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1.
A rich dinoflagellate cyst assemblage has been recovered from an outcrop of the Gurpi Formation, the hydrocarbon source rock in the South Iranian Basin. Key dinoflagellates recorded in the section studied provide a means of correlation with zonation schemes for Australasia and north-west Europe. These include Eucladinium kaikourense, Nelsoniella aceras, Odontochitina spp., Cannosphaeropsis utinensis, Palaeocystodinium denticulatum and Dinogymnium spp. The assemblage points to a late Santonian–late Maastrichtian age for the Gurpi Formation. Dinoflagellate and planktonic foraminiferal evidence indicates the presence of a hiatus spanning the uppermost Maastrichtian to at least the lowermost Danian at the base of a glaucony-rich layer separating the Gurpi Formation from the overlying Pabdeh Formation. Palynofacies and lithofacies profiles suggest that the sediments were deposited in an open, relatively deep marine outer ramp environment belonging to ramp facies 8 and 9.  相似文献   

2.
The planktic foraminifera of the Chuangde Formation (Upper Cretaceous Oceanic Red Beds, CORBs) as exposed at Tianbadong section, Kangmar, southern Tibet has been firstly studied for a detailed for a detailed biostratigraphy elaboration. A rich and well-preserved planktic foraminifera were recovered from the Chuangde Formation of the Tianbadong section and the Globotruncanita elevata, Globotruncana ventricosa, Radotruncana calcarata, Globotruncanella havanensis, Globotruncana aegyptiaca, Gansserina gansseri and Abathomphalus mayaroensis zones have been recognized. The planktic foraminiferal assemblage points to an early Campanian to Maastrichitian age for the CORBs of the eastern North Tethyan Himalayan sub-belt, which also provides a better understanding of the shifting progress of the Indian Plate to the north and the evolution of the Neotethyan ocean. The lithostratigraphy of the Chuangde Formation of the Tianbadong section comprises two lithological sequences observed in ascending succession: a lower unit (the Shale Member) mainly composed of purple (cherry-red, violet-red) shales with interbedded siltstones and siliceous rocks; and an upper unit (the Limestone Member) of variegated limestones. The strata of the Chuangde Formation in the Tianbadong section are similar to CORBs in other parts of the northern Tethyan Himalaya area of Asia (Gyangze, Sa’gya, Sangdanlin, northern Zanskar, etc.). The fossil contents of the Chuangde Formation in the sections (CORBs) studied provide a means of correlation with the zonation schemes for those of the northern Tethyan Himalayan sub-belt and the Upper Cretaceous of the southern Tethyan Himalayan sub-belt. Paleogeographic reconstruction for the Late Cretaceous indicates that the Upper Cretaceous Chuangde Formation (CORBs) and correlatable strata in northern Zanskar were representative of slope to basinal deposits, which were situated in the northern Tethyan Belt. Correlatable Cretaceous strata in Spiti and Gamba situated in the southern Tethyan Belt in contrast were deposited in shelf environments along the Tethyan Himalayan passive margin. CORBs are most likely formed by the oxidation of Fe(II)-enriched, anoxic deep ocean water near the chemocline that separated the oxic oceanic surface from the anoxic.  相似文献   

3.
Cenomanian/Turonian boundary (upper Sarvak Formation) benthic foraminiferal assemblages were analyzed to reconstruct oxygen level, primary productivity, and water turbulence in the Izeh Zone, Zagros Basin. The interplay between environmental perturbations during the Oceanic Anoxic Event 2 (OAE2) and regional tectonic activities in the Zagros Basin resulted in formation of various benthic foraminiferal assemblages in the study section. The OAE2 interval at the region of study starts with extinction of rotaliporids at the onset of δ13C positive excursion (peak “a”), which is associated with population of infaunal benthic foraminifera (especially Bolivina alata). The following interval at the onset of Whiteinella archaeocretacea Biozone is characterized by the total absence of benthic taxa and dominance of planoheterohelicids (“Heterohelix shift”) in the black shale strata, indicating expansion of oxygen minimum zone and unhospitable conditions for both benthic and planktic foraminifera. The upper part of OAE2 interval (including δ13C peaks “b” and “c”) coincides with harbinger of Neo-Tethys closure in the Arabian Plate, causing a compressional tectonic regime, and creation of uplifted terrains in the basin. The relative sea level started to locally fall in this succession, which was accompanied by a better ventilation of seafloor, lower TOC contents, and reappearance of benthic foraminifera.  相似文献   

4.
Open marine sediments deposited during the Cenomanian–Turonian transition are well exposed in the Spanish Baños de la Hedionda section (Betic Cordillera, South Iberian Palaeomargin). Analysis of foraminiferal assemblages and geochemical proxies allow inferences on the impact of the Oceanic Anoxic Event 2 (OAE2) in this area of the western Tethys. Three main intervals have been identified corresponding to different lithological units and biozones. (1) The top of the Capas Blancas Member (Rotalipora cushmani Biozone) represents the pre-extinction phase with diverse foraminiferal assemblages and well developed water-column tiering, well-oxygenated, oligotrophic deep-waters and oxygenated to poorly oxygenated, mesotrophic surface-waters. Foraminiferal opportunist species point to a minor event with dysoxic conditions preceding the OAE2. (2) The black radiolaritic shales (Whiteinella archaeocretacea Biozone) consist of a foraminiferal-barren interval, except for the lowermost centimetres where planktic surface-dweller opportunists are common. Redox sensitive elements (Cr/Al, V/Al, U/Th, MoEF, Moaut, UEF and Uaut) and increased TOC values reflect oxygen depleted conditions related to the OAE2. The increase in P/Ti values at the base of this stratigraphic interval indicates an abrupt increase in productivity. High concentrations of radiolarians are congruent with high surface productivity probably related to changes in oceanic circulation and enhanced upwelling currents, as well as subsequent shallowing of the oxygen-minimum zone. The increase in MoEF and Moaut towards the top of the black radiolaritic shales indicates temporal euxinic conditions. (3) A slow, bottom-up recovery of foraminiferal assemblages is inferred at the base of the Boquerón Member (Helvetoglobotruncana helvetica Biozone), with seafloor recolonization by benthic foraminifera being recorded previous to the water column colonization by planktic forms, mainly by intermediate-dwellers typical of mesotrophic waters. The subsequent proliferation of surface-dweller opportunists and deep-dweller opportunists adapted to mesotrophic to eutrophic conditions, and the decrease in planktic foraminiferal diversity, may indicate the persistence of poorly oxygenated conditions in the water column towards the lower-middle part of the H. helvetica Biozone.  相似文献   

5.
Planktic and benthic foraminifera including uvigerinids are documented from the Upper Bhuban Formation, exposed at Thingdawl village, Kolasib district, Mizoram. The foraminiferal assemblage is poorly preserved and consists of index fossils useful for precise biochronology and interpretation of the depositional environment. A total of ten benthic and six planktic foraminiferal species are described. Six species belong to the genus Uvigerina, four to Ammonia, one species each of Globorotalia, Globigerinoides, Clavatorella, Praeorbulina and two species of the genus Orbulina. Based on lithological and foraminiferal assemblage, middle neritic to upper part of outer neritic paleobathymetry is inferred for the deposition of this part of the Upper Bhuban Formation. The foraminiferal assemblage suggests late Early Miocene to early Middle Miocene, (∼16 Ma) equivalent to planktic foraminiferal zones N8-N9 for the Upper Bhuban Formation.  相似文献   

6.
《Cretaceous Research》2012,33(6):705-722
Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ13C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ13C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ13C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.  相似文献   

7.
Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ13C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ13C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ13C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.  相似文献   

8.
The foraminiferal fauna from two holes near the coastal section of marls and limestones at Torquay is dominated by inner to mid‐shelf benthic forms, especially the cibicidids, discorbids and miliolids. Planktonic species are rare and rarely age‐diagnostic. A cluster analysis of the species occurrences and relative abundances identified four assemblages, A to D up‐section. These assemblages also closely correspond to lithological changes characterising lithostratigraphic units: Angahook Formation (assemblage A), lower and upper Jan Juc Formation (B, C) and Puebla Clay (D). Biofacies trends based on the relative abundances of inner and outer neritic taxa led to the recognition of third‐order sequences and boundaries equivalent to TB1.1 to TB1.4, confirming a previous identification on sedimentological grounds. They demonstrate that foraminiferal assemblages were directly influenced by third‐order sea‐level fluctuations and can be used to predict third‐order sequences. The long‐uncertain regional Oligocene‐Miocene boundary is placed at the Jan Juc Formation—Puebla Clay contact, across which there was a major faunal change. Several benthic forms disappeared, at least temporarily: Cibicidoides perforatus, Amphistegina lessonii and Pararotalia mackayi. The typical Miocene planktonic taxa Globoquadrina dehiscens and Globoturborotalita brazieri made their first appearance. These events were associated with an increase in inner neritic benthos signalling a low sea‐level, consistent with the contemporaneous global pattern.  相似文献   

9.
The Jianshan section in Jianhe County,Guizhou Province,is a very important auxiliary section to the candidate stratotype at the boundary between Cambrian Series 2 and Series 3.Twenty four samples collected from the Jianshan section of the Kaili Formation were analyzed for the concentrations of trace element and rare-earth elements(REEs) by inductively coupled plasma mass spectrometry.The trace element and REE geochemical characteristics of the Kaili Formation at the Jianshan section show that trace element composition and REE distribution patterns across the section are covariant.Notably,at 44 m above the base of the Kaili Formation,most trace element contents and REEs concentrations are lower than those values observed below 44 m.Above 44 m,an increasing trend in the concentration of both trace elements and REEs is observed.These geochemical data indicate important,but subtle,changes at the 44 m horizon.If this is,in fact,verified by work in progress,geochemical criteria can provide additional supports for defining the top surface of Cambrian Series 2,especially in sections where Oryctocephalus indicus is absent.Trace element ratios,specifically Ni/Co,V/Cr,V/(Ni+V) and Th/U as well as Ce and Eu anomalies indicate that the sedimentary environment of the Kaili Formation at the Jianshan section was oxidizing,that benthic redox conditions were less oxic,and may have been influenced by freshwater as compared to the Balang area.Moreover,the Eu anomalies observed in the Kaili Formation are similar to the negative Eu anomalies observed in post-Archean sedimentary rocks,and reveal fluctuations in oxygen content from the bottom to the top of the Kaili Formation.  相似文献   

10.
Knudsen, K. L., Jiang, H., Kristensen, P., Gibbard, P. L. & Haila, H. 2011: Early Last Interglacial palaeoenvironments in the western Baltic Sea: benthic foraminiferal stable isotopes and diatom‐based sea‐surface salinity. Boreas, 10.1111/j.1502‐3885.2011.00206.x. ISSN 0300‐9483. Stable isotopes from benthic foraminifera, combined with diatom assemblage analysis and diatom‐based sea‐surface salinity reconstructions, are used for the interpretation of changes in bottom‐ and surface‐water conditions through the early Eemian at Ristinge Klint in the western Baltic Sea. Correlation of the sediments with the Eemian Stage is based on a previously published pollen analysis that indicates that they represent pollen zones E2–E5 and span ～3400 years. An initial brackish‐water phase, initiated c. 300 years after the beginning of the interglacial, is characterized by a rapid increase in sea‐surface and sea‐bottom salinity, followed by a major increase at c. 650 years, which is related to the opening of the Danish Straits to the western Baltic. The diatoms allow estimation of the maximum sea‐surface salinity in the time interval of c. 650–1250 years. After that, slightly reduced salinity is estimated for the interval of c. 1250–2600 years (with minimum values at c. 1600–2200 years). This may be related to a period of high precipitation/humidity and thus increased freshwater run‐off from land. Together with a continuous increase in the water depth, this may have contributed to the gradual development of a stratified water column after c. 1600 years. The stratification was, however, particularly pronounced between c. 2600 and 3400 years, a period with particularly high sea‐surface temperature, as well as bottom‐water salinity, and thus a maximum influence of Atlantic water masses. The freshwater run‐off from land may have been reduced as a result of particularly high summer temperatures during the climatic optimum.  相似文献   

11.
The limestone–marlstone (or limestone–calcareous shale) bedding couplets of the lower Bridge Creek Member of the Greenhorn Formation coincide with Oceanic Anoxic Event 2 and the Cenomanian–Turonian stage boundary at 93.9 Ma, and are characterized by fluctuations in microfossil and macrofossil biofacies, and organic carbon. Since G.K. Gilbert (1895), these strongly alternating lithofacies have been attributed to climate and/or productivity cycles. Heretofore, only the calcareous shale and marlstone parts of the Bridge Creek bedding couplets have been quantitatively analyzed for planktic and benthic foraminiferal assemblages. In this study, foraminiferal assemblages extracted from the hard limestone beds are comparable with the muddier lithologies thereby allowing a quantitative evaluation of the foraminiferal response to cyclically changing conditions in the U.S. Western Interior Sea (WIS) that resulted in the deposition of these lithologic couplets. The results reveal a modest cyclical response of foraminiferal assemblages extracted from limestone beds compared to adjacent calcareous shale or marlstone. These include the absence of planktic planispiral morphotypes (Globigerinelloides), increase in the proportion of planktic biserial and triserial morphotypes (Heterohelix and Guembelitria, respectively), and an increase in the proportion of benthics relative to total foraminifera (decrease in percent planktics) in the limestone beds. Such conditions suggest that the limestones may have been more productive than the adjacent shales and marlstones. Reduced surface salinity and greater stratification of the upper water column may have also contributed to the differences in assemblages preserved in the marlstones and calcareous shales. The onset of OAE 2 in the late Cenomanian is marked by an abrupt benthic oxygenation event (‘Benthonic Zone’) as Tethyan waters were drawn well north into the WIS, and cool Boreal waters spread across northwest Europe, known as the Plenus Cold Event. At this time, the WIS became an important ocean gateway for surface ocean circulation with rising sea level that helped facilitate the development and spread of OAE 2. A cyclonic (counterclockwise) gyre circulation in the WIS during deposition of the lower part of the Bridge Creek was driven by the difference between precipitation in the north and evaporation in the south. The gyre is represented by two modes, strong and weak, responsible for deposition of the limestone and marlstone, respectively. For the middle and upper parts of the studied section representing the plateau of OAE 2 and subsequent peak transgression of the WIS, the counterclockwise gyre was driven less by E-P gradient but by the amount of surface runoff from both margins of the WIS with deposition of limestone beds during the wetter (strong) phase and marlstones during the drier (weak) phase. Highest levels of TOC redevelop after OAE 2 in the early Turonian with the incursion or development of an oxygen minimum zone at the time of peak transgression.  相似文献   

12.
Intertrappean beds exposed between upper and lower traps of the Government and Sunnamrayalu quarries of Rajahmundry were analyzed based on benthic and planktic foraminifera, ostracodes and algae observed in thin sections. Planktic foraminifera indicate deposition occurred in the early Danian Parvularugoglobigerina eugubina (P1a) zone shortly after deposition of the lower trap flows. The most diverse planktic assemblages were deposited in limestones of the middle intertrappean interval and indicate an upper P1a age, or subzone P1a(2), as marked by the co-occurrence of P. eugubina, Globoconusa daubjergensis, Parasubbotina pseudobulloides and Subbotina triloculinoides. Reworked late Maastrichtian planktic foraminifera are common in a limestone interval and suggest erosion of uplifted Cretaceous sediments. Benthic foraminiferal assemblages indicate deposition occurred predominantly in shallow inner shelf to brackish environments. Similarly, ostracodes indicate variable environments ranging from inner neritic to brackish with fresh water influx, as also indicated by the presence of fresh water algae. These data confirm an overall deepening from restricted shallow marine to estuarine, lagoonal and finally open marine conditions followed by abrupt emersion and paleosoil deposition prior to the arrival of the upper trap flows at or near the base of C29n.  相似文献   

13.
New carbon (δ13C) isotope records calibrated by planktonic bioevents provide general support for a late Campanian age assignment of the Shiranish Formation (Fm.) and its boundaries in the Dokan section (NE Iraq). The Shiranish Fm. is characterised at the base by a mid-Campanian unconformity as can be interpreted by absences of nannofossil zones CC20-21. The Shiranish Fm. then spans nannofossil biozones CC22-CC23a (UC15d-eTP to UC16aTP). Results obtained on carbon isotopes suggest that diagenesis affected and compromised a few carbonate samples in the uppermost 50 m of the section. However, once these samples are discarded, pristine trends suggest that the top of the section records a negative carbon isotope excursion that is interpreted as CMBa-c events that straddle the Campanian–Maastrichtian boundary. This interpretation is supported by the lowermost occurrence of planktic foraminifers Rugoglobigerina scotti and Contusotruncana contusa some 30 m above the base of the negative excursion and 10 m below a positive excursion identified as the Maastrichtian M1+ event. Discrepancies in the stratigraphic range of several planktic foraminifer bioevents are highlighted and advocate for the need of many more integrated records of planktic foraminifer and nannofossil biostratigraphy alongside carbon isotope stratigraphy in the eastern Tethys in order to improve regional and global schemes.  相似文献   

14.
The Early Maastrichtian (Late Cretaceous) Qahlah Formation is the oldest stratigraphical unit lying upon the eroded top of the obducted Semail Ophiolite, of Santonian–Campanian age, in the Northern Oman Mountains. It crops out as a series of low hills on the western flank of the mountains, which are located along the boundary between the United Arab Emirates and the Sultanate of Oman. This paper presents the results of an investigation into the bio- and lithofacies of the Qahlah Formation in the Jabal Huwayyah section (at the boundary between the Al Ain area of the United Arab Emirates and the Buraimi area of Oman). The data collected were used to determine the age and depositional environment of the rock unit studied. In addition, the section is lithostratigraphically correlated with other sections in the study area. Microfossils from Jabal Huwayyah are represented mainly by the larger benthic foraminifera Loftusia elongata, L. morgani, Pseudorbitolina marthae, Orbitoides media, Omphalocyclus macroporus and Lepidorbitoides minor. These foraminifera are reported for the first time from this section. The diversity and abundance of fauna, particularly within the calcareous siltstone bed near the top of the formation (unique to the Huwayyah area), reflect a shallow marine depositional environment.  相似文献   

15.
In 2005, Hurricane Katrina produced a distinct, acute ecological disturbance of the benthic invertebrate community of Lake Pontchartrain, LA, USA. The bivalve Rangia cuneata and other community dominants were lost from 50% (815 km2) of the lake bottom. The storm surge directly killed benthic organisms and produced salinity stratification that caused episodes of detrimental low dissolved oxygen concentration at depths >3.7 m. Past disturbance of the bottom by shell dredging and intrusion of higher salinity bottom water through deep shipping channels appears to have contributed to the severity of this impact. Colonization by tolerant opportunistic taxa occurred, but low rainfall after Katrina has slowed the recovery of the typical rangia community. A decrease in water transparency and an increase in turbidity and chlorophyll a were associated with the loss of clam biomass. Other hurricanes may have produced less obvious but similar effects on smaller spatial and temporal scales.  相似文献   

16.
The North West Shelf is an ocean‐facing carbonate ramp that lies in a warm‐water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long‐period swells and large internal tides, resulting in preferentially accumulating coarse‐grained sediments. Circulation is dominated by the south‐flowing, low‐salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward‐flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea‐level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid‐ramp. The most conspicuous stranded particles are ooids and peloids, which 14C dating shows formed at 15·4–12·7 Ka, in somewhat saline waters during initial stages of post‐Last Glacial Maximum (LGM) sea‐level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner‐ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine‐derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid‐ramp sediment is heterozoan in character, but sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene outer‐ramp sediment is mainly pelagic, veneering shallow‐water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at ≈ 200 m water depth suggest periodic upwelling or Fe‐redox pumping, whereas enhanced near‐surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at ≈ 140 m water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner‐ramp sediments can contain important heterozoan elements, mid‐ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid‐ramp sedimentation mimicking maximum flooding intervals, and outer‐ramp planktic productivity can generate locally important fine‐grained carbonate sediment bodies. Changing oceanography during sea‐level rise can profoundly affect sediment composition, sedimentation rate and packaging.  相似文献   

17.
在野外露头和镜下薄片观察分析的基础上,对重庆东部地区下寒武统龙王庙组83个碳酸盐岩样品的碳、氧同位素进行测试,并分析了其所保留的海水原始信息的有效性。结果表明,扣除无效数据后,重庆地区龙王庙组δ13C值分布在-4.300‰~2.694‰之间,平均值为-0.031‰;δ18O值分布在-9.880‰~-0.100‰之间,平均值为-7.396‰;纵向上,碳同位素值整体呈现先降低后升高的趋势,底部、中上部及顶部为正值且变化幅度小;中下部整体为负值且波动幅度大,正、负漂移事件频发。古环境恢复结果显示,龙王庙期重庆东部地区整体处于海相沉积环境,海水盐度在龙王庙组早期较低,晚期较高;海水温度主要介于20~30,℃之间,属温暖或炎热的亚热带气候;龙王庙期共经历3期海退—海侵作用,海平面上升、海洋生产力增加、有机碳快速埋藏使得海洋中13C含量升高,反之则使其降低。龙王庙组沉积中期与末期,δ13C值负偏,水体较浅,盐度较高,是白云岩发育的最有利阶段。  相似文献   

18.
The present study encompasses a detailed investigation of downcore Holocene foraminifera and their assemblages to comprehend the ecology of the Bay of Bengal and compare it with that of the South China and Sulu Seas, at the same depth for all the three water bodies. Based on temperature and dissolved oxygen profiles, benthic foraminiferal abundance, and species diversity values, it is inferred that the Bay of Bengal is much better ventilated than either the South China or the Sulu Sea. The planktic/benthic (P/B) ratios are extremely low when compared with those reported elsewhere in the world. The absolute dominance of benthic foraminiferal species over their planktic counterparts is attributed to the effect of fragmentation and dissolution of the latter, as they are relatively more susceptible to this process. The very low P/B values are also indicative of water depth below the lysocline in this part of the Bay of Bengal.  相似文献   

19.
 The interval spanning the Paleocene–Eocene (P/E) transition in the Possagno section consists of 1 m of red marls, including a 4-cm-thick, dark-red "dissolution" clay, which represents the Paleocene/Eocene boundary event. The Possagno section is much more condensed than other Tethyan and North Atlantic sections previously studied; however, in this section the most significant biotic, isotopic and sedimentological events across the P/E boundary can be recognized. The Possagno section spans the following planktic foraminiferal subzones: upper part of M. gracilis Subzone, A. berggreni Subzone, A. sibaiyaensis Subzone and probably lowermost part of P. wilcoxensis Subzone. The quantitative analysis indicates a major increase of low-latitude acarininids, including compressed tropical acarininids just above the boundary clay. This acarininid incursion begins just below the boundary clay but reaches its maximum just above the clay. The planktic foraminiferal faunal turnover is gradual except for the acarininid incursion. The isotopic results show a negative excursion in ∂13C values at the small benthic foraminifera mass extinction event. The acarininid maximum diversity coincides with this isotopic excursion, and reflects an increase in surface seawater temperature. Despite being very condensed, the Possagno section allows us to further confirm that the different biotic, isotopic and sedimentological events recognized in the Spanish sections (Alamedilla, Campo, Caravaca, Zumaya) are not local in nature and allows the establishment of a detailed chronostratigraphic framework to define the P/E boundary stratotype. Received: 8 April 1998 / Accepted: 12 April 1999  相似文献   

20.
The distribution of archaeal lipids, including archaeol and glycerol dibiphytanyl glycerol tetraethers (GDGTs), in dolomite concretions and surrounding sediment from the Monterey Formation (Miocene) and the Sisquoc Formation (Miocene-Pliocene) were examined to distinguish planktic from benthic contributions. For this purpose, dolomites with positive δ13C values (+7‰ to +13‰) were chosen; such highly positive values point to pronounced methanogenesis of benthic archaea in the sedimentary column. At first glance, distributions and relative abundances of GDGTs in both dolomites and background sediment were similar, resembling patterns of marine planktic crenarchaea. A contribution of benthic euryarchaea to the GDGT pool became evident only from variations in the δ13C values of different biphytanes obtained after ether cleavage of GDGTs. Whereas bi- and tricyclic biphytanes had an isotopic signal typical of planktic archaea (δ13C −23.6‰ to −20.5‰ and −23.4‰ to −21.2‰, respectively) for both dolomite and background sediment, acyclic and monocyclic biphytanes showed lower values for dolomite samples (−25.1‰ to −22.6‰ and −27.6‰ to −24.7‰, respectively), indicating a contribution of lipids from benthic archaea. The isoprenoid diether archaeol (δ13C −23.9‰ to −22.9‰), assigned to euryarchaea, was only detected in dolomite samples, also reflecting additional input from sedimentary archaea, probably autotrophic methanogens. The occurrence of lipids derived from methanogenic archaea agrees with the strong 13C-enrichment of dolomites and with mineral formation taking place in the zone of archaeal methanogenesis. This implies that the lipid biomarker inventory of sedimentary strata needs to be interpreted carefully, as it is often not straightforward to discriminate between input from the water column and sedimentary microbial activity.  相似文献   

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