认识偏心率周期的地层古气候意义

介绍了偏心率周期在地层和古气候研究方面的新发展.现有地球轨道模式对250Ma以来的轨道运算误差能控制在0.2%之内, 使基于偏心率周期来划分地层年代成为可能.新的国际标准地层年代表以405ka长偏心率周期为基础来划分主要地层界线.新生代将包括E1-E162偏心率长周期, 底界年龄(65.5±0.3) Ma.这一地层年代表的建立, 标志着轨道地层学时代的到来.偏心率的100ka短周期和405ka长周期在诸多地质记录中都有反映, 特别是来自深海钻孔的物理化学古气候指标.很多古气候重大事件往往发生在偏心率周期的弱振幅时期, 表明弱振幅时期易受其他因素的干扰影响, 这些因素包括碳储库、冰盖和海平面变化、电磁场, 以及区域构造重组等等.越来越多的研究发现碳同位素在偏心率周期上与地球轨道驱动相关, 且常领先于氧同位素的变化, 表明热带碳循环过程是影响全球气候变化的关键因素之一.      

中更新世气候转型与100ka周期研究

中更新世气候转型是第四纪气候变化中最重要的特征之一，它是指全球气候的主导周期在中更新世时从41ka转变为100ka，且气候波动的幅度也加大。经典的Milankovitch假说不能完全解释中更新世气候转型的原因以及100ka周期在气候记录中的强烈表现，因为太阳辐射与气候记录之间存在着相当的差异，尤其是二者在变化幅度上不匹配。近年来围绕这一转型过程的时代和原因获得了一些新的进展，主要是针对中更新世气候转型的时间、对气候记录中100ka周期的重新检讨以及非太阳辐射因素在这一转型过程中所起的作用。其它可能的转型原因包括大冰盖、温室气体、地球轨道面倾角、冰盖基底、构造隆升等。     

四川盆地中二叠统茅口组米兰科维奇旋回及高频层序

四川盆地中二叠统茅口组形成于中二叠世中晚期。通过分析四川盆地中二叠统茅口组露头剖面岩性特征、对比广元上寺长江沟露头剖面碳氧同位素变化趋势与沉积层序发育特征,将茅口组划分为2个可全盆地追踪的Ⅱ型三级层序,并分析了茅口组三级层序的主控因素。在此基础上,对钻穿中二叠统茅口组典型井的自然伽马能谱测井曲线开展频谱分析,识别出多种具有米兰科维奇旋回特征的高频旋回,并计算出茅口组沉积的平均速率及沉积时限等相关参数。最后,采用数字滤波消除掉其他次要旋回因素的影响而仅保留与主控因素相关的旋回信息,建立茅口组的高频层序划分方案。结果表明,茅口组三级层序在形成过程中主要受控于构造升降及全球海平面变化;茅口组米兰科维奇旋回特征明显,其中长偏心率旋回(413.0 ka)和短偏心率旋回(123.0 ka)分别是形成四级层序(准层序组)和五级层序(准层序)的主控因素,与之对应的平均旋回厚度在龙17井区分别为13.44m和4.31m,在安平1井区分别为16.03m和4.68m;茅口组大约发育15个四级层序,其发育时限大约为6.11Ma;根据构造背景曲线和长偏心率旋回曲线的叠加曲线划分高频层序,其高频层序界面更加接近实际地层的发育情况。     

试论短周期幕式构造沉降对陆相断陷盆地高频沉积旋回的控制

近年来,在构造背景稳定的海相地层中,基于米兰科维奇旋回控制下的高频旋回逐渐成为进行地层精细划分、定年和探讨盆地演化的重要手段,并被逐渐尝试着应用到陆相断陷盆地中。但陆相断陷盆地为构造盆地,断裂构造理论以及大量地表、地下的构造、沉积现象都表明,在高频旋回沉积过程中可能存在着短周期幕式构造沉降的影响,这就使得在陆相断陷盆地中应用米兰科维奇旋回理论必须要考虑和去除这种影响。     

基于米氏旋回的黔西盘县上二叠统煤系高频层序研究

上二叠统记录了地质历史时期最大规模的生物灭绝事件和最深刻的环境变化。对上二叠统的层序地层格架进行精细 描绘,建立高分辨率的地层序列,是深入了解此次事件及其演化的基础和关键。基于钻测井、岩心观测及地球化学分析测 试结果等资料综合分析,并运用小波分析技术,对黔西盘县上二叠统煤系进行了米氏旋回的识别和划分,结果表明,研究 区上二叠统煤系记录了稳定的米兰科维奇旋回,天文轨道周期对其沉积过程具有明显影响,由长、短偏心率、地轴斜率和 岁差周期引起的地层旋回厚度分别为16.06~17.24 m、5.39~5.70 m、2.11~2.15 m、1.12~1.21 m,长偏心率周期对地层中沉积 旋回的控制和影响最强。对长、短偏心率周期进行滤波分析后,建立了上二叠统煤系“浮动”天文年代标尺,为约束同沉 积火山事件层（Tonstein） 的形成及其持续时限提供了年代学依据。以区域等时对比效果明显的长偏心率旋回为标尺,并结 合旋回沉积序列、旋回界面特征等,将上二叠统煤系划分为4个三级层序,并进一步划分为16个四级层序（对应于中期旋 回）,建立了研究区高频层序地层格架。     

Metre‐scale cycles in shallow water carbonate successions: Milankovitch and stochastic origins

Metre‐scale cycles are a common feature in Precambrian and Phanerozoic shallow water carbonate successions, and astronomically forced changes in sea‐level (Milankovitch cycles) may have been an important driver controlling their deposition. Nevertheless, the degree to which potentially low amplitude astronomically paced sea‐level oscillations may have controlled carbonate accumulation in deep time is unclear. In this study, a stochastic model of carbonate accumulation demonstrates how metre‐scale exposure‐bound sequences can be generated under conditions of random sea‐level change. These sequences have characteristic durations close to Milankovitch cycles, despite the absence of any astronomical control on their formation. Metre‐scale sequences with sub‐Milankovitch (millennial‐scale) durations can also be generated by the model, potentially shedding light on the origin of sub‐Milankovitch sequences such as those recorded on the Middle Triassic Latemar platform of Northern Italy. Sensitivity tests demonstrate how shallow water carbonates may be very sensitive to weak (i.e. low amplitude) astronomically forced sea‐level oscillations. Notably, strong statistical evidence (P < 0·01) for astronomical cycles can be preserved in modelled successions even when astronomical forcing contributes <1% of the sea‐level variance on million year timescales. Taken together, metre‐scale cycles with Milankovitch‐scale durations in ancient carbonate successions may reveal very little about the amplitude, or even the existence, of astronomical forcing as a sea‐level driver.     

Contrasting environmental effects of astronomically driven climate change on three Eocene hemipelagic successions from the Basque–Cantabrian Basin

Several processes can contribute to the formation of hemipelagic limestone–marl alternations as a consequence of astronomically driven climate change. The aim of this study was to decipher which environmental factors governed the formation of three Eocene hemipelagic successions of the Basque–Cantabrian Basin using a comprehensive set of physical and bulk carbonate geochemical data (bed thickness, mineralogy, %CaCO₃, δ¹³C and δ¹⁸O). The results show that the significance of several environmental processes varied depending on the palaeogeographic setting and eccentricity‐modulated precessional seasonality. In the Sopelana starved deep‐sea basin, limestones were formed as a consequence of high pelagic carbonate productivity during periods of warm seawater and sluggish circulation, which corresponded with periods of low seasonality (summers at aphelion); conversely, marls accumulated when pelagic carbonate productivity decreased during periods with cooler waters and more vigorous circulation, which occurred when seasonality was higher (summers at perihelion). In the Gorrondatxe submarine fan fringe, marls accumulated when high seasonality produced significant continental rainfall and run‐off, causing the dilution of pelagic carbonate sedimentation with terrigenous supplies. In the Oyambre upper slope, marls also accumulated when seasonality was high, as pelagic carbonate productivity decreased due to both the expansion of low‐salinity waters on the ocean surface and the increase in continentally derived nutrients, which caused detrimental seawater conditions for calcareous plankton. Both in Gorrondatxe and Oyambre, limestones accumulated when boreal summer at aphelion caused low seasonality, which allowed relatively stable conditions to prevail. At minimum eccentricity, when precession‐driven seasonality contrast diminished, changes in pelagic carbonate productivity were significant in the three sections. On the contrary, at maximum eccentricity, when seasonality peaked due to summers occurring at perihelion, the effects of other environmental processes, such as continental and oceanic currents, became influential. However, the influence of these processes minimized when summertime coincided with aphelion at maximum eccentricity and seasonality was weakest.     

频谱分析法在识别米兰科维奇旋回及高频层序中的应用——以塔里木盆地塔中－巴楚地区下奥陶统鹰山组为例

在巴楚地区露头高频层序研究及塔中地区钻测井三级层序划分的基础上,对塔里木盆地塔中地区塔中162井和塔中43井下奥陶统鹰山组层序OSQ2的自然伽马能谱测井ln（Th/K）曲线进行了滤波处理、快速傅里叶变换处理、小波时频分析以及调谐处理,识别出具有米兰科维奇旋回特征的高频旋回。结果表明：斜度（黄赤交角）旋回最为明显,周期为37.0 ka,是以向上变浅为主的六级米级旋回的主要控制因素,其主旋回平均厚度在台地边缘相带的塔中162井及塔中43井中分别为4.55 m及3.97 m,在巴楚地区半局限-开阔台地相带中为2～4 m;95 ka的短偏心率周期是形成五级准层序的主要控制因素,而大致代表四级准层序组形成时限的413 ka长偏心率周期在地层记录中表现并不明显。据高频层序叠置关系分析及平均主旋回个数初步估算,塔中-巴楚地区下奥陶统鹰山组层序OSQ2的形成时限大约为4.92 Ma,塔中43井鹰山组层序OSQ2受后期构造隆升而被剥蚀约192  m。结合四级层序（准层序组）发育时限（413 ka）及其叠置关系,以及与其调谐的小波变换曲线特征,初步将塔中-巴楚地区鹰山组层序OSQ2划分出12个准层序组。自然伽马能谱测井ln（Th/K）曲线包含沉积古水深相对变化的信息,是反映气候变化生成的米兰科维奇旋回层序的良好指标。     

利用采样定理与沉积速率确定旋回分析最佳采样间隔

旋回地层学方法近年来被成功应用于年代确定及重大地质事件天文影响因素的判别.采样是旋回分析中最重要的一步, 目前大多使用地球物理、地球化学替代性指标, 采样频率过高, 会大大增加测量和计算的工作量, 同时也会增加随机干扰或其他非气候因素的干扰; 采样频率过低, 可能识别不出其中所包含的米兰柯维奇旋回成分.为确定一个最佳的采样间隔, 通过对80~100 Ma理论日照量曲线及两个实测剖面3种采样间隔(密集采样间隔与约等于一个岁差周期沉积厚度四分之一和一半的采样间隔)数据分别进行谱估计并比较谱估计结果.发现在满足采样定理的前提下, 以一个岁差周期沉积厚度的约一半作为采样间隔, 既可以分析出全部的米兰柯维奇旋回信号, 又具有最少的工作量, 是旋回分析的最佳采样间隔.实际采样中需根据平均沉积速率来确定这个最佳采样间隔.      

Milankovitch forcing of bioturbation intensity in deep-marine thin-bedded siliciclastic turbidites

Trace fossils have been used to infer a Milankovitch-type control on pelagic and hemipelagic sediments, but there have been no comparable studies in deep-marine siliciclastic turbidite successions. Here, we present the results of a quantitative analysis of trace-fossil abundance and intensity in an essentially continuous, Middle Eocene, 230-m-long (Well A6) core, comprising very thin- and thin-bedded siliciclastic turbidites in the deep-marine Ainsa basin, Spanish Pyrenees. After removing the sediment slides and debris flow deposits, as these represent geologically instantaneous events approximately two orders of magnitude thicker than the typical laminated turbiditic sediments in the core, spectral analysis was conducted on the bioturbation intensity data. Spectral analysis of bioturbation intensity in the A6 core suggests, for the first time from a siliciclastic turbidite succession at a tectonically active plate margin (thrust-top basin in a foreland basin), a cyclicity that is interpreted to reflect the 41-kyr and an ~ 112-kyr (possibly an average of the 95- and 125-kyr) Milankovitch frequencies. We propose that this drove environmental changes in bottom-water conditions in the Ainsa basin, most likely leading to critically varying levels of oxygenation (stratification) of bottom waters. Our age model for the Ainsa basin (~ 4 km of deep-marine sediments in ~ 10 Myr) yields an average sediment accumulation rate of ~ 40 cm kyr^− 1, that is consistent with that inferred from the spectral analysis (~ 30 cm kyr^− 1) for fine-grained sedimentation. An implication of these results is that global environmental change, probably glacio-eustasy, acted as a driver on deep-marine siliciclastic sedimentation patterns during the Middle Eocene.