Patterns and rates of fluid flow during burial diagenesis of the Middle Jurassic Lincolnshire Limestone, eastern England

A phase of ferroan burial calcite from the Middle Jurassic Lincolnshire Limestone exhibits a systematic spatial arrangement of oxygen isotopic characteristics. Mean δ¹⁸O values of the ferroan calcites from each of 15 core and outcrop localities over a study area 25 × 25 km were obtained. These values show a marked depletion from west to east across the study area of approximately 3‰, such that the oxygen isotopic composition of the ferroan calcites can be contoured. The systematic change in oxygen isotopic composition across the study area is believed to have recorded the thermal gradient in the limestone during ferroan calcite precipitation. This thermal gradient can be partially attributed to approximately 200 m of differential burial of the Lincolnshire Limestone across the study area during the Chalk deposition, with a maximum burial of 550 m to the east of the area at this time. A component of up-dip fluid flow (from east to west) through the formation is required to generate the temperature enhancements above those predicted for conduction alone by simple differential burial. Using a finite-difference step computer program, rates of fluid flow during ferroan calcite precipitation are calculated to be approximately 25 m/year. This rate of fluid flow is considerably greater than rates usually predicted for buried sedimentary basins. The causes of such rapid, probably relatively short-lived flow-rates may be the sudden dewatering of adjacent shales, the release of overpressure within the formation of interest, seismic pumping, or fluid circulation round a supracrustal convective loop.     

Wave pressures and uplift forces on and scour around submarine pipeline in clayey soil

Experimental investigations are carried out on wave-induced pressures and uplift forces on a submarine pipeline (exposed, half buried and fully buried) in clayey soil of different consistency index both in regular and random waves. A study on scour under the pipeline resting on the clay bed is also carried out. It is found that the uplift force can be reduced by about 70%, if the pipeline is just buried in clay soil. The equilibrium scour depth below the pipeline is estimated as 42% of the pipe diameter for consistency index of 0.17 and is 34% of the pipe diameter for consistency index of 0.23. The results of the present investigations are compared with the results on sandy soil by Cheng and Liu (Appl. Ocean Res., 8(1986) 22) to acknowledge the benefit of cohesive soil in reducing the high pore pressure on buried pipeline compared to cohesionless soil.     

Numerical prediction on the scour burial of cylinder object freely resting on the sandy seabed in the East China Sea using the DRAMBUIE model1

After experiencing 8-day combined tidal current, circulation and wave actions, scour depth surrounding cylinder object freely resting on sandy seabed in the East China Sea (ECS) in January is numerical...     

Burial fluxes and sources of organic carbon in sediments of the central Yellow Sea mud area over the past 200 years

Long-term changes of composition,sources and burial fluxes of TOC(total organic carbon) in sediments of the central Yellow Sea mud area and their possible affecting factors are discussed in this paper.Firstly,similarity analysis is employed to confirm that the carbon burial features resulted from two collected cores are typical in the central Yellow Sea mud area where YSWC(Yellow Sea Warm Current) is prevalent.On this basis,the burial flux of TOC here was considered to be 235.5–488.4 μmol/(cm2?a) since the first industrial revolution,accounting for about 70%–90% among burial fluxes of TC(total carbon) in the sediments.Compared TOC/TC ratio in the two cores with that in other marine sediments worldwide,we suggest that the growth of calcareous/non-calcareous organisms and dissolution of IC(inorganic carbon) are important factors controlling the TOC/TC ratio in sediment.Results of two-end mixed model based on δ13C data indicate that marine-derived organic carbon(OCa)is the main part among total burial organic carbon which accounts for a ratio over 85%.Due to the high TOC/TC ratio in the two cores,TC in the sediments also mainly exists as OCa,and the proportion of OCa is about 60%–80%.Away from the shore and relatively high primary production in upper waters are the main reasons that OCa is predominant among all burial OC in sediments of the central Yellow Sea mud area.Burial of OC in this mud area is probably mainly influenced by the human activities.Although the economic development during the late 19 th century caused by the first industrial revolution in China did not obviously increase the TOC burial fluxes in the sediments,the rise of industry and agriculture after the founding of new China has clearly increased the TOC burial flux since 1950 s.Otherwise,we also realize that among TC burial fluxes,TIC account for about 10%–30% in sediments of the central Yellow Sea mud area,so its burial could not be simply ignored here.Distinct from TOC burial,long-term TIC burial fluxes variations relate with climate changes more closely:the East Asian summer monsoon may influence the strength of the Huanghe River(Yellow River) flood,which could further affect the transport of terrestrial IC from land to the central Yellow Sea as well as the burial of these IC in the sediments.     

Geometry of scour hole around, and the influence of the angle of attack on the burial of finite cylinders under combined flows

Experiments were conducted to investigate the geometry of the scour hole and flow structure around short cylinders under the action of waves alone (WA) and combined flows (CF). The study is aimed at better understanding the dynamics of isolated objects on a sandy floor under oscillatory flows as occurs in shallow water regions in coastal areas. Flow velocities within the fluid core were recorded and 3D mapping of the bottom was performed with sub-aquatic acoustic sensors. Experiments were conducted for cylinder Reynolds wave number and Keulegan-Carpenter number within the ranges 10⁴R_e1.7×10⁵ and 2KC71, respectively. The present experimental evidence shows that the geometric characteristics of the scour hole (length and width) depend primarily on the Keulegan-Carpenter number (KC) and the cylinder aspect ratio (a_r=L_c/D). The effect of variation in the angle of attack of the flow with respect to the cylinder main axis was also investigated. Initial orientations of zero and ninety degrees were found to be stable while cylinders with intermediate initial orientations tended to orientate their main axes perpendicular to the flow direction. The final angle of orientation was found to be primarily a function of the Shields parameter, θ, and the initial angle of attack, α_i.     

Efficient trapping of organic carbon in sediments on the continental margin with high fluvial sediment input off southwestern Taiwan

Accumulation rates of marine and terrigenous organic carbon in the continental margin sediments off southwestern Taiwan were estimated from the measured concentrations and isotopic compositions of total organic carbon (TOC) and previously reported sedimentation rates. Surficial sediments were collected from the study area spanning from the narrow shelf near the Kaoping River mouth to the deep slope with depths reaching almost 3000 m. The average sediment loading of Kaoping River is 17 Mt/yr, which yields high sediment accumulation rates ranging from 0.08 to 1.44 g cm⁻² yr⁻¹ in the continental margin. About half of the discharged sediments were deposited on the margin within 120 km of the river mouth. Carbon isotopic compositions of terrestrial and marine end-members of organic matter were determined, respectively, based on suspended particulate matter (SPM) collected from three major rivers in the southwestern Taiwan and from an offshore station. All samples were analyzed for the TOC content and its isotopic composition (δ¹³C_org). The SPM samples were also analyzed for the total nitrogen (TN) content. TOC content in marine sediments ranges from 0.45% to 1.35% with the highest values on the upper slope near the Kaoping River mouth. The TOC/TN ratio of the SPM samples from the offshore station is 6.8±0.6, almost identical to the Redfield ratio, indicating their predominantly marine origin; their δ¹³C_org values are also typically marine with a mean of −21.5±0.3‰. The riverine SPM samples exhibit typical terrestrial δ¹³C_org values around −25‰. The δ¹³C_org values of surficial sediments range from −24.8‰ to −21.2‰, showing a distribution pattern influenced by inputs from the Kaoping River. The relative contributions from marine and terrestrial sources to sedimentary organic carbon were determined by the isotope mixing model with end-member compositions derived from the riverine and marine SPM. High fluvial sediment inputs lead to efficient trapping of organic carbon over a wide range of water depth in this continental margin. The marine organic accumulation rate ranges from 1.6 to 70 g C m⁻² yr⁻¹ with an area weighted mean of 4.2 g C m⁻² yr⁻¹, which is on a par with the mean terrestrial contribution and accounts for 2.3% of mean primary production. The depth-dependent accumulation rate of marine organic carbon can be simulated with a function involving primary productivity and mineral accumulation rate, which may be applicable to other continental margins with high sedimentation rates. Away from the nearshore area, the content of terrigenous organic carbon in surficial sediments decreases with distance from the river mouth, indicating its degradation in marine environments.     

Burial and thermal history of the West Bashkirian sedimentary basins

The GALO system is applied to the numerical reconstruction of burial and thermal histories of the West Bashkirian lithosphere from the Riphean to the present. An analysis of the variation in tectonic subsidence of the basin during its development is utilized to estimate approximately the mantle heat flow variations. Our variant of basin evolution suggests that after cooling in the Early Riphean, the rather weak thermal reactivations have not led to considerable heating of the lithosphere in the study region. Surface heat flow decreased from relatively high values in the Early Riphean (60–70 mW/m² in the eastern area and 40–50 mW/m² in the western part) to present-day values of 32–40 mW/m². In spite of the relatively low temperature regime of the basin as a whole, a syn-rifting deposition of more than 10 km of limestone, shale and sandstone in the Riphean resulted in rather high temperatures (180–190 °C) at the base of present-day sedimentary blanket in the eastern area. In agreement with the observed data, computed present-day heat flow through the sediment surface increases slightly from 32 to 34 mW/m² near the west boundary of the region to 42 mW/m² near the boundary of the Ural Foldbelt, whereas the heat flow through the basement surface decreases slightly from 28–32 to 24–26 mW/m² in the same direction. The mantle heat flow is only 11.3–12.7 mW/m², which is considerable lower than mean heat flow of the Russian Platform (16–18 mW/m²) and comparable with the low heat flow of Precambrian shields.     

层间氧化带与埋藏型古地表氧化带的鉴别特征

本文结合新疆５１２矿床层间氧化带的特征,论述了层间氧化带及埋藏型古地表氧化带的主要特点,并得出层间氧化带的鉴别特征：后成因,对个别含水层的依存性,剖面上呈舌状,沿地层倾向具有明显的分带性及一定的矿物组合等。     

石膏对白云岩溶解影响的实验模拟研究

表生到埋藏成岩作用的温度与压力（４０-１３０℃、常压－３０ＭＰａ）条件下，含膏与不含膏白云岩的溶解实验证明:在表生与相对浅埋藏的温压条件（低于７５℃，２０ＭＰａ）下，石膏（或硬石膏）的存在可不同程度地加速白云岩的溶解，随着实验温度和压力的升高，石膏（或硬石膏）对白云岩溶解的这种积极作用逐渐降低。在相对深埋藏的温压条件（高于７５℃、２０ＭＰａ）下，石膏（或硬石膏）的存在显著阻止白云岩的溶解，随着实验温度和压力的继续升高，石膏（或硬石膏）对白云岩溶解的这种消极作用也逐渐增加。从实验的这种结果可以预测，在近地表条件下和埋藏成岩作用的早期阶段，由溶解作用造成的含膏白云岩地层的次生孔隙将比不含膏的白云岩地层更为发育，因而在经历了古风化作用的地层中，含膏白云岩层更易形成良好的储层；与之相反，在相对高温高压的深埋藏成岩阶段，不含膏的白云岩地层中将更容易因酸性水的溶解作用而形成次生孔隙。因而在非蒸发沉积环境中形成的白云岩体（如正常海沉积环境的灰岩中的白云岩透镜体）更易因深埋藏溶蚀作用而形成良好的储层。     

近百年来洱海沉积物有机碳埋藏时空变化

湖泊沉积物有机碳埋藏是陆地碳循环中重要的环节,对全球碳平衡起着重要的作用.洱海是云贵高原第二大淡水湖,目前对洱海生态环境变化、重金属与营养盐污染等已展开了较为深入的研究,但对近百年来沉积物有机碳埋藏时空变化与驱动因素缺乏系统的分析.本文通过对洱海北部湖区典型岩芯（编号EH2012）以及其他湖区13个沉积岩芯中有机碳（OC）含量的分析,结合湖泊生态环境与气候因子变化,研究了近百年来沉积物有机碳埋藏时空变化特征与影响因素.C/N摩尔比值表明洱海沉积物中OC以湖泊内源为主,不同湖区岩芯中OC含量变化趋势基本相似.以EH2012岩芯为例,OC含量历史变化可分为3个阶段：1970年以前,表现出较稳定的低值;1970-2000年,OC含量逐渐增加;2000年以来,OC含量增加更加明显并达到近百年来的最大值.近百年来,EH2012岩芯有机碳累积速率（OCAR）呈逐渐增加趋势,变化范围为7.9~87.2 g/（m²·a）,平均OCAR（OCAR）为24.1 g/（m²·a）;OCAR变化与OC含量和区域气温呈显著正相关,全球变暖与营养驱动下的湖泊生产力提高可能是近年来OCAR增加的主要原因之一.基于重金属Cd污染初始时间建立的时标,1982年以来不同湖区OCAR为17.1~44.7 g/（m²·a）,采用克里金插值得到的全湖OCAR均值为31.4 g/（m²·a）;不同湖区OCAR与磷累积速率呈显著正相关,指示了湖泊营养水平对有机碳埋藏空间变化的重要影响.