Larger Benthic Foraminifera Research Articles

Facies and depositional settings of the Middle Eocene-Oligocene carbonates in Kutch

Middle Eocene Fulra Limestone and Oligocene Maniyara Fort Formation represent platform carbonate deposits of Kutch at the north-western margin of India. These carbonates contain larger benthic foraminifera, including Alveolina, Assilina, Discocyclina, Lepidocyclina, Miogypsina, Nummulites and Spiroclypeus. This study presents paleodepositional and paleobathymetric interpretations for both formations using benthic foraminifera in combination with lithological association, sedimentary structures and early diagenetic features. The six carbonate facies comprising the Fulra Limestone indicate a depositional spectrum ranging from bar-lagoon to mid-ramp depositional conditions. It records several shallowing upward cycles, leading to emergence and formation of paleokarst. The four carbonate facies of the Maniyara Fort Formation represents deposition within the inner ramp setting in bar-lagoon and patch-reef environment, while intervening fine siliciclastics correspond to episodes of relative sea level fall. Nummulitic accumulations form low-relief bars within the fair weather wave base in both the formations. The depositional setting of the Paleogene carbonate in Kutch broadly resembles Eocene platformal deposits in the circum-Tethys belt.

Stratigraphy and larger benthic foraminifera of Middle Eocene to Middle Miocene rocks along the Tobruk-Al Bardia scarps, northeastern Cyrenaica, Libya

Stratigraphy and larger benthic foraminifera of Middle Eocene to Middle Miocene rocks along the Tobruk-Al Bardia scarps, northeastern Cyrenaica, Libya

The enigmatic Palaeocene-Eocene coralline Distichoplax: Approaching the structural complexities, ecological affinities and extinction hypotheses

Abstract The alluring but scarcely understood Palaeocene-Eocene coralline alga Distichoplax has been examined in detail from multiple carbonate sections outcropping in Mawmluh near Cherrapunji (East Khasi Hills) in Meghalaya, N-E India. Despite having a wide geographical distribution Distichoplax still remains a mysterious taxon with faint knowledge pertaining to its evolution, ecology and causes of ultimate extinction. The commonly known feather-shaped and zipper-shaped cell arrangements need detailed reassessment considering the morphological intraspecific variability. The threefold objective of this paper is to analyse the complex cell/anatomical variations in Distichoplax, provide some insights into its ecological affinity, and most importantly propose hypotheses on the possible causes of its untimely demise in sharp contrast to other coralline algae distinguished by a long evolutionary record continuing to the present-day. The abundance of Distichoplax varies from monodominant assemblages to their co-dominance with geniculate coralline algae, non-geniculate Sporolithon and larger benthic foraminifera (orthophragminids) before facing sharp decline and ultimate extinction. Rare fairly well-preserved uniporate conceptacles and absence of cell fusions link Distichoplax to the subfamily Lithophylloideae of the family Corallinaceae. Very poor incidence of reproductive features, significant blooms of the species during the Thanetian (Late Palaeocene) and its eventual disappearance in the Eocene indicate Distichoplax may have been excessively dependent on vegetative propagation, and perished due to the climate adversities like the Palaeocene-Eocene Thermal Maximum (PETM) and biotic pressures like the Larger Foraminiferal Turnover (LFT).

New quantitative data on Omphalocyclus from the Maastrichtian in Northern Iraq

Omphalocyclus is a well-known genus of large benthic foraminifera; species in the genus mainly occur in the argillaceous limestones and carbonates of the Aqra Formation of the early to late Maastrichtian age from Kurdistan foreland basin in Northern Iraq. Its presence is a significant indicator of its paleogeographic distribution in the Middle East. The Aqra formation was divided by lithology into lower, middle, upper, and transitional parts using Google Earth satellite images. Evolutionary changes were documented in successive Omphalocyclus assemblages for the first time in Iraq, and a clear increase in embryon size was found towards the end of the Maastrichtian. This evolutionary development was followed by an increase in the number of epi-embryonic chamberlets. The earliest known Omphalocyclus specimens in the studied strata predominantly have three to four primary epi-embryonic chamberlets and one to two accessory epi-embryonic chamberlets, and these occur in horizons attributable to the Pseudotextularia intermedius zone. The gain of radial stolons in the tritoconch wall may have led to accessory epi-embryonic in consecutive Omphalocyclus populations in horizons attributable to the Racemiguembelina fructicosa zone; accordingly, only a few additional epi-embryonic chamberlets arise from the tritoconch. Epi-embryonic chamberlets tend to be relatively large and comprise a large part of the embryon around its dense external enclosure. Notable differences in embryon size, the number of epi-embryonic chamberlets, as well as other morphological features in successive populations, two prominent species were distinguished: O. cideensis in the early Maastrichtian strata and O. macroporus in the late Maastrichtian. Omphalocyclus specimens exhibited the same evolutionary age and a similar embryonic developmental stage, according to a morphometric analysis of O. macroporus embryon size. However, tests of some of these specimens from the uppermost Aqra Formation were ellipsoidal, biconcave, and centrally depressed, and other tests showed disk forms and central depressions with a well-rounded equatorial margin. These were interpreted as two new subspecies: O. macroporus ellipsoides subsp. nov. and O. macroporus maukabensis subsp. nov.

Ecological tolerances of Miocene larger benthic foraminifera from Indonesia

To provide a comprehensive palaeoenvironmental reconstruction based on larger benthic foraminifera (LBF), a quantitative analysis of their assemblage composition is needed. Besides microfacies analysis which includes environmental preferences of foraminiferal taxa, statistical analyses should also be employed. Therefore, detrended correspondence analysis and cluster analysis were performed on relative abundance data of identified LBF assemblages deposited in mixed carbonate-siliciclastic (MCS) systems and blue-water (BW) settings. Studied MCS system localities include ten sections from the central part of the Kutai Basin in East Kalimantan, ranging from late Burdigalian to Serravallian age. The BW samples were collected from eleven sections of the Bulu Formation on Central Java, dated as Serravallian. Results from detrended correspondence analysis reveal significant differences between these two environmental settings. Cluster analysis produced five clusters of samples; clusters 1 and 2 comprise dominantly MCS samples, clusters 3 and 4 with dominance of BW samples, and cluster 5 showing a mixed composition with both MCS and BW samples. The results of cluster analysis were afterwards subjected to indicator species analysis resulting in the interpretation that generated three groups among LBF taxa: typical assemblage indicators, regularly occurring taxa and rare taxa. By interpreting the results of detrended correspondence analysis, cluster analysis and indicator species analysis, along with environmental preferences of identified LBF taxa, a palaeoenvironmental model is proposed for the distribution of LBF in Miocene MCS systems and adjacent BW settings of Indonesia.

The Paleogeographic Evolution of the Orthophragminids of the Paleogene

AbstractOrthophragminids are larger benthic foraminifera (LBF) and, together with the nummulitids, were the major rock-forming foraminifera from the middle Paleocene to the late Eocene. Today, porous, LBF-bearing, Paleogene limestones, which occur globally from the Pacific and Atlantic margins of the Americas to the Indo-Pacific, form potentially valuable oil reservoirs, and their biota have formed the basis of the definition of three paleobiogeographic provinces, namely those of the Americas, Tethys, and the Indo-Pacific. The orthophragminids of the western part of the Tethyan Province have been studied extensively, however, the other provinces are less well characterized, and until now the origin and paleogeographic development of this group have not been fully articulated. New material described here allows the clear definition of a fourth, South African paleobiogeographic province, and, when combined with refined biostratigraphic dating based on new material from the Americas, Europe, South Asia and SE Asia, enables their paleogeographic and biostratigraphic evolution to be determined. Critically, the occurrence of cosmopolitan planktonic foraminifera (PF) within LBF assemblages enables the first occurrences of various LBF forms within each province to be dated relative to well-calibrated planktonic zones (PZ). From this, we infer that, like the previously studied lepidocyclinids and nummulitids, the orthophragminids originated in the Americas during the Paleocene, probably between the late Danian (PZ P1c, 63.5 Ma) and the early Selandian (PZ P3a, 61.6 Ma). By the middle Paleocene, the orthophragminids had migrated across the Atlantic to the previously isolated West African coast at the extreme of Tethys, probably during global sea-level low stands at 60.3 Ma and again at 56.4 Ma. Subsequently, the American Province again became isolated. In the Tethys, the orthophragminid migrations followed two paths: northeastward through the Tethyan corridor in the late Paleo-cene (Thanetian), and south in the earliest Eocene (Ypresian) to South Africa. The Tethyan forms evolved during the Eocene into many lineages, which in turn migrated, after a few million years of their first appearances into the Indo-Pacific, where they again became isolated and diversified further. Meanwhile the South African forms remained similar to their American ancestors in both small size and external ornamentation, while their internal evolution closely followed that of Tethys forms, as exhibited by three species of Nemkovella and Discocyclina described here from South Africa (Nemkovella mcmilliana n. sp., Discocyclina davyi n. sp. and D. africana n. sp.).

Mg/Ca and Sr/Ca as novel geochemical proxies for understanding sediment transport processes within coral reefs

Sediment transport is a key driver of reef zonation and biodiversity, where an understanding of sediment dynamics gives insights into past reef processes and allows the prediction of geomorphic responses to changing environmental conditions. However, modal conditions within the back-reef seldom promote sediment transport, hence direct observation is inherently difficult. Large benthic foraminifera (LBF) have previously been employed as ‘tracers’ to infer sediment transport pathways on coral reefs, as their habitat is largely restricted to the algal flat and post-mortem, their calcium carbonate test is susceptible to sediment transport forces into the back-reef. Foraminiferal test abundance and post-depositional test alteration have been used as proxies for sediment transport, although the resolution of these measures becomes limited by low test abundance and the lack of variation within test alteration. Here we propose the novel use of elemental ratios as a proxy for sediment transport. Two species, Baculogypsina sphaerulata and Calcarina capricornia, were analysed using a taphonomic index within One Tree and Lady Musgrave reefs, Great Barrier Reef (Australia). Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to determine Mg/Ca and Sr/Ca and these ratios were compared with taphonomic data. Decreases in test Mg/Ca accompany increases in Sr/Ca in specimens from algal-flat to lagoonal samples in both species, mirroring trends indicated by taphonomic values, therefore indicating a relationship with test alteration. To delineate mechanisms driving changes in elemental ratios, back-scattered electron (BSE) images, elemental mapping and in situ quantitative spot analyses by electron microprobe microanalysis (EPMA) using wavelength dispersive X-ray spectrometers (WDS) were performed on un-altered algal flat and heavily abraded tests for both species. EPMA analyses reveal heterogeneity in Mg/Ca between spines and the test wall, implying the loss of appendages results in a decrease in Mg/Ca. BSE imaging and WDS elemental mapping provided evidence for cementation, facilitated by microbial-boring as the primary cause of increasing Sr/Ca. These novel proxies hold advantages over taphonomic measures and further provide a rapid method to infer sediment transport pathways within back-reef environments.

Relative sea-level change in western New Guinea recorded by regional biostratigraphic data

We present new biostratigraphic analyses of approximately 200 outcrop samples and review biostratigraphic data from 136 public domain exploration wells across western New Guinea. Biostratigraphic ages and palaeodepositional environments were interpreted from occurrences of planktonic and larger benthic foraminifera, together with other fossils and environmental indicators where possible. These data were compared with existing geological maps and exploration well data to reconstruct the palaeogeography of western New Guinea from the Carboniferous to present day. In addition, we used the known bathyal preferences of fossils to generate a regional sea-level curve and compared this with global records of sea-level change over the same period. Our analyses of the biostratigraphic data identified two major transgressive-regressive cycles in regional relative sea-level, with the highest sea levels recorded during the Late Cretaceous and Late Miocene and terrestrial deposition prevalent across much of western New Guinea during the Late Paleozoic and Early Mesozoic. An increase in the abundance of carinate planktonic foraminifera indicates a subsequent phase of relative sea-level rise during a regional transgressive event between the Late Jurassic and Late Cretaceous. However, sea-levels dropped once more during a regressive event between the Late Cretaceous and the Paleogene. This resulted in widespread shallow water carbonate platform development in the Middle to Late Eocene. A minor transgressive event occurred during the Oligocene, but this ceased in the Early Miocene, likely due to the collision of the Australian continent with intra-Pacific island arcs. This Miocene collision event resulted in widespread uplift that is marked by a regional unconformity. Carbonate deposition continued in platforms that developed in shallow marine settings until these were drowned during another transgressive event in the Middle Miocene. This transgression reached its peak in the Late Miocene and was followed by a further regression culminating in the present day topographic expression of western New Guinea.

Micropaleontological and taphonomic characteristics of mass transport deposits in the northern Gulf of Eilat/Aqaba, Red Sea

Submarine mass transport deposits (MTDs) are a well-known phenomenon in tectonically active regions. Evidence for such deposits is commonly found in the continental slope sedimentary records, as distinct units with coarser grain size compared to the usual and continuous pelagic sedimentation. The Gulf of Eilat/Aqaba is located between the southernmost end of the Dead Sea transform and the spreading center of the Red Sea, and is considered as an active tectonic region.In this study, an innovative approach using symbiont-bearing Larger Benthic Foraminifera (LBF) to identify MTDs in the Gulf of Eilat/Aqaba (GEA) sedimentary record is presented. The abundance, size and preservation state of LBF shells were analyzed in two radiocarbon dated sediment cores collected at different deposition environments, at water depth of 532m and 316m.The microfaunal and taphonomic results show that the coarse units are characterized by a generally higher numerical abundance of LBF, dominated by Operculina ammonoides, Amphistegina papillosa and Amphistegina bicirculata. These benthic assemblages are found in deeper depths than their original habitat, ranging between 50 and 120m, in accordance with their symbionts light requirements. In the coarse units, LBF >1mm appear in high frequency, up to 161 specimens per g sediment, and poorly preserved shells are also abundant, containing up to 247 specimens per g sediment. In addition, these units also contain high numbers of yellowish and blackish colored LBF shells, as opposed to null in the non-disturbed units, and unlike their natural white color.The large shell size indicates that high energy is involved in the displacement of the sediments. The poor state of preservation also suggests a turbulent flow during transportation, which requires a high-energy triggering mechanism. The color alteration is probably associated with a diagenetic process related to increasing burial time/depth, also supported by the stratigraphic older ages of the MTDs, suggesting a long burial before the sediments were displaced. In addition, according to the dating of the record, some units correlate with historical and pre-historical earthquakes, reinforcing LBF species as a reliable proxy for mass transport events.

Reef calcifiers are adapted to episodic heat stress but vulnerable to sustained warming.

Shallow marine ecosystems naturally experience fluctuating physicochemical conditions across spatial and temporal scales. Widespread coral-bleaching events, induced by prolonged heat stress, highlight the importance of how the duration and frequency of thermal stress influence the adaptive physiology of photosymbiotic calcifiers. Large benthic foraminifera harboring algal endosymbionts are major tropical carbonate producers and bioindicators of ecosystem health. Like corals, they are sensitive to thermal stress and bleach at temperatures temporarily occurring in their natural habitat and projected to happen more frequently. However, their thermal tolerance has been studied so far only by chronic exposure, so how they respond under more realistic episodic heat-event scenarios remains unknown. Here, we determined the physiological responses of Amphistegina gibbosa, an abundant western Atlantic foraminifera, to four different treatments––control, single, episodic, and chronic exposure to the same thermal stress (32°C)––in controlled laboratory cultures. Exposure to chronic thermal stress reduced motility and growth, while antioxidant capacity was elevated, and photosymbiont variables (coloration, oxygen-production rates, chlorophyll a concentration) indicated extensive bleaching. In contrast, single- and episodic-stress treatments were associated with higher motility and growth, while photosymbiont variables remained stable. The effects of single and episodic heat events were similar, except for the presumable occurrence of reproduction, which seemed to be suppressed by both episodic and chronic stress. The otherwise different responses between treatments with thermal fluctuations and chronic stress indicate adaptation to thermal peaks, but not to chronic exposure expected to ensue when baseline temperatures are elevated by climate change. This firstly implies that marine habitats with a history of fluctuating thermal stress potentially support resilient physiological mechanisms among photosymbiotic organisms. Secondly, there seem to be temporal constraints related to heat events among coral reef environments and reinforces the importance of temporal fluctuations in stress exposure in global-change studies and projections.

DIVERSITY AND DISTRIBUTION OF LIVING LARGER BENTHIC FORAMINIFERA FROM CORAL REEF ENVIRONMENTS, SOUTH ANDAMAN ISLAND, INDIA

The waters around the Andaman and Nicobar Islands (ANI) sustain a rich fauna, including larger benthic foraminifera (LBF), which are presently not well-studied. We documented the diversity and distribution of LBF at six reef sites (4–30 m) in South Andaman. We recorded a total of 16 LBF species belonging to 12 genera and 7 families. Amphistegina lessonii, A. radiata, and Calcarina spengleri were found at five of the sites surveyed. The density of living larger benthic foraminifera was 5–17 specimens per cm 2 . The Shannon-Wiener Diversity Index (H) ranged from 0.90–2.06, the Margalef Species Richness Index (D) from 0.39–1.62 and Pielou’s Equitability Index (J) from 0.77–1.12. These relatively high index values reflect the carbonate environment, characterized by a diverse and relatively even distribution of larger benthic foraminiferal assemblages in the South Andaman Islands.

Patterns of Sediment Transport Using Foraminifera Tracers across Sand Aprons on the Great Barrier Reef

ABSTRACT Fellowes, T.E.; Gacutan, J.; Harris, D.L.; Vila-Concejo, A.; Webster, J.M., and Byrne, M., 2017. Patterns of sediment transport using foraminifera tracers across sand aprons on the Great Barrier Reef. Sediment dynamics exert large control over coral reef geomorphological evolution and are vital to understanding past and present geomorphic responses. Large benthic foraminifera (LBF) live in the algal reef flats, and their tests (shells) are transported post-mortem by waves and currents onto back-reef environments, including sand aprons. This study investigated the patterns of transport linking surficial and downcore sediments in samples from three sand aprons with different wave exposures at One Tree Reef on the southern Great Barrier Reef (Australia). Six LBF genera represented up to 32% of the sediments analysed. Lagoonward transport increased LBF test abrasion and sediment bulk density. Sediment grain size and LBF abundance in sediments also decreased with lagoonward transport. Sediment transpo...

Impact of tectonic and volcanism on the Neogene evolution of isolated carbonate platforms (SW Indian Ocean)

Understanding the impact of tectonic activity and volcanism on long-term (i.e. millions years) evolution of shallow-water carbonate platforms represents a major issue for both industrial and academic perspectives. The southern central Mozambique Channel is characterized by a 100km-long volcanic ridge hosting two guyots (the Hall and Jaguar banks) and a modern atoll (Bassas da India) fringed by a large terrace. Dredge sampling, geophysical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that submarine flat-top seamounts correspond to karstified and drowned shallow-water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults. Microfacies and well-constrained stratigraphic data indicate that these carbonate platforms developed in shallow-water tropical environments during Miocene times and were characterized by biological assemblages dominated by corals, larger benthic foraminifera, red and green algae. The drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the Early Pliocene and seems closely linked to (1) volcanic activity typified by the establishment of wide lava flow complexes, and (2) to extensional tectonic deformation associated with high-offset normal faults dividing the flat-top seamounts into distinctive structural blocks. Explosive volcanic activity also affected platform carbonates and was responsible for the formation of crater(s) and the deposition of tuff layers including carbonate fragments. Shallow-water carbonate sedimentation resumed during Late Neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion. Latest carbonate developments ultimately led to the formation of the Bassas da India modern atoll. The geological history of isolated carbonate platforms from the southern Mozambique Channel represents a new case illustrating the major impact of tectonic and volcanic activity on the long-term evolution of shallow-water carbonate platforms.

Siliciclastic input and sedimentation rate as controls on paleoecological distribution of Lower Miocene benthic carbonate producers in a fan–delta: Zagros foreland basin, Iran

This paper aimed to study Lower Miocene (Burdigalian) mixed carbonate–siliciclastic deposits within an Upper Cenozoic synorogenic conglomerate–dominated succession in north of Shalamzar in the Zagros foreland basin, Iran. The deposits are composed of nine facies: foraminiferal mudstone, silty mudstone, sandy mudstone, fossiliferous sandy mudstone, fossiliferous argillaceous mudstone, fossiliferous calclithite, coral limestone, calcareous claystone and hybrid sandstone. The facies represent a mixed carbonate– siliciclastic shelf–type fan–delta. The subenvironments of the fan–delta include muddy pro–delta, sandy delta– front, clastic proximal mouth bar and a subordinate delta plain. Siliciclastic input and sedimentation rate controlled the paleoecological distribution of different benthic carbonate–producing fauna in the fan–delta during its progradation into a shelf marine environment. Input of siliciclastic deposits and sedimentation rate limited the diversity and development of corals and controlled their colonization and growth morphologies in the sandy delta–front. Siliciclastic input (including plant materials and coal debris) and sedimentation rate controlled the trophic habitats of many gastropods and their abundance and distribution in the sandy delta– front and clastic proximal mouth bar. Also, increased siliciclastic input favored abundance of larger benthic foraminifera in most parts of the fan–delta with the exception of the muddy pro–delta.

Variation in sensitivity of large benthic Foraminifera to the combined effects of ocean warming and local impacts

Large benthic foraminifera (LBF) are crucial marine calcifiers in coral reefs, and sensitive to environmental changes. Yet, many species successfully colonise a wide range of habitats including highly fluctuating environments. We tested the combined effects of ocean warming, local impacts and different light levels on populations of the common LBF Amphistegina lobifera collected along a cross-shelf gradient of temperature and nutrients fluctuations. We analysed survivorship, bleaching frequency, chlorophyll a content and fecundity. Elevated temperature and nitrate significantly reduced survivorship and fecundity of A. lobifera across populations studied. This pattern was exacerbated when combined with below optimum light levels. Inshore populations showed a consistent resistance to increased temperature and nitrate levels, but all populations studied were significantly affected by light reduction. These findings demonstrated the capacity of some populations of LBF to acclimate to local conditions; nonetheless improvements in local water quality can ultimately ameliorate effects of climate change in local LBF populations.

Symbiosis and microbiome flexibility in calcifying benthic foraminifera of the Great Barrier Reef

BackgroundSymbiosis is a phenomenon that allows organisms to colonise a wide range of environments and occupy a variety of ecological niches in marine environments. Large benthic foraminifera (LBF) are crucial marine calcifiers that rely on photo-endosymbionts for growth and calcification, yet the influence of environmental conditions in shaping their interactions with prokaryotic and eukaryotic associates is poorly known.ResultsHere, we used next-generation sequencing to identify eukaryotic photosynthesizing and prokaryotic microbes associated with the common LBF Amphistegina lobifera across a physio-chemical gradient on the Great Barrier Reef (GBR). We collected samples from three reef sites located in the inner-, mid- and outer-shelf regions of the northern section of the GBR. Results showed the consistent presence of Bacillaryophyta as the main eukaryotic taxa associated with A. lobifera across all reef sites analysed; however, the abundance and the diversity of prokaryotic organisms varied among reef sites. Inner-shelf specimens showed the highest diversity of prokaryote associates, with a total of 231 genotypes in their core microbiome. A total of 30 taxa were identified in the core microbiome across all reef sites. Within these taxa, Proteobacteria was the most abundant bacteria present. The presence of groups such as Actinobacteria was significantly correlated with inner-shelf populations, whereas the abundance of Bacteroidetes and Firmicutes was associated with A. lobifera collected from mid- and outer-shelf reef sites.ConclusionsWe found that benthic foraminifera form stable and persistent symbiosis with eukaryotic partners, but flexible and site-specific associations with prokaryotic microbes that likely influence the ecological success of these crucial calcifying organisms on the GBR.

Banatia aninensis n. gen., n. sp., a new complex larger benthic foraminifer from the upper Barremian of Romania

A new larger benthic foraminifera is described as Banatia aninensis n. gen., n. sp. (Family Pfenderinidae Smout & Sugden) from upper Barremian Urgonian-type shallow-water carbonates of the Reşita – Moldova Nouă Zone, southwestern part of Romania. The low to medium trochospiral test of Banatia n. gen. is characterized by marginally undivided chambers and a wide axial part. The latter is made up of pillars continuous between successive chambers and a labyrinthic endoskeleton (plates and pillars) with a fine canal system between. Banatia n. gen. is compared with Pfenderina Henson, Pseudopfenderina Hottinger, and Accordiella Farinacci. The new taxon occurs in algal-foraminiferal wackestones interpreted as deposits of an internal lagoonal realm. So far unrecorded in the literature, the taxon might be paleogeographically restricted (endemic).

Microfacies analysis of larger benthic foraminifera-dominated Middle Eocene carbonates: a palaeoenvironmental case study from Meghalaya, N-E India (Eastern Tethys)

Larger benthic foraminifera (LBF) show significant abundance and diversity in the Palaeogene carbonate sediments of Meghalaya, N-E India, but have previously received less attention from the palaeoenvironmental perspective. LBF are important contributors to recent as well as fossil shallow marine, tropical carbonate settings. They find wide application in biostratigraphy and palaeoenvironmental reconstructions. The larger foraminiferal turnover (LFT) during the Palaeocene-Eocene transition is very important with respect to their evolution in several parts of the world including the Eastern Tethys. The present microfacies analysis documents the status of LBF in the Middle Eocene sediments from the Prang Formation of the Sylhet Limestone Group in Meghalaya, N-E India. Five major facies types (MFTs)—miliolid grainstone-packstones, alveolinid-rotaliid grainstone-packstones, nummulitid-alveolinid grainstone-packstones, coralline algal-nummulitid packstone-wackestones and coralline algal wackestones have been recorded in the current study. Evaluation of the palaeoenvironmental parameters aids in understanding the seascape of this Eastern Tethyan domain. It is suggested that an oligotrophic nutrient regime supported the rapid evolution and dominance of the LBF. Most notable is the prolific augmentation in Alveolina and Nummulites populations. High surface water temperatures during the Late Palaeocene-Early Eocene global warming episode possibly persisted to a certain level during the Middle Eocene and continued to favour the larger foraminifera as the major carbonate producers instead of the vulnerable corals.

Paleogene Nummulitid biostratigraphy of the Kohat and Potwar Basins in north-western Pakistan with implications for the timing of the closure of eastern Tethys and uplift of the western Himalayas

The Paleogene larger benthic foraminifera (LBF) of the Kohat and Potwar basins, Pakistan are very useful in dating shallow marine sediments and cessation of marine sedimentation that provides constraint for crucial information on the initial age of India-Asia collision. We record important Paleogene LBF species in multiple sections of the two basins. We performed the biometric analysis of the nummulitid species useful for taxonomic purposes. We recognize six Larger Benthic Foraminiferal Zone (BFZ Zone 1-6) in the Kohat Basin. Among these Zones the first three (BFZ Zone 1-3) occur also in the Potwar Basin and the adjoining Trans Indus Ranges (TIR). The correlation of BFZ 1-6 Zones with the previous local and regional LBF biostratigraphic schemes in the Eastern Tethys (Pakistan-India) and Western Tethys (European Basins) resulted in recognition of useful index taxa for developing a regional stratigraphic framework during Paleogene. In the BFZ 1 Zone, The last occurrence (LO) of Miscellanea miscella and first occurrence (FO) of Assilina dandotica in the BFZ 2 Zone mark the Late Paleocene (Late Thenetian) Early Eocene (Lower Illerdiuan1) Boundary. The co-occurrence of A. pustulosa, Al. vredenburgi, Al. globula and Al. pasticillata in the BFZ 2 Zone characterizes the Early Eocene (Lower IlIerdian 1-Middle Illerdian 1) sediments. The synchronous FO of N. atacicus and N. globulus is an excellent global biostratigraphic marker of the Early Eocene (Middle Illerdian 1 Middle Illerdian 2) Boundary and the FO of O. complanatus is a useful biostratigraphic marker for the Early Eocene (Lower Cuisian 2-Middle Cuisian) Boundary in the BFZ 3 Zone. Mammalian bones found at the base of Koldana Formation in the Kohat Basin represent Early Eocene (Upper Cuisian), which is in agreement with the LBF biostratigraphy of the underlying Middle Cuisian strata. The FO of A. exponens in the BFZ 4 Zone record the Middle Eocene (Middle Lutetian 1) sediments while FO of N. beaumonti in the BFZ 5 Zone marks the Middle Lutetian 1Middle Lutetian 2 Boundary. The FO of A. cancellata in the BFZ 6 Zone marks Middle Lutetian 2Upper Lutetian Boundary. The implications of our LBF study are that cessation of marine sedimentation in both Pakistani basins occurred in Early Eocene (Middle Cuisian ≈BFZ 3) around 50-49.5 Ma. Notably, marine conditions returned in the Kohat Basin in Middle Eocene (Middle Lutetian 1 ≈ BFZ 4) due to an eustatic sea level rise. The final cessation of marine sedimentation, causing closure of the Eastern Tethys seaway in the Kohat Basin occurred in Middle Eocene (Upper Lutetian ≈BFZ 6) around 41.2 Ma, probably a result of some form of post-collisional stress.

Evaluation of oxygen isotope and Mg/Ca ratios in high‐magnesium calcite from benthic foraminifera as a proxy for water temperature

AbstractOxygen isotope ratios (δ18O) and Mg/Ca ratios in organically produced calcium carbonate (calcite and aragonite) by marine species have been used as proxies for paleotemperature. The relationship between the components of large benthic foraminiferal tests (high‐magnesium calcite) and seawater temperature has been little investigated yet. We conducted culture experiments using two species of reef‐dwelling large benthic foraminifera under six temperature conditions (21–30°C). We selected the perforate species Calcarina gaudichaudii, and the imperforate species Amphisorus kudakajimensis, both of which are host to algal symbionts, for the experiments. The results indicated the significant correlation of Mg/Ca ratios (C. gaudichaudii and A. kudakajimensis) and δ18O (C. gaudichaudii) with temperature. The test weight of both species showed temperature dependency, with maxima at 27°C or 29°C and subsequent decline at 30°C. After correction for the contribution of the initial tests, which can be calculated from the weight ratios of the initial and cultured tests, the relationships of the Mg/Ca ratios and δ18O to temperature were obtained with the least squares best fit method as follows: C. gaudichaudii: T = 18.5–4.52 (δ18Oc–δ18Osw), A. kudakajimensis: T = 25.0–4.96 (δ18Oc–δ18Osw), and C. gaudichaudii and A. kudakajimensis: Mg/Ca = 2.85 T + 81.20. We concluded that species‐specific calibration is unnecessary for Mg/Ca‐temperature relationship, although it is necessary for δ18O related to the different calcification mechanisms of imperforate (A. kudakajimensis) and perforate (C. gaudichaudii) species.