Larger Benthic Foraminiferal Assemblages Research Articles

Larger benthic foraminiferal assemblages and their response to Middle Eocene Climate Optimum in the Kohat Basin (Pakistan, eastern Tethys)

The Middle Eocene Climatic Optimum (MECO) at ∼40 Ma is a significant global warming event associated with pronounced changes in the hydrosphere, atmosphere, and biosphere. The Kohat Formation in the Kohat Basin (eastern Tethys, Pakistan) is studied for identifying the response of larger benthic foraminifera (LBF) to MECO. The LBF assemblages in the Kohat Formation, covering from the Shallow Benthic Zones (SBZ) 15 to 17, suggest middle Lutetian to early Bartonian in age. Microfacies analyses indicate a lagoonal (inner carbonate ramp facies belt) to open marine (middle carbonate ramp facies belt) paleodepositional environment of the Kohat Formation. A distinct positive δ13C shift marks the stratigraphic position of the MECO in this formation. At the Peak-MECO event that is marked by the onset of the positive carbon isotope excursion (CIE), no evident compositional variation in the LBF assemblages is observed. However, significant changes in the LBF assemblages with the local first and last occurrences of some LBF genera can be observed in the Post-MECO and CIE recovery phase. These changes are verified by the sudden disappearance of Alveolina and orthophragminids and initial dominance of larger shell-size Nummulites fabianii, Heterostegina, and Linderina species accompanied by an increase in the species diversity. Here, we argue that the change in the observed LBF assemblages in the uppermost part of the Kohat Formation might be related to a larger foraminiferal turnover occurring during the Post-MECO event and corresponds to the CIE recovery phase.

Inner Shelf Benthic Foraminifera of the South China Sea, East Coast Peninsular Malaysia

Abstract The distributional patterns of modern benthic foraminifera from the inner shelf of the southern South China Sea, off the east coast of peninsular Malaysia, are documented for the first time. The study area from Tanjung Sedili, Johor in the south to Marang, Terengganu, in the north was selected for a sand-resource study by the Minerals and Geoscience Department, Malaysia in 1993. Twenty-four surface sediment samples from <50 m water depth contained 266 foraminiferal species belonging to 6 orders, 49 families, and 117 genera, including 32 agglutinated, 130 calcareous hyaline, and 104 calcareous porcelaneous species. Two biofacies were distinguished by cluster analysis. Biofacies A was characterized by high relative abundance of Amphistegina papillosa and few other larger benthic foraminiferal (LBF) taxa in sandier sediments. Biofacies B was characterized by Pseudorotalia schroeteriana and other small rotaliids that were found in muddy sediments. The following features of foraminiferal assemblages and sediments reflect the strong fluvial/terrestrial influence on this tropical shelf environment: the overall moderate foraminiferal diversity, dominance of rotaliids over miliolids, overall dominance of smaller foraminiferal assemblages by Elphidium crispum, the limited diversity of LBF, the limited abundance of A. lessonii and the dominance of LBF assemblages by A. papillosa at relatively shallow depths. These features indicate at least intermittently turbid waters with limited light penetration and the dominance of the shelf sediments by siliciclastics, with mean percent carbonates <35%.

Global evolution and paleogeographic distribution of mid-Cretaceous orbitolinids.

Members of the Larger Benthic Foraminiferal (LBF) family Orbitolinidae occurred from the Cretaceous to the Paleogene, however, they were most diverse during the mid-Cretaceous, and dominated the agglutinated LBF assemblages described from limestones of that period. Various orbitolinid species have been used to zone and date lithologies formed in the shallow, warm waters of the Aptian to the early Cenomanian, and many, sometimes inaccurate, generic and sub-generic nomenclatures have been proposed to differentiate the often-subtle morphological changes that orbitolinids exhibit over time. Also, until now, it has not been possible to develop an effective global overview of their evolution and environmental development because descriptions of specimens from Asia have been relatively rare. Following our recent study of over 1800 orbitolinid-rich thin sections of material from 13 outcrops of Langshan limestone, from the Southern Tibetan Plateau, and from the Barito Basin, South Kalimantan, Indonesia, it has been possible to compare the stratigraphic ranges of these orbitolinids with previously described Tethyan and American forms, based on the use of a planktonic zonal (PZ) scheme, itself tied to the most recent chronostratigraphic scale. This has allowed the reconstruction of the phylogenetic and paleogeographic evolution of the orbitolinids from their Valanginian origin in the Tethys. Although the Tethys remained the paleogeographic centre for the orbitolinids, it is inferred here for the first time that a bi-directional paleogeographic migration of some orbitolinid genera occurred from the Tethys to the Americas and also to the Western Pacific region. Our observations and dating suggest that global marine regressions in the Aptian were coincident with, and may well have facilitated, these orbitolinid transoceanic migrations. Migration stopped however after rising sea level in the early Albian appears to have again isolated these provinces from each other. Tectonic forces associated with the subduction of the Farallon Plate and further sea level raises led to the opening of the Western Interior Seaway in North America, which correlates with, and may have been the cause of, the middle Albian (top of PZ Albian 2) extinction of the American orbitolinids. The extinction of the orbitolinids revealed that the Western Pacific province was split into two sub-provinces, with extinction occurring at the end of the early Albian (top of PZ Albian 1) in the Northwest Pacific sub-province, and at the end of the Albian (top of PZ Albian 4) in the subprovince that is today South East Asia (on the margins and west of the Wallace Line). The final near extinction of the orbitolinids occurred at the end of the Cenomanian in the Tethyan province, which coincides with, and may have been caused by, global anoxic oceanic events that correlate with a near-peak Mesozoic eustatic sea level high-stand that led to the overall global collapse of the paleotropical reef ecosystem at that time.

Mid-Eocene (Bartonian) larger benthic foraminifera from southeastern Turkey and northeastern Egypt: New evidence for the palaeobiogeography of the Tethyan carbonate platforms

Larger benthic foraminiferal assemblages from the mid-Eocene (Bartonian) sedimentary successions of the Tethyan carbonate platforms have been studied in southeastern Turkey and northeastern Egypt. In the Hazro–Diyarbakir section (SE Turkey), small-medium miliolids and textularinids are identified from the lower intervals of the Hoya Formation, whereas alveolinids and soritids (porcellaneous) and orbitolinids (agglutinated) increase in diversity and abundance in the upper intervals. The Dictyoconus aegyptiensis (Chapman) and Somalina stefaninii Silvestri are recorded for the first time from the Hoya Formation. The larger benthic foraminiferal assemblage from the Hoya Formation shows a significant similarity to those reported from the Observatory Formation (coeval with the Sannor Formation) in the Cairo–Suez district (NE Egypt). The studied foraminiferal assemblages imply restricted lagoonal–tidal flat palaeoenvironments. Palaeobiogeographically, the larger benthic foraminiferal assemblages recorded in southeastern Turkey and northeastern Egypt carbonate platforms display a strong affinity to the Arabian, Middle East and African platforms. The position of the global sea-level and the plate tectonic organization of the studied region during the Bartonian were the main factors that facilitated faunal exchange within the carbonate platforms.

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.

Micropalaeontological evidence for the late Oligocene Oi‐2b global glaciation event at the Zarabanda section, Spain

We present an integrated micropalaeontological study (using smaller and larger benthic foraminifera, planktonic foraminifera and calcareous nannofossils) of the late Oligocene from the Zarabanda section (western Tethys), in order to reconstruct the palaeoenvironmental turnover and to precisely establish the chronology of the palaeoclimatic events. Planktonic foraminifera show that the Zarabanda section is apparently continuous and spans the planktonic foraminiferal zones O5 (upper part), O6 and O7, and the calcareous nannofossil zones CP19b and CN1a (lower part). The quantitative analysis of smaller benthic foraminiferal assemblages enabled us to determine a middle–lower bathyal depth of deposition for most part of the section. The smaller benthic foraminifera show a bloom of neritic species, an increase in the percentages of cool‐water species, a decrease of the Planktonic/Benthic ratio, generic richness, heterogeneity and diversity all around a 10m thick succession of calcarenites, in the lower part of the planktonic foraminifera O6 Zone and the middle part of the calcareous nannofossil CP19b Zone. These changes coincide with the major turnover in the larger benthic foraminiferal assemblages; the last occurrence of Eulepidina dilatata and the first occurrence of Nephrolepidina morgani. These variations in the smaller and larger benthic foraminiferal assemblages may be associated with the major expansion of the Antarctic Ice Sheet at approximately 26.7Ma, which is known as the Oi‐2b global glaciation event.

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.

Latitudinal changes in larger benthic foraminiferal assemblages in shallow‐water reef sediments along the Ryukyu Islands, Japan

Abstract The taxonomic diversity of hermatypic corals decreases with increasing latitude, which correlates with sea‐surface temperatures. However, little is known about latitudinal changes in the taxonomic diversity and biogeographic patterns of larger benthic foraminifera, although their physiological requirements are similar to those of hermatypic corals because of their symbiotic relationships with microalgae. The present study examined how the abundance and taxonomic composition of larger foraminiferal assemblages in shallow‐water reef sediments change with latitude along the Ryukyu Islands (Ryukyus), which are located near the northern limit of coral‐reef distributions in the western Pacific Ocean. Three islands from different latitudes in the Ryukyus were selected to investigate latitudinal changes in larger foraminiferal assemblages: Ishigaki Island (24°20′N, 124°10′E), Kudaka Island (26°09′N, 127°54′E) and Tane‐ga‐shima Island (30°20′N, 131°E). Four sediment samples were taken at each of three topographic sites (beach, shallow lagoon and reef crest) on the reef flat of each island. Foraminiferal tests of a 2.0‐ to 0.5‐mm size fraction were selected, identified and counted. The variations in foraminiferal abundance in reef sediments from three latitudinally different islands exhibit two contrasting trends along reef flats: a shoreward decrease on Ishigaki and Tane‐ga‐shima Islands and a shoreward increase on Kudaka Island. A total of 25, 24 and 13 foraminiferal taxa were identified in Ishigaki, Kudaka and Tane‐ga‐shima Islands, respectively. Baculogypsina sphaerulata, Neorotalia calcar and Amphistegina spp. were dominant (i.e. >3% of foraminiferal assemblages) in the three islands. Calcarina gaudichaudii and Calcarina hispida were common on Ishigaki and Kudaka Islands but were absent on Tane‐ga‐shima Island. Larger foraminiferal assemblages from three different reef‐flat environments on Ishigaki Island can be distinguished, whereas those from the three environments on Kudaka and Tane‐ga‐shima Islands are similar in composition. These latitudinal changes in larger foraminiferal assemblages in reef sediments may possibly be caused by variations in the topography of reef flats, distributions and standing crops of living foraminifers on reef flats, and the northern limit of some calcarinid species in the northern Ryukyus.