【作者】 刘娟；

【导师】 黄春菊；

【作者基本信息】 中国地质大学 ， 地质学， 2024， 博士

【摘要】 晚古新世至早始新世是地质历史时期中典型的温室气候期,温室气体浓度的增加被认为是导致该时期全球温度升高的主要因素,这与当前人类正面临的因CO₂、CH₄、N₂O等温室气体浓度增加而导致的一系列全球生态环境问题在形成机制上颇具相似性。这些全球变暖的地质实例为理解当前气候变化的驱动机制提供了有用类比。然而,由于陆相沉积的不连续和年代学的不确定,阻碍了我们对于古新世-始新世气候变化在不同时间、空间范围内的理解。太阳活动驱动了千年到十年时间尺度上的气候变化,但由于保存条件和地层分辨率的限制,太阳活动研究大多集中于第四纪时期,鲜有老地层记录报道。学者们在地质记录中已经发现了万年和千年际的周期,但轨道与亚轨道之间的联系和驱动机制尚未开展研究。另外,天文作用力对于有机碳埋藏的影响早已引起不同学者的关注,他们分别从湖相、海相沉积物等开展研究工作,并取得了一系列研究成果。但是,目前对于天文轨道周期对有机碳埋藏的驱动机制还存在争议。本论文基于中国东部苏北盆地的古近系湖相沉积物开展研究,利用岩石地层学、磁性地层学、旋回地层学等对该盆地溱页1（QY-1）钻孔384 m的岩心进行研究,在高精度年代框架的约束下,探讨这一时期苏北盆地轨道-亚轨道驱动的气候演变过程,并对其驱动机制进行了深入分析。同时为了更深入探讨天文轨道周期对有机碳埋藏的驱动机制,对搜集到的已发表的始新世以来南襄盆地和渤海湾盆地的总有机碳（TOC）数据进行了分析。通过以上研究,主要获得了以下几点新认识:1.建立了苏北盆地东南部溱潼凹陷QY-1钻孔阜宁组的高精度天文年代标尺。在有机碳含量较高的岩心样品中采集了5个样品进行铼锇同位素测年,获得的等时线年龄为59.12±4.05 Ma。利用岩心中采集的397个古地磁样品进行测试分析,建立的磁极性柱与GPTS2020对比获得了该钻孔在3680-4064 m的沉积时间约为57.1-52.79 Ma,其中3692.73-371.3 m对应C24n.2r+C24n.2n+C24n.1r极性段,3714-3750 m对应C24n.3n极性段,3750.7-4064 m对应C24r极性段,平均沉积速率约为8.9 cm/kyr。以自然伽马（GR）作为替代性指标,通过数据预处理,频谱分析等,依据沉积速率纵向变化把QY-1井分为三段进行天文旋回分析。频谱图在3634-3790 m、3790-3970 m和3970-4064 m三段分别有对应于～35 m、～55 m和～28m的沉积旋回的显著峰值,结合古地磁估算的沉积速率并根据COCO和e COCO估算的沉积速率变化,其分别对应405 kyr长偏心率周期。分别对～35 m、～55 m和～28 m进行高斯带通滤波,共识别出11个405 kyr的长偏心率旋回。用405 kyr长偏心率周期进行天文调谐,建立了QY-1钻孔的年代模型,获得了时间域数据序列,结合古地磁分析结果,建立了该钻孔52.56-57.10 Ma（3634-4064 m）高精度的天文年代框架,该钻孔沉积物的平均沉积速率约为9.5 cm/kyr。2.苏北盆地QY-1钻孔古气候替代指标时间序列揭示了轨道-亚轨道周期共同驱动了晚古新世-早始新世苏北盆地的古气候变化,并且发现亚轨道周期振幅变化受轨道周期所调制。通过对苏北盆地QY-1钻孔GR数据的旋回分析结果显示,晚古新世-早始新世东亚气候受偏心率周期主控,低纬度夏季日照通过偏心率周期调制岁差周期振幅的变化控制了盆地内的湖平面变化和古湖泊的沉积演化。同时,我们的研究表明,在这种温室状态下,气候系统足够敏感,能够对与～200 kyr偏心率周期相关的日照微小变化做出反应。通过分析钻孔记录的～187 kyr和～1008 yr的古气候变化,发现了～20 kyr岁差周期,～10 kyr半岁差周期,～5 kyr四分之一岁差周期以及～2 kyr,～1 kyr,～500 yr,～88 yr,～22 yr,～11 yr的太阳活动周期,说明苏北盆地古气候变化受到轨道和亚轨道周期的共同驱动,晚古新世-早始新世时期太阳活动无明显变化。同时研究发现～20 kyr岁差周期是～2 kyr太阳活动周期的振幅调制信号,～10 kyr半岁差周期是～1 kyr太阳活动周期的振幅调制信号,说明地球表层气候系统不但受到像偏心率周期调控岁差周期振幅变化这样的驱动机制,可能同时还受到轨道周期调制亚轨道周期振幅变化以及半岁差周期调制1 kyr周期振幅变化的驱动机制。3.随着晚古新世-早始新世（57.10-52.56 Ma）全球极热事件的结束,气候干热程度逐渐下降,稳定性相对增加,东亚气候响应了这一气候变化。在晚古新世-早始新世（57.10-52.56 Ma）时期,随着全球碳同位素负偏的幅度逐渐减小,气候也会随之发生变化,GR数据的递归分析显示54.1 Ma之后苏北盆地区域气候的稳定性有所增加。另外,来自MS、Ti等古气候替代指标的证据,表明苏北盆地在晚古新世-早始新世气候经历了前期的炎热干旱到后期温暖湿润。4.探讨了有机碳埋藏的气候变化驱动机制,揭示了不同级别轨道周期的叠加可能通过非线性气候效应诱发碳循环的扰动进而驱动了有机碳差异的富集过程。为了探究有机碳埋藏的气候变化驱动机制,本研究综合了苏北盆地、南襄盆地、渤海湾盆地的TOC数据,发现这三个盆地的TOC数据序列都存在显著的1.2Myr长斜率周期,说明斜率的周期性变化导致了太阳辐射的纬度变化进而经向温度梯度也发生周期性变化,进一步驱动了TOC有规律的堆积和埋藏,由于后期沉积作用及生物扰动,导致其具有显著的1.2 Myr长周期。利用沉积噪音模型模拟的古湖平面变化的结果分析,发现TOC的高值对应于古湖平面的高值,说明较高的湖泊水位会加剧底层水的缺氧,从而有利于有机碳的发育和保存。我们的研究结果强调了斜率的周期性变化对于中低纬度水文及有机碳埋藏影响的重要性。通过与La04理论1.2 Myr长斜率周期和2.4 Myr长偏心率周期振幅变化进行对比,发现TOC的高值与1.2 Myr长斜率周期的振幅高值有关,此时2.4 Myr长偏心率周期振幅处于高值或低值。据此,我们推断当偏心率和斜率周期的轨道参数配置超过阈值时,即使在不稳定的环境中,生产力通量也能达到净碳埋藏通量的阈值。TOC与古气候替代指标的相关性分析也进一步说明古生产力是控制有机质富集的最关键因素。更多还原

【Abstract】 The Late Paleocene to Early Eocene represents a quintessential greenhouse climate phase in geological history,where an increase in atmospheric greenhouse gases has been considered as the principal factor leading to the period’s global temperature rise.This bears a notable similarity in formation mechanisms to the array of global environmental issues currently faced by humanity,stemming from the increase in greenhouse gas concentrations（CO₂,CH₄,N₂O,etc.）.These similarities offer useful analogs for understanding the climatic driving mechanisms behind current global warming.However,the discontinuity of terrestrial sediments and chronological uncertainties have hindered our understanding of Paleocene-Eocene climate changes across different times and spatial scales.The sun,as Earth’s primary external energy source,plays the most significant role in Earth’s climate.Research indicates that solar activity drives climate changes over interannual-century or even millennium scale,but due to preservation conditions and stratigraphic resolution limitations,the studies on solar activity are mainly focused on the Quaternary,and it is lack of studies in the older strata.Scholars have identified millennial and inter-millennial cycles in the geological record,yet the links and driving mechanisms between orbital and sub-orbital scales remain unexplored.In addition,the influence of astronomical forcing on organic carbon burial has been attracted the attention of scholars.Many researchers have been carried out on Lacustrine and marine sediments and achieved a series of fruitful results.However,the driving mechanism of astronomical orbital cycles on organic carbon burial is still controversial.Based on the Paleogene lake sediments from the Subei Basin in Eastern China,this thesis investigates the chronology of Paleogene boreholes in the Subei Basin and reconstructing the paleo-climate at orbital and suborbital scales and analyzes their driving mechanisms through gamma-ray,geochemical elements,magnetic susceptibility,and total organic carbon.In order to investigate the driving mechanism of orbital cycle on organic carbon burial,published total organic carbon data were also collected and analyzed in this study.The main conclusions are as follows:1.A high-precision absolute astronomical time scale was established for the Funing Formation in borehole QY-1 of the Qintong Depression in the southeast of the Subei Basin.The Rhenium-Osmium（Re-Os）dating gives an isochronal age of 59.12±4.05 Ma.Magnetostratigra imply that the 3680-4064 m interval across E_f²spans～52.79-57.1 Ma.Specifically,3692.73 to～3713.3 m corresponds to the polarity intervals of C24n.2r+C24n.2n+C24n.1r,～3714 to 3750 m corresponds to the C24n.3n polarity interval,and 3750.7 to 4064 m corresponds to the C24r polarity interval.Based on this chronological framework,the average sedimentation rate for this borehole section is approximately 8.9 cm/kyr.To explore whether Milankovitch cycles might also be present in Subei Basin,we carried out high-resolution gamma ray（GR）from downhole logging analysis of an 430 m borehole（QY-1）from the Subei Basin,China.Spectral analyses of the GR data reveal cycles with～35 m,～55 m and～28 m wavelengths at3634-3790 m,3790-3970 m,3790-4064 m,respectively.Based on the average accumulation rate,these cycles all represent 405 kyr eccentricity.A total 11 of 405 kyr long eccentricity cycles were identified by Gaussian bandpass filtering of the GR data.Anchoring of this floating time scale at 4064 m to an age of 57.101 Ma,an absolute astronomical time scale（ATS）spanning 57.1-52.56 Ma（3634-4064 m）was established.According to this chronological framework the average sediment rate of this 430-m core succession is about 9.5 cm/kyr.2.The change of paleoclimate during late Paleocene to early Eocene was driven by orbital and suborbital cycles in the Subei Basin,and suborbital cycles amplitude variations were modulated by orbital cycles.Spectral analysis of the GR data in our study indicates that the dominant forcing that controlled paleoclimate change were precession and eccentricity cycles during the late Paleocene to early Eocene.Eccentricity can affect regional insolation via its modulation of precession.At the same time,we show that the climate system during this greenhouse state was sensitive enough to respond to the small changes in insolation associated with 200 kyr eccentricity cycles.Time series analysis reveals evidence for～20 kyr precession cycles,～10 kyr half-precessional cycles,～2 kyr and～1 kyr solar activity cycles in the Fe series and～500 yr,～88 yr,～22 yr,and～11 yr solar activity cycles in the P series,respectively.According to this we inferred that the variation of climate in the Subei Basin during the late Paleocene-early Eocene was controlled by suborbital and orbital forcing,and this also confirms that solar activity did not change significantly during this period.In addition,it is found that the period of about～20 kyr is the amplitude modulation period signal of the solar activity cycle of about 2 kyr,while the climate change signals of 10 kyr half-precession cycle is the solar activity cycle～1 kyr amplitude modulation period signals,indicate that the Earth’s climate is not only driven by such an eccentricity modulated precession cycle,but also by the orbital cycles modulation of the suborbital cycles amplitude,as well as half-precession cycle modulation of the millennium cycle amplitude variation.3.With the end of the great global hyperthermal event from the late Paleocene-early Eocene（57.1-52.56 Ma）,the climate was gradually becoming less dry and hot,and the climate of East Asia has responded to this climate change.As the global negative carbon isotope shift decreases,the climate changes,and recursive analyses of GR data show an increase in the stability of the regional climate after 54.1 Ma.In addition,evidence from paleo-climate proxies such as MS and titanium（Ti）suggests that the climate of the Subei Basin during the late Paleocene to early Eocene experienced hot and dry in the early period to warm and humid in the later period.4.The mechanism of organic carbon burial was explored,revealing that the superposition of different orbital cycles may have induced perturbations in the carbon cycle through non-linear climatic effects,which in turn drove the process of differential enrichment of organic carbon.To investigate the mechanism of organic carbon burial,this study synthesizes TOC data from the Subei Basin,Nanxiang Basin and Bohai Bay Basin,and finds that the late Paleocene-Eocene TOC evolution shows a dominant 1.2 Myr long obliquity cycle.This suggests that the obliquity cycle influences low latitudes through the longitude gradient,which in turn drives the TOC to have a significant 1.2 Myr cycle.Changes in paleo-lake level simulated by a sedimentary noise model revealed that high values of TOC corresponded to high values of paleo-lake level,suggesting that higher lake levels exacerbate anoxia in the bottom water,thereby favoring the development and preservation of organic carbon.Our results emphasize the importance of the influence of obliquity on hydrology and organic carbon burial at low and middle latitudes.A comparison with the 1.2 Myr long obliquity cycles and 2.4 Myr long eccentricity cycles of La04 shows that high values of TOC are associated with high values of 1.2 Myr obliquity cycles,when 2.4 Myr eccentricity cycles is at high or low values.Accordingly,we infer that when the eccentricity and obliquity parameter configurations exceed thresholds,productivity fluxes can reach the threshold for fluxes carbon burial fluxes even in unstable environments.Correlation analyses of TOC with paleo-climate proxies further suggest that paleo-productivity is the most critical factor controlling organic matter enrichment.更多还原