Alkenones in Cretaceous black shales, Blake-Bahama Basin, western North Atlantic

We here report the discovery of unusual distributions of long-chain alkenones (C₃₇-C₄₂) in two Cretaceous black shales from the Blake-Bahama Basin, western North Atlantic. These sediments are Cenomanian (c. 95 Ma) and mid-Albian (c. 105 Ma) in age, thus significantly extending the geological range of these compounds. The precise source of these lipids is, as yet, unknown, although they may derive from an ancient ancestor of Emiliania huxleyi.     

Late Quaternary climatic events and sea-level changes recorded by turbidite activity, Dakar Canyon, NW Africa

The relationship of sea-level changes and short-term climatic changes with turbidite deposition is poorly documented, although the mechanisms of gravity-driven sediment transport in submarine canyons during sea-level changes have been reported from many regions. This study focuses on the activity of the Dakar Canyon off southern Senegal in response to major glacial/interglacial sea-level shifts and variability in the NW-African continental climate. The sedimentary record from the canyon allows us to determine the timing of turbidite events and, on the basis of XRF-scanning element data, we have identified the climate signal at a sub-millennial time scale from the surrounding hemipelagic sediments. Over the late Quaternary the highest frequency in turbidite activity in the Dakar Canyon is confined to major climatic terminations when remobilisation of sediments from the shelf was triggered by the eustatic sea-level rise. However, episodic turbidite events coincide with the timing of Heinrich events in the North Atlantic. During these times continental climate has changed rapidly, with evidence for higher dust supply over NW Africa which has fed turbidity currents. Increased aridity and enhanced wind strength in the southern Saharan-Sahelian zone may have provided a source for this dust.     

北大西洋ODP1049C孔Aptian-Albian期高频旋回大洋红层的成因:矿物学证据

北大西洋ODP171B航次1049C孔Aptian-Albian期沉积以出现大洋红层与灰色、白色沉积物高频旋回为特征。为了探讨大洋红层的成因,本文进行了矿物学、地球化学、沉积学等方面的研究。漫反射光谱、磁化率和活性铁数据表明,赤铁矿、针铁矿的出现是导致样品由白色向红色转变的矿物学原因。棕色和橙色样品中出现赤铁矿和针铁矿的特征峰,FeR/FeT平均值分别为0.23和0.24,明显区别于其他颜色的样品。磁化率与红层成良好的正相关关系,说明铁氧化物矿物含量的变化是导致磁化率变化的原因。X射线衍射结果表明,不论颜色如何,样品中均含伊利石、高岭石、蒙脱石、绿泥石等粘土矿物,其分布与样品颜色没有直接关系,很可能反映当时物源区气候稳定。ODP1049C孔岩芯沉积物出现橙色、棕色、白色、灰色等颜色的高频变化,颜色过渡接触界线清晰,说明导致红色变化的赤铁矿和针铁矿是沉积期低温氧化的产物。棕色样品中CaCO3含量最低,推测红层所对应的氧化条件是由于较低的有机质堆积速率造成的。     

Characteristics of Milankovitch cycles recorded in Eocene strata in the eastern depression of North Yellow Sea Basin,North China

As cyclical orbital movements of Earth, Milankovitch cycles can be recorded in sedimentary strata. The time they reflect can be used to accurately divide and compare strata. Milankovitch cycles recorded in strata enrich the stratigraphic theory, especially the theories of cycle stratigraphy, and thus they are widely used in geological survey engineering nowadays. This study explored the characteristics of the Milankovitch cycles recorded in the eastern depression of the North Yellow Sea Basin, highlighting their control over high-frequency stratigraphic sequences. The Eocene Milankovitch cycles in the depression were calculated based on the method proposed by J. Laskar, and their parameters primarily include eccentricity cycles of 125 ka and 99 ka, obliquity cycles of 51 ka and 39 ka, and precession cycles of 23 ka and 19 ka. Spectral analysis of gamma-ray (GR) and spontaneous potential (SP) log curves of the Eocene strata was carried out to divide and compare stratigraphic sequences, revealing that the spectral peaks correspond well to astronomical cycles. This indicates that the strata in the depression fully record Milankovitch cycles. Furthermore, there are long-, medium-, and short-term stratigraphic cycles in the eastern depression, with a thickness of 13.03–15.89 m, 3.70–5.21 m, and 2.17–2.94 m, respectively. The sedimentation rates of the Eocene strata were calculated to be 121.2–127.12 m/Ma accordingly. From the uplift to the center of the lacustrine basin along the slope in the eastern depression, both the sedimentation duration and the sediment thickness increase, while the sedimentation rate remains relatively stable. The Eocene strata can be divided into six stages of high-frequency sequences by continuous wavelet transformation, namely E₆–E₁ from bottom to top. The sedimentation duration and sedimentation rates of the sequences were calculated using spectral analysis with each of the sequences as a separate window. Moreover, the impacts of climate change on the sedimentary environment in the eastern depression were analyzed. It can be concluded that E₆ was a lowstand system tract, E₅ and E₄ were lacustrine expansion system tracts, E₃ was a highstand system tract, and E₂ and E₁ were lacustrine contraction system tracts. All these verify that Milankovitch cycles serve as an effective approach for the analysis of sedimentary cycles.     

North Atlantic control on precipitation pattern in the eastern Mediterranean/Black Sea region during the last glacial

Based on proxy records from western Black Sea cores, we provide a comprehensive study of climate change during the last glacial maximum and late-glacial period in the Black Sea region. For the first time we present a record of relative changes in precipitation for NW Anatolia based on variations in the terrigenous supply expressed as detrital carbonate concentration. The good correspondence between reconstructed rainfall intensity in NW Anatolia and past western Mediterranean sea surface temperatures (SSTs) implies that during the glacial period the precipitation variability was controlled, like today, by Mediterranean cyclonic disturbances. Periods of reduced precipitation correlate well with low SSTs in the Mediterranean related to Heinrich events H1 and H2. Stable oxygen isotopes and lithological and mineralogical data point to a significant modification in the dominant freshwater/sediment source concomitant to the meltwater inflow after 16.4 cal ka BP. This change implies intensification of the northern sediment source and, with other records from the Mediterranean region, consistently suggests a reorganization of the atmospheric circulation pattern affecting the hydrology of the European continent. The early deglacial northward retreat of both atmospheric and oceanic polar fronts was responsible for the warming in the Mediterranean region, leading simultaneously to more humid conditions in central and northern Europe.     

新疆吐鲁番盆地古近纪气候事件的研究

吐鲁番盆地是天山山脉中的一个山间盆地,古近系保存完好,是研究中国西北地区古近纪气候变迁的良好地区。通过对吐鲁番盆地连木沁剖面古近系多种气候代用指标（Fe₂O₃、FeO、CaCO₃、有机碳、有机碳同位素）的研究,探讨了吐鲁番盆地古近纪的气候变迁和重要气候事件。利用Fe3+/Fe2+值,恢复了吐鲁番盆地古近纪的气温：古新世的年均气温为22.35℃,始新世为21.82℃,渐新世为15.69℃,并且在渐新世的早期和晚期发生较大幅度的降温事件。对地层中CaCO3（碳酸钙）含量的研究表明,古新统和始新统碳酸钙平均含量分别为468％和666％,指示该沉积时期气候相对比较湿润,而渐新统达15.01％,指示渐新世气候明显干旱化,并且渐新世的早期和晚期气候更为干旱。有机碳同位素（δ¹³Corg）值分布于C3植物的范围内,表明其受C3植物的影响。对地层中的δ¹³Corg值研究显示,该值与地层中碳酸钙含量呈负相关,与气温呈正相关,即该值偏负时,地层中碳酸钙含量增加,气温降低。各种气候指标指示吐鲁番盆地古新世和始新世气候比较温暖湿润,而渐新世气候干旱温凉。     

内蒙古乌拉特后旗乌兰剖面记录的早白垩世火灾事件

古火灾事件研究对于重建当时的古气候及古地理具有重要的意义,白垩纪是全球火灾事件最盛行的时期之一,但在我国目前报道较少。本文通过对内蒙银根-额济纳旗盆地东部乌兰剖面早白垩世砂岩中发现的大量炭屑的显微形貌特征观察和多环芳烃(PAHs)测定,发现炭屑燃烧痕迹明显,木质细胞结构保存完好;同时在样品中检出了燃烧源的多环芳烃(PAHs)菲、芘、苯并荧蒽、苯并[e]芘等,证实炭屑是火灾产物,燃烧温度可达520℃以上。结合沉积环境分析、孢粉资料和岩石磁化率测量,认为乌兰剖面上3个炭屑密集层代表该区在早白垩世经历了3个大火频发时期,炭屑是火灾过程中松柏类和衫科类等高大乔木燃烧后的残留物,并推测大部分炭屑是由周边地区发生火灾堆积,后期通过快速搬运而来,结合前人在霍林河煤田发现的早白垩世火灾事件,推断我国华北地区在早白垩世大火频发,是早白垩世北半球中纬度火灾带的组成部分。     

塔里木盆地西部白垩纪—古近纪海相地层框架及对重大地质事件的记录

塔里木盆地西部白垩纪—古近纪发生了大规模的海侵事件,形成一个喇叭状向西开口的海湾,该海湾属于东特提斯洋的一个分支。该地区白垩纪—古近纪海相地层记录了东特提斯洋演化和一系列重大地质事件,具有重要的研究价值,但对其地层的研究仍相对薄弱,对重大地质事件的研究还不够深入。本文拟通过详细的岩石地层、生物地层和其他地层方法,完善地层划分与对比框架,并在此基础上讨论Cenomanian/ Turonian界线大洋缺氧事件(OAE2)、白垩纪/古近纪界线(K/Pg)、古新世—始新世极热事件(PETM)、特提斯海进与海退等一系列重大地质事件。塔里木盆地白垩纪—古近纪海相或海陆过渡相地层自下而上为克孜勒苏群、库克拜组、乌依塔克组、依格孜牙组、吐依洛克组、阿尔塔什组、齐姆根组、盖吉塔格组、卡拉塔尔组、乌拉根组和巴什布拉克组,上述地层中含有丰富的有孔虫、介形虫、钙质超微、沟鞭藻、孢粉、双壳类、腹足类等化石,以及少量菊石、腕足类、海胆和鲨鱼牙齿等化石。综合的生物地层和年代地层研究表明,克孜勒苏群的时代为早白垩世Barremian-Albian期,库克拜组—依格孜牙组的时代为晚白垩世Cenomanian-Maastrichtian期(Cenomanian/Turonian界线可能位于库克拜组三段),吐依洛克组的时代为白垩纪—古近纪过渡期;阿尔塔什组的时代为古新世早中期,齐姆根组为古新世晚期—始新世最早期,盖吉塔格组—乌拉根组的时代为中始新世中晚期,巴什布拉克组的时代为晚始新世,但不排除最上部进入渐新世早期。塔里木盆地的海侵开始于克孜勒苏群中上部沉积期(Albian晚期—Aptian早期),但规模很有限,大规模的海侵始于晚白垩世Cenomanian早期;从晚白垩世—古新世,共经历了5次大规模的海侵—海退事件;大约41 Ma前后,海水退出盆地南部的昆仑山山前,34 Ma前后,海水退出盆地北部的天山山前。上述海侵—海退事件可能受构造和全球海平面变化的双重影响,但构造事件对海侵的启动和结束可能更具决定性的影响。阐述了塔里木盆地西部白垩纪—古近纪海相地层所记录的OAE2、K/Pg界线、PETM和特提斯海侵—海退等事件,其中笔者及团队第一次在塔里木盆地西部齐姆根组中所发现和报道的PETM事件,将有助于揭示全球近岸地区PETM的特征和生物-环境响应。在未来的研究中,需要进一步厘清塔里木盆地西部地层序列,建立更加精细的生物地层和年代地层框架,加强对PETM和特提斯海侵—海退等重大地质事件的研究。     

Organic matter characterisation of the Selli Level (Umbria-Marche Basin, central Italy)

The Selli Level is a marker-bed in the Umbria-Marche Apennines that represents the regional sedimentary expression of the Lower Aptain Oceanic Anoxic Event 1a. This one to three-metre-thick interval shows a remarkable uniformity on a regional scale, with a green to grey marly lower part overlain by black shales. Bulk geochemical studies and an examination of palynofacies were carried out on three sections in order to characterise the distribution and nature of the organic matter in the Selli Level which has been poorly known hitherto. The organic content is medium (0.5 to 2.9% TOC) in the basal part and higher (4.5% on average, and up to 18% TOC) in the black shales. Pyrolysis and palynological data indicate that the organic matter is mainly of marine origin. The geochemical characteristics of the Selli Level are compared to other, more or less organic-rich marker-beds in the Cretaceous succession of the Umbria-Marche Apennines.

Abstract

Le Niveau Selli est un niveau-repère des Apennins d'Ombrie-Marches qui correspond à l'enregistrement régional de l'événement anoxique océanique 1a. Ce niveau, épais de un à trois mètres, présente une remarquable uniformité à l'échelle régionale avec une partie inférieure argileuse, verdâtre à grise, surmontée par des black shales. Des analyses géochimiques et l'examen des palynofacies ont été entreprises sur trois coupes afin de caractériser le contenu en matière organique du niveau Selli, jusqu'à présent mal connu. Les teneurs en carbone organique sont moyennes (0,5 à 2,9% COT) dans la partie basale et élevées (4,5% en moyenne et jusque 18% COT) dans les black shales. Les résultats de pyrolyse et des observations palynologiques indiquent que la matière organique est essentiellement d'origine marine. Les caractéristiques géochimiques du niveau Selli sont comparées aux autres niveaux repères, plus ou moins riches en carbone organique, qui émaillent la série du Crétacé du basin d'Ombrie-Marches.     

北黄海东部次盆地下白垩统沉积环境及沉积充填

下白垩统是北黄海东部次盆地目前最重要的油气勘探层位,准确认识下白垩统的沉积特征,对下一步油气勘探战略选区及优化勘探部署具重要意义。依据钻测井、地震资料,利用古生物、地球化学、地球物理等手段,研究了下白垩统的沉积相类型,分析了古气候、古盐度、古水动力、古物源等沉积环境,最终探讨了三级层序地层格架内下白垩统的沉积充填及演化。结果表明,北黄海东部次盆地下白垩统发育扇三角洲-湖泊沉积体系。该时期以亚热带炎热半干旱气候为主,夹短暂温湿气候。古水体为淡水-半咸水性质,接受来自北部、东部、西南等多个方向的物源供给。早期地层分布局限,以滨浅湖亚相、扇三角洲相沉积为主;之后,湖泊水体范围扩大,发育扇三角洲平原-扇三角洲前缘-滨浅湖亚相沉积序列;中晚期地层受区域抬升作用的影响,仅残存于盆地西北部,以滨浅湖相沉积为主。     

Chronology of late Holocene climatic events in the northern North Atlantic based on AMS 14C dates and tephra markers from the volcano Hekla,Iceland

A combination of AMS¹⁴C dating and tephrochronology has been used to date late Holocene oceanographic events in a 335 cm marine record, covering about 4600 cal. yr with sedimentation rates exceeding 80 cm 1000 yr⁻¹. The core site is located 50 km offshore on the northern Icelandic shelf. Tephra markers from Iceland serve to correlate the marine and terrestrial records. Especially notable is the presence of three geochemically correlated tephra markers from the Icelandic volcano Hekla (Hekla 4, Hekla 3 and Hekla 1104). Benthic and planktonic foraminiferal abundance and distribution as well as the petrography of the sand fraction of the muddy shelf sediments are used as palaeoceanographic proxies. The foraminiferal assemblages reflect a general cooling trend during the last 4600 yr. A marked drop in sea‐surface temperatures is registered at about 3000 cal. yr BP, corresponding to the level of the Hekla 3 tephra. There is faunal indication of temperature amelioration during the Medieval Warm Period and a cooling again during the Little Ice Age. Periods of ice rafting events are indicated by ice rafted debris (IRD) concentrations, e.g. at around 3000 cal. yr BP and during the Little Ice Age. The former event occurred just prior to the deposition of the Hekla 3 tephra marker, the largest Holocene Hekla eruption. A correlation with terrestrial climatic events in Iceland is presented. A standard marine reservoir correction of 400 ¹⁴C yr appears to be reasonable, at least during periods with high influence of water masses from the Irminger Current on the northern Icelandic shelf. An increase to ca. 530 ¹⁴C yr may have occurred, however, when water masses derived from the East Greenland Current were dominant in the area. Copyright © 2000 John Wiley & Sons, Ltd.     

Late Cretaceous marine fossils and seawater incursion events in the Songliao Basin,NE China

The Songliao Basin is the largest non-marine oil-bearing basin in China. Because of the absence of substantial evidence, the hypothesis of seawater incursion events into the Songliao Basin remains controversial. The presence of marine fossils could provide direct proof to support this supposition. Here, we report new discoveries of foraminifera, calcareous nannofossils, brackish dinoflagellates, and other marine and brackish-water fossils to support the suggestion of seawater incursion events in the Songliao Basin. Relatively abundant benthic and planktonic foraminifera, calcareous nannofossils, marine and brackish-water dinoflagellates, fish, and bivalves have been discovered in Members 1 and 2 of the Nenjiang Formation, a few foraminifera and brackish-water dinoflagellates have been found in the lower Qingshankou Formation, and just a few brackish-water bivalves have been found in the uppermost Qingshankou Fm. Based on the presence of marine molecular fossils and other evidence, we suggest that relatively large seawater incursion events occurred during the sedimentation of the lower Nenjiang Fm., relatively smaller seawater incursions occurred during the deposition of the lower Qingshankou Fm., and possibly a very small seawater incursion occurred during the sedimentation of the uppermost Qingshankou Fm. These seawater incursion events in the Songliao Basin were controlled by regional tectonic activity, evolution of the palaeo Songliao Lake, and global sea level change. These periodic seawater incursions brought marine biota into the palaeo Songliao Lake.     

北海盆地维京地堑新近纪下切谷发育主控因素及其内部充填特征*

下切谷是陆地上一种常见的侵蚀地貌,古今均十分发育,但是不同盆地、不同时代发育的下切谷,无论是在沉积充填特征还是平面展布形态等多个方面都大相径庭。为此,作者依据曼宁公式进行理论推导,再结合分析北海盆地维京地堑发育的典型下切谷实例以及前人进行的相关水槽实验结果总结认为: 地形坡度、基准面下降速率及幅度与下切谷所侵蚀地层的岩性是影响下切谷发育模式的最主要因素。这些因素共同控制了: (1)下切谷的弯度指数及宽深比,(2)下切谷平面上发育密度及其规模,(3)与下切谷伴生的陆棚三角洲、陆棚边缘三角洲沉积厚度及展布范围。海侵过程中下切谷的沉积充填类型及岩性,受平均基准面上升速率及沉积物供应量的共同控制,根据沉积充填类型及岩性的不同可将其划分为富砂型陆棚三角洲充填及富泥型河口湾充填两类。     

Late Pleistocene sea-level oscillations (MIS 10–2) recorded in shallow marine and coastal plain sediments of the southern Wanganui Basin, New Zealand

The northern Wanganui Basin, New Zealand, is one of the key global sites for understanding marine cyclic sedimentation during the Quaternary. This paper presents the first evidence of marine cyclic sedimentation from its central-southern parts. Sedimentological, micropalaeontological and palynological analyses on a 280-m-deep borehole encountered units dating back to MIS 10. The sequence includes four marine cycles spanning MIS 9–5, which are overlain by terrestrial fluvial aggradation surfaces dating from MIS 4–2. Each marine unit represents a progressively shallowing depositional environment from the mid-shelf to coastal plain. This is overlain by a terrestrial sequence of lowstand fluvial terraces. Localized fault movements appear to have influenced the sedimentary character of the sequence during MIS 7a and 5e producing basement highs which provided protection to the shoreline. The cyclothems described in this paper now extend the already extensive, previously described record from MIS 17–10 to produce a combined eustatic record of Quaternary sea level change within the basin to MIS 5. They also provide an excellent example of the sedimentary response of a coastal basin to a progressive loss of sedimentation accommodation space.     

The relative utility of foraminifera and diatoms for reconstructing late Holocene sea-level change in North Carolina, USA

Foraminifera and diatoms preserved in salt-marsh sediments have been used to produce high-resolution records of Holocene relative sea-level (RSL) change. To determine which of these microfossil groups is most appropriate for this purpose we investigated their relative utility from salt marshes in North Carolina, USA. Regional-scale transfer functions were developed using foraminifera, diatoms and a combination of both (multi-proxy) from three salt marshes (Oregon Inlet, Currituck Barrier Island and Pea Island). We evaluated each approach on the basis of transfer-function performance. Foraminifera, diatoms and multi-proxy-based transfer functions all demonstrated a strong relationship between observed and predicted elevations (r²_jack > 0.74 and RMSEP < 0.05 m), suggesting that they have equal utility. Application of the transfer functions to a fossil core from Salvo to reconstruct former sea levels enabled us to consider relative utility in light of ‘paleo-performance’. Fossil foraminifera had strong modern analogues, whilst diatoms had poor modern analogues making them unreliable. This result reflects the high diversity and site-specific distribution of modern diatoms. Consequently, we used foraminifera to reconstruct RSL change for the period since ∼ AD 1800 using a ²¹⁰Pb- and ¹⁴C-based chronology, and we were able to reconcile this with tide-gauge records.     

松辽盆地东南隆起区白垩系营城组二段沉积相分析及层序划分

依据吉林九台的野外地质勘察资料,通过分析营城组二段内部建筑结构、岩性、岩相等地质特征,运用层序地层学的理论和研究方法,对松辽盆地营城组二段砾岩进行精细的层序地层研究和层序划分对比。野外砾岩露头精细解剖揭示以辫状水道沉积最为发育,厚层砾岩由多期辫状水道垂向切叠而成,可划分为三级基准面旋回层序（1个长期、3个中期、8个短期）。     

北淮阳晓天盆地早白垩世玄武岩地球化学:富集地幔的证据

产出于北淮阳地区晓天盆地的早白垩世玄武岩的主量元素具有碱性系列和亚碱性系列过渡的特征。其微量元素特征表现为Rh、Ba、Th、K、Pb的显富集和Nb、Ti的亏损。锶和钕同位素的初始比值分别为0．7076，0.7080和—13．9，—12．7。这些特征表明所研究玄武岩来自富集的源区，很可能是古老的扬子型岩石圈地馒。与长江中下游和大别造山带中同时代幔源岩石相比表明，虽然总体上都表现为富集特征，但具有明显的不均一性。长江中下游地幔源区最亏损，可能反映来自亏损的软流圈地慢和富集的岩石圈地幔的物质的混合。大别造山带的源区最富集，代表古老岩石圈的贡献。北准阳地区玄武岩的源区特征与大别造山带的非常相似，但富集程度略低。     

准噶尔盆地石南31井区下白垩统清水河组一段辫状河三角洲相及沉积演化

针对准噶尔盆地腹部石南31井区下白垩统清水河组一段三角洲相沉积的不同认识,根据砾岩的成层性、组构、结构成熟度、孔隙发育等特征,以及砂岩的结构、构造、粒度特征、砂体的空间展布特征,笔者认为清水河组一段是较强水动力条件下牵引流沉积的产物,其沉积环境属辫状河三角洲。通过对岩心的详细观察与描述,结合测井相分析,以石南31井区211口井的单井相分析为基础,依据连井相剖面分析和平面沉积相综合研究,认为该区辫状河三角洲的各亚相发育齐全,并识别出了辫状河道、河漫、水下分流河道、支流间湾、河口砂坝、远砂坝、前三角洲泥、前三角洲沟道砂等微相。在清一段早期（K₁q²₁）,研究区以湖泊相沉积为主,仅东南部为辫状河三角洲沉积。其后,研究区南北两个方向的辫状河三角洲相向推进（K₁q^1-3-4₁ 、K₁q^1-3-3₁）;继而,东南部的辫状河三角洲平原消失,研究区内仅见三角洲前缘（K₁q^1-3-2₁）沉积;K₁q^1-3-1₁沉积时期以细粒沉积为主,而沉积相格局则大致同前。同样,K₁q^1-2₁沉积时期,北方发育辫状河三角洲平原及前缘,东南方仅发育三角洲前缘。清一段晚期（K₁q^1-1₁）,由于湖平面快速上升,全区均演化为前辫状河三角洲亚相,主要由前三角洲泥构成,并且发育条带状分布的沟道砂沉积。