The sedimentary expression of oceanic anoxic event 1b in the North Atlantic

The Aptian/Albian oceanic anoxic event 1b contains the record of several perturbations in the global carbon cycle and multiple black shale levels, particularly in the Western Tethys. The local lithological expression of an oceanic anoxic event depends on palaeogeographical and depositional setting as well as on regional palaeoclimate. Marine sediments at a particular location may therefore be more or less organic‐rich (or not at all) and they may consist of different lithologies. In most studies, however, much of the lithological variability associated with oceanic anoxic events is left unaccounted for and, thus, the exact processes leading to the enrichment of organic matter in these marine sediments and their subsequent preservation in the geological record are unknown. This study focuses on the local sedimentary processes behind the deposition of organic‐rich sediments at Deep Sea Drilling Project Site 545 and Ocean Drilling Program Sites 1049 and 1276 in the North Atlantic during oceanic anoxic event 1b. Although specifically dealing with the sediments deposited during this particular event at these localities, it is expected that the same processes were responsible for determining the exact sedimentary products at localities in similar settings, as well as during other similar events in the Mesozoic. Here, it is shown that the deposition of organic‐rich sediments during oceanic anoxic event 1b was a consequence of the enhanced productivity favoured by upwelling and by riverine nutrient input, or aeolian fertilization of the euphotic zone depending on geographical location. Slope instability processes resulted in the transfer of part of these organic‐rich sediments from the shelf to deep sea depocentres as mud‐laden organic‐rich turbidites, especially in the northern North Atlantic. The so‐called ‘black shales’ are much more varied than their name implies. The end product of sedimentation during an oceanic anoxic event at a particular location is commonly the result of several equifinal processes acting on a local scale rather than the direct result of basinal or even global mechanisms.     

古海洋生产力指标研究进展

寻求反映古海洋生产力变化的指标是古海洋学研究的重要内容。生物沉积物、营养元素、微量元素和同位素、古生物资料等与古海洋生产力具有一定的关系,常用于古海洋生产力的重建工作:(1)生物沉积物主要包括有机碳、生物碳酸盐和生物硅质沉积等,其埋藏速率在很大程度上受到古海洋生产力的控制,常常用作古海洋生产力的替代性指标;(2)在地质时间尺度上,古海洋生产力主要受到大洋中营养元素可利用程度的控制,因此营养元素的相关记录可以提供古海洋生产力状况的重要信息;(3)海洋中某些微量元素的地球化学行为或者与有机质的改造有关,或者受到氧化还原条件变化的控制,或者受到某种浮游生物的生长的影响,从而能够直接或者间接地指示古海洋生产力的变化;(4)海相碳酸盐和有机质的碳同位素记录,海相有机物的氮同位素比值也用以再造古生产率等古海洋学参数;(5)可以利用对微体化石丰度值及其组合反映古生产力的变化。     

白垩纪中期海相富有机碳沉积的地球生物学背景

白垩纪中期(约125～88Ma)海相富有机碳沉积广泛发育于全球各主要洋盆和沉降区,是许多大油田的重要烃源岩。文中从地球生物学的角度探讨了当时海相富有机碳沉积的地质背景和主控因素,认为在白垩纪中期全球事件频发的特殊背景下,保存条件(大洋缺氧)和生产力的提高共同控制了有机质的富集。大洋缺氧不但加快了海洋生物的灭绝和更替,使有机质大量堆积,而且促进了营养元素(P、N、Fe等)的再生和厌氧自养型海洋微生物(如蓝细菌、绿硫细菌和古菌类等)的极度繁盛,提高了生产力水平;而高的生产力又反过来加剧了海洋的缺氧。新的研究成果(Os同位素证据)表明,白垩纪大规模海底火山作用引发了持续的温室效应和海水循环的静滞、紊乱以及弱上升流发育,进而导致生产力的提高和大洋缺氧事件(OAE)的发生,最终造成有机质的广泛堆积和保存。     

Nature and geochemistry of surface sediments of the mangrove environment along the Egyptian Red Sea coast

Grain size and geochemical analyses have been carried out on 58 samples collected from different areas of mangrove environment of the Egyptian Red Sea coast. The texture characteristics of mangrove sediments show that the mean size decreases in sediments of coastal areas while increase in island sediments and the sediment type generally changes from sand to slightly gravelly muddy sand. The sediments are composed mainly of poorly sorted, negatively nearly symmetrical and leptokurtic fine sands. Cluster analysis showed that, distribution of gravel, sand and mud fractions is related to bottom facies and type of sediment source. Generally, sand fraction is the main category among the three constituents. Geochemically the factor controlling the carbonate content of studied sediments includes material supply of biogenic and terrigenous components. Therefore, sediments of Hamata area, km 17 south Safaga, Wadi El-Gemal Island and Abu-Minqar Island are terrigenous sediments, while sediments of km 37 north Quseir are highly carbonates. In general organic matter is higher in the mangrove sediments relative to adjacent areas. The high organic matter in the mangrove sediments is primarily due to the high supply from primary productivity, terrestrial and reworked sediments. Texture is the main controlling factor for the organic matter enrichment. Spatial variations in the phosphorus content in the different studied localities are related to the sources of phosphorus to the area. In comparison, cluster analysis indicates that phosphorus might be transported from terrestrial source to the sea through wadis draining the excavated Upper Cretaceous phosphate rocks. Also, the abundance of phosphorus content may be attributed to phosphatization of calcareous skeletons. Our observations provide knowledge on the physico-chemical effects and will be useful in the management and suitable development of the areas under study. In addition it represent database in the future.     

Phosphorus distribution in sediments of the Delaware River estuary

In order to determine the primary factors related to the accumulation of phosphorus in estuarine sediments, a study of phosphorus fractions in sediments of the Delaware River Estuary was undertaken. A correlation matrix between the phosphorus fractions, determined by serial extraction, and 14 sediment variables was computed. Total phosphorus and total inorganic phosphorus in the sediment-phosphorus reservoir decreases with increasing salinity. This variation is correlated with decreasing iron oxyhydroxide content in the sediment. Neither clay content nor calcium carbonate content appear to be significantly correlated with variation in total inorganic phosphorus content in the fine-grained sediments of this estuary. Although calcium phosphate is concluded to be a major constituent of the sediment-phosphorus reservoir, there was no evidence found that it is authigenic in this environment.     

The Paragenesis of Organic Matter,Phosphorus, and Uranium in Marine Sediments

Using some uranium deposits and recent U-bearing sediments as examples, it is shown that all U-bearing rocks are characterized by an association of organic matter and calcium phosphate, irrespective of the quantitative relationship between these components. A considerable proportion of these components was delivered into sediments with remains of marine planktonic and nektonic organisms. Along with organic matter, calcium phosphate played a significant role in uranium concentration. This is related to a high sorption ability of the calcium phosphate. Uranium accumulated during diagenesis as a result of diffusion exchange between bottom and interstitial waters. The combination of anoxic bottom environment with high bioproductivity in upper aerated waters, a typical phenomenon in oceanic upwelling zones, is the most favorable factor of uranium concentration in the sedimentary process. This determines the stable paragenetic association of organic matter, phosphorus, and uranium in marine sediments, such as black shales and organogenic phosphate deposits.     

海平面变化在湖南西部桑植地区栖霞组富有机碳沉积物形成中的作用

湖南西部桑植地区中二叠统栖霞组地层旋回性明显,有机质也呈周期性变化.研究其有机质聚集堆积控制因素将有助于理解海平面变化在富有机碳沉积物形成过程中的作用.选择其中一个旋回作为研究目的层段,通过黄铁矿形态以及地球化学参数有机碳TOC、硫同位素、DOP以及微量元素的研究发现,初级生产力参数Ba、Ni、Cu和Zn的变化与TOC含量的变化一致,有机质聚集堆积主要受海洋表层初级生产力的控制,底部水体氧化还原条件与初级生产力有关.有机质的堆积最终归因于高频相对海平面变化,海平面快速上升带来丰富的营养物质,提高海洋表层生物生产力,海底有机质的分解消耗大量氧气,氧需求量的增加形成底部水体贫氧-厌氧环境.      

Kerogen: chemical structure and formation conditions

The available data on the composition of the pyrolysis products of kerogen from the Mesozoic carbonaceous strata of the Russian Plate evidence that changes in the contents of total organic carbon (TOC) lead to a regular change of the mechanisms of organic-matter (OM) conservation in sediments. Each mechanism prevails for particular TOC contents. The initial increase in the TOC content of rocks is accounted for by the fact that the higher is the biologic productivity of the basin, the higher is the portion of nonmineralized organic matter. This is due mainly to the mechanism of selective accumulation of the most stable biochemical components such as algaenan. The appearance of H₂S first in the pore waters of sediment and then in the water column increases the degree of preservation of initial OM at the expense of its sulfurization. This process runs first in the lipid and then in the carbohydrate fractions of initial OM.     

泰国湾百年来有机碳埋藏记录及环境响应

受热带季风气候和周边陆源输入的影响,低纬泰国湾海—陆相互作用强烈,是研究海洋沉积有机碳与陆源输入、海洋初级生产力等气候环境变化响应关系的理想区域.通过对泰国湾泥质区T43柱样中总有机碳(TOC)、总氮(TN)、稳定碳同位素(δ13C)以及粒度等指标的分析,基于210Pb建立的高分辨年代地层框架,重建了该区百年来有机碳的...     

Phosphate-rich sedimentary rocks: Significance for organic facies and petroleum exploration

Phosphorus-bearing rocks and sediments can be divided into two genetically distinct classes: phosphatic shales or limestones and phosphorites. Phosphatic shales are primary sediments in which phosphate nodules or micronodules have formed diagenetically by precipitation of calcium phosphates derived mainly from organic phosphorus. The nodules form in reducing environments at shallow depths within the sediments, where loss of phosphate by diffusion to the overlying water column is minimized. Highly biogenic sediments containing large amounts of organic matter and some fine clastic debris provide ideal environments for the formation of phosphate nodules.Phosphorites, in contrast, represent concentrated accumulations of reworked phosphate nodules which originated in phosphatic shales or limestones. Currents, wave action, recrystallization, and erosion and resedimentation are important mechanisms in the concentration process.Phosphatic shales and limestones may become excellent oil source rocks if thermal maturity is achieved. They are useful facies indicators for anoxic or nearly anoxic depositional environments, and are often associated with restricted basins, or, during certain geologic periods, with broad shelves developed during transgressions. Phosphorites, in contrast, are often correlated with sea-level regressions or uplifts. They are modest source rocks because of their low organic carbon contents and the fact that they were reworked under oxidizing conditions. Nevertheless, because phosphorites are derived from, and often grade into, phosphatic shales, they also are of potential utility in the search for oil source beds.     

A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene

A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., C_org:S ratio, C_org:P_org ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters.The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N₂ production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent.     

Spatial and temporal variations in terrestrially-derived organic matter from sediments of the Delaware estuary

Terrestrially-derived organic matter in sediments of the Delaware Estuary originates from riverine transport of soils and fresh litter, sewage and industrial wastes, and marsh export of organic matter. The quantity, composition, and spatial distribution of terrigenous organic matter in sediments was determined by elemental (C and N), lignin, and stable carbon isotope analyses. Sediments in the upper Delaware Estuary had low organic carbon content and high lignin content. In contrast, sediments in the lower Delaware Estuary had high organic carbon content and low lignin content. There was a slight decrease in the proportion of syringyl and cinnamyl phenols relative to vanillyl phenols between the upper estuary and lower estuary. Differences in lignin and stable carbon isotope compositions between sediments of the Delaware Estuary and sediments of the Broadkill River estuary (an adjoining salt-marsh estuary) supported previous observations that marshes do not export substantial quantities of organic matter to estuaries. Additional results suggested that lignin-rich sediments were concentrated in the upper estuary, most likely in the zone of high turbidity. Furthermore, algal material diluted lignin-rich sediments, particularly in the lower estuary. The weaker algal signal in bottom sediments compared to that in suspended particulate matter suggested algal material was decomposed either in the water column or at the sediment-water interface. Physical sorting of sediments prior to deposition was also indicated by observations of compositional differences between the upper and lower estuary bottom sediments. Finally, seasonal variations in primary productivity strongly influenced the relative abundance of terrestrial organic matter. In fall, however, the proportion of lignin was greatest because of a combination of greater inputs of terrestrially-derived organic matter, lower river discharge, and a decrease in algal biomass.     

铁元素对海相沉积物早期成岩作用的影响

铁元素是地壳中丰度最高的元素之一,在海相沉积物成岩过程中起着非常重要的作用.铁元素作为海洋初级生产力的微营养元素,影响海相沉积物中有机质的输入,其在早期成岩阶段与硫化物和磷元素的耦合关系,可以促进或者降低有机质的保存;铁元素在海相沉积物早期成岩过程中可以改变孔隙水的化学性质,影响亚稳定碳酸盐矿物的保存,导致碳酸盐沉积物...     

In-situ analyses of phosphorus contents of carbonate minerals: Reconstruction of phosphorus contents of seawater from the Ediacaran to early Cambrian

The Ediacaran and Cambrian periods were one of the most important periods for the evolution of life. The biodiversity drastically expanded in the period. However, the origins are still ambiguous because surface environmental changes through the time have not been well understood yet. We conducted in-situ analyses of the phosphorus contents of carbonate minerals with a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) to estimate the phosphorus contents of seawater from the Ediacaran to the early Cambrian. Carbonate rocks contain not only the carbonate minerals but also detrital and authigenic materials such as silicate and phosphate minerals. Therefore, the whole rock compositions of carbonate rocks are not directly related with seawater composition. To avoid the influence of the involvement of the phosphate minerals, we performed the microscopic observation, elemental mapping of Si, Ca, Mg, Fe, and P contents with an electron probe microanalyzer (EPMA), and investigation of time profiles of signal intensities of Ca, Sr, Mn, P, La and Ba with the LA-ICP-MS. Especially, samples with low Mn/Sr ratios and primary textures such as oolites are suitable to estimate the primary phosphorus contents of the carbonates.The chemostratigraphy of the phosphorus contents of carbonates from the drill core and outcrop samples displays that the phosphorus contents decrease from ca. 400 ppm in the Ediacaran through ca. 200 ppm around the terminal Ediacaran and the beginning of the Cambrian to ca. 50 ppm in the early Cambrian. Previous works on ⁸⁷Sr/⁸⁶Sr chemostratigraphy from the Ediacaran to the Cambrian sections suggested relatively high continental influx in the middle Ediacaran, and around the Precambrian–Cambrian (PC/C) boundary. The high phosphorus content in the Ediacaran was possibly due to the high continental flux. On the other hand, previous works on chemostratigraphy of carbon isotope values of carbonate carbon from the Ediacaran to the Cambrian sections showed some large negative anomalies in the Ediacaran and around the Precambrian–Cambrian (PC/C) boundary, and suggested that the negative anomalies were caused by remineralization and respiration of dissolved organic matter. The degradation of the organic matter also accounts for the high phosphorus contents in the Ediacaran. The high phosphorus content of seawater favors enhancement of primary productivity and formation of phosphorites. The high phosphorus contents in the seawater possibly led to the emergence of the large, and motile organism through the enhancement of primary productivity and the consequent increase of oxygen content of the seawater.     

Paläoklima und paläoozeanische Verhältnisse im SW-Pazifik während der letzten 6 Millionen Jahre (DSDP - Site 594, Chatham Rücken, östlich Neuseeland)

Detailed sedimentological investigations were performed on sediments from DSDP-Site 594 (Chatham Rise, east of New Zealand) in order to reconstruct the evolution of paleoclimate and paleoceanographic conditions in the Southwest Pacific during the last 6 million years. The results can be summarized as follows:

1. High accumulation rates of biogenic opal and carbonate and the dominance of smectites in the clay fraction suggest increased oceanic productivity and an equable dominantly humid climate during the late Miocene.
2. During Pliocene times, decreasing contents of smectites and increasing feldspar/quartz ratios point to an aridification in the source area of the terrigenous sediments, culmunating near 2.5 Ma. At that time, accumulation rates of terrigenous components distinctly increased probably caused by increased sediment supply due to intensified atmospheric and oceanic circulation, lowered sea level, and decreased vegetation cover.
3. A hiatus (1.45 to 0.73 Ma) suggests intensified intermediate-water circulation.
4. Major glacial/interglacial cycles characterize the upper 0.73 Ma. During glacial times, oceanic productivity and terrigenous sediment supply was distinctly increased because of intensified atmospheric and oceanic circulations and lowered sea level, whereas during interglacials productivity and terrigenous sediment supply were reduced.
5. An increased content of amphibols in the sediments of Site 594 indicates increased volcanic activities during the last 4.25 Ma.

     

河西猪野泽沉积物有机地化指标之间的关系及古环境意义

探讨了河西走廊石羊河终端湖泊--猪野泽晚冰期以来沉积物总有机碳(TOC)、碳氮比(C/N)、有机碳同位素(δ13C)三种有机地球化学指标之间的关系及古环境意义.结果表明:猪野泽QTH01剖面沉积物TOC指标主要代表湖泊及流域的植被初级生产力;C/N值受到周围区域表土C/N值和沉积物成岩过程的双重影响,总体较低,并不能直...     

南四湖及主要入湖河流沉积物中磷的赋存形态研究

应用沉积物磷形态SMT提取方法,分析了南四湖及主要入湖河流丰水季节表层沉积物、独山湖及微山湖区沉积岩芯中磷的含量及形态组成变化,研究了磷的埋藏演化过程与人为污染特征.结果表明,老运河表层沉积物中磷的含量最高,为13068.3 mg/kg,以OP及NaOH-P为主,其富集系数分别为6.5和9.0,主要是与来自济宁市的废水污染有关.其余表层沉积物以HCl-P为主;受人为污染的影响,NaOH-P、OP产生一定程度的富集,其富集系数分别为1.0.3.7、1.3～6.0;部分沉积物中高含量的有机质加重了OP的富集.独山湖及微山湖沉积岩芯中以HCl-P和OP为主;20世纪60年代末期以来,随着湖泊生产力的提高及早期成岩过程中有机质的矿化分解,独山湖及微山湖沉积岩芯中TOC含量自下而上呈指数增加,这也导致了沉积物中OP的增加;独山湖及微山湖沉积岩芯6～0 cm和5～0 cm较高的OP及NaOH-P含量,指示了20世纪80年代中期以来磷的人为污染过程.     

有机质供给和保存在烃源岩形成中的控制作用

烃源岩形成研究一直集中于海相盆地的“生产力模式”和“保存模式”之争, 湖相盆地研究较少.利用二连盆地烃源岩有机碳、碳酸盐碳同位素、热解参数、还原硫等地球化学分析数据, 表征了湖盆古生产力和陆源有机质输入, 研究了有机质供给和保存在烃源岩形成中的控制作用, 发现不同的湖盆中有机质供给和保存对烃源岩形成的控制作用明显不同.有机质供给量大的湖盆原始有机质含量高, 烃源岩形成主要受有机质供给控制, 受有机质保存条件影响小, 即使在含氧的水底也会有部分有机质来不及氧化而富集形成烃源岩; 有机质供给量小的湖盆原始有机质含量低, 氧化环境中大部分有机质被降解难以形成烃源岩, 只有还原环境下有机质才能得到较好的保存, 有机质保存是控制烃源岩形成的主要因素.      

Phosphorus Cycle in the Ocean

The behavior of phosphorus is considered at major stages of the sedimentary cycle beginning with supply sources for its delivery into the ocean to precipitation and its sedimentation, localization and subsequent diagenetic redistribution in bottom sediments. River runoff represents the main phosphorus source in the ocean. It delivers annually about 1.5 Mt of dissolved phosphorus and more than 20 Mt of suspended phosphorus into the ocean. Up to 80% of the suspended phosphorus incorporated in the lithogenic material precipitates within submarine continental margins. Phosphorus dissolved in seawater repeatedly participates in biogeochemical processes owing to its assimilation by phytoplankton that annually consumes from 1.5 to 2.5 Gt of phosphorus. Dissolved phosphorus is incorporated in organic remains and precipitates from seawater by a biogenic mechanism, too. Only a part of phosphorus settled onto the bottom is buried in sediments. Due to reducing diagenetic processes, up to 30–40% of the primarily precipitated phosphorus diffuses from the upper layer of sediments into bottom water. Diffusion flux into the ocean significantly exceeds the supply of dissolved phosphorus from river runoff. The absolute mass phosphorus dispersed in sediments is several orders of magnitude greater than the mass concentrated in phosphorite deposits. However, the majority of phosphorite formation epochs coincide with the intensification of total phosphorus accumulation in marine sediments in conditions of humid climate, intense chemical weathering of rocks on continents, and considerable expansion of the oceanic shelf area.