Biostratigraphy and paleoecology of the Oligo-Miocene succession in Fars and Khuzestan areas (Zagros Basin,SW Iran)

Paleontological and biostratigraphical studies on carbonate platform succession from southwest Iran documented a great diversity of shallow-water benthic foraminifera during the Oligocene–Miocene. Larger foraminifera are the main means for the stratigraphic zonation of carbonate sediments. The distributions of larger benthic foraminifera in two outcrop sections (Abolhayat and Lali) in the Zagros Basin, Iran, are used to determine the age of the Asmari Formation. Four assemblage zones have been recognized by distribution of the larger benthic foraminifera in the study areas. Assemblage 3 (Aquitanian age) and 4 (Burdigalian age) have not been recognized in the Abolhayat section (Fars area), due to sea-level fall. The end Chattian sea-level fall restricted marine deposition in the Abolhayat section and Asmari Formation replaced laterally by the Gachsaran Formation. This suggests that the Miocene part of the formation as recognized in the Lali section (Khuzestan area) of the Zagros foreland basin is not present in the Abolhayat outcrop. The distribution of the Oligocene larger benthic foraminifera indicates that shallow marine carbonate sediments of the Asmari Formation at the study areas have been deposited in the photic zone of tropical to subtropical oceans. Based on analysis of larger benthic foraminiferal assemblages and microfacies features, three major depositional environments are identified. These include inner shelf, middle shelf and outer shelf. The inner shelf facies is characterized by wackestone–packstone, dominated by various taxa of imperforate foraminifera. The middle shelf is represented by packstone–grainstone to floatstone with a diverse assemblage of larger foraminifera with perforate wall. Basinwards is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and flat nummulitidae and lepidocyclinidae. Planktonic foraminifera wackestone is the dominant facies in the outer shelf.     

Compound‐specific isotope analysis of benthic foraminifer amino acids suggests microhabitat variability in rocky‐shore environments

The abundance and biomass of benthic foraminifera are high in intertidal rocky‐shore habitats. However, the availability of food to support their high biomass has been poorly studied in these habitats compared to those at seafloor covered by sediments. Previous field and laboratory observations have suggested that there is diversity in the food preferences and modes of life among rocky‐shore benthic foraminifera. In this study, we used the stable nitrogen isotopic composition of amino acids to estimate the trophic position, trophic niche, and feeding strategy of individual foraminifera species. We also characterized the configuration and structure of the endobiotic microalgae in foraminifera using transmission electron microscopy, and we identified the origin of endobionts based on nucleotide sequences. Our results demonstrated a large variation in the trophic positions of different foraminifera from the same habitat, a reflection of endobiotic features and the different modes of life and food preferences of the foraminifera. Foraminifera did not rely solely on exogenous food sources. Some species effectively used organic matter derived from endobionts in the cell cytoplasm. The high biomass and species density of benthic foraminifera found in intertidal rocky‐shore habitats are thus probably maintained by the use of multiple nitrogen resources and by microhabitat segregation among species as a consequence.     

Diversity and geographic distribution of benthic foraminifera: a molecular perspective

The diversity and distribution of modern benthic foraminifera has been extensively studied in order to aid the paleoecological interpretation of their fossil record. Traditionally, foraminiferal species are identified based on morphological characters of their organic, agglutinated or calcareous tests. Recently, however, new molecular techniques based on analysis of DNA sequences have been introduced to study the genetic variation in foraminifera. Although the number of species for which DNA sequence data exist is still very limited, it appears that morphology-based studies largely underestimated foraminiferal diversity. Here, we present two examples of the use of DNA sequences to examine the diversity of benthic foraminifera. The first case deals with molecular and morphological variations in the well-known and common calcareous genus Ammonia. The second case presents molecular diversity in the poorly documented group of monothalamous (single-chambered) foraminifera. Both examples perfectly illustrate high cryptic diversity revealed in almost all molecular studies. Molecular results also confirm that the majority of foraminiferal species have a restricted geographic distribution and that globally distributed species are rare. This is in opposition to the theory that biogeography has no impact on the diversity of small-sized eukaryotes. At least in the case of foraminifera, size does not seem to have a main impact on dispersal capacities. However, the factors responsible for the dispersal of foraminiferal species and the extension of their geographic ranges remain largely unknown. Special Issue: Protist diversity and geographic distribution. Guest editor: W. Foissner.     

Discovery of agglutinated benthic foraminifera in Devonian black shales and their relevance for the redox state of ancient seas

Agglutinated foraminifera are benthic organisms that occur in marginal marine to bathyal environments. Though some taxa can live in oxygen deficient environments, they require at least some oxygen in order to persist at the seafloor. The discovery that they occur widely in Late Devonian black shales has a bearing on the boundary conditions required for episodes of extensive carbon sequestration in marine sediments and their connection to atmospheric composition and global climate. Devonian black shales of the eastern US have been studied extensively to determine the fundamental controls on carbon burial, and a range of mechanisms has been proposed. Finding agglutinated benthic foraminifera in these black shales refocuses the debate about their origin and points to limitations of earlier models.     

Ocean basin and depth variability of oxygen isotopes in cenozoic benthic foraminifera

One of the longest, most detailed quantitative records of oceanographic change in the Cenozoic is that provided by oxygen isotope measurements made on the tests of foraminifera. As indicated by measurements on benthic foraminifera, the deep waters of the world ocean have undergone an overall cooling of about 10°C in the Cenozoic. This change has been neither monotonic nor gradual. Rather, it is evidenced by a few, relatively rapid increases in the ¹³O content of the benthic shells. These “steps” in the isotopic record have been associated with major evolutionary changes in the mean state of the deep ocean. The variance around this mean state has also changed through the Cenozoic. From relatively high variance in the Middle Eocene, the oceans showed low variance in the Late Eocene and Oligocene. In the Miocene the variance of the isotopic measurement again increased, reaching a maximum of short duration in the Middle Miocene. This maximum as well as that which occurred during the Late Pliocene and Quaternary, may be attributable to fluctuations in the isotopic composition of the oceans caused by growth and decay of large ice sheets.In the Late Miocene the benthic oxygen isotopes in Atlantic sites less than 3000 m deep have a higher variance than sites at similar depths in the Pacific and Indian Oceans. It is thought that this high variance results from long-term changes in the importance of the cool and salty North Atlantic Deep Water relative to that of the warmer and less saline Antarctic Intermediate Water at Atlantic sites between 1000 m and 3000 m water depth. Such significant differences in benthic isotopic variance between the ocean basins have been demonstrated only in post-Middle Miocene intervals.     

Thanetian–Ilerdian coralline algae and benthic foraminifera from northeast India: microfacies analysis and new insights into the Tethyan perspective

Thanetian–Ilerdian carbonate deposits from the Lakadong Limestone in Assam Shelf, Meghalaya, northeast India, are studied with respect to microfacies distributions and controlling ecological factors on dominant biogenic components. Palaeoenvironmental implications are inferred following the detailed analysis of microfacies characterized by rich assemblages of coralline red algae and benthic foraminifera. The carbonate sediments have been interpreted as lagoonal to outer shelf facies. It is envisaged that the analysed benthic communities thrived in a meso‐oligotrophic regime above the fair‐weather wave base. The Lakadong Limestone constitutes a well‐preserved record of Late Palaeocene–earliest Eocene shallow marine carbonate ecosystem and has high potential to decipher its response to an interval of distinct changes in climate and tectonic settings. The abundance of oligotrophic larger benthic foraminifera in the Lakadong Limestone is comparable to the foraminiferal assemblages of west Tethys. The phylogenetic changes (‘Larger Foraminiferal Turnover’, LFT) and subsequent rapid radiation of typical Eocene larger benthic foraminifera (Alveolina, Nummulites) usually observed in the west Tethys have also been observed in the upper part of the Lakadong Limestone. The eastward migration of Eocene foraminifera from the west coincided with the India‐Asia collision and global warming events at the Palaeocene–Eocene boundary that may have produced a wide array of modifications in biogeography, seasonal run‐offs and ocean circulation pathways. The data indicate that rapid rate of migration from west before the onset of geographic barriers and/or timely restoration of pan‐Tethyan environmental conditions ensured the incidence of these forms in the earliest Eocene sediments.     

南黄海近岸底栖有孔虫分布与环境因子转换函数的建立

本文对南黄海西北陆架边缘的300个表层沉积物样品中的底栖有孔虫数据,与相应站位的水深、底层水温、盐度、沉积物粒度、有机碳含量等环境参数作了分析研究。除趋势对应分析(DCA)、典型对应分析(CCA)表明有孔虫种群分布与水深、夏季底层水温(Ts)和冬季底层水盐度(Sw)之间存在着显著的数值对应关系。利用加权平均偏最小二乘回归方法(WA-PLS)建立了底栖有孔虫数据与上述环境因子之间的数值转换函数。水深、Ts与Sw转换函数的预测均方根误差(RMSEP)分别是10.8%、11.5%和12.5%,与利用其他指标建立的此类转换函数水平(8%—20%)相当。利用此类统计建模方法建立近岸底栖有孔虫分布与浅海环境因子之间的数值关系是一项新的尝试,结果表明在我国近海底层水体古环境研究中底栖有孔虫转换函数是一种具有一定潜力的研究方法。     

Benthic foraminifera across the Cretaceous–Tertiary (K–T) boundary: a review

The response of the Earth’s biota to global change is of fundamental interest to paleontologists, but patterns of change in paleontologic data are also of interest to a wider spectrum of Earth scientists in that those patterns are of great significance in constraining hypotheses that attempt to explain physical changes in the Earth’s environment. The Cretaceous–Tertiary (K–T) boundary is a case in point. Some paleontologists have criticized the bolide impact hypothesis, not because they deny the impact but because the proposed effects of that impact do not always conform to the available paleontological data. Benthic foraminifera are of particular interest in this context because it has been suggested for over 20 years that shallow-water benthic foraminifera were affected more severely than deep-water benthic foraminifera by events at the K–T boundary. This observation adds to the fact of planktonic foraminiferal extinction and indicates that K–T boundary environmental effects were largely restricted to shallow waters. In this paper I review all published works on smaller benthic foraminifera at the K–T boundary and conclude the following. (1) Shallow-water benthic foraminifera were not more severely affected than deeper dwelling species. True extinction, as opposed to local extinction and/or mass mortality, is generally quite low no matter what the water depth. (2) The data are not sufficient in quality, quantity and geographic range to conclude that there is a latitudinal pattern of extinction. (3) In general, biotic changes (such as they are) begin before the boundary in shallow and intermediate depth waters and at the boundary in deep water. Disagreements about the placement of the boundary and the presence, absence and duration of hiatuses hinder more precise conclusions. (4) There appears to be preferential survivorship of epifaunal species into the early Danian with a short interval dominated by infaunal taxa in the earliest Danian. This pattern can best be explained by short-lived input of increased amounts of organic matter at the boundary followed by a sudden collapse of primary productivity and, hence, major reduction or cessation of organic flux to the seafloor. In summary, based on the current dataset, smaller benthic foraminifera, no matter whether they lived in shallow or deep waters, high or low latitudes, or infaunal or epifaunal microhabitats, survived the environmental events across the K–T boundary quite well. Mass extinction does not characterize this group of organisms at this time.     

Applications of nummulitids and other larger benthic foraminifera in depositional environment and sequence stratigraphy: an example from the Eocene deposits in Zagros Basin,SW Iran

The Tale-Zang Formation in Zagros Mountains (south-west Iran) is a Lower to Middle Eocene carbonate sequence. Carbonate sequences of the Tale-Zang Formation consist mainly of large benthic foraminifera (e.g. Nummulites and Alveolina), along with other skeletal and non-skeletal components. Water depth during deposition of the formation was determined based on the variation and types of benthic foraminifera, and other components in different facies. Microfacies analysis led to the recognition of ten microfacies that are related to four facies belts such as tidal flat, lagoon, shoal and open marine. An absence of turbidite deposits, reefal facies, gradual facies changes and widespread tidal flat deposits indicate that the Tale-Zang Formation was deposited in a carbonate ramp environment. Due to the great diversity and abundance of larger benthic foraminifera, this carbonate ramp is referred to as a “foraminifera-dominated carbonate ramp system”. Based on the field observations, microfacies analysis and sequence stratigraphic studies, three third-order sequences in the Langar type section and one third-order sequence in the Kialo section were identified. These depositional sequences have been separated by both type-1 and type-2 sequence boundaries. The transgressive systems tracts of sequences show a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts of sequences contain predominantly imperforate foraminifera.     

Deep-sea palaeoceanography of the Maldives Islands (ODP Hole 716A), equatorial Indian Ocean during MIS 12-6

Deep-sea benthic foraminifera, planktic foraminifer Globigerina bulloides and pteropods have been quantitatively analysed in 451 samples from Ocean Drilling Program (ODP) Hole 716A, to understand both surface and deep-sea palaeoceanographic changes in the equatorial Indian Ocean basin during the late Quaternary (∼444–151 Kyrs). Benthic foraminifera were analysed from >125 μm size fraction whereas Globigerina bulloides and pteropods were analysed from >150 μm size fraction. Factor analysis of most dominant benthic foraminiferal species over the studied time span made it possible to identify three biofacies characterizing distinct deep-sea environmental settings at Hole 716A. The environmental interpretation of each species is based on the ecology of recent deep-sea benthic foraminifera. The faunal record indicates fluctuating deep-sea conditions including changes in surface productivity, organic food supply and deep-sea oxygenation linked to changing wind intensities. These changes are pronounced on glacial-interglacial time scales driven by summer monsoon winds.     

Oxygen isotopic evidence for the depth stratification of tertiary and cretaceous planktic foraminifera

Oxygen isotope analyses of Tertiary and Cretaceous planktic foraminifera indicate that species have been stratified with respect to depth in the water column at least since Albian time. There is a relationship between morphology and depth habitat. Species with globigerine morphology have consistently occupied shallower depths than have species with globorotalid morphology. Biserially arranged species occupied both shallow and deep levels in the water column. On the average, it appears that ancient species with shallow habitats have been more susceptible to dissolution and have been preserved less well than species dwelling in deeper habitats. This relationship is similar to that observed for Recent planktic foraminifera. Comparison of carbon isotope ratios of adult and juvenile forms indicates that either the source of the carbon found in the shell or the carbon isotopic fractionations which occur during calcite secretion change during the development of individual foraminifera. The carbon isotopic ratios do not provide a reliable means for reconstructing the depth habitats of ancient species. Temperature-depth profiles for tropical Tertiary oceans have been reconstructed from the isotopic temperatures of planktic and benthic foraminifera. The vertical thermal structure of Oligocene oceans resembled that of modern oceans most closely. Those of Paleocene and Maastrichtian times differed most from that of modern oceans.     

Benthic foraminifera and their role to decipher paleoenvironment during mid-Cretaceous Oceanic Anoxic Events - the “anoxic benthic foraminifera” paradox

Oceanic Anoxic Events (OAEs) are a widespread, recurring and well-studied feature of the mid-Cretaceous greenhouse climate, reflecting severe perturbations in Earth's climate and oceanography. To investigate bottom-water environments before, during and after these OAEs and to understand the dynamic fluctuations of seafloor environments, benthic foraminifera are a commonly employed tool. Here, a review of the response of benthic foraminiferal assemblages to OAEs (i.e., OAEs 1a, 1b, 1d, 2, and 3) will be given. I will discuss in detail how benthic foraminifera can be used to identify the paleoclimatic and paleoceanographic mechanisms responsible for OAE formation. The main feature of benthic foraminiferal assemblages during oceanic anoxic events is a dramatic decrease in both diversities and abundances or their total absence. However, some studies record the permanent occurrence of benthic foraminifera throughout OAEs or short-term repopulation events. These patterns clearly indicate that mid-Cretaceous OAEs are complex events. The paradox of anoxic (and even euxinic) conditions during OAEs coincident with the occurrence of benthic foraminifera (here called the “anoxic benthic foraminifera” paradox) is most readily explained by the sampling (and therefore temporal) resolution of typical foraminiferal studies that have homogenized hundreds to thousands of years in a single sample.     

Palaeoenvironmental conditions preceding the Messinian Salinity Crisis: A case study from Gavdos Island

The Messinian pre-evaporitic sedimentary succession of Gavdos Island (Metochia section) is a nearly uninterrupted succession of marine sediments, dominated by finely laminated diatomaceous marls, which are cyclically alternating with clayey diatomites and white diatomites. The qualitative and quantitative analysis of the planktonic foraminiferal fauna allowed the recognition of nine bioevents, which have been astronomically dated for the Mediterranean. The base of the diatomitic succession in Gavdos Island is dated at 6.722 Ma and the top at 6.015 Ma. The studied section contains benthic foraminiferal genera characteristic of an outer shelf to slope environment. The qualitative and quantitative analysis of this microfauna revealed three benthic foraminiferal fossil assemblages and the occurrence of allochthonous species transported into the bathyal environment by current activity. The cyclical pattern of the benthic foraminifera assemblages indicates that the studied sediments have been affected by repeated episodes of basin restriction characterized by low diversity benthic foraminifera populations, and a limited planktonic foraminifer association typified by shallow, surface-dwelling forms. This restriction was partly due to Antarctic cooling, which produced palaeo-Mediterranean sea-level oscillations during the Early Messinian, as a prelude to closure of the Atlantic connections. The relative impact of climatic versus tectonic control on sedimentation patterns within this basin is discussed.     

Molecular evidence for an independent origin of modern triserial planktonic foraminifera from benthic ancestors

Gallitellia vivans is the only Recent representative of the triserial planktonic foraminiferal family Guembelitriidae. The origin and evolution of this interesting albeit poorly known family are enigmatic. To elucidate the phylogenetic relationships between G. vivans and other planktonic foraminifera, we sequenced the small subunit ribosomal DNA (SSU rDNA) for comparison to our extensive database of planktonic and benthic species. Our analyses suggest that G. vivans represents a separate lineage of planktonic foraminifera, which branches close to the benthic rotaliids Stainforthia and Virgulinella. Both genera resemble Gallitellia in general morphological appearance, having elongate triserial tests at least in their early ontogenic stages. The divergence time of G. vivans is estimated at ca. 18 Ma (early Miocene), suggesting an origin independent from the Cretaceous and Paleogene triserial planktonic foraminifera. Our study thus indicates that modern triserial planktonic foraminifera are not related to the Cretaceous–Paleogene triserial species, and that the sporadic occurrences in the fossil record are not the result of poor preservation, but reflect multiple transitions from benthic to planktonic mode of life.     

Facies,depositional sequences,and biostratigraphy of the Oligo-Miocene Asmari Formation in Marun oilfield,North Dezful Embayment,Zagros Basin,SW Iran

The Asmari Formation in Marun oilfield (south-west Iran), is about 440 m-thick marine carbonate succession with subordinate siliciclastic rocks, characterized by abundant benthic foraminifera (perforate and imperforate). Foraminiferal biostratigraphy indicates that this unit is Oligocene–Miocene in age. The distribution of benthic foraminifera and other components have led to the recognition of three siliciclastic and ten carbonate facies that were deposited in inner ramp (shoreline, tidal flat, restricted and open lagoon and shoal), middle and outer ramp sub-environments. Based on vertical facies trends, three third-order sequences in the Oligocene and three third-order sequences in the Miocene sediments have been identified. These depositional sequences are bounded by both type 1 and type 2 sequence boundaries. The transgressive systems tracts (TST) of sequences show deepening-upward facies trend with a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts (HST) have a shallowing-upward facies trend and contain predominantly imperforate foraminifera. Deposition of these depositional sequences (DS) were controlled by both eustasy and tectonic subsidence.     

Paleoceanographic changes during the past 1.9 Myr at DSDP Site 238, Central Indian Ocean Basin: Benthic foraminiferal proxies

Deep-sea benthic foraminifera have been quantitatively analyzed in samples (> 125 μm size fraction) from Ocean Drilling Program (ODP) Site 238, to understand paleoceanographic changes in the Central Indian Basin over the past 1.9 Myr. Factor and cluster analyses of the 25 highest-ranked species made it possible to identify five biofacies, characterizing distinct deep-sea environmental settings. The environmental interpretation of each biofacies is based on the ecology of recent deep-sea benthic foraminifera. The benthic faunal record indicates fluctuating deep-sea conditions in environmental parameters including oxygenation, surface productivity and organic food supply. These changes appear to be linked to Indian summer monsoon variability, the main climatic feature of the Indian Ocean region. The benthic assemblages show a major shift at ∼ 0.7 to 0.6 Ma, marked by major turnovers in the relative abundances of species, coinciding with an increased amplitude of glacial cycles. These cycles appear to have influenced low latitude monsoonal climate as well as deep-sea conditions in the Central Indian Ocean Basin.     

Molecular evidence for an independent origin of modern triserial planktonic foraminifera from benthic ancestors

Gallitellia vivans is the only Recent representative of the triserial planktonic foraminiferal family Guembelitriidae. The origin and evolution of this interesting albeit poorly known family are enigmatic. To elucidate the phylogenetic relationships between G. vivans and other planktonic foraminifera, we sequenced the small subunit ribosomal DNA (SSU rDNA) for comparison to our extensive database of planktonic and benthic species. Our analyses suggest that G. vivans represents a separate lineage of planktonic foraminifera, which branches close to the benthic rotaliids Stainforthia and Virgulinella. Both genera resemble Gallitellia in general morphological appearance, having elongate triserial tests at least in their early ontogenic stages. The divergence time of G. vivans is estimated at ca. 18 Ma (early Miocene), suggesting an origin independent from the Cretaceous and Paleogene triserial planktonic foraminifera. Our study thus indicates that modern triserial planktonic foraminifera are not related to the Cretaceous–Paleogene triserial species, and that the sporadic occurrences in the fossil record are not the result of poor preservation, but reflect multiple transitions from benthic to planktonic mode of life.     

Benthic foraminifera in deep trenches of the Kara Sea and the relation of <Emphasis Type="Italic">Saccorhiza ramosa</Emphasis> (Brady) to the distribution of natural radionuclides

The abundance and structure of the foraminifera community have been investigated. The role of agglutinating and secreting species in bottom sediments of the deep trenches of the Kara Sea has been studied. It has been found that the abundance and dominance of the agglutinated foraminifera Saccorhiza ramosa depend on the depth of the sea and are related to the genetic type of origin of such sediments. A correlation has been revealed between the distribution of S. ramosa and the concentrations of natural radionuclides (²²⁶Ra, ²³²Th, and ²¹⁰Pb) in bottom sediments. The degree of dominance of S. ramosa in the community of benthic foraminifera increases along with natural radioactivity.