Cyclic palaeokarst surfaces in Aptian peritidal carbonate successions (Taurides, southwest Turkey): internal structure and response to mid-Aptian sea-level fall     

&#x;smail mer Y&#x;lmaz  Demir Alt&#x;ner 《Cretaceous Research》2006,27(6):814-827

The sedimentology and cyclic stratigraphy of palaeokarst structures in Aptian peritidal carbonate successions are interpreted using field and laboratory microfacies analyses of closely spaced samples from measured outcrop stratigraphic sections in southwest Turkey. Cycles displaying shallowing-upward metre-scale cyclicity are generally composed of lime mudstones/wackestones/packestones at the bottom and stromatolites or lime mudstones with charophytes and ostracods at the top. Subaerial exposure structures such as in situ karst breccias, dissolution vugs/pipes, mud cracks and sheet cracks are encountered at the top of the cycles. The presence of limestone layers between the successive karst breccia levels indicates that they are in situ palaeokarst structures, not recent karstifications or deep penetration from the upper palaeokarst surface down to the older strata. Palaeokarst breccia deposits are interpreted as mantling breccia formed as a result of epikarstification. Three main palaeokarst levels are recorded in nearly all sections. The sedimentology of the palaeokarst breccias, their position in cyclic peritidal carbonates and the biostratigraphic framework are used to trace the record of the global mid-Aptian sea-level fall in the southwest Taurides. The successive occurrences of three karst breccia levels close to the mid-Aptian sea-level fall correspond to falling periods of high-amplitude sea-level fluctuation within a late high-stand or early fall condition of a third-order sea level.  相似文献   

Abrupt climate change: An alternative view   总被引：2，自引：0，他引：2  

Carl Wunsch 《Quaternary Research》2006,65(2):191-203

Hypotheses and inferences concerning the nature of abrupt climate change, exemplified by the Dansgaard-Oeschger (D-O) events, are reviewed. There is little concrete evidence that these events are more than a regional Greenland phenomenon. The partial coherence of ice core δ¹⁸O and CH₄ is a possible exception. Claims, however, of D-O presence in most remote locations cannot be distinguished from the hypothesis that many regions are just exhibiting temporal variability in climate proxies with approximately similar frequency content. Further suggestions that D-O events in Greenland are generated by shifts in the North Atlantic ocean circulation seem highly implausible, given the weak contribution of the high latitude ocean to the meridional flux of heat. A more likely scenario is that changes in the ocean circulation are a consequence of wind shifts. The disappearance of D-O events in the Holocene coincides with the disappearance also of the Laurentide and Fennoscandian ice sheets. It is thus suggested that D-O events are a consequence of interactions of the windfield with the continental ice sheets and that better understanding of the wind field in the glacial periods is the highest priority. Wind fields are capable of great volatility and very rapid global-scale teleconnections, and they are efficient generators of oceanic circulation changes and (more speculatively) of multiple states relative to great ice sheets. Connection of D-O events to the possibility of modern abrupt climate change rests on a very weak chain of assumptions.  相似文献   

Limestone–marl alternations as environmental archives and the role of early diagenesis: a critical review     

Hildegard Westphal 《International Journal of Earth Sciences》2006,95(6):947-961

Limestone–marl alternations and other micritic calcareous rhythmites have long appealed to sedimentologists, as they appeared to directly reflect high-frequency environmental change. In particular, when orbital forcing gained popularity amongst sedimentologists and paleoclimatologists in 1980s, such rhythmites seemed to offer an ideal tool for high-resolution chronostratigraphy and environmental reconstruction. However, in spite of the fact that orbital forcing has become a routine interpretation of calcareous rhythmites, and that the processes of formation of calcareous rhythmites are considered well understood, research in the past 10 years again has questioned their primary origin and their direct interpretability. Detailed petrographic, paleontological, and geochemical data from numerous successions through geological time provided the basis for testing whether or not the regular alternation of limestone beds and marl or shale interlayers represents bimodally fluctuating environmental conditions in a direct way. In particular, these data, supplemented by box model simulations, imply that post-depositional alteration (diagenesis) has the potential to not only seriously distort primary environmental signals, but also to mimic primary signals. This questions the use of micritic calcareous rhythmites for high-resolution chronostratigraphy and for environmental interpretations where independent data of diagenetically inert parameters are not available. Diagenetic changes appear to have a yet widely underestimated influence on the appearance of limestone–marl alternations and other calcareous rhythmites. The aim of the present review is to summarize new approaches and give an overview of our research results in this field of the past decade. This review also aims at pointing to still enigmatic aspects that need to be addressed before the interpretation of micritic calcareous rhythmites can be considered a reliable tool for high-resolution chronostratigraphy and paleoenvironmental interpretation. “This difference [between shale and limestone] does not find definite expression in the chemical composition but appeals to the eye.” Gilbert (1895)  相似文献   

云南鹤庆盆地近1Ma来的气候序列重建初探   总被引：5，自引：3，他引：5       下载免费PDF全文

童国榜  刘志明  王苏民  羊向东  王书兵 《第四纪研究》2002,22(4):332-339

云南鹤庆盆地近1Ma来的孢粉植物群很好地揭示了本区的环境演化过程。本文利用植物生态位知识,通过模糊数学方法对鹤庆(QH)孔孢粉序列所记录的古气候信息进行重塑,建立了近1Ma来的年均温、年降水量及年较差序列,并将本区气候演化分为4个气候旋回,32个气候期。该序列可与黄土-古土壤序列及海洋^δ18O阶段对比,但冷暖波动的幅度及降温时段持续的长短有其特色,表明本区气候既受全球气候变化的控制,又受青藏高原阶段隆升的制约;近1Ma来气温的波动幅度呈递增趋势,0.30MaB.P.尤其显著;冷、暖段间年均温的变化幅度由第一气候旋回的7℃增大到第三气候旋回的9℃(按4次平滑值计算),而样点的实际最大差值由8℃增至16℃;最强烈的降温发生在第二气候旋回中的0.45～0.14MaB.P.时间段,这与玉龙雪山冰川活动规模最大相一致;降温时年降水量及年较差值增大,证实近1Ma来,西南季风区的气候组构与东南季风区不同。  相似文献   

变化中的地球所面临的挑战:全球变化的科学理解(英文)   总被引：7，自引：0，他引：7  

BerrienMOOREⅢ  张雪芹 《地学前缘》2002,9(1):41-47

简要介绍了变化中的地球所面临的挑战 ,并在描述地球系统科学近年来所取得成就的基础上给出了全球变化的科学理解。同时也简要介绍了世界正在面临的主要环境问题 ;描述了气候系统研究所达成的共识、面临的挑战及所取得的研究进展 ;并从大气、海洋和陆地间的CO2 交换、转换、不确定性等方面阐述了碳循环 ;然后 ,简单描述了全球水循环研究进展 ;最后 ,展望了在变化的地球中生存所要面临的挑战与机遇。  相似文献   

强水动力湖泊夏季分层期氮的生物地球化学循环初步研究:以贵州红枫湖南湖为例   总被引：9，自引：1，他引：9  

肖化云  刘丛强  李思亮  王仕禄 《地球化学》2002,31(6):571-576

湖泊在夏季由于藻类生长而消耗大量硝酸盐,水体硝酸盐含量一般要低于春季。而红枫湖南湖水体硝酸盐含量却高于春季(比平均含量高0.83mg/L),说明尚有其他重要的硝酸盐来源。据估算,南湖水体硝酸盐含量升高0.83mg/L约需要1.66×105kg硝酸盐,另外有约10.1×105kg硝酸盐随下泻水输出南湖,再加上夏季藻类生长(生产的chla量约为640kg)所消耗的硝酸盐3.52×105kg,共消耗硝酸盐15.28×105kg。扣除河流输入的4.42×105kg硝酸盐,南湖尚存在约10.86×105kg硝酸盐的亏空。利用氮稳定同位素示踪技术,结合硝酸盐及叶绿素a(chla)含量、溶解氧(DO)等的变化,认为这部分硝酸盐来自湖泊中下部(斜温层)有机质的大量矿化(硝化),是水动力驱动高DO的上部水体下沉从而引起下部有机质(硝化)的结果。南湖这种强水动力湖泊整个夏季分层期氮的生物地球化学循环是斜温层有机质矿化(硝化)释放硝酸盐和变温层藻类生长同化硝酸盐为有机质同时发生的特殊类型。  相似文献   

Paleoecology of a Middle Wisconsin Deposit from Southern California     

R. Scott Anderson  Mitchell J. Power  Susan J. Smith  Kathleen Springer  Eric Scott 《Quaternary Research》2002,58(3):310

Analysis of a buried deposit in the Diamond Valley of southern California has revealed well-preserved pollen, wood, and diatom remains. Accelerator mass spectrometry dates of 41,200±2100 and 41,490±1380 ¹⁴C yr B.P. place this deposit in marine isotope stage 3. Diatoms suggest a shallow lacustrine environment. Pollen data suggest that several plant communities were present near the site, with grassland, scrub, chaparral, forest, and riparian communities represented. Comparison with modern pollen suggests similarities with montane forests in the nearby San Bernardino and San Jacinto ranges, indicating vegetation lowering by at least 900 m elevation and temperatures 4°–5°C cooler than today. An increase in high-elevation conifer pollen documents climatic cooling near the profile top. Early-profile diatoms are typical of warm water with high alkalinity and conductivity, whereas later diatoms suggest a higher flow regime and input of cooler water into the system. We suggest that the sequence is part of the cooling phase of an interstadial Dansgaard–Oeschger cycle. Records of the middle Wisconsin period are rare in southern California, but the Diamond Valley site is similar to records from Tulare Lake in the San Joaquin Valley and the ODP Site 893A record from Santa Barbara Basin. It is probable that the Diamond Valley assemblage is a local expression of a vegetation type widespread in the ranges and basins of southwestern California during the middle Wisconsin.  相似文献   

两个冰期-间冰期旋回的黄土记录及其古气候意义   总被引：9，自引：14，他引：9  

熊尚发  刘东生  丁仲礼 《地理科学》2002,22(1):18-23

北京邻区枣沟、斋堂和赤峰黄土－古土壤剖面粒度和磁化率变化在幅度和相位上都有明显的差异。在两个冰期－间冰期旋回，磁化率记录具有冰量变化模式，而粒度记录具有轨道周期特点，粒度和磁化率之间的相位关系在从冰期向间冰期转换和冰期向间冰期过渡的过程中，既有滞后也有超前的表现，很难单纯以成土作用来解释。由于黄土粒度主要反映粉尘源区环境和古风场强度的变化，而磁化率主要记录的是沉积区的成壤作用，反映夏季风的强度变化，因此剖面粒度和磁化率变化的不协调说明古风场和粉尘源区变化与粉尘沉积区气候变化之间不统一。这可能意味着控制夏季风强度变化和控制冬季风强度变化的机制存在差异。  相似文献   

河南东濮凹陷古近系沙河街组盐岩沉积特征及成因   总被引：13，自引：6，他引：13       下载免费PDF全文

胥菊珍  蒋飞虎  张孝义  尹哲 《古地理学报》2003,5(2):162-170

东濮凹陷是渤海湾盆地的一个次级构造单元，其古近系的两个沉积旋回发育了四套盐岩，分别位于沙河街组第三段的第四亚段(S34)、第三亚段(S33)、第二亚段(S32)和沙河街组的第一段(S1)。主要分布在凹陷北部。每套盐岩又由若干个盐韵律组成，每一单韵律由盐岩和泥岩组成。两套等时对比标志层（深湖沉积）之间盐岩与砂泥岩存在着相变关系。盐岩是在深盆背景下干旱成因的，盐类物质来自地下深处上涌的卤水，分层卤水是盐形成的基本形式，上涌通道是基底大断裂，动力基础是岩浆活动。盐的阻隔作用使其对下部地层的温度、压力均有影响，这种影响有利于加快盐下地层中的有机质成熟与演化。盐又是良好的盖层，且在高温、高压下盐呈塑性状态。勘探实践看出东濮凹陷的岩性油气藏与盐岩的沉积有密切的关系，因此搞清盐岩的分布规律对东濮凹陷下步勘探有着极为重要的意义。  相似文献   

Modelling of oxygen and nitrogen cycling as a function of macrophyte community in the Thau lagoon     

M. Plus  A. Chapelle  P. Lazure  I. Auby  G. Levavasseur  M. Verlaque  T. Belsher  J. -M. Deslous-Paoli  J. -M. Zaldívar  C. N. Murray 《Continental Shelf Research》2003,23(17-19):1877

A three-dimensional model coupling physical and biological processes for the whole Thau lagoon (Mediterranean coast of France) was developed in order to assess the relationships between macrophytes and the oxygen and nitrogen cycles. Ten species have been inserted as forcing variables in the model. Plankton dynamics, shellfish cultivation impact and mineralization of organic matter are also considered, as well as nutrient and oxygen exchanges between the sediment and the water column.Simulations with and without the macrophytes have shown that the system can be characterized as having a highly structured pattern involving lagoon nitrogen and oxygen cycles. This pattern is created by the combined influence of macrophytes, watershed and oyster farming.The model has been also used to assess the total annual macrophyte production at the whole lagoon scale. Comparisons with phytoplankton production and with results from other temperate lagoons have underlined the high productivity of the Thau lagoon supported by active nutrient regeneration.  相似文献