53.3 d) has frequently been used to quantify recent fluvial sediment inputs to estuaries and coastal marine environments, providing an important tool to understand the timing and pathways of sediment dispersal. This study utilizes a 2-year high-resolution time series collected from the Copper River, Alaska, to examine the seasonal fluctuation in 7Be activity of suspended sediments near the river mouth. We show that there is considerable variability (particulate 7Be = ≤ 3.3–47 Bq kg−1) throughout the hydrograph, with a significant negative correlation to discharge and suspended sediment concentration. This variability is interpreted to be driven by a combination of fluctuations in atmospheric deposition, water aging, and dilution by 7Be-depleted sediments, reflecting erosion within the streambed/watershed and/or differing freshwater/sediment source. Particularly within this high-latitude setting, seasonal differences in provenance (snowpack/glacial melting vs. rainfall) likely influence 7Be activities. Importantly, 7Be was below detectable limits in river suspended sediments during peak discharge periods in both years, indicating that the flood signal in adjacent coastal environments will be negligible at this time. These results suggest that 7Be will not be an effective tracer of fluvial sediment deposition in some coastal environments.

Sedimentological and geochemical investigation of sediment cores collected in the deep (>400 m) central basin of Prince William Sound, along with geochemical fingerprinting of sediment source areas, are used to identify earthquake-generated sediment gravity flows. Prince William Sound receives sediment from two distinct sources: from offshore (primarily Copper River) through Hinchinbrook Inlet, and from sources within the Sound (primarily Columbia Glacier). These sources are found to have diagnostic elemental ratios indicative of provenance; Copper River Basin sediments were significantly higher in Sr/Pb and Cu/Pb, whereas Prince William Sound sediments were significantly higher in K/Ca and Rb/Sr. Within the past century, sediment gravity flows deposited within the deep central channel of Prince William Sound have robust geochemical (provenance) signatures that can be correlated with known moderate to large earthquakes in the region. Given the thick Holocene sequence in the Sound (~200 m) and correspondingly high sedimentation rates (>1 cm year−1), this relationship suggests that sediments within the central basin of Prince William Sound may contain an extraordinary high-resolution record of paleoseismicity in the region.

Collectively, the modern Ayeyarwady (Irrawaddy) and Thanlwin (Salween) rivers deliver >600 Mt/yr of sediment to the sea. To understand the fate of Ayeyarwady and Thanlwin river-derived sediments to the sea, we conducted a 14-day geophysical and geological survey in the northern Andaman Sea and eastern Bay of Bengal in December 2017. Overall, ~1500-km of high-resolution Chirp-sonar profiles and 30 sediment cores from the shelf were acquired. This paper presents the results of the processed high-resolution profiles together with sediment analyses. Our findings indicate: 1) There is little modern sediment accumulating on the shelf immediately off the Ayeyarwady River mouths. In contrast, a major mud wedge with a distal depocenter, up to 60 m in thickness, has been deposited seaward in the Gulf of Martaban, extending to ~130 m water depth into the Martaban Depression. Further, 2) There is no evidence showing that modern sediment has accumulated or is transported into the Martaban Canyon; 3) There is a mud drape/blanket wrapping around the narrow western Myanmar Shelf in the eastern Bay of Bengal. The thickness of the mud deposit is up to 20 m nearshore and gradually thins to the slope at −300 m water depth, and likely escapes into the deep Andaman Trench; 4) The estimated total amount of Holocene sediments deposited offshore is ~1290 × 109 tons. If we assume this has mainly accumulated since the middle Holocene highstand (~6000 yr BP) like other major deltas, the historical annual mean depositional flux on the shelf would be 215 Mt/yr, which is equivalent to ~35% of the modern Ayeyarwady-Thanlwin rivers derived sediments; 5) Unlike other large river systems in Asia, such as the Yangtze and Mekong, this study indicates a bi-directional transport and depositional pattern controlled by the local currents that are influenced by tides, and seasonally varying monsoons winds and waves. Organic carbon biomarkers and isotope compositions show a gradual changing pattern with the along-shelf transport from the river to the Gulf of Martaban in the east and to the Bay of Bengal in the west.

A fundamental goal of the Earth Science community is to understand how perturbations on Earth's surface are preserved in the stratigraphic record. Recent Source to Sink (S2S) studies of the Waipaoa Sedimentary System (WSS), New Zealand, are synthesized herein to provide a holistic perspective of the processes that generate, transport and preserve sedimentary strata and organic carbon on the Waipaoa margin in the late Quaternary. Rapid uplift associated with subduction processes and weak sedimentary units have conspired to generate rapid rates of incision and erosion in the Waipaoa catchment since the Last Glacial Maximum (LGM). We show that although much of the sediment exported offshore during this time interval originated from valley excavation, a substantial portion emanated from hillslopes, mostly through deep-seated landslide and earthflow processes that were vigorous during periods of rapid fluvial incision just prior to the Pleistocene–Holocene transition. Lacustrine sediments deposited in naturally-dammed 7-ky-old Lake Tutira provide a record of Holocene environmental controls on upper catchment sedimentation in the WSS, with 1400 storms identified. Storm frequency is modulated by the waxing and waning of atmospheric teleconnections between the tropics and Antarctica. Furthermore, clear long-term changes in sediment yield are evident from the Lake Tutira record following human settlement as conversion to pasture is accompanied by a 3-fold increase in the long-term lake sediment accumulation rate. Whereas there is ample evidence that Waipaoa River flood deposits are routinely deposited offshore in the sheltered confines of Poverty Bay, over the longer term, waves and currents subsequently resuspend and transport these deposits both landward (sandy fraction) and seaward (finer fraction). Thus, the timing of sediment supply to areas of net sediment accumulation is more often driven by wave events that are not associated with river flooding. Therefore, we conclude that asynchronicity of river-sediment delivery and of wave resuspension in most instances precludes the direct preservation of flood events in the stratigraphic record of the Waipaoa Shelf. Over the longer term, the sediment package preserved on the shelf and slope since the LGM can be explained in large measure by sequence-stratigraphic models forced by varying sea level and ongoing tectonic deformation of the margin. As sea level rose, sediment supply to the slope was reduced by about a factor of 5 due to shelf trapping. Despite this reduction, turbidites are found at similar frequency throughout the LGM–Present, as the dominant trigger appears to be subduction earthquakes, with large ones having a return interval of about 200 ± 100 years. Sediment-budget exercises that consider both modern (river discharge versus centennial accumulation rates) and post-LGM (terrestrial production versus offshore isopachs) mass balances indicate that about half of the total sediment production from the Waipaoa escapes the study area. Moreover, a coupled sediment transport-hydrodynamic model and observations of textural trends on the shelf indicate that a large fraction of the sediment is carried outside the study area along the shelf to the northeast by the river plume or by combined current/wave activity. Therefore, we conclude that the WSS is an open system with sediment escape from the present day through the LGM. The organic matter associated with sediment as it moves from upland source to marine sink is a product of particle history, and provides a record of materials that have cycled over timescales of days to millions of years. The ubiquity of fossil Organic Carbon (OC) in both the terrestrial and marine realms of the Waipaoa attests both to the chronic nature of its source, crumbling mudstones further destabilized by land use, and its biogeochemical recalcitrance. Modern OC persists by virtue of its continual production along the S2S transit, and is buried and preserved within the adjacent marine depocenters. The Waipaoa contrasts with dispersal systems on wide, energetic shelves (e.g., the Amazon and Fly Rivers) where sediment is extensively refluxed in oxygenated overlying water resulting in the biogeochemical incineration of particulate OC. The Waipaoa, like other small mountainous rivers on active margins, exhibits a high riverine OC preservation efficiency (> 50%) in its marine depocenters because of the relatively rapid, event-driven accumulation of sediment.

The determination of sediment accumulation rates in environments with temporal variations in texture is challenging using traditional radioisotope methods, largely due to low activities associated with coarse sediments. This study used Englebright Lake, an impoundment in northern California, as a model system to examine the application of plutonium isotopes in lacustrine environments where the interlayering of coarse and fine sediments complicates the geochronology. Inductively coupled plasma mass spectrometry was used to quantify plutonium isotopes and low limits of detection allowed for the measurement of plutonium in sand, clay, and silt fractions. Although measurable levels of plutonium were found in sand fractions, over 75 % of the total plutonium activity was found in fine-grain-size fractions (

Three main rivers—the Ganges, Brahmaputra, and Meghna—coalesce in the Bengal basin to form the world’s largest delta system, which serves as filter and gateway between the Himalayan collision and vast Bengal fan repository. New insights into the Holocene construction of the Ganges-Brahmaputra-Meghna delta, with a focus on river sedimentation, channel migration, and avulsion history, are presented here using the Sr geochemistry of bulk sediments as a provenance tracer. The sediment load of each river transmits a distinct Sr signature owing to differences in source rocks from the Himalaya, Tibet, and local regions, allowing for effective tracking of river channels and stratigraphic development within the delta. In the early Holocene, vigorous delta aggradation occurred under rapid sea-level rise and high river discharge and supported the construction of sand-dominated stratigraphy by laterally mobile, braided-stream channels. However, the vertically (i.e., temporally) uniform, but geographically distinct, Sr signatures from these deposits indicate that the Ganges, Brahmaputra, and Meghna fluvial systems remained isolated from one another and apparently constrained within their lowstand valleys. By the mid-Holocene, though, delta stratigraphy records spatially and temporally nonuniform Sr signatures that hallmark the onset of avulsions and unconstrained channel migration, like those that characterize the modern Ganges and Brahmaputra fluvial systems. Such mobility developed in the mid-Holocene despite declining discharge and sea-level rise, suggesting that earlier channel behavior had been strongly influenced by antecedent topography of the lowstand valleys. It is only after the delta had aggraded above the valley margins that the fluvial systems were able to avulse freely, resulting in numerous channel reorganizations from mid-Holocene to present. These channel-system behaviors and their role in delta evolution remain coarsely defined based only on this initial application of Sr-based provenance tools, but the approach is promising and suggests that a more complete understanding can be achieved with continued study.

Recent sediment dispersal and accumulation on the Northern Andaman Sea continental shelf, off the Ayeyarwady (Irrawaddy) and Thanlwin (Salween) Rivers, are investigated using seabed, water column, and high-resolution seismic data collected in December 2017. 210Pb and 137Cs derived sediment accumulation rates are highest (up to 10 cm y−1) in the mid-shelf region of the Martaban Depression, a basin that has formed on the eastern side of the N-S trending Sagaing fault, where rapid progradation of a muddy subaqueous delta is occurring. Landward of the zone of highest accumulation, in the shallow Gulf of Martaban, is a highly turbid zone where the seabed is frequently mixed to depths of ~1 m below the sediment water interface. Frequent resuspension in this area may contribute to the formation of extensive fluid muds in the water column, and consequent re-oxidation of the shallow seabed likely reduces the carbon burial efficiency for an area where the rivers are supplying large amounts of terrestrial carbon to the ocean. Sediment cores from the Gulf of Martban have a distinctive reddish brown coloration, while x-radiographs show sedimentary structures of fine silt laminations in mud deposits, which indicates strong tidal influences. The seaward part of the Martaban Depression off the modern subaqueous delta is covered by relict sediment, with no apparent connection between the modern sediment deposit and the offshore Martaban canyon. On the western side of the Sagaing fault, the “Mouths of the Ayeyarwady” represent a large promontory where the subaerial delta has prograded seaward some ~200 km along the eastern flank of the Indo-Burman range since the Holocene maximum transgression. The shelf area off the “Mouths of the Ayeyarwady” presently exhibits low sediment accumulation rates (~1 cm y−1) and a relative coarse (sandy) texture. The reduced accumulation and coarse texture in the western shelf region at the present time is attributed to frequent wave resuspension, and subsequent transport of finer river-derived sediment eastward (into the Martaban Depression) during the SW Monsoon. A mud drape (accumulation rates

Holocene sediments buried on the continental shelf seaward of the Waipaoa River, northeastern New Zealand, preserve a stratigraphic record of terrestrial environmental change. The well characterized, long-term record of storms, volcanism, and human disturbance in this region provides an opportunity to examine how such changes are reflected in the character of organic carbon (OC) buried on the continental margin. Complimentary evidence obtained from analyses of the 14 C content and the elemental and stable carbon isotopic composition of different sedimentary fractions, including charcoal, wood, and clay-sized isolates, indicates that these perturbations led to mobilization of OC components with variable storage histories in the watershed. Charcoal transported in the modern Waipaoa River and buried in offshore depocenters includes a highly aged component that has apparently been slowly released from storage in soil or alluvial terraces. The charcoal fraction ages become dramatically younger in sediments deposited after the Taupo volcanic eruption (1717 cal yr BP) and then Polynesian settlement (ca. 700 cal yr BP), both signaling biomass burning. The mean age of woody plant fragments and clay-bound OC deposited on the shelf also varies over time, with older material having accumulated in the middle Holocene and since human occupation. Deposition of older-than-average wood fragments and clay-bound OC between about 5 and 3.6 kyr BP correlates with evidence for a period of increased storm frequency in the region and may reflect the enhanced delivery of aged soil and sedimentary rock mobilized from hill slopes via earthflows and/or deep seated landslides. Similarly, the deposition of older wood and clay-bound OC commensurate with the first anthropogenic disturbance ca. 700 years ago is consistent with accelerated mass wasting due to deforestation. At the same time, a change in the elemental and stable isotopic composition of bulk and clay-bound OC buried on the shelf may reflect increased marine primary productivity and/or mobilization of OC from deep levels in soil profiles. Deforestation of the Waipaoa headwaters by European settlers beginning in the middle 19th century is evinced by a sharper rise in the age of clay associated OC buried offshore. Today, deep gully incision into fractured sedimentary bedrock is a major source of sediment and kerogen to the river, and this process has left its mark on the age of sedimentary OC delivered to the adjacent margin.

Seasonal patterns of seabed erosion and deposition in a dynamic estuarine environment, the York River (USA), are examined through acoustic studies and seabed sampling to identify the dominant environmental controls on sediment mobility. Areas near Clay Bank in the York River were surveyed monthly using a dual-frequency sonar for a 12 month period. In addition, six stations within these areas were sampled during the surveys. Strong spatial and temporal variations of a soft mud layer were identified, providing new insight to recurring seasonal and spatial patterns in deposition and erosion in the middle reaches of the York River estuary. Sub-bottom data indicated maximum thickness (5–22 cm ± 4 cm) of the soft mud layer during spring and winter from April 2008 through March 2009. Minimum thicknesses (1–12 cm ± 4 cm) of the soft mud layer were observed during summer and fall. Maximum thickness coincided with the occurrence of the secondary turbidity maximum near Clay Bank in the York River. Spatial variations in the soft-mud layer thickness were also identified between and within the York River secondary channel, the inactive oyster reef area, the south west flank of the main channel, and the main channel. The upriver secondary channel, the south west channel flank, and the main channel areas were physically dominated. The downriver secondary channel area was biologically dominated and the inactive oyster reef area reflected both physical and biological processes. Water content trends showed no correlation with seabed stability in this area. Observed changes in soft mud thicknesses are interpreted to result from both biological and physical factors, which vary markedly between winter/spring and summer/fall seasons, and among the different environments of the estuary's channel.

Sediment geochronology of coastal sedimentary environments dominated by sand has been extremely limited because concentrations of natural and bomb-fallout radionuclides are often below the limit of measurement using standard techniques. ICP-MS analyses of 239+240Pu from two sites representative of traditionally challenging (i.e., low concentration) environments provide a “proof of concept” and demonstrate a new application for bomb-fallout radiotracers in the study of sandy shelf-seabed dynamics. A kasten core from the New Zealand shelf in the Southern Hemisphere (low fallout), and a vibracore from the sandy nearshore of North Carolina (low particle surface area) both reveal measurable 239+240Pu activities at depth. In the case of the New Zealand site, independently verified steady-state sedimentation results in a 239+240Pu profile that mimics the expected atmospheric fallout. The depth profile of 239+240Pu in the North Carolina core is more uniform, indicating significant sediment resuspension, which would be expected in this energetic nearshore environment. This study, for the first time, demonstrates the utility of 239+240Pu in the study of sandy environments, significantly extending the application of bomb-fallout isotopes to coarse-grained sediments, which compose the majority of nearshore regions.

Modern sediment dispersal and accumulation on the Poverty Shelf off the Waipaoa River, New Zealand, is investigated using X-radiographic, radio-isotopic and physical property analyses of nearly 200 box and Kasten cores collected in January 2005 aboard the R/V Kilo Moana . The high-yield, small mountainous Waipaoa River empties onto a tectonically active, narrow margin and represents an important analog for shelf sedimentation in similar environments worldwide. X-radiographs and bulk-density measurements from a multi-sensor core logger show three distinct facies on the shelf where physical and/or biological factors dominate strata formation. Spatial distribution of these facies delineates a radial pattern with distance from the river mouth, from areas dominated by stratified layers on the inner shelf to mottled muds on the outer shelf, demonstrating a continuum of controls from wave reworking to episodic and extreme wet event sedimentation, despite a tectonic framework that supports rapid accumulation in two recently identified shelf depocenters. Analysis of short-lived 7 Be ( t 1/2 = 53 days) reveals a broad spatial distribution across- and along-shelf in surface sediments, suggesting rapid transport from the Waipaoa source. However, beryllium-7 inventories are consistent with centennial-scale trends observed by other researchers using longer-lived 210 Pb analyses, with highest inventories in the shelf depocenters. Box core bulk-density analyses show lower than average bulk densities within these depocenters, and higher than average bulk densities on the inner shelf and between the depocenters. These observations confirm rapid deposition of low-density muds in the depocenters, with higher bulk-density, possibly physically reworked sediments, remaining off the mouth of Poverty Bay and between the depocenters. Based on this single observational period, there appears to be no fundamental difference between seasonal and longer-term accumulation patterns on the Poverty Shelf.

The combination of high sediment yields and the prevalence of tectonically controlled accommodation on collision margins such as that adjacent to the Waipaoa River, NZ, create the potential for these areas to contain high-resolution records of natural and anthropogenic signals. This study describes modern (100-yr) sedimentation patterns off the Waipaoa and quantifies a sediment budget for the continental shelf, which is compared to long-term Holocene trends. 210 Pb and 239,240 Pu geochronologies reveal three main shelf depocenters. Two of these depocenters are located in subsiding synclinal basins on the mid-shelf, landward of the actively deforming Ariel and Lachlan anticlines. The depocenters exhibit accumulation rates ranging from 0.75 to 1.5 cm yr − 1 and display both steady-state and non-steady-state 210 Pb activity profiles. Textural characteristics of the non-steady-state cores indicate the possible preservation of flood event layers. The third depocenter is located near the shelf break and has accumulation rates as high as 1.0 cm yr − 1 . The inner shelf and central mid-shelf are characterized by low, uniform 210 Pb activity profiles and low accumulation rates, indicating that sediment is bypassing the inner shelf region and being deposited on the mid- to outer shelf. The modern sedimentation patterns seen in this study are similar to those for the Holocene, suggesting that regional tectonics are the major influence on Poverty shelf sedimentation. A modern, bulk sediment budget estimates that 3.6 ± 0.9 × 10 6 t yr − 1 of sediments remains on the shelf, amounting to only ~ 25% of the 15 Mt of sediments discharged from the river per year. This indicates massive export of sediments from the study area to the adjacent slope or along the shelf. In contrast, studies of the Mid-Late Holocene sediment budget in the same area indicate that the sediment input and shelf trapping have been roughly in balance. When compared with the modern budget, this suggests an extraordinarily rapid shift from shelf trapping to shelf bypassing, most likely driven by increasing sediment discharge in response to deforestation.

Multiple sediment transport and reworking processes influence fine, cm-scale strata formation and long-term accumulation on the Waiapu River shelf, New Zealand. Gravity cores collected during two cruises, in August 2003 and May 2004, were analyzed using 7 Be and 210 Pb geochronologies, bulk carbon, δ 13 C, X-radiographs, and grain-size to investigate sediment mixing and accumulation patterns. The presence of 7 Be on the inner- and mid-shelf regions ( 210 Pb accumulation rates revealed that the middle to outer shelf (50–130 m) acted as fine sediment repositories on longer time scales. Excess 210 Pb accumulation rates were high, with an area weighted average of 1.1 ± 0.1 cm/yr and ranging between 0.2 and 3.5 cm/yr, yet were localized such that only an estimated ~ 23% (ranging between 17 and 38%) of the fluvial load was retained on the shelf between 40 and 200 m depths over the last 80 to 100 yr. Sediments not retained on the shelf were either transported to deeper waters or along the shelf beyond the sampling area. Several cores collected from the high sediment accumulation zone on the middle to outer shelf exhibited non-steady state excess 210 Pb profiles, suggesting that multiple transport processes influenced fine-scale strata formation. Layers of low excess 210 Pb activity and predominantly terrestrial δ 13 C and C/N values were likely formed during floods, when sediments were rapidly deposited and buried on the shelf. These event layers were sufficiently thick (up to ~ 20 cm), such that all or a portion of the initial flood layer immediately transited through the surface mixing layer, ensuring preservation in the sediment record. Sediments inter-bedded with these event layers reflected a relatively marine source indicating either that they were not deposited rapidly or were significantly bioturbated. A gradient of physical and biological mixing signatures, extending radially from the Waiapu River mouth, suggested that high background accumulation rates and flood deposits negatively impacted the preservation of biological structures and enhanced preservation of event-produced beds.

The joint US–NZ Source-to-Sink initiative aims to unravel the tectonic, climatic and other processes that shape the landscape from the mountains to deep ocean. Of two focus areas chosen world-wide, the Waipaoa Sedimentary System (WSS) of north-eastern New Zealand was identified as an example of a mid-latitude, high sediment input system. It resides within the westerly wind belt and extends across the convergence zone of the Australian and Pacific plates. Accordingly, the tectonic and climatic drivers are prominent and their influences on the WSS are decipherable as is documented in the suite of Source-to-Sink studies presented in this special issue of Marine Geology. The WSS is forced at several temporal and spatial scales, which collectively appear as short-term events (storms, earthquakes and volcanic eruptions) superimposed on long-term, behavioural shifts (tectonic uplift and eustatic sea level change). Overall, the WSS is undergoing uplift, which has been contributing to valley incision since the Last Glacial Maximum. Uplift is both gradual and intermittent, the latter relating to fault- and subduction-related earthquakes. Such seismic activity destabilises the landscape and, together with frequent storms, generates large amounts of sediment. As this sediment passes through the system, part is captured in traps formed by local subsidence of the coastal plain and middle continental shelf where large thicknesses of Holocene and probably older sediment are sequestered (e.g., ~200 m under the coastal plain). Nevertheless, substantial sediment still reaches the continental slope to accumulate in canyon heads and slope basins with other deposits redistributed down-slope in mass flows. Major volcanic eruptions also affect the sediment flux via direct input from ash-fall and by enhanced erosion following volcanic destruction of terrestrial vegetation. Frequent storms of varying intensity play a key role in the generation and transfer of sediment. Major events such as Cyclone Bola of 1988, inundated the shelf with sediment, but identifiable flood deposits were preserved only locally, attesting to the effectiveness of shelf physical and biological mixing regimes. Despite the storms, earthquakes and volcanic eruptions, the largest perturbation of the WSS, at least in the last ~2400 years, has been the human deforestation of the landscape. The resultant influx of sediment overwhelmed the shelf’s storage capacity. Instead of capturing 90% the fluvial input, as in pre-human times, the modern shelf retains only ~25%, the remainder escaping to the slope and elsewhere.

Recent studies of continental margins suggest that small, high-yield rivers are capable of generating shelf sediment-gravity flows, an idea that fundamentally alters our understanding of material flux from the continents to the ocean. Discharge measurements indicate that the Waiapu River, North Island, New Zealand reaches hyperpycnal concentrations (> 36 kg m− 2) on a yearly basis. This study contrasts shelf-edge basins with a broad trough along the shelf-edge off the Waiapu River, testing whether there is evidence that shelf sediment-gravity flows propagate to topographic lows. Observations and measurements through geochemical and sedimentological analyses of sediment cores, EM1002 swath bathymetry, and Chirp sub-bottom profiles suggest differing transport modalities on the outer shelf. In general a southern trough-shaped region exhibits high terrigenous inputs and non-steady-state 210Pb profiles, whereas the northern basins contain steady-state 210Pb profiles and increased marine influence. Sediment-gravity flows dominate accumulation in the southern region, whereas within the northern portion, surface plume sedimentation is indicated. Overall this study suggests that sediment-gravity flows could be bypassing the northern basins, perhaps a result of oceanographic influences and bathymetric steering as they seek a more direct route across the shelf.

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

A process-oriented sedimentary facies model is developed for the York River estuary, a sub-estuary of the lower Chesapeake Bay. This facies model was based on 210Pb and grain-size profiles, as well as X-radiographs taken from kasten cores and box cores collected in a series of across-river transects. Throughout most of the energetic microtidal York River, the seabed is characterized by physical mixing to depths of 25–200 cm. A strong cross-estuary gradient in processes is observed with one side, including channel, flank and shoal, dominated by frequent deep erosion and redeposition (physical mixing), while physical mixing is reduced on the other side, resulting in a greater preservation of biological mixing signatures. Within the physically dominated side of the river, the mixed layer is characterized by ‘stair-stepped’ 210Pb profiles with one or more segments (∼25–200 cm thick) of nearly uniform excess activity. X-radiographs reveal that, although a record of limited biogenic sediment modification is preserved, sedimentary structures within the mixed layer are dominated by centimeter to decimeter scale units of finely to coarsely laminated strata bounded by hiatal surfaces. This demonstrates that mixing results primarily from erosion, resuspension and deposition. Reduced salinity limits the number of benthic species in the York River. Physical disturbance leads to an impoverishment of this community, which is composed primarily of small, opportunistic species with a paucity of larger macrofauna. As a result, mixing in the biologically dominated side of the river is generally on the order of a few centimeters, but may be as deep as 40 cm, and 210Pb geochronology yields low biodiffusion rates (0.43–3.35 cm2 yr−1). X-radiographs reveal the presence of some laminations which suggest that although the mixing is controlled by biological processes the mixing intensity is relatively low. Based on 210Pb geochronologies, residence time estimates for particles within the mixed layer are on the order of centuries. Residence time calculations based on the sediment mass in the physically mixed layer is equivalent to 70 yr of river sediment yield, consistent with century-scale residence times from core data. The frequency and intensity of seabed mixing appears to differ between the lower and upper river. The lower York River is wider and deeper, and is more susceptible to large storms and sea surges, which we suspect drives much of the recorded seabed mixing. Within the upper river, longer-term events (storms) may cause the deepest mixing, but much of this record is destroyed by shorter-term, high-frequency events which produce shallow to mid-depth (

Seabed and water column observations from the microtidal York River subestuary were used to examine temporal changes in this fine-grained environment and compare the impact of infrequent disturbance events (annual to interannual time scales) with more frequent events (fortnightly to seasonal time scales). An experimental across-channel transect is described by time-series observations of seabed radiochemical profiles, X-radiographs, sidescan sonar, and current meter data for 1999. Four morphological subenvironments were distinguished based on the average maximum 137Cs penetration depth and the character of excess 210Pb profiles: southern shoal, flank, secondary channel, and main channel. Various combinations of long- and short-term events (e.g. spring–neap tidal cycle, spring freshet, storms) influence subenvironment sediment structure and physical mixing depths. Shoal sediments were eroded/deposited on the spring–neap tidal cycle during the early spring months, whereas the other three subenvironments were not noticeably influenced by tides on a fortnightly basis. Shortly after the peak currents of the annual spring freshet, sediments were temporarily deposited on the shoal and flank. Major events, such as Hurricane Floyd, did not affect this section of the river except as a result of subsequent longitudinal sediment transport along the estuary main channel that resulted in rapid sediment erosion and deposition of ∼100 cm in ∼1 month. The formation and infilling of longitudinal furrows have previously been implicated as contributing to shallow (25 cm) seabed mixing in the secondary channel. However, interannual variations in river discharge and estuarine circulation likely control sediment supply to this section of the river and may augment the location and frequency of furrow formation within the secondary channel and the main channel. Overall, the data indicate that the seabed at the upper York transect is frequently physically disturbed such that bioturbation structures are not preserved in surface sediments (0–30 cm). However, the presence of bioturbation structures below 30 cm in some of these subenvironments suggests either that this section of the river has experienced long-term changes in the dominant mixing mechanism or that the sedimentary stratigraphy is event-driven.

Abundant sediment supply and accommodation space in the Bengal Basin have led to the development of a major Late Quaternary delta sequence. This sequence has formed in a tectonically active setting and represents an important example of a high-energy (marine and fluvial), high-yield continental margin deposit. Recent studies have detailed the delta’s stratigraphy and development, noting that tectonics and sediment supply control the Ganges–Brahmaputra more significantly than in many other delta systems. These ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonics and sediment-supply have had on deltaic processes and sequence character. Unique and differing stratigraphies are found within thedeltasystem,suchthatfine-grainedsedimentpreservationisfavoredinareasofactivetectonicprocessessuchasfolding,block faulting, and subsidence. Coarse-grained deposits dominate the stratigraphy under the control of high-energy fluvial processes, and mixed fine–coarse stratigraphies are found in areas dominantly influenced by eustatic sea-level change. Overlaid upon these spatially varying stratigraphic patterns are temporal patterns related to episodic events (e.g., earthquakes and rivers avulsions) and long-termchangesinclimate andsediment supply.Modelingisalsousedtoinvestigatethe influenceofavariable sedimentsupply on sequence character. Results show that the timing and magnitude of sediment input, relative to sea-level rise, is a significant control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence. D 2002 Elsevier Science B.V. All rights reserved.

Late Quaternary sediments of the Bengal basin contain a history of river switching and climate change as revealed from sand- and clay-size mineralogy of boreholes and modern riverbed grabs. Epidote to garnet ratios (E/G) in sand fraction sediments are diagnostic of source, with high (>1) E/G indicating Brahmaputra provenance and low (

Sediment cores from the Ganges–Brahmaputra delta in Bangladesh were examined for sedimentological character, clay mineralogy, elemental trends (C, N, S), and 14C geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late Holocene. A widespread facies succession from Muddy Sand to Interbedded Mud records progradation of shoal–island complexes and the transition from subtidal to intertidal conditions. Mangrove-vegetated islands and peninsulas represent the final phase of progradation; a Mottled Mud that is deposited by penetration of turbid coastal water into the mangroves during high water events. Organic matter preservation is generally low (

Sediment deposition and accretion rates in a Virginia tidal freshwater marsh were measured to provide insight to the processes and time scales that are important for maintaining marsh surface elevation. Short-term sediment deposition rates (biweekly to monthly) measured using sediment collection tiles were spatially and temporally variable. Rates were greatest near a tidal creak and decreased along a transect extending toward the marsh interior. When integrated across the entire marsh, annual sediment deposition (as organic carbon) averaged 517±353 g C m−2 y−1 and was sufficient to balance the effects of existing relative sea level rise and marsh respiration rates. At the creekbank, the highest deposition rates were measured during summer although rates were relatively constant over time at the interior sites. Similar spatial and temporal patterns were obtained when deposition rates were calculated from 7Be inventories (monthly time scale). Sediment inventories of7 Be were greater than those supported atmospherically, indicating that the spatial patterns of sedimentation were not due to sediment erosion and redistribution within the marsh. Accretion rates calculated from 137Cs (decadal scale) and14 C dating (centuries to millennia) were substantially less than annual deposition rates, with a decrease in accretion rate with increasing time scale. Mineralization rates of recently deposited sediments (measured as O2 consumption) indicated that sediment metabolism could potentially remove ∼30% of recently deposited carbon within one month of deposition. The metabolism of a labile sediment fraction could explain a portion of the observed decrease in accretion rate with increasing time scale, with the remainder due to periodic storm-induced erosion and historical variability in sediment deposition rates.

Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence development. However, little is known about past changes in fluvial sediment loads despite the recognition of significant variation under changing climatic regimes. In this study we present the first quantified estimate of sediment discharge for a major river system under conditions of an intensified early Holocene monsoon. Development of the Ganges-Brahmaputra River delta began ca. 11000 yr B.P., when rising sea level flooded the Bengal basin, thereby trapping most of the river's discharge on the inner margin. Chronostratigraphic data from these deltaic deposits are used to calculate the rates of sediment storage on the margin, which provide a minimum estimate of the river's past sediment load. Results reveal that ∼5 × 1012 m3 of sediment was stored in the Bengal basin from ca. 11000 to 7000 yr B.P., which corresponds to a mean load of 2.3 × 109 t/yr. In comparison, modern sediment load of the Ganges-Brahmaputra is ∼1 × 109 t/yr, ranking it first among the world's rivers and underscoring the significance of a two-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene strongly suggests its relation to a stronger than present southwest monsoon in South Asia. Similar patterns of high monsoon-related sediment discharge have been noted throughout the tropics and subtropics, suggesting a widespread fluviosedimentary response, the potential magnitude of which is showcased by the Ganges-Brahmaputra system.

The Sepik River, with an estimated annual sediment load of ∼85×106 t yr−1, empties directly into a submarine canyon that transverses a narrow continental shelf (

Borehole data from the Ganges–Brahmaputra delta system unveil an intriguing Late Quaternary history controlled by immense sediment discharge, tectonics, and eustasy. Sea level first intersected a major portion of the lowstand surface ∼10,000–11,000 cal yr BP, marking the onset of sediment trapping and delta growth. Despite rapid sea-level rise (>1 cm/yr), sediment load was sufficient to maintain relative shoreline stability during this time. By ∼7000 cal yr BP, accommodation was quickly infilled with slowed sea-level rise, leading to upstream channel migration and widespread dispersal of sands. This forced coastal progradation along the western delta front where the Ganges was situated, and much of the river's fine-grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf. Concurrently, Brahmaputra sediments were sequestered to an inland tectonic basin, thus starving the adjacent shoreline and leading to transgression along the eastern delta. By ∼5000 cal yr BP, the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river-mouth estuary. Present delta configuration appears to have developed by ∼3000 cal yr BP. Evolution of the tectonically active Ganges–Brahmaputra (G–B) system reveals important similarities and differences with other deltas. Overall facies succession follows that of basic models, progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain. However, the timing, thickness, and controls of these deposits differ. Immense sediment discharge from the Ganges and Brahmaputra rivers allowed initial delta growth 2000–3000 yr earlier than the global average, despite debouching onto a canyon-incised, high-energy margin. Subsequently, a thick (∼50 m) transgressive systems tract was formed during the early Holocene phase of delta aggradation, contrasting with extensive shoreline retreat documented along most margins. Highstand progradation of coastal and shelf sequences ensued by the middle Holocene, but subsiding inland basins also favored accumulation of a thick (to 40 m) highstand sequence in the lower floodplains. Unlike many other delta systems, subsidence in the subaerial G–B delta is not dominated by compaction, but rather by plate-driven tectonic processes that generate rates up to 4 mm/year. Overall, the huge sediment load, tectonic subsidence, major seismic events, and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta, shelf, and deep-sea Bengal Fan throughout the Late Quaternary.

Sediments and pore waters from two sites in the urbanized Elizabeth River, Virginia were sampled for levels of polycyclic aromatic hydrocarbons (PAHs). Pore water PAH concentrations were similar between Sites 1 and 2, despite sediment PAH concentrations being much greater at Site 2. Organic carbon-normalized distribution coefficients (KOC)obs for all PAHs were significantly higher at Site 2 compared to Site 1, but varied ∼2–3 orders of magnitude between the top and bottom of the core in the sediments at Site 1. PAH distribution between sediments and pore waters at Site 1 were most notably related to the bimodal sediment deposition patterns as depicted by X-radiograph depth profiles, and PAH isomer concentration ratios. Sedimentary organic carbon/nitrogen ratios, lignin-phenol concentrations, soot carbon, and particle surface area both before and after organic digestion, were also analyzed in order to assess the role of sediment geochemistry on PAH (KOC)obs. Different factors were determined to control particle surface area at each site, offering insight into explaining observed PAH distribution coefficients. At Site 1, sediment organic matter was inaccessible for digestion and perhaps PAH partitioning as well. Sorption of PAHs to mineral surfaces explains low values for PAH distribution coefficients observed in the deeper sediments in this core. At Site 2, large and invariant (KOC)obs may result from sediments comprised of particles (e.g., woody debris) infused with creosote and coated with organic matter sequestering PAHs within the particle matrix. Our results indicate there is significant heterogeneity in PAH distribution coefficients in estuarine sediments, which may be attributed to sediment deposition patterns, PAH source(s), and sediment geochemistry.

Floodplain deposits are recognized as potentially large sinks for fluvial sediments, yet spatial and temporal patterns of accumulation are not known for many river systems. In the tectonically active Bengal Basin, the Ganges and Brahmaputra rivers coalesce and have formed a large floodplain/delta complex, where widespread overbank flooding may deposit a significant portion of the river's estimated sediment discharge. To investigate the magnitude and distribution of this process, accumulation rates were determined by 137Cs and 210Pb radioisotope geochronology at 60 sites located in three regions of the Bengal Basin differing in age, physiography, and river influence. These geochronological methods have been applied to floodplain environments in only a few recent studies, and the present research offers a revised approach for heterogeneous systems. Patterns of sediment deposition in the Bengal Basin reveal three dominant controls on floodplain accretion: channel processes, overbank flooding, and surface runoff. Accretion is most rapid in the river braidbelt and adjacent floodplain, decreasing rapidly with distance from the main channel. Accumulation rates increase again in low-lying distal basins, where several meters of annual precipitation and associated runoff rework surrounding floodplain surfaces and transport remobilized sediments to local catchments. The extent and magnitude of this process indicate its comparable importance with overbank deposition for floodplain accumulation. A sediment budget for the study area reveals that at least 15% of the estimated 1 billion tonnes of fluvial sediment discharge is stored annually and not reaching the oceans as previously assumed. Extrapolated to the remaining ∼60,000 km2 of floodplain not covered in this project, the sequestered load could rise to >30% of annual sediment discharge.

Biologically dominated lower Chesapeake Bay and the physically dominated York River subestuary are contrasted in terms of the dynamics of sediment mixing, strata formation and sea-bed particle residence times. Two lower bay sites were examined; both are located within the bay stem plains and are characterized by muddy sand and an abundance of large, deep-dwelling organisms. X-radiographs indicate extensive biological reworking of sediments, with no long-term preservation of physical stratification.210Pb profiles reveal low sediment accumulation rates at both lower bay sites ( 80vs6–30 cm2year−1). In contrast, the York River site, located within a partially-filled palaeochannel, is predominantly mud with a depauperate benthic community dominated by small, short-lived, shallow-dwelling organisms. Although210Pb accumulation rates at the York River site (

The Ganges-Brahmaputra is among the world's three largest river systems in terms of sediment load, but, until now, no high-resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers' mouths. The other two (Amazon, Huanghe) discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy. High-resolution seismic reflection profiles of the Bengal shelf reveal similar stratigraphy: topset beds dip gently (0.036 °) and diverge offshore; more steeply dipping foreset beds (0.190 °) converge farther seaward; and relatively thin, gently dipping bottomset beds (0.022 °) extend across the outer shelf, overlying an erosional surface presumed to be of Late Pleistocene age. Sediment accumulation rates are highest in the foreset region (≥ 5 cm/year) and reduced in the bottomset region (

Excess 234Th and 210Pb seabed inventories were measured in cores collected from the Amazon continental shelf to examine particle scavenging and seabed dynamics. Typical excess 210Pb inventories range from 100 to 300 dpm cm−2, and the total excess 210Pb inventory for the Amazon shelf was determined to be 2.7 × 1017 dpm. The 210Pb measurements indicate that particle-reactive species are scavenged not only from the Amazon River but also from the lateral advection of offshore water. In order to sustain the 210Pb inventories, the volume of water supplied by the lateral advection from offshore must be approximately five to ten times the water discharge of the Amazon River. This lateral advection supplies about 67% of the total excess 210Pb to the Amazon continental shelf with relatively small contributions from riverine input (31%), atmospheric fallout (2.3%), and in-situ production (0.1%). The 234Th inventories were measured on four cruises, which occurred during periods of differing river discharge, wind stress, and flow rates of the North Brazil Current. The 234Th excess seabed inventories show large spatial and seasonal variability, with a range from 0 to 22 dpm cm−2, Ratios of predicted 234Th inventories over observed 234Th inventories indicate that a large portion of the inventories on the inner and mid shelf occur in the fluid mud layer or the suspended sediments. The 234Th excess seabed inventories are controlled by resuspension, cycling the inventories between the water column and the seabed. 210Pb and 234Th excess seabed inventories are compared directly using corrections for the different time scales and offshore concentrations. This approach indicates that for most of the shelf, the inventories of the shorter-term tracer 234Th are less than predicted by the inventories of the longer-term tracer (210Pb). There are two explanations for this trend. The first is that a larger portion of the 234Th inventory occurs in the fluid muds or the water column relative to 210Pb. The second is that the supply of offshore water, scavenging efficiency, and/or deposition have been lower over the two year study period relative to the last one hundred years.

Sedimentary processes on the continental shelf and shoreline northwest of the Amazon River mouth were investigated as part of A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds) during four field expeditions between 1989 and 1991. Periodic deposition and resuspension of seabed layers as much as a meter thick dominate sedimentary processes for most of the inner shelf and for the shoreface and foreshore north of Cabo Cassipore. Strata forming as a result of this process consist of decimeter-thick mud beds separated by hiatal (scour) surfaces. The volume of sediment resuspended seasonally from the inner shelf surface layer (SL) is of the same order of magnitude as the annual input from the river, indicating that resuspension is an important control on suspended-sediment distributions in shelf waters. Most resuspension from the SL occurs during February–May (the period of maximum wind stress), which is also the time of rapid deposition on the mudflats, suggesting that sediment resuspended from the SL could contribute to shoreface and foreshore accretion for the northern portion of the study area. In addition, some of the sediment resuspended from the SL is transported seaward periodically in the form of near-bottom fluid-mud flows. This results in non-steady-state input of certain particle-reactive trace metals, which is reflected in the occurrence of quasi-cyclic210Ph profiles in the foreset region of the subaqueous delta. As determined using228Ra/226Ra geochronology, sediment accumulation rates in this region are 10–60 cm y−1. Farther seaward, in the bottomset region, accumulation rates decrease and there is increased evidence of biological activity preserved in sedimentary structures. However, episodic (but reduced) sediment input from fluid-mud flows also extends to this region, affecting the fauna and fine-scale stratigraphy.

A recent study of the subaqueous delta and coastal plain near the mouth of the Amazon River provides insight to the geological record created there and elsewhere. A compound clinoform structure is forming across the Amazon shelf. The uppermost portion is the shoreline, whose aggradation brings the modern sedimentary deposit to sea level and produces a deposit 5–10 m thick. It contains sediments accumulating primarily in shallow subtidal areas, intertidal mudflats and mangrove forests, and progradation occurs by overlapping of northward-extending mudcapes. Through these processes, the coastal plain has been widened by 10–100 km during the Holocene. The shoreline deposits are prograding across topset strata of the modern subaqueous delta, which is the lowermost and dominant portion of the compound clinoform structure. The subaqueous delta extends to a water depth of 70 m, with a depositional break between topset and foreset strata at 30–40 m. Advective sediment input to the foreset region causes high accumulation rates, which control the geometry and progradation of the clinoform structure. On time scales of 102−103 y, physical processes (e.g. waves, currents) have changed and the upper portions of the coastal plain and subaqueous delta have been eroded. One expression of this is a widespread unconformity recorded within late Holocene strata on the inner shelf. Over longer time scales (103−104 y) sea-level changes have led to more extensive erosion and only the lower 20 m of the compound clinoform structure is preserved, overlain by a transgressive sand layer. On other continental margins with different regional characteristics (e.g. more rapid subsidence) larger fractions of the clinoform structures could be preserved.

In order to understand the formative processes and resulting stratigraphy on the Amazon shelf and adjacent shoreline of Amapa, geological investigations were undertaken as part of A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds). The design of the study provided results of a multifaceted nature: integrated with observations in other disciplines; focused on sedimentary processes; representative of fluctuating conditions on several time scales; and broadly distributed on the shelf, including regions not investigated in the past. On short time scales, most muddy sediment is transported northwest of the river mouth. From there, it moves seaward as fluid mud to cause rapid accumulation of foreset beds, and moves northwestward to prograde the northernmost Amapa shoreline and to supply sediment to the Guianas. Fluid muds cover the shelf as far northward as ~3.5 °N and allow strong tides to propagate to shore, where tidal currents cause most of the Amapa shoreline to undergo erosion today. Averaged over decades and centuries, about half of the Amazon sediment discharge accumulates on the adjacent shelf, and another significant fraction (~one-sixth) leads to accretion of the northernmost Amapa shoreline and northwestward bypassing of sediment. The remaining sediment is hypothesized to be trapped in deltaplain deposits of the lower Amazon River system. The foreset region and shoreline represent the two common loci of sediment accumulation that alternate their predominance on time scales of millennia, and lead to a two-stage progradation of coastal-plain and subaqueous-deltaic deposits. This muddy regressive sedimentation during high sea level is replaced by formation of erosional sand layers during low sea level and transgressive conditions. Future research in the study area should address important considerations that were delineated by the present study, including: mechanisms of shoreline accretion; the Holocene history recorded in topset and coastal-plain strata; the role in local sedimentation played by the large shoal extending from Cabo Norte; and the entrapment of Amazon sediment by the delta plain.

The continental shelf southeast and northwest of the Amazon River mouth is overlain by a sediment layer of about a meter in thickness, characterized by 210Pb profiles consisting of one or more segments with nearly-uniform excess activity. Uniform 210Pb activity profiles in shelf environments commonly are a result of intense biological reworking of sediments; however, biological activity is minimal in this area. Hence, this surficial layer (SL) is a result either of physical reworking of the sediments, or of rapid (possibly episodic) deposition of sediment. Factors controlling the formation of the SL were examined during the AmasSeds project between August 1989 and October 1991. Primarily using 210Pb (t1/2 = 22.3 yr) activity profiles and X-radiography, the thickness and character of the SL were assessed at four locations reoccupied during four seasonal periods corresponding to various riverine and oceanographic conditions (e.g., discharge stage, wind stress); one of these stations was reoccupied during semidiurnal and fortnightly periods. Additionally, replicate sediment cores were obtained at stations in order to assess small-scale spatial variability of geochemical and sedimentological characteristics in the seabed. The SL thicknesses in replicate sediment cores obtained at each site were found to vary within ±5 cm. Normalized thickness of the SL decreased by an average of 17 cm between August 1989 and February 1990, a period of increasing wind stress during falling-to-rising river discharge. The SL thickness subsequently increased slightly (8 cm) from February to May 1990, and increased again (27 cm) between May 1990 and October 1991. The maximum SL thicknesses were observed in October, a period of low wind stress and minimal river input, suggesting that the observed seasonal changes are dictated by oceanographic conditions as well as by sediment discharge. Variations in SL thickness are reflected by changes in suspended sediment (mostly as fluid muds), as the highest suspended sediment inventories and most extensive fluid muds are observed during the period when SL thickness is at a minimum. Although a significant SL was present during all of the sampling periods, changes in the character of 210Pb profiles at two of the stations suggest that substantial, perhaps complete, removal and redeposition of the SL occurred some time between February and May. Preserved strata (i.e., below the SL) is characterized by greater evidence of bioturbation and increased sand content, but otherwise is similar to that of the SL. Long-term preservation of strata is thought to occur through episodic suspension and redeposition of the SL, with net accumulation of decimeter-thick, hiatus-bounded layers.

The recovery of quasi-cyclic 210Pb profiles from the foreset region of the Amazon subaqueous delta has raised questions concerning the non-steady-state flux of 210Pb to this region of the shelf. Radioisotopic profiles (210Pb, 234Th, 228Ra/226Ra, 137Cs) from seven kasten cores, along with the analysis of grain size, organic carbon, and clay-sized mineralogy are used to investigate the time scale and the controlling mechanisms of the 210Pb cyclicity. A common assumption used in 210Pb geochronology is that the initial specific activity is constant with time; an assumption that is invalid in this region. Variations of particle affinity for 210Pb have been shown to affect the initial specific activity reaching the seabed in various other studies. However, investigations of grain size, organic carbon, and mineralogy have revealed no relationship with the fluctuating 210Pb activity in this area. A new geochronometer, 228Ra/226Ra, is used to determine sediment accumulation rates over a 30-year time scale, which range from 10 to 60 cm/yr in the study area. Spectral analysis of the 210Pb cyclicity has revealed that the major component of 210Pb cyclicity has a frequency of approximately 2–3 years. In addition, components of the 210Pb cyclicity occur on annual and semi-annual frequencies. The controlling mechanism for the 210Pb cyclicity on these time scales appears to be the seaward transport and accumulation of low-activity fluid muds from the topset region.

Sedimentary structures on the Bengal shelf are examined using X-radiographic, thin-section, and scanning-electron-microscopic (SEM) techniques, which reveal contrasting depositional processes for the inner and middle shelf as well as for either side of a major submarine canyon indenting the western shelf margin. The inner shelf is characterized by tidally produced interlaminated silts and muds that reveal a neap-spring cyclicity. The cyclicity is more pronounced for the winter season, when high-discharge/monsoon conditions do not obscure the tidal signature. On the middle shelf, alternating beds of silts and muds probably represent high-discharge and low-discharge conditions, respectively. Silty beds on the western margin of the submarine canyon are thinner than those found on the eastern margin. The western margin receives predominantly fine-grained Ganges sediments, in contrast to the eastern area which receives both Ganges and coarser Brahmaputra sediments; these observations indicate that the submarine canyon presently intercepts westward along-shelf sediment transport. Muddy beds and laminae that appear internally homogeneous in the X-radiographs and exhibit faint bedding in thin sections often are composed of resistant (protruding) and less-resistant (recessed) microlaminae as observed with the SEM. On the inner shelf, resistant microlaminae are composed of fine-grained polymineralic, equant particles. The less-resistant microlaminae are composed of large, platy illite grains rimmed by smaller platy or lath-like illite particles. Resistant and less-resistant microlaminae within middle-shelf muddy sediments have the same textural characteristics observed on the inner shelf; however, both types essentially are monomineralic. On the middle shelf the smallest particles within the less-resistant microlaminae exhibit an open flocculated structure that may result from higher salinity, and/or decreased bottom shear stress associated with tidal currents. Biological structures predominantly occur on the western inner and middle shelf, particularly in intervals of high smectite concentrations. This relationship may exist because of a preferential organic affinity for expandable minerals, providing a favorable substrate for benthic organisms.

An examination of suspended sediment dispersal in the coastal ocean off the Ganges-Brahmaputra river system using shipboard and satellite data reveal distinct patterns of sediment distribution and sediment motion which help explain the dynamics of the world's largest sediment dispersal system. During the low-discharge period, suspended sediment concentration distribution is mainly a function of tidal energy and the sediments are primarily transported as a suspension population. Satellite and sea-bed data indicate that transport of river sediments is largely restricted to an area landward of about the 20-isobath during the low-discharge period. A simplistic estimate of residual transport and its separation into tidal and non-tidal components indicate that non-tidal transport contributes about 47% of the total transport and has transport directions mostly southwestward during the northeast monsoon. The tidal residual transport, which contributes about 53% of the total transport, however, does not show any particular direction for most of the stations, although some landward stations show shoreward transport. Although few water-column measurements were made during the high-discharge period, data from this and previous studies suggest that the dominant transport direction is to the southwest for most of the year.

Recent rates of carbon burial in two deep-sea basins, the Sulu and South China Seas, are contrasted to determine factors influencing organic carbon preservation in deep-sea sediments. The Sulu Sea is an oxygen-deficient, warm-water basin, whereas the adjacent South China Sea is more typical of open ocean conditions. High-resolution sea-bed profiles of 14C reveal that accumulation rates for the Sulu Sea are 2–3 times higher than the South China Sea at any given water depth (e.g., 1.3 g cm−2 kyr−1 versus 4.3 g cm−1 kyr−1, respectively, at 3000 m). Higher accumulation rates for the Sulu Sea are attributed to higher terrigenous input and better carbonate preservation in the warmer bottom waters. Benthic mixing rates determined using 210Pb geochronology generally are lower in the Sulu Sea (0.004–0.254 cm2 yr−1) as compared with the South China Sea (0.17–1.57 cm2 yr−1), especially at the 3000 m and 4000 m sites in the Sulu Sea where benthic mixing is negligible. Despite the higher bottom-water oxygen levels in the South China Sea, organic carbon concentration profiles are strikingly similar for the two basins at any given water depth. With similar concentration profiles, higher accumulation rates in the Sulu Sea result in equivalently higher carbon preservation rates. As primary production is thought to be similar for both basins (if anything slightly higher for the South China Sea), the production rate can not explain the observed differences in carbon preservation. In addition, benthic mixing rates apparently do not play a major role in carbon preservation as extremely low rates at the deeper Sulu Sea would predict relatively low carbon preservation, contrary to our observations. We conclude that sediment accumulation rate is the dominant control on carbon preservation in this area of the western equatorial Pacific.

Shelf sediment dispersal seaward of the Ganges-Brahmaputra river system is examined during January/February (low discharge) through analysis of sediment cores and surface grab samples for clay-size mineralogy. Illite is the dominant mineral in surficial sediments landward of the 50-m isobath, comprising >40% of the clay-size assemblage. Kaolinite concentrations of surficial sediments generally are greater than 20% throughout the area; however, very high abundances occur off Bhola Island (25–33%) and the Chittagong coast (34–39%), probably as a result of extensive shoreline erosion of cultivated areas enriched in kaolinite. Smectite and illite/smectite concentrations, reflecting input from distributaries of the Ganges, are enriched on the western inner shelf and along the western margin of the “Swatch of No Ground”, a major submarine canyon west of the present river mouth. Although surficial chlorite abundances on the inner shelf in water depths

We used the Sacramento-San Joaquin River Delta CA (Delta, hereafter) as a model system for understanding how human activities influence the delivery of sediment and total organic carbon (TOC) over the past 50–60 years. Sediment cores were collected from sites within the Delta representing the Sacramento River (SAC), the San Joaquin River (SJR), and Franks Tract (FT), a flooded agricultural tract. A variety of anthropogenic tracers including 137Cs, total DDE (∑DDE) and brominated diphenyl ether (BDE) congeners were used to quantify sediment accumulation rates. This information was combined with total organic carbon (TOC) profiles to quantify rates of TOC accumulation. Across the three sites, sediment and TOC accumulation rates were four to eight-fold higher prior to 1972. Changes in sediment and TOC accumulation were coincident with completion of several large reservoirs and increased agriculture and urbanization in the Delta watershed. Radiocarbon content of TOC indicated that much of the carbon delivered to the Delta is “pre-aged” reflecting processing in the Delta watershed or during transport to the sites rather than an input of predominantly contemporary carbon (e.g., 900–1400 years BP in surface sediments and 2200 yrs BP and 3610 yrs BP at the base of the SJR and FT cores, respectively). Together, these data suggest that human activities have altered the amount and age of TOC accumulating in the Delta since the 1940s.

Sedimentary processes active in the Wilmington Canyon area during the past century are documented from geochronologic, sedimentologic, and radiographic analysis of sediment cores collected using primarily D.S.R.V. Alvin . Site selection for the Alvin dives, and for collection of box and kasten cores, was achieved using Sea MARC and SeaBeam data, allowing accurate sampling of numerous subenvironments from this extensively dissected area of the continental margin. 210 Pb sediment accumulation rates from intercanyon areas are highest on the upper slope, 1.7 mm/yr (140 mg/cm 2 /yr), and decrease with water depth to about 0.4 mm/yr (36 mg/cm 2 /yr) on the upper rise. Within submarine canyons, accumulation rates vary dramatically, from a maximum of > 6 mm/yr (> 500 mg/cm 2 /yr) to less than 0.3 mm/yr (22 mg/cm 2 /yr). Higher accumulation rates in canyons occur as a result of additional supply of older sediment, probably from bioerosion of the canyon walls. This is supported by observations from D.S.R.V. Alvin , reduced 210 Pb activities, and indurated mud clasts observed in several canyon cores. Extremely low accumulation rates in Wilmington and South Wilmington canyons are probably the result of strong episodic boundary currents. Non-steady-state 210 Pb profiles and sedimentologic data indicate small-scale mass wasting in South Heyes Canyon and on the lower slope and rise just north of Wilmington Canyon.