Gas-induced erosion features in the North Sea

Pockmarks are prominent features on the seafloor at two main regions of the North Sea, the South Fladen area and the Norwegian Trench. The pockmarks vary in size from 5 m across to about 150 m and in depth from 1 m to about 20 m. Some of the features consist of several very small ‘unit-pockmarks’. These are probably gas-induced erosion features and found only in soft, finegrained, marine and glaciomarine sediments. They are probably caused by shallow- or deep-seated gas (or fluid) release through the seabed, whereby the finest particles are thrown into suspension and redistributed by nearbottom currents. On the North Sea Plateau a small pockmarked area has recently been found. A close relationship exists here between mobile gas in the upper sediments, faulting in the soft sediments and the morphology of the seabed. Other morphological features in the North Sea such as ‘coast parallel depressions’ and ‘elongated depressions’ or terraces are also interpreted here as gas-induced erosion products. This applies, yet again, to the ‘mottled seabed’ features, which appear as patches of high reflectivity on side scan sonar records from the North Sea Plateau; they correspond to shallow seabed depressions on the deep towed boomer records. Judging from reported observations from several shallow seas world wide volatile transport through the seabed is probably quite common and several morphological features are undoubtably induced by this process. Several erosion features of similar origin are probably common, also, in deep ocean basins.     

A linear erosion/elevation measuring instrument (LEMI)

The Linear Erosion/Elevation Measuring Instrument (LEMI) offers an alternative to erosion pins for assessing changes in the ground surface that need not disturb the soil at the point of measurement. The device, fabricated from two carpenter's levels, is mounted on each of a series of support rods implanted along a hillslope profile. The vertical distance between the LEMI and the surface may be recorded to the nearest one-half millimetre. Testing indicates that the use of the LEMI sacrifices about one millimetre of measurement replicability while obtaining undisturbed site data.     

超百年一遇强降雨对厦门岛沙滩影响研究

2018年5月7日厦门岛遭遇了超百年一遇的强降水,对岸滩造成了严重影响。文章采用现场测量、无人机航拍等方法,通过对冲毁岸滩进行现场调查与测量,发现厦门岛东南部10.78km的滨海沙滩岸段,共有10个岸段的沙滩发生明显冲刷,受损岸线总长度为6.1km(占比57%),形成了28条冲刷沟,沙滩遭受的侵蚀面积为20286m²,冲刷量超过4万m³。分析其原因,主要是海岸带开发利用时,不透水面增多且具有一定的坡度,加之后方陆域地下排洪管涵排水,当短期强降水发生时,局部地形集中汇水后急速冲向海岸,导致海岸侵蚀垮塌和后退,沙滩冲刷成沟而流失大量海砂。针对不同沙滩的冲刷程度与影响机制,提出了相应的整治修复方案,可为今后沙滩维护保养提供参考。     

Regional soil erosion in response to land use and increased typhoon frequency and intensity,Taiwan

Reservoir sedimentation data and sediment yields from Taiwanese rivers show increased soil erosion in response to both 20th century changes in land use and a more recent increase in typhoon frequency and intensity. Decadal variations of up to 5- to 20-fold in suspended-sediment rating curves demonstrate supply-limited transport and correspond to increased sediment delivery from hillslopes due to changes in land use, regional ground shaking during the Chi-Chi earthquake, and post-2000 changes in typhoon frequency and intensity. While accelerated erosion in central Taiwan after the Chi-Chi earthquake has been documented previously, our results show that periods of increased upland erosion also occurred earlier, in response to 20th century changes in land use. Analyses of rainfall records and typhoon frequency for the period 1900–2009 further point to an island-wide increase in erosion rates corresponding to increased typhoon frequency and intensity after 1990.     

南黄海盆地南部坳陷南五凹三垛组剥蚀量研究

南黄海盆地南五凹自白垩纪以来经历了4期构造运动,其中古近纪末期三垛运动最为强烈,造成大范围的剥蚀。结合井资料,综合利用泥页岩声波时差法、镜质体反射率Ro差值法,从点上对三垛组剥蚀量进行了恢复;然后依托二维地震剖面,利用地层趋势法对该期剥蚀量进行了区域成图。结果表明,该事件中南五凹最大剥蚀厚度在凹陷西部,一般为700~950 m;东部剥蚀厚度一般为300~500 m,整体剥蚀厚度小于新近系地层补偿厚度。三垛组剥蚀量的研究对认识南五凹的成藏史及油气分布规律具有重要意义。     

Mesozoic–Tertiary exhumation history of the Altai Mountains, northern Xinjiang, China: New constraints from apatite fission track data

This study uses apatite fission track (FT) analysis to constrain the exhumation history of bedrock samples collected from the Altai Mountains in northern Xinjiang, China. Samples were collected as transects across the main structures related to Palaeozoic crustal accretion events. FT results and modeling identify three stages in sample cooling history spanning the Mesozoic and Tertiary. Stage one records rapid cooling to the low temperature part of the fission track partial annealing zone circa 70 ± 10 °C. Stage two, records a period of relative stability with little if any cooling taking place between 75 and 25–20 Ma suggesting the Altai region had been reduced to an area of low relief. Support for this can be found in the adjacent Junngar Basin that received little if any sediment during this interval. Final stage cooling took place in the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. This last stage, linked to the far field effects of the Himalayan collision, most likely generated the surface uplift and relief that define the present-day Altai Mountains.     

Geochemical characteristics of water and sediment from the Dal Lake, Kashmir Himalaya: constraints on weathering and anthropogenic activity

Two hundred and forty water samples (in four seasons) and seventeen sediment samples have been analyzed to monitor the natural and anthropogenic influences on the water and sediment chemistry of the Dal Lake, Kashmir Himalaya. The scatter diagrams [(Ca+Mg)/total cations (TZ⁺), (Ca+Mg)/HCO₃, (Ca+Mg)/(HCO₃₊SO₄), (Na+K)/TZ⁺; (Ca+Mg)/(Na+K)] and the geological map of the study area suggest predominance of carbonate and silicate weathering. Lower pH and high total dissolved solids, electrical conductivity and values in the Gagribal basin and in some patches of other basins reflect anthropogenic inputs in the form of sewage from surrounding population, houseboats, hotels, etc. The Dal Lake is characterized by high chemical index of alteration (CIA: 87–95), reflecting extreme weathering of the catchment area. Relative to the average carbonates, the lakebed sediments are enriched in Al, Ti, Zn, Cu and Co and depleted in Ni and Mn. Compared to the post-Archean Shale the sediments have higher Al, Zn and Cu contents and lower Ni and Co. There are distinct positive anomalies of Al, Mn, Zn and Cu and negative anomalies of Ni and Pb with respect to the upper continental crust. Geoaccumulation index (I _geo) and the US Environmental Protection Agency sediment quality standards indicate that the Gagribal basin and some patches of the Nagin basin are polluted with respect to Zn, Cu and Pb. These data suggest that the Dal Lake is characterized by differential natural and anthropogenic influences.     

Unexpected fast rate of morphological evolution of geologically-active continental margins during Quaternary: Examples from selected areas in the Italian seas

In the last few decades, seafloor imagery systems have drastically changed our vision of a mostly regular and depositional marine landscape, evidencing how erosive and mass-wasting processes are widespread in the marine environments, with particular reference to geologically-active areas. Most of the previous studies have focused on the characterization of these features, whereas a very few ones have tried to estimate what is the extent and order of magnitude of erosion rates in these areas. In this paper, we show several examples from some of the most geologically-active margins off Southern Italy aimed to a) quantify the spatial extent of such processes, b) better understand the role of submarine erosion in the morphogenesis of the coastal sector, and c) try to roughly estimate the order of magnitude of erosion rates in these areas. The results are impressive, with mass-wasting features widespread from coast down to −2600, affecting from the 52% up to 97% of the whole continental slope. Because of the narrow or totally lacking shelves in these areas, mass-wasting processes often occur close to the coast and match embayment of the coast, so indicating a key role in the morphogenesis of coastal sector, with significant implication on the related geohazard. Finally, based on a morphological approach integrated by available stratigraphic constraints we have roughly estimated average erosion rates in these areas, ranging from (at least) some mm/year to a few cm/year, i.e., some hundreds of meters up to kilometers eroded in each eustatic cycle. Despite the large uncertainties of these estimates as well as their spatial and temporal variability in response to regional and local factors, the obtained values are very high and they should be considered for future model of margin evolution, source-to-sink computation and marine/coastal geohazard assessment.     

渤海海域庙西凹陷中南洼古近纪盆地原始形态恢复

庙西凹陷中南洼位于渤中凹陷与胶辽隆起过渡带, 受胶辽隆起差异隆升作用控制, 古近系孔店组、沙四段、沙三段以及东营组沉积之后各组遭受了不同程度的剥蚀。 本文综合 利用声波时差法、镜质体反射率法以及地层厚度变化率法恢复了庙西凹陷中南洼各关键期剥蚀厚度, 再现了庙西凹陷中南洼古近系演化过程。 结果表明, 该区古近纪各沉积期在沉积之后普遍遭受了阶段性抬升剥蚀, 致使各期盆地边界形成多个扩大-收缩旋回, 盆地原形并非小而浅;孔店期-东营期原始盆地沉积边界较现今盆地边界可向东外推 18～25 km;孔店期盆地为统一凹陷, 孔店期之后中、南部洼陷区开始出现差异性隆升-沉降作用, 沙四段沉积期继承孔店期的特点, 差异进一步加大, 沙三段沉积期南、北两个次洼彻底分离, 形成两个孤立洼陷, 东营组披覆沉积两洼之上, 并遭受区域性剥蚀。     

坡面侵蚀过程定量研究进展

坡面是土壤侵蚀最基本的地貌单元,定量研究坡面侵蚀能为研究土壤侵蚀规律、确定坡面重点侵蚀部位、建立土壤侵蚀预报模型提供科学依据。总结了溅蚀、片蚀、细沟侵蚀、浅沟侵蚀的定量研究进展,简述了土壤侵蚀模型的研究进展,对目前的其它研究方法进行了评述。