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21.
Global controls on the oceanographic influences on the nature of carbonate factories are broadly understood. The details of the influences of changes in temperature and nutrients across individual carbonate shelves are less well constrained, however. This study explores spatial and temporal variations in chemical oceanography along and across the Yucatan Shelf, a modern carbonate ramp, and how these factors relate to variable bottom character, sediment and sediment geochemistry. In‐situ sensors and remote‐sensing data indicate the sporadic presence of cool, upwelled water with low dissolved oxygen and elevated Chlorophyll‐a. This current‐driven, westward flow of upwelled water is most evident in a zone just offshore of the northern peninsular shoreline, but its influence wanes ca 75 km offshore and as the shore turns southward. The impacts of this water mass include a transitional photozoan–heterozoan assemblage with biosiliceous components, relict grains and common thin Holocene sediment accumulations nearshore; further offshore are coralgal reefs and expansive sand plains. Geochemical proxies of bulk sediment, including high δ18O and elevated HREE/LREE (heavy rare‐earth element/light rare‐earth element) ratios near, and downcurrent of, the upwelling source, are interpreted to represent the signal of nearshore, westward movement of the cool and nutrient‐rich, upwelled water. Collectively, these data emphasize how local processes such as upwelling and longshore transport can variably influence carbonate sediment accumulations and their geochemical signatures, both along and across individual shelves. These data and insights provide an analogue for the influences of spatial variability of water masses in the geological record, and for accurate interpretation of stratigraphic changes of sedimentary and geochemical proxy data in carbonate archives. 相似文献
22.
桂林洞穴滴水对应CaCO3沉积物的稳定同位素特征与环境意义 总被引:2,自引:0,他引:2
桂林地区降水及茅茅头大岩滴水滴速和现代碳酸盐的碳氧同位素研究表明:(1)滴水沉积的δ18O达到了同位素沉积平衡,洞穴滴水的δ18O与同期降雨的δ18O值呈正相关关系;(2)在旱季和湿季交替时,滴水δ18O值滞后于降雨δ18O,受活塞效应的影响;(3)δ13C难以达到同位素沉积平衡,主要受滴水动力学影响;(4)同一洞穴不同滴水对环境的响应不同,如果洞穴顶板覆盖较厚,水分在土壤和岩层中滞留时间较长,滴水则反映受到平滑作用的降雨,反之则波动较大. 相似文献
23.
陕西富平地区上奥陶统赵老峪组为深水盆地边缘至深水斜坡沉积环境,发育深水原地沉积、等深流沉积及重力流沉积。以野外实测剖面、岩相特征等资料为基础,结合米兰科维奇理论、天文地层学、古海洋学以及古气候学,系统研究该地区等深流强度与古气候、海平面、古盐度的关系。结果表明:Fischer图解可以客观、准确地反映深水沉积地层相对海平面变化;深水沉积及等深流沉积中不同级次旋回垂向组合关系记录了米兰科维奇特性,即小级别旋回在垂向上有序地按照1∶4或者1∶5的形式叠加构成较大级别旋回;等深流强度受米兰科维奇旋回影响,呈多周期弱→强→弱递变;等深流强化周期约为0.1Ma。 相似文献
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Fabio Trincardi Alessandro Amorosi Alessandro Bosman Annamaria Correggiari Fantina Madricardo Claudio Pellegrini 《Basin Research》2020,32(2):402-418
Reconstructions of ancient delta systems rely typically on a two-dimensional (2D) view of prograding clinothems but may miss their three-dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high-resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high-resolution multi-beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth-bar) and muddy seaward-dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high-resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one-year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut-and-fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically. 相似文献
28.
Peter Copeland 《Basin Research》2020,32(6):1532-1546
Placing geologic events in a temporal framework is essential to telling the story of Earth history. However, clastic sedimentary rocks can be difficult to date in an absolute reference because they are made up of grains that are older than the rock in which they are now found, and some clastic rocks do not contain fossils that allow precise reference to the Geologic Timescale. For such rocks, the isotopic dating of detrital minerals can be used to estimate the time of deposition; the clastic rock must be younger than the youngest grain analysed. However, many researchers eschew this simple and straightforward approach in favour of schemes that estimate the maximum allowable depositional age as the weighted mean of the age of several grains, chosen by a variety of selection criteria. This is a mistake; in the absence of a geochemical resemblance apart from the similarity of their age, detrital grains should not be assumed to have originated in the same system and therefore any averaging or other manipulation of such data is statistically invalid and produces results without geologic significance. In the absence of interbedded volcanic rocks or index fossils, dating of detrital minerals can be an important aid in understanding the time of deposition of clastic rocks, but the best estimate will come from taking note of the youngest single grain and not by inappropriately averaging data. 相似文献
29.
Lacustrine basins and their deposits are good paleoclimate recorders and contain rich energy resources. Shelf-margin clinoforms do exist in deep lacustrine basins, but with striking differences from those in deep marine basins, caused by a correlation between the river-derived sediment supply and the lake level. This study uses empirical relationships to calculate the water and sediment discharge from rivers and coeval lake level during wet–dry cycles at 10 s of ky time scale. Sediment supply and lake-level changes are used for a stratigraphic forward model to understand how lacustrine clinoforms develop under different climate conditions. The results show that both wet and dry cycles can be associated with thick deep-water fan deposits, supporting the existing climate-driven lacustrine model proposed based on field data (e.g. Neogene Pannonian Basin and Eocene Uinta Basin). The wet period with high sediment supply and rising lake level creates the highly aggradational shelf, progradational slope and thick bottomset deposits. This is contrary from marine basin settings where the presence of rising shelf-margin trajectory commonly indicates limited deep-water fan deposits. This work suggests marine-based stratigraphic models cannot be directly applied to lacustrine basins. 相似文献
30.
对日本海西南陆坡对马暖流主流轴下方高沉积速率沉积物柱状样,进行了沉积学和地球化学系统研究,以探讨全新世以来对马暖流在研究区的形成与演化,并得出了与附近地区古海洋学方法非常相近的结果,从而为今后研究区的对马暖流演化研究提供了沉积学和地球化学指标。据此,该柱状样中所记录的对马暖流演化可划分为5个主要阶段。全新世初期(9.6kaBP),对马暖流开始进入日本海,其强度较弱,同时,伴随着富营养东海沿岸水的影响,海底环境由还原性向氧化性转变;6.5kaBP时,对马暖流控制下的日本海南部现代对流模式最终建立起来,东海沿岸水的影响基本消失,随后,对马暖流明显减弱;3.0kaBP时,对马暖流再次增强并基本达到现在水平,且在随后的3ka里保持总体稳定,期间在2.0~0.7kaBP存在一次较弱的减弱过程。 相似文献