A discriminatory diagram of massive versus stratified deposits based on the sedimentation and bedload transportation rates. Experimental investigation and application to pyroclastic density currents

Dense gas-particle jets similar to collapsing eruption columns were generated by large-scale experiments. The column collapse resulted in a ground-hugging current forming stratified layers with bedding similar to natural pyroclastic density current deposits. At the impact of the collapsing column on the ground, a thick, massive bed was formed due to a high sedimentation rate that dumped turbulence due to high clast concentration. Down-current, flow expansion favoured turbulence and dilute gas-particle current that formed thin rippled layers deposited under traction. Experiments fed with fine ash (median size 0·066 mm) formed deposits without tractional structures, because fine particles, as other sedimentary fine material, is cohesive and exposes a limited surface to the shear stress. Experimental outcomes show that massive beds are formed where the sedimentation rate per unit width S_rw exceeds the bedload transportation rate Q_b by two orders of magnitude. A lower ratio generates traction at the base of the flow and formation of shear structures that increase in wavelength and height with a decreasing flux. This study presents a diagram that provides a useful addition for facies analysis of pyroclastic density currents, provided that deposits representing sustained sedimentation can be identified in the field. In the diagram a decrease in the S_rw/Q_b ratio corresponds to an increase in bedform size. Application of the diagram for hazard assessment purposes allows the reconstruction of the mass eruption rate of the Agnano–Monte Spina eruption at Campi Flegrei, which is the main variable defining the intensity of past eruptions, and of the Bingham rheology of the massive underflow of the Mercato pyroclastic density current at Vesuvius.     

Upper Cretaceous bottom current deposits,north-east Greenland

Reported ancient bottom current deposits in deep marine settings are scarce and most of them remain contentious. This study describes sedimentological, ichnological and petrographical characteristics of a drill core that covers ca 10 Myr of Upper Cretaceous stratigraphy at Hold with Hope, north-east Greenland. The core is divided into four facies associations, which are interpreted to reflect deposition from bottom currents, turbidity flows and hemipelagic settling in slope and/or near slope environments. The evidence for bottom current influence is three-fold. Firstly, pervasive indications of winnowing such as marine bioclast-rich lags and outsized clasts on ‘mud on mud’ contacts are suggestive of low-sediment concentration flows capable of transporting up to pebble-sized clasts. Common Mn–Fe–Mg rich carbonate matrix cements and various types of hiatal chemogenic lag deposits showing glauconite, apatite and carbonate clasts also point to condensation, prolonged exposure at the sediment–water interface and recurrent phases of sea-floor erosion. Secondly, such deposits can show indicators for tidal processes such as double mud-drapes, tangential bottom sets in dune-scale cross-bedding and cyclic rhythmites. Thirdly, inverse to normal grading at various scales is common in fully marine, commonly seafloor-derived sediments. Ichnological data indicate considerable taxonomic variability in the bottom current deposits, but recurrent fabrics are characteristically dominated by morphologically simple burrows such as Thalassinoides and Planolites, with secondary Phycosiphon, Nereites, Zoophycos and/or Chondrites. In general, opportunistic taxa are common whereas mature composite ichnofabrics are rare. The omission surfaces are locally burrowed with stiffground to firmground trace fossil suites. The results contribute to establishing sedimentological, ichnological and mineralogical criteria for recognition of bottom current deposits as well as to the understanding of the Late Cretaceous palaeoenvironmental evolution of the Arctic region.     

Spatial-temporal variability of submesoscale currents in the South China Sea

Spatial and seasonal variabilities of submesoscale currents in the northeastern South China Sea are investigated by employing a numerical simulation with a horizontal resolution of 1km. The results suggest that submesoscale currents are widespread in the surface mixed layer mainly due to the mixed layer instabilities and frontogenesis. In horizontal, submesoscale currents are generally more active in the north than those in the south, since that active eddies, especially cyclonic eddies, mainly occur in the northern area. Specifically, submesoscale currents are highly intensified in the east of Dongsha Island and south of Taiwan Island. In temporal sense, submesoscale currents are more active in winter than those in summer, since the mixed layer is thicker and more unstable in the winter. The parameterization developed by Fox-Kemper et al. is examined in terms of vertical velocity, and the results suggest that it could reproduce the vertical velocity if mixed layer instability dominates there. This study improves our understanding of the submesoscale dynamics in the South China Sea.     

辽东湾中部西岸浅水海域冬季实测海流分析

2015年12月在辽东湾中部西岸浅水海域进行了8个站、连续半个月的坐底ADCP海流剖面观测,通过对分层潮流和余流分析,得到该海域的海流特征如下:1)实测海流以潮流特征占主导,潮流特征为规则半日潮流,优势分潮为M2;M2椭圆长轴大小为25~50 cm/s、方向多为NE-SW向,具有显著的往复流特征。2)观测期间的平均余流为1~10 cm/s,方向多为SW向,平均余流在水平和垂向上的空间差异明显,日均余流波动剧烈;表层余流方向与局地风向具有很好的同步一致性,且距岸较近站位的表层余流受风影响更大;中、底层余流与风的相关性较差。本文得到的余流方向不支持冬季辽东湾北部的边界顺时针环流的存在。     

天津港主航道连续观测点潮流和余流特征分析

基于天津港主航道连续观测点31 d的实测海流资料,利用调和分析对主航道潮流和余流特征进行研究,同时结合同步风速资料研究风对表层余流的影响。结果表明:(1)航道附近属于弱流海区,表层平均流速为31.4 cm/s,流速总体上由表至底逐渐减小,流速方向大致集中在NW—SE向。(2)观测海域潮流以正规半日往复潮占主导,优势分潮为M₂,浅水分潮较为显著,涨潮流流速大于落潮流流速。(3)观测期间表层平均余流流速为2.8~13.8 cm/s,随着深度增加余流流速逐渐减小,方向大多为NW向。该站表层余流受风的影响显著,东南风将使余流方向偏向西北。     

Coastal currents adjacent to the Caeté Estuary,Pará Region,North Brazil

We examined and compared tidal currents and water column structure between a near-shore station (12 km from the coast) and an offshore station (32 km from the coast) adjacent to the Caeté River, Pará Region, Brazil. Although the coastal system of Pará is largely influenced by local tides and wind, we found substantial differences in the dominant forcing agents between stations. Water column dynamics at the near-shore station were largely affected by local tidal processes, while differences between surface and bottom layer flows also indicated the importance of gravitational circulation at this station and a substantial influence of the adjacent Caeté River discharge. In comparison, at the offshore station, water column structure was largely influenced by a semi-diurnal tidal flow, an along-coastal current flow (mainly associated with the North Brazil Current) and the dynamics of local wind flow. The near-shore station at low tide showed a high level of stratification; at high tide such stratification was reduced. In comparison, stratification was only apparent within the upper 6 m at the offshore station, the rest of the water column was relatively well-mixed. The stratification within Station 1 at low tide was a result of the bi-directional movement of water discharged from the Caeté River, with lower salinity surface water and high salinity bottom water resulting in an estuarine-like circulation environment. The spatial variability and lack of correlation in current flow and water column structure between the near-shore and offshore stations suggest that a flow field resulting from differences in local circulation, tidal variability and wind persistence separate areas. We argue that this separation may indicate that the offshore station is located in a transition region between the Caeté River waters and the local coastal area.     

舟山海域高频地波雷达表层流探测试验

2015年4月7-30日,在浙江省舟山近海海域开展了“嵊山-朱家尖”小型阵列变频高频地波雷达系统的海上比测试验,通过雷达观测数据与定点ADCP海流资料的比对检验了地波雷达表层流探测性能。径向流比对结果显示,测点与雷达法向夹角越小,距离雷达距离越近,径向流比测结果越好,雷达探测的结果越可靠。嵊山站径向流与ADCP观测结果的各站总体平均误差为7.98 cm/s,平均均方根误差为15.34 cm/s,平均相关系数为0.89,朱家尖站径向流与ADCP观测结果的各站总体平均误差为6.24 cm/s,平均均方根误差为12.36 cm/s,平均相关系数为0.81。根据矢量流比对结果显示,矢量流速与ADCP观测结果的各站总体平均误差为4.82 cm/s,平均均方根误差为15.03 cm/s,平均相关系数为0.44。设置在嵊山、朱家尖两个雷达站双站探测的核心区域（两个雷达站连线的中垂线上,并且与两个雷达站构成一个近似直角三角形）的站点比测结果更加理想,当流速大于0.25 m/s时,对于核心区域平均后的流向均方根误差为24.9°。     

Forward-modeling of co-evolution of turbidity currents,sediment transport,and cyclic steps in the Rio Muni Basin

Previous quantitative studies of field-scale cyclic steps are mostly based on analysis of field data. Such studies have shed light on the erosion/deposition patterns over these morphological features as well as the magnitudes of the turbidity current parameters back estimated using the measured geometry data. However, it remains unclear to what extent such back estimated hydraulic features and erosion/deposition patterns can be numerically reproduced by process-based numerical models. Here, a tw...     

Three-dimensional numerical modeling of cohesive sediment transport and wind wave impact in a shallow oxbow lake

It was observed that in some closed inland lakes sediment transport was dominated by wind-induced currents, and the sediment resuspension was primarily driven by wind-induced waves. This paper presents the development and application of a three-dimensional numerical model for simulating cohesive sediment transport in water bodies where wind-induced currents and waves are important. In the model, the bottom shear stresses induced by currents and waves were calculated, and the processes of resuspension (erosion), deposition, settling, etc. were considered. This model was first verified by a simple test case consisting of the movement of a non-conservative tracer in a prismatic channel with uniform flow, and the model output agreed well with the analytical solution. Then it was applied to Deep Hollow Lake, a small oxbow lake in Mississippi. Simulated sediment concentrations were compared with available field observations, with generally good agreement. The transport and resuspension processes of cohesive sediment due to wind-induced current and wave in Deep Hollow Lake were also discussed.     

The impact of internal waves on upper continental slopes: insights from the Mozambican margin (southwest Indian Ocean)

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high-resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex-shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small-scale sedimentary features generated by internal waves are superimposed on large-scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd