The existence and function of tidally dominated and predominantly allochthonous marshes are ultimately contingent upon the operation of hydrodynamic and sedimentary processes within constraints ...imposed by the available accommodation space and sediment supply. This paper re-interprets published data relating to contemporary vertical marsh growth and sea-level rise in the context of the conceptual model relating elevation, sedimentation, sea-level rise, sediment supply and tidal range. This analysis is supported by numerical mass balance modelling of the equivalent parameter space and of the sensitivity of marsh hydroperiod and sedimentation to sea-level and sediment supply forcing. The effect of autocompaction on the translation of sedimentation into elevation change is also considered.
Parameter space modelling provides a framework for the interpretation of field data and affords indicative insights into marsh resilience to change. It is argued that the assessment of marsh response to external environmental forcing should be based not on empirical comparisons of sedimentation versus sea-level rise but on the estimation of sediment supply, and the efficiency with which this is depleted by deposition, as metrics of marsh resilience. This implies a shift towards more intensive process studies aimed at elucidating more fully the linkages between tidal marshes and adjacent estuarine and coastal systems.
Model results also indicate significant variability in marsh sedimentation associated with 18.6 yr tidal modulation and meteorological processes at short-term scales. Such variability further complicates the interpretation of sedimentation or elevation change data obtained from monitoring programmes of short duration. Longer-term monitoring is of value, however, as a means of identifying important mechanisms of climate and sediment supply forcing that may contribute to the formation and maintenance of marsh sedimentary sequences.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This paper provides a description and evaluation of the sedimentary facies and environments associated with a range of glacier thermal and dynamic regimes, with additional consideration given to the ...tectonic context. New and previously published data are evaluated together, and are presented from modern terrestrial and marine glacial sedimentary environments in order to identify a set of criteria that can be used to discriminate between different glacier thermal regimes and dynamic styles in the sedimentary record. Sedimentological data are presented from a total of 28 glaciers in 11 geographical areas that represent a wide range of contemporary thermal, dynamic and topographic regimes. In the context of “landsystems”, representatives from terrestrial environments include temperate glaciers in the European Alps, Patagonia, New Zealand, the Cordillera Blanca (Peru), cold glaciers in the Dry Valleys of Antarctica and the Antarctic Peninsula region, and polythermal valley glaciers in Svalbard, northern Sweden, the Yukon and the Khumbu Himal (Nepal). The glaciomarine environment is illustrated by data from cold and polythermal glacier margins on the East Antarctic continental shelf, and from a polythermal tidewater glacier in Svalbard, along with general observations from temperate glaciers in Alaska. These data show that temperate glacial systems, particularly in high-relief areas, are dominated by rockfall and avalanche processes, although sediments are largely reworked by glaciofluvial processes. Debris in polythermal glaciers is both thermally and topographically influenced. In areas of moderate relief, debris is mainly of basal glacial origin, and the resulting facies association is dominated by diamicton. In high-relief areas such as the Himalaya, the debris load in polythermal glaciers is dominated by rockfall and avalanche inputs, resulting in extensive accumulations of sandy boulder-gravel. Cold glaciers are dominated by basal debris-entrainment, but sediments are little modified from the source materials, which are typically sandy boulder-gravel from older till, and sand (from glaciofluvial, glaciolacustrine and aeolian sources). Similar facies associations, but with different facies geometry and thickness occur in equivalent glaciomarine settings. Application of these concepts can aid the interpretation of glacier thermal regime (and hence palaeoclimate) in Quaternary and ancient glacial systems.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Quantifying the rates of biogeochemical processes in marine sediments is essential for understanding global element cycles and climate change. Because organic matter degradation is the engine behind ...benthic dynamics, deciphering the impact that various forces have on this process is central to determining the evolution of the Earth system. Therefore, recent developments in the quantitative modeling of organic matter degradation in marine sediments are critically reviewed. The first part of the review synthesizes the main chemical, biological and physical factors that control organic matter degradation in sediments while the second part provides a general review of the mathematical formulations used to model these processes and the third part evaluates their application over different spatial and temporal scales. Key transport mechanisms in sedimentary environments are summarized and the mathematical formulation of the organic matter degradation rate law is described in detail. The roles of enzyme kinetics, bioenergetics, temperature and biomass growth in particular are highlighted. Alternative model approaches that quantify the degradation rate constant are also critically compared. In the third part of the review, the capability of different model approaches to extrapolate organic matter degradation rates over a broad range of temporal and spatial scales is assessed. In addition, the structure, functions and parameterization of more than 250 published models of organic matter degradation in marine sediments are analyzed. The large range of published model parameters illustrates the complex nature of organic matter dynamics, and, thus, the limited transferability of these parameters from one site to another. Compiled model parameters do not reveal a statistically significant correlation with single environmental characteristics such as water depth, deposition rate or organic matter flux. The lack of a generic framework that allows for model parameters to be constrained in data-poor areas seriously limits the quantification of organic matter degradation on a global scale. Therefore, we explore regional patterns that emerge from the compiled more than 250 organic matter rate constants and critically discuss them in their environmental context. This review provides an interdisciplinary view on organic matter degradation in marine sediments. It contributes to an improved understanding of global patterns in benthic organic matter degradation, and helps identify outstanding questions and future directions in the modeling of organic matter degradation in marine sediments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The river‐reservoir continuum drives a separation of sedimentation along the longitudinal hydrodynamic gradients, potentially causing imbalanced stoichiometric sedimentation. However, there are still ...uncertainties regarding the contribution of imbalanced stoichiometric sedimentation to methane emissions along this continuum. A 2‐year field survey and in situ experiments were conducted in China's Three Gorges Reservoir (TGR), a river‐valley‐dammed reservoir. Sediments were trapped and collected to analyze for particulate carbon (C), nitrogen (N), and phosphorus (P) concentrations of different sizes in the summer flooded and winter dry seasons along the mainstem. Large amounts of sediments were deposited in the middle part of the TGR, particularly in the summer flood season. Hydrodynamic gradients structured imbalanced stoichiometric sedimentation patterns. Averaged particulate C:N:P proportions in the sediment layer along the upper, middle, and lower parts of the TGR in the first 30 years of reservoir operation were estimated to be 68:7:1, 97:8:1, and 151:13:1, respectively. The middle part of the TGR, being the “control point” of methane accumulation based on seasons, exhibited a shift in stoichiometry and stable isotope signatures, indicating that there was likely a shift in sources of POM. P, specifically in smaller size particles (<10 μm), seemed to be the potential key driver that regulates particulate C:N:P in the sediment of the TGR. Its sedimentation, primarily in the middle part of the TGR, contributed to the significant decreases in particulate C/P ratios and could possibly regulate long‐term CH4 accumulation in the reservoir.
Key Points
The middle part of the Three Gorges Reservoir received bulk deposition of upstream sediments and was the “control point” of methane accumulation
The longitudinal gradients of sediment stoichiometry and stable isotope reflect a shift of POM sources in the middle part of the reservoir
Sedimentation of particulate P was likely the potential key driver that drove particulate C:N:P in sediment and regulated CH4 accumulation
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The Mekong River, largely undeveloped prior to 1990, is undergoing rapid dam construction. Seven dams are under construction on the mainstem in China and 133 proposed for the Lower Mekong River and ...tributaries. We delineated nine distinct geomorphic regions, for which we estimated sediment yields based on geomorphic characteristics, tectonic history, and the limited sediment transport data available. We then applied the 3W model to calculate cumulative sediment trapping by these dams, accounting for changing trap efficiency over time and multiple dams on a single river system. Under a “definite future” scenario of 38 dams (built or under construction), cumulative sediment reduction to the Delta would be 51%. Under full build‐out of all planned dams, cumulative sediment trapping will be 96%. That is, once in‐channel stored sediment is exhausted, only 4% of the predam sediment load would be expected to reach the Delta. This scenario would have profound consequences on productivity of the river and persistence of the Delta landform itself, and suggests that strategies to pass sediment through/around dams should be explored to prevent the consequences of downstream sediment starvation.
Key Points
The Mekong River basin is being rapidly developed, with 140 dams built, under construction or planned
We estimated sediment starvation with the 3W model for three scenarios of future dam building
Full build‐out of proposed dams would trap 96% of the river's predam sediment load
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 <1 cm/yr) is present on the northwestern part of the delta, where some sediment likely escapes the shelf to the deeper Bay of Bengal. In contrast with the Gulf of Martaban, sediments in this mud drape show olive grey coloration, while sedimentary structures are dominated by mottled sandy mud with shells, and occasional sand layers. The mud drape is likely derived from a mixture of local rivers draining westward from the Indo-Burman range, and a contribution from the Ayeyarwady system delivered during the NE monsoon. Overall, the shelf offshore the Ayeyarwady and Thanlwin rivers represents a compound subaqueous delta with marked differences from east to west that are controlled by a combination of oceanographic and tectonic factors.
•Ayeyarwady sediment transport on the shelf is controlled by tectonic and oceanographic factors.•Wave resuspension limits sediment accumulation off the western headland.•Sediment is trapped in the Gulf of Martaban, a wave sheltered embayment.•Sediment accumulation rates are highest (up to 10 cm y−1) in the Martaban depression.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Sediment deposition in water reservoirs has major implications for storage capacity, reservoir lifetime, and water quality. Changes in rainfall patterns and land use will consequently alter the rate ...of erosion and therefore have a direct effect on sedimentation rates. This literature review employed a systematic mapping approach to collate and describe evidence of contemporary sedimentation trends for impounded reservoirs and natural lakes with emphasis on studies which analysed impacts on water storage capacity. Fourteen studies determined an overall increase in sedimentation rate, 13 identified a recent decline and another 5 reported mixed results. Interestingly, 83.3% of the articles that studied natural lakes found an increase in recent contemporary sedimentation, while 54.5% of the articles on impounded reservoirs indicated recent declines in sediment deposition. Land use change was the main causative factor responsible for sedimentation rate increase followed by the combined effects of land activities and climate change. Soil and sediment management strategies, implemented in and upstream of some impounded reservoirs, have proved to be effective in mitigating and remediating reservoir sedimentation. From the 147 papers preselected, only 33 contain sufficient sedimentation data to infer recent rate trends with only about 45% of these articles reporting quantities of storage capacity loss caused by sedimentation. Across these 33 studies, assessments of sedimentation and associated storage capacity loss are compromised by the limited spatiotemporal resolution of current measurement methods, reinforcing the requirement to develop new, more robust techniques to monitor sedimentation and storage capacity changes.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The Cenozoic terrestrial, intermontane Qaidam Basin on the northeastern edge of the Tibetan Plateau contains >12km of sedimentary rocks that potentially document the accommodation of India-Asia ...convergence and the growth of the plateau. The chronology remains incomplete, hindering cross-basin correlation between lithostratigraphic units and their further interpretation. Here we present a high-resolution magnetostratigraphy spanning >5km of Paleogene-Neogene sequence at Dahonggou in the Northern Qaidam Basin. Based on correlation with the geomagnetic polarity time scale (GPTS), we have dated the section to being between ~52 and ~7Ma. The bottom conglomeratic unit, ranging from >52Ma to ~44Ma, was deposited in high-energy environments (e.g., alluvial fan or braided river), reflecting the earliest deformation and uplift of the basin-bounding Qilian Shan fold-thrust belt in response to India-Asia collision. In addition, we identified two major increases in sedimentation rate at 25–16Ma and after ~9.5Ma and three phases of lesser increases at 52–44Ma, 38–33Ma, and 14.6–12.0Ma. These increases in sedimentation rate are consistent with regional thermochronology and basin analysis studies, which revealed enhanced motion on basin-bounding thrust faults. We argue that these accelerated sedimentation rates indicate pulsed tectonism in the northeastern Tibetan Plateau. The pulse at 25–16Ma may further relate to phases of strong rainfall linked to an intense monsoon at that time.
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•Magnetostratigraphy dates a section ranging from ~52 to 7Ma.•Changes in sedimentation rate correlate with deformations at ~52–46, ~38–33, ~25–16, ~14.6–12, and <~9.5Ma.•Pulse of sedimentation at ~25–16Ma may also reflect a strong summer monsoon.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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