渤海湾表层沉积物元素地球化学分布特征与影响因素

对取自渤海湾的307个表层沉积物进行了元素测试和粒度分析。渤海湾表层沉积物的常微量元素呈现4种组合:富集于粗粒沉积区的SiO_2、Na_2O亲碎屑元素组合;富集于细粒沉积区的以Al_2O_3和重金属元素为代表的亲黏土元素组合;与缺氧环境有关的MnO、V、TOC元素组合和与河流输入有关的陆源CaO、TiO_2元素组合。粒度粗细主导了渤海湾元素含量分布的整体格局;海域河流物源供应不同较大影响了渤海湾南部(富Na_2O、CaO和SiO_2)和北部(富Ba和P_2O_5)在元素含量上的差异;高流速潮流对海底的冲刷再分配导致曹妃甸南侧Sr、Ca元素的条带状富集异常;有机质在细粒沉积区的富集导致缺氧环境的形成和K_2O、Mn、V、自生黄铁矿的海洋自生化学沉积;人类活动导致以Pb为代表的重金属污染在河流入海口、港口及沿岸海域的元素分布异常;曹妃甸沙坝内侧的泻湖(海洋钙质生物沉积)与沙坝外侧水下岸坡(陆源碎屑沉积)的截然不同的物源,导致了独特地貌沉积环境下元素分布的局部差异。     

Global synchroneity of a positive carbon isotope excursion at the Cenomanian/Turonian boundary: Validation by calcareous microfossil biostratigraphy of the Yezo Group, Hokkaido, Japan

Abstract Isotopic analyses of organic carbon from the mid-Cretaceous sequence in Hokkaido, Japan, revealed a 2‰ positive excursion of δ¹³C values at the biostratigraphically defined Cenomanian/Turonian (C/T) boundary recognized in the Yezo Group. The planktonic foraminiferal Whiteinella archaeocretacea Zone, which is known to bracket the Cenomanian/Turonian boundary elsewhere in the world, was recognized in the Oyubari area of central Hokkaido based on the distribution of commonly occurring planktonic foraminifera. In the Tappu area of northwestern Hokkaido, where diagnostic planktonic foraminifera are rare but calcareous nannoplankton occur commonly, the interval coeval with the W. archaeocretacea Zone can also be established by recognizing the conjoined last appearance levels of Corollithion kennedyi and Axopodorhabdus albianus, both calcareous nannoplankton species. Carbon isotope profiles exhibit a similar pattern with comparable peaks and troughs occurring in the same stratigraphic position in the sequences. A prominent, positive 2‰ shift of δ¹³C values, here called ‘δ¹³C spike’ occurs in the middle of the W. archaeocretacea Zone in the Oyubari area and just above the conjoined last appearances of the two above-mentioned nannoplankton taxa in the Tappu area. The Cenomanian/Turonian boundary can be drawn just above the peak position of the spike in both sections. The Rock Eval analyses and biomarker analyses of organic carbon indicate that organic carbon subjected to our isotope analyses is of terrestrial origin. Therefore, the observed 2%o shift should reflect changes in the isotopic composition of the atmospheric CO₂. A unique layer composed predominantly of sand-grain sized spumellarian Radiolaria is present immediately above the δ¹³C spike both in the Oyubari and Tappu areas, suggesting an increasing availability of both nutrients and silica in surface waters.     

Middle and late Cenomanian oceanic anoxic events in shallow and deeper shelf environments of western Morocco

The response of shallow‐water sequences to oceanic anoxic event 2 and mid‐Cenomanian events 1a and 1b was investigated along the west African margin of Morocco north of Agadir (Azazoul) and correlated with the deep‐water sequence of the Tarfaya Basin (Mohammed Beach) based on biostratigraphy, mineralogy, phosphorus and stable isotopes. In the deeper Mohammed Beach section results show double peaks in δ¹³C_org for mid‐Cenomanian events 1a and 1b (Rotalipora reicheli biozone, lower CC10a biozone), the characteristic oceanic anoxic event 2 δ¹³C excursion (Rotalipora cushmani extinction, top of CC10a biozone) and laminated (anoxic) black shale. In the shallow environment north of Agadir, a fluctuating sea‐level associated with dysoxic, brackish and mesotrophic conditions prevailed during the middle to late Cenomanian, as indicated by oyster biostromes, nannofossils, planktonic and benthonic foraminiferal assemblages. Anoxic conditions characteristic of oceanic anoxic event 2 (for example, laminated black shales) did not reach into shallow‐water environments until the maximum transgression of the early Turonian. Climate conditions decoupled along the western margin of Morocco between mid‐Cenomanian event 1b and the Cenomanian–Turonian boundary, as also observed in eastern Tethys. North of Agadir alternating humid and dry seasonal conditions prevailed, whereas in the Tarfaya Basin the climate was dry and seasonal. This climatic decoupling can be attributed to variations in the Intertropical Convergence Zone and in the intensity of the north‐east trade winds in tropical areas.     

The impact of anoxia on pelagic macrofauna during the Toarcian Oceanic Anoxic Event (Early Jurassic)

Extreme environmental change during the Toarcian Oceanic Anoxic Event had widespread impacts on marine biota. This study provides new evidence, from the Yorkshire coast sections, UK, that the event was associated with periods of elevated fish and ammonite mortality. Using a synthesis of pelagic macrofaunal changes, benthic macrofaunal data and geochemical proxies we show that there are stratigraphical correlations between: (1) pelagic macrofaunal ranges and abundance, (2) benthic macrofaunal abundance, and (3) geochemical proxies that indicate deoxygenation. We identify eight stratigraphical intervals of differing character. Results suggest two major phases of relatively persistent deoxygenation with photic zone euxinia. The cyclostratigraphic timescale indicates that each phase lasted at least tens of thousands of years. Belemnite migration during the event probably resulted from increased seawater temperatures and low food supply similar to that observed for many marine taxa, including squid, within the present-day oceans.     

A Fresh Look at the Geobiology & Sedimentary Environments of the Hypersaline Great Salt Lake,USA

正1 Introduction The Great Salt Lake(GSL)is the defining hydrological feature of the Great Basin,North America’s largest desert--and it is the largest waterbody in the western USA.Despite the early(Eardley,1938)recognition of"bioherms,"algal layers,and mats covering hundreds of square km of lake area,these features have not been the     

Response and recovery of the Comanche carbonate platform surrounding multiple Cretaceous oceanic anoxic events,northern Gulf of Mexico

The ubiquity of carbonate platforms throughout the Cretaceous Period is recognized as a product of high eustatic sea-level and a distinct climatic optimum induced by rapid sea-floor spreading and elevated levels of atmospheric carbon-dioxide. Notably, a series of global oceanic anoxic events (OAEs) punctuate this time-interval and mark periods of significantly reduced free oxygen in the world's oceans. The best records of these events are often from one-dimensional shelf or basin sections where only abrupt shifts between oxygenated carbonates and anoxic shales are recorded. The Comanche Platform of central Texas provides a unique opportunity to study these events within a well-constrained stratigraphic framework in which their up-dip and down-dip sedimentologic effects can be observed and the recovery of the platform to equilibrium states can be timed and understood. Stable isotope data from whole cores in middle Hauterivian through lower Campanian mixed carbonate-siliciclastic strata are used to construct a 52-myr carbon isotope reference profile for the northern Gulf of Mexico. Correlation of this composite curve to numerous global reference profiles permits identification of several anoxic events and allows their impact on platform architecture and facies distribution to be documented. Oceanic anoxic events 1a, 1b, 1d, and 2 occurred immediately before, after, or during shale deposition in the Pine Island Member, Bexar Member, Del Rio Formation, and Eagle Ford Group, respectively. Oceanic anoxic event 3 corresponds to deposition of the Austin Chalk Group. Platform drowning on three occasions more closely coincided with globally recognized anoxic sub-events such as the Fallot, Albian-Cenomanian, and Mid-Cenomanian events. This illustrates that the specific anoxic event most affecting a given carbonate platform varied globally as a function of regional oceanographic circumstances.Using chemo- and sequence-stratigraphic observations, a four-stage model is proposed to describe the changing facies patterns, fauna, sedimentation accumulation rates, platform architectures, and relative sea-level trends of transgressive-regressive composite sequences that developed in response to global carbon-cycle perturbations. The four phases of platform evolution include the equilibrium, crisis, anoxic, and recovery stages. The equilibrium stage is characterized by progradational shelf geometries and coral-rudist phototrophic faunal assemblages. Similar phototrophic fauna typify the crisis stage; however, incipient biocalcification crises of this phase led to retrogradational shelf morphologies, transgressive facies patterns, and increased clay mineral proportions. Anoxic stages of the Comanche Platform were coincident with back-ground deposition of organic-rich shale on drowned shelves and heterotrophic fauna dominated by oysters or coccolithophorids. Eustatic peaks of this stage were of moderate amplitude (∼30 m), yet relative sea-level rises were greatly enhanced by reduced sedimentation rates. In the recovery stage, heterotrophic carbonate factories re-established at the shoreline as progradational ramp systems and sediment accumulation rates slowly increased as dysoxia diminished. Full recovery to equilibrium conditions may or may not have followed. Geochemical and stratigraphic trends present in the four stages are consistent with increased volcanism along mid-ocean ridges and in large-igneous provinces as primary drivers of Cretaceous OAEs and the resulting transgressive-regressive composite sequences.     

羌塘盆地双湖地区曲色组冷泉碳酸盐岩及其地质意义

羌塘盆地南部双湖地区曲色组地层发育大量碳酸盐岩结核,这些结核多呈丘状、椭球状、透镜状、似层状、脉状、树枝状产出。其物质组分主要为泥微晶碳酸盐矿物,少量粘土矿物、石英及草莓状、半自形黄铁矿等。草莓状黄铁矿平均粒径在5.0μm左右,内部可见葵花状构造。发育凝块状、气孔状和渗漏孔等特殊构造。产双壳类、菊石类、蠕虫状或树枝状生物及超微生物化石,生物密度极高。碳同位素明显负偏,硫同位素则明显正偏,其特征与现代海底天然气水合物的渗漏、释放所形成的冷泉碳酸盐岩机理一致,因而推测为古代海底天然气渗漏喷发形成。大量海底天然气泉口的存在,可能表明羌塘盆地双湖地区早侏罗世大洋缺氧事件与海底天然气水合物喷发存在极大关联。     

Carbon-isotope records of the Early Jurassic (Toarcian) oceanic anoxic event from the Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps) sections: palaeoceanographic and stratigraphic implications

The Toarcian oceanic anoxic event ( ca 183 Ma) coincides with a global perturbation marked by enhanced organic carbon burial and a general decrease in calcium carbonate production, probably triggered by changes in the composition of marine plankton and elevated carbon dioxide levels in the atmosphere. This study is based on high-resolution sampling of two stratigraphic successions, located in Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps), Italy, which represent expressions of the Toarcian oceanic anoxic event in deep-water pelagic sediments. These successions are characterized by the occurrence of black shales showing relatively low total organic carbon concentrations (compared with coeval strata in Northern Europe), generally < 2%, and low hydrogen indices. On this basis, they are similar to other Toarcian black shales described from the Tethyan region. The positive and negative carbon-isotope records from the two localities permit a high-resolution correlation such that ammonite biostratigraphy information from Valdorbia can be transferred to those parts of the Monte Mangart section that lack these fossils. Spectral analyses of δ¹³C_org values and of CaCO₃ percentages from the sedimentary records of both the Valdorbia and Monte Mangart sections reveal a strong cyclic pattern, best interpreted as an eccentricity signal which hence implies a duration of ca 500 kyr for the negative carbon-isotope excursion. Based on the carbon-isotope curves obtained, the high-resolution correlation between the Italian successions and a section in Yorkshire (Northern Europe) confirms the supposition that the apparent mismatch between the dating of the Toarcian oceanic anoxic event in the Boreal and Tethyan realms is an artefact of biostratigraphy.     

Multiple sulfur isotope chemostratigraphy across the Permian–Triassic boundary at Chaotian,China: Implications for a shoaling model of toxic deep-waters

Previous studies on multiple sulfur isotopes (³²S, ³³S, and ³⁴S) in sedimentary pyrite at the end-Permian suggested a shoaling of anoxic/sulfidic deep-water contributing to the extinction. This scenario is based on an assumption that the sedimentary sulfur cycle was largely controlled by benthos activity, though a stratigraphic correlation between the sulfur records and ichnofabrics of the sediments at the end-Permian has not yet been examined. We report the multiple sulfur isotopic composition of pyrite in the Permian–Triassic boundary interval at Chaotian, South China. Our data can be generally explained by a mixing of sulfur in sulfide from two different sources: one produced via sulfate reduction in an open system with respect to sulfate and the other produced in a closed system. In particular, the former with the substantially low δ³⁴S (<−40 ‰) and high ∆³³S (up to +0.100 ‰) values was likely produced via water-mass sulfate reduction or via sulfate reduction in oxic sediments with common burrows. The frequent occurrence of small pyrite framboids (mostly <5 μm in diameter) in the Lopingian (Late Permian) Dalong Formation of deep-water facies supports the enhanced water-mass sulfate reduction in an anoxic deep-water mass. The negative ∆³³S values are observed only in the oxic limestones, and no substantial ∆³³S change is observed across the extinction horizon despite of the disappearance of bioturbation. Our results are apparently inconsistent with the previous shoaling model. We expand the model and infer that, when the deep-water was sulfidic and its shoaling rate was high, a substantial amount of hydrogen sulfide (H₂S) was supplied onto the shelf via the shoaling; that resulted in the positive ∆³³S value of the bulk sediments. The observed ∆³³S variation on a global scale suggests a substantial variation in H₂S concentration and/or in upwelling rate of shoaling deep-waters during the Permian–Triassic transition.     

从有孔虫分析西藏南部白垩纪海平面升降

白垩纪是地质历史中的最大海侵时期。海平面的升降改变了海洋的物理、化学及生物因素,从而影响了有孔虫的演化与发展。根据对有孔虫丰度、分异度及演化类型的研究,认为西藏白垩纪最大海侵时期形成于赛诺曼期与土仑期的界线附近。此后海平面总的处于下降趋势,只是在康尼亚克期至三冬期海平面又有回升。马斯特里赫特末期有孔虫的大量灭绝反映了白垩纪与第三纪界线事件的影响。