During the Late Campanian, sea-level fluctuation occurred in the form of two chert bands and the topmost conglomeratic phosphate beds (CF8a zone). A great transgression event occurred associated with the global warming (CF8b zone) trend indicated by large double-keeled foraminifera as Globotruncana aegyptiaca Nakkady of oligotrophic conditions. Through the event of CF6/CF5 zones, a gradual increase in the relative abundance of Gublerina rajagopalani Govindan and Planoheterohelix planata (Cushman) and other heterohelicids have been shown to tolerate and thrive in a wide range of environmental conditions as in high-stress environments. Sea-level fall at the CF6 zone and the overlying CF5 boundary marked a warming climate for the Middle Maastrichtian Event. In the latest CF4 records, the beginning of the decrease in planktic/benthic ratio, globotruncanids, rugoglobigerinids, and heterohelicids indicating a fall in sea level coincided with the CF4/CF3 and the development of dark grey shales in a regressive regime. The observed low abundance of planktic specimens may be due to the presence of pyrite with black shale interval suggesting low oxygen condition. The latest zones CF3, CF2, and CF1 are generally characterized by heterohelicids blooms specially Planoheterohelix globulosa (Ehrenberg), and a gradual decrease in diversity associated with the warming phase before the K/Pg boundary event, implying high biotic stress to even absent of Guembelitria cretacea species through CF3–CF1 zones. Pseudotextularia elegans (Rzehak) occurs in the zones CF4 and CF2 warming of phases 1 and 2 of Deccan Trap Volcanic. The absence of P. elegans (Rzehak) in the zone (CF3) is postulated due to a tectonic cause (maximum cooling of Deccan Trap Volcanic) whereas its absence in the zone (CF1) was due to regression of phase 2.
The Duwi Basin is located in the south-central part of Egypt, facing the northern part of the Red Sea. The Egyptian central platform  is characterized by a series of platforms and different troughs with different depths containing different Cretaceous sections. Gebel Duwi Basin is one of the important NW deep troughs (Figure 1). The upper Campanian–Maastrichtian (C/M) interval of the Dakhla Formation (i.e., Hamama Member) in the Gebel Duwi section is still poorly documented as compared to the Sinai and the Western Desert, despite numerous Late Cretaceous biostratigraphic studies performed all over the Egyptian stable shelf  by various researchers and investigators on different fossils groups.
The Dakhla Formation at the Duwi Basin has not been calibrated for the biostratigraphy of different foraminiferal groups. This formation has been investigated, and its planktic foraminiferal assemblages characterize the micropaleontological analyses, most precise biostratigraphy, and contribute to the database of a comprehensive interpretation of the development of the Duwi Basin through the Late most C/M interval. The planktonic foraminiferal biostratigraphic schemes especially, the Maastrichtian interval have been largely developed based on the Tethyan deposits in Italy, Tunisia, and the Blake Nose (subtropical North Atlantic) [2,3,4,5,6]. To test the applicability of that scheme to the Egyptian southern Tethyan realm at Gebel Duwi, it is necessary to initiate stratigraphic documentation of foraminiferal biostratigraphy.
The planktic foraminiferal species population changes indicate increased high-stress environments beginning from the Late Campanian to the Lower Maastrichtian. Refs [2,3,7,8] illustrated that the Deccan eruptions increased the temperature of the seawater by 3–4°C and the temperature of surface waters by 4–7°C causing transgressions of the sea during CF4 and CF2. This volcanism released about 19 trillion tones of H2SO4 and 270 billion tons of HCl into the atmosphere, causing acid rains, and reducing the pH level of the shallow ocean waters .
Specific objectives of this work include establishing a detailed database of lithological and planktic foraminiferal species from the Duwi Basin section for determining some paleoecologic aspects within the water column based on the relative densities of important species, identifying characteristics of specific biozonation of the Maastrichtian stage and reconstruction changes in specific species as zonal markers for the Tethyan sections at least in Egypt.
2 Geologic setting and lithology
The depositional history of the Duwi Basin started in the Early Cretaceous times, which has been related to the tectonic evolution of the North African passive continental margin [10,11]. The Upper Cretaceous sediments outcropping at Gebel Duwi are represented by Quseir, Duwi, and Dakhla Formations [12,13,14,15,16] of the eastern-south Tethyan biogeographic realm.
The basement complex is unconformably overlain by the Pri-Rift sediments of the northwestern margin of the Red Sea that filled these hanging wall synclines with Paleozoic, Cretaceous, and Paleogene sediments [14,17]. The lower part of these sediments is Nubian facies, overlain by the 500–700 m thick interbedded shales, sandstones, and limestones of the Quseir, Duwi, Dakhla, Tarawan, Esna, and Thebes Formations [12,14].
The Gebel Duwi section is the most complete Maastrichtian sequence of the Egyptian Eastern Desert localities . The regions of central and southern Egypt after Coniacian were subjected to sea-level fluctuations throughout the Late Campanian. During the Early Maastrichtian sea-level transgression, open marine conditions were established in the Eastern Desert (including the Gebel Duwi section), where the erosion was generally limited to other localities of the stable shelf. The Dakhla Formation at the extreme eastern side close to the Red Sea in Gebel Duwi covers the Cenomanian–Late Campanian stages of both Quseir and Duwi Formations [15,16].
Said  introduced and described a section of 130 m thick gray shales becoming marl at the base with calcareous sandy and silty beds of the scarp north Mut section at Dakhla Oasis, Western Desert. Authors of refs [13,19] fixed the Maastrichtian age to the lower part and Early Danian of the upper part of the Dakhla Formation. Meanwhile, authors of refs [20,21] ratified that the age of the Dakhla Formation extends to the Selandian Stage. Author of ref. , at Gebel Duwi, divided this formation into a lower Hamama Member of the Maastrichtian age and the upper Beidaa shale Member of the lower Paleocene. Author of ref. , in Gebel Duwi, redefined the Maastrichtian Hamama Member as 50 m thick and made up of marl beds overlying the Duwi Formation and underlying the about 85 m thick Lower Paleocene Beidaa Member. The Hamama Member (Lower Dakhla Formation) is unconformably overlying the Duwi Formation and unconformably underlain by the Beidaa Member (upper part of the Dakhla Formation). Two chert bands and topmost conglomeratic phosphate beds separate the uppermost Duwi Formation and the Dakhla Formation. Lithologically, the lower part of the Hamama Member consists of dark gray to black organic-rich calcareous clays that change to grayish, greenish, yellow marls and intervening shales in lower beds, few limestone thin ledges at the mid-section, and monotonous greenish to grey marl and shales with gypsum veins at upper levels. The top of the Hamama Member is the erosional surface of a hiatus that contains reworked fossils in a clay matrix due to local shallowing tectonic activity, sea-level fluctuation, and regression [19,23].
3 Materials and methods
High-resolution sampling was done to analyze planktic foraminiferal assemblages from the Hamama Member (lower part of the Dakhla Formation in the Gebel Duwi section). The collected 45 rock samples represent 50 m thick defined section belonging to the late C/M interval. In the laboratory, 100 g of each sample was washed through a 63 µm sieve, dried in an oven at a temperature below 50°C, and then sorted for picking out the foraminiferal species. For each sample, the planktic numbers are separated from the benthic numbers to calculate the planktic/benthic (P/B) ratio. Accordingly, at least 300 planktic foraminifers are selected from each sample. The same number or more was considered for P/B ratio calculation from the fractions of ≥100 µm and ≥63 µm. Data of the specimens count are presented as relative abundance percent to extract all wanted curves of the selected species and morphotypes, and morphogroups are plotted against the stratigraphic succession.
Picked planktic specimens are grouped, identified, and scanned (Scanning electron microscope (SEM) micrographs) using an SEM from Alexandria University (JSM-IT200, Series JEOL). The original samples are preserved with the author’s collections deposited at the Menoufia University Museum Collection (Figures 2–5).
Despite the good distribution and very good excellent mode of preservation of the planktic foraminifera, there is no published record of this microfossil group dealing with the more complete Maastrichtian interval section of the Gebel Duwi section. For this reason, it is important to conduct a detailed biostratigraphic scheme for the planktic foraminiferal analysis (Figures 6–8) to assign the precise age dating to the Hamama Member (lower part of the Dakhla Formation). The planktic foraminiferal contents of the Gebel Duwi section are typical Tethyan characters (Table 1). The zone and subzones which have been identified and documented here clearly show a similar sequence of correlatable bioevents with some amendment or adjustment or modulation or even permutation according to our materials.
Note: Dark color means barren interval.
4.1 Globotruncana aegyptiaca zone (CF8)
The Globotruncana aegyptiaca zone is subdivided into two subzones (CF8a and CF8b) to yield a higher resolution time control .
4.1.1 Globotruncana aegyptiaca (CF8a)
Definition: It is defined as an interval from the first appearance datum (FAD) of the Globotruncana aegyptiaca Nakkady to the FAD of the Rugoglobigerina hexacamerata Brönnimann with modification following ref.  of the Late Campanian stage. This biozone comprises the uppermost layers of the underlying Duwi Formation and the lowest beds of the Dakhla Formation (Hamama Member).
Age: Late Campanian.
Thickness: 6.0 m (samples 1 and 2)
Remarks: Ref.  illustrated that between 72.48 and 71.00 Ma, the FAD of Globotruncana aegyptiaca Nakkady was the marker of the earliest Maastrichtian age [25,27,28], while some authors considered that the FAD of Globotruncana aegyptiaca Nakkady lies within the Late Campanian [2,5,24,29,30,31].
4.1.2 Rugoglobigerina hexacamerata (CF8b)
Author: Ref. .
Definition: This subzone is defined by the FAD of Rugoglobigerina hexacamerata Brönnimann at the base and the FAD of Gansserina gansseri (Bolli).
Age: Late Campanian.
Thickness: 5.40 m (samples 3–7)
Remarks: We can offer here the first appearance of Globotruncanella compressiformis (Pessagno) as another foraminiferal index species of the Late Campanian besides Rugoglobigerina hexacamerata Brönnimann at least in the stratigraphy of Gebel Duwi section.
4.2 Gansserina gansseri zone (CF7)
Authors of ref.  divided the Gansserina gansseri zone into three subzones: the Late Campanian Rugoglobigerina rotundata subzone CF7a, and the two Early Maastrichtian Rugoglobigerina scotti CF7b subzones and Planoglobulina acervulinoides CF7c subzones (Table 2).
Definition: According to several authors in the published literature, the FAD of Rugoglobigerina rotundata slightly postdates the FAD of Gansserina gansseri [27,34,35,36], in this case, the Rugoglobigerina rotundata subzone could be correlated to the lower part of the Gansserina gansseri zone.
4.2.1 Rugoglobigerina scotti subzone (CF7b)
Author: Ref. .
Definition: The subzone is defined by the FAD of Rugoglobigerina scotti (Brönnimann) at the base and the FAD of Contusotruncana contusa (Cushman) at the top.
Age: Early Maastrichtian.
Thickness: 4.30 m (samples 8–11)
Remarks: The C/M boundary has not been exactly defined. Authors of refs [38,39] illustrated that the C/M boundary is placed at the top of Baculites eliasi ammonite zone (macrofossils), and strontium isotope data correlations between the Kronsmoor section of Germany, English Chalk, and the United States Western Interior.
On the other hand, Authors of ref.  estimate an age of 71.6 ± 0.7 Ma for the C/M stage boundary. However, in the Tercis-Les-Basins section (Landes, France), the ammonite datum event Pachydiscus neubergicus (Von Hauer) was introduced as the marker of the C/M boundary as compared with FAD of Rugoglobigerina scotti (Brönnimann) and Contusotruncana contusa (Cushman) in 72 Ma [35,41,42].
Authors of ref.  informally used the datum of Rugoglobigerina hexacamerata Brönnimann and Planoglobulina carseyae (Plummer) at 71 Ma for the C/M boundary based on biostratigraphic correlation with the geomagnetic time scale at DSDP site 525 (northwestern Tunisia). Author of ref.  placed the FAD of Rugoglobigerina pennyi Brönnimann for the C/M boundary. Meanwhile, this taxon was recorded by authors of ref.  throughout the Maastrichtian, but not in the Campanian. It is worth mentioning that the FAD of Gublerina cuvillieri was adopted by author of ref.  and confirmed by authors of refs [5,47], as a reliable marker for placing the C/M boundary.
4.3 Contusotruncana contusa (CF6)
Author: Ref. .
Definition: It is defined by the first appearance of the nominate taxon C. contusa (Cushman) at the base of 70.15 Ma [4,49] and the last appearance of Globotruncana linneiana (d’Orbigny) at the top. This zone is equal to the CF6 zone of refs [2,3,24].
Age: Early Maastrichtian.
Thickness: 6.80 m (samples 12–18)
Remarks: An overflow of faunal turnover characterizes this zone as commented by authors of refs [2,3]. According to ref. , the first appearance of Pseudotextularia intermedia (De Klasz) was at 69.83 Ma level and Pseudotextularia elegans (Rzehak) at 69.55 Ma. The presence of Globotruncana linneiana (d’Orbigny) marks the Early Maastrichtian zone CF6. In the present material of the Gebel Duwi section, the Pseudotextularia intermedia (De Klasz) taxon is completely absent as a marker taxon after Contusotruncana contusa (Cushman) as investigated by the authors of refs [2,3], but it was noticed within the Late Maastrichtian interval of the Duwi section (CF3 zone).
4.4 Pseudotextularia intermedia (CF5)
Author: Ref. .
Definition: Pseudotextularia intermedia (De Klasz) is absent in the present study, so the definition by authors of refs [2,3] is used to define zone CF5 in the present work. The biostratigraphic interval extended from the last appearance datum (LAD) of the Globotruncana linneiana (d’Orbigny) at the base to the FAD of Racemiguembelina fructicosa (Egger) at the top.
Age: Late Early Maastrichtian.
Thickness: 6.40 m (samples 19–23)
Remarks: The authors recommended renaming this zone due to the absence of Pseudotextularia intermedia (De Klasz) in the study area. On the other hand, Authors of ref.  recorded P. intermedia (De Klasz) in Campanian rocks (zone CF10) in the Jorband section (north Iran). The interval of the CF5 zone [2,3] is represented by a community barren of Pseudotextularia intermedia (De Klasz) taxon through Maastrichtian of the Dakhla Formation of Central Egypt and Western Desert .
4.5 Racemiguembelina fructicosa (CF4)
Author: Ref. .
Definition: Authors of ref.  defined this zone by the first appearance of Racemiguembelina fructicosa (Egger) at the base and Abathomphalus mayaroensis (Bolli) at the top. In the present study, the authors follow the definition given in refs [2,3,24], the first appearance of Racemiguembelina fructicosa (Egger) marks the base of this zone and the first appearance of Pseudogumbelina hariaensis Nederbragt marks the top. The first occurrence of Racemiguembelina fructicosa (Egger) was at 69.02 Ma level, just slightly above the first occurrence of Abathomphalus mayaroensis (Bolli) taxon at 69.33 Ma. Racemiguembelina fructicosa zone marks the Early/Late Maastrichtian stage .
Age: Latest Early Maastrichtian
Thickness: 12.50 m (samples 24–32)
Remarks: The boundary between Racemiguembelina fructicosa zone (CF4) and the overlying Pseudoguembelina hariaensis zone (CF3) is marked by abrupt thin layers of some dark gray shales represented by the majority of very small (dwarfed) planktic species of compressed tests and impregnated by reddish colors as Globigerinelloides spp., mostly decrease in the number of globotruncanids except Globotruncana esnehensis Nakkady and flooding of heterohelicids which they considered more opportunistic taxa.
Several Late Campanian species disappeared in the late boundaries of this zone, such as Archaeoglobigerina cretacea (d’Orbigny) and others. Globotruncanita conica (White) appeared through this zone at a level of 68.75 Ma, and Racemiguembelina powelli at level 69.14 Ma .
Pseudotextularia elegans (Rzehak) indicates a connection of the basin with the warm Tethys . The dark grey shales towards the upper interval of zone CF4 record the beginning of the decrease in P/B in globotruncanids, rugoglobigerinids, Globigerinelloides, and heterohelicids (Figures 7 and 8) implying a fall in sea level and development of black shale in a regressive regime.
4.6 Pseudoguembelina hariaensis (CF3)
Author: Ref. 
Definition: This zone is defined by the FAD of the Pseudoguembelina hariaensis Nederbragt at the base and the FAD of Pseudogumbelina palpebra Brönnimann and Brown at the top. Also, the top of this zone is defined as the LAD of G. gansseri (Bolli) [2,3,24] and is equal to the CF3 zone.
Age: Late Maastrichtian.
Thickness: 8.90 m (samples 33–39)
Remarks: Authors of refs [52,53,54,55] recorded that the world’s major sea-level fall hiatus coinciding with the CF4/CF3 boundary interval may cause common dwarfed specimens in several horizons and extinctions of Globotruncana bulloides Vogler at level 67.87 Ma and Planoglobulina carseyae (Plummer). The smaller size fractions <63 µm contain the first occurrence of common Pseudoguembelina hariaensis Nederbragt, a new generation of heterohelicids, Globigerinelloides, absent of Guembelitria cretacea and rare Globotruncanita stuarti (de Lapparent) and Globo. conica (White). This zone is characterized by declined Gansserina gansseri (Bolli), which is a very rare or absent taxon.
The faunal changes indicate continued and even increasing high-stress environments during the CF3 zone [56,57]. The top of the CF3 zone is marked by the extinction of Globotruncana linneiana (d’Orbigny) (at level 66.96 Ma) according to ref. . Authors of ref.  recorded Globotruncana esnehensis Nakkady with Pseudoguembelitria hariaensis Nederbragt in the subtropical North Atlantic. The observed low abundance of planktic specimens between zones CF4 and CF3 (Figure 7) may be due to the presence of pyrite with black shale interval suggesting low oxygen condition. This event coincides with global environmental perturbations of sea-level fall at ∼ 66.25 Ma [52,58] and is recognized in the Indian and Tethyan oceans [53,54,55,59,60].
4.7 Pseudoguembelina palpebra (CF2)
Definition: In the present study, it is defined as a biostratigraphic interval extended from the FAD of the Pseudoguembelina palpebra Brönnimann and Brown at the base and the FAD of Plummerita hantkeninoides (Brönnimann) at the top of the biozone.
Age: Late Maastrichtian.
Thickness: 0.70 m (samples 40–43)
Remarks: Unfortunately, new species have stopped appearing through this zone interval, and thereby we have used the last appearance and occurrence (i.e., complete extinction) of Globotruncana linneiana (d’Orbigny) and Pseudoguembelina palpebra Brönnimann and Brown and Gansserina gansseri (Bolli) and the only first appearance of Plummerita hantkeninoides (Brönnimann) with an estimated duration of ∼180 at the top limit of this zone [55,61].
Zone CF2 summarizes the faunal diversity and species abundance data related to high-stress environments of Deccan volcanism phase 2 (Late Maastrichtian). During the Late Maastrichtian stage, the warm water Tethyan planktic foraminifera Pseudotextularia elegans (Rzehak) occurs in the Racemiguembelina fructicosa zone (zone CF4), and Pseudoguembelina palpebra zone (zone CF2). The absence of P. elegans (Rzehak) in the Pseudoguembelina hariaensis zone (zone CF3) is related to tectonic cause whereas its absence in the Plummerita hantkeninoides zone (zone CF1) was due to regression.
4.8 Plummerita hantkeninoides (CF1)
Author: Ref. 
Definition: This zone is defined by the total range of the nominated taxon, Plummerita hantkeninoides (Brönnimann). This zone is equal to the CF1 zone of ref. .
Age: Latest Maastrichtian.
Thickness: 0.70 m (samples 44–45)
Remarks: The biggest surprise is the lack of “Guembelitria blooms” , which are present in several sections of Al-Kef, Elles (in Tunisia, [55,64,65], Israel, Spain, Italy, Brazos (Texas), northern Mexico, Bulgaria, Kazakhstan, Denmark, Madagascar, southern middle latitudes, high latitudes, southern ocean, and others  and in the Indian Ocean. In the Qareiya area in Central Egypt, authors of refs [56,63] recorded low Guembelitria abundance indicating that the Late Maastrichtian of Egypt experienced similar high-stress conditions as Danian data. The high content of small heterohelicids indicates highly abundant blooms in the Duwi section. Like the case of the Gebel Duwi section, authors of ref.  recorded no abundance of Guembelitria cretacea in the Late Maastrichtian of SW Iran.
The maximum Maastrichtian cooling at 66.25 Ma is followed by rapid warming of 3–4°C in the oceans and 7–8°C on land in zones CF2 and CF1 [2,3,67,68,69] correlative with Deccan phase 2 . The climate warm events in zones CF2 and CF1 are accompanied by decreased planktic foraminiferal diversity and increased dissolution effects [57,71,72,73].
5.1 Species richness
The major increase in species richness had no significant effect on the relative abundance of long-ranging species of Globigerinelloides and caused no significant extinctions. Globotruncanita stuarti (de Lapparent) and Globotruncana orientalis El-Naggar are common in Late Maastrichtian but do not exceed 3% of each of the total population. Species diversification of the planktic foraminifera during the Late Maastrichtian (CF4), which results in the highest species richness characterizing the studied section is more than 50 species (Table 1) (Figure 8).
The Latest Campanian species richness varies from 30 to 20% in zone CF8a to CF8b respectively, the Latest Maastrichtian ranges from 15% in zone CF7 and CF6 to 13% in zone CF5 and 40% in zone CF4. The Late Maastrichtian ranges from 16 to 13% in zones CF2 and CF1 (Figure 8).
Species diversification of the planktic foraminifera during the Late Maastrichtian (CF4), which results in the highest species richness characterizing the studied section is more than 50 species, also in the interval between C8a and C8b, species diversification of the planktic foraminifera is very high and nearly equal to those in CF4, so there are two important intervals of highest diversification.
5.2 Species population changes
5.2.1 Campanian (CF8a and CF8b)
Sea level fluctuation occurred in the form of two chert bands and topmost conglomeratic phosphate beds (zone CF8a), which separate between the uppermost Duwi Formation and the lowermost of the Dakhla Formation (Hamama Member). The Latest Campanian stage CF8b zone belongs to the lower stratigraphic interval of the Hamama Member. The keeled species of Globotruncana, Contusotruncana, and Globotruncanita show strong fluctuations through all the Campanian intervals (Figure 7) adopting typical Tethyan assemblages [74,75]. During the Late Campanian, a great transgression event occurred associated with global warming (CF8b zone) trend indicated by large double-keeled foraminifera as G. aegyptiaca Nakkady of oligotrophic conditions [76,77,78].
There is an increase in depth with a high rate of speciation in planktic components indicating the time of expansion of the Egyptian continental shelf and continued into the Maastrichtian (CF8b) . The most important first-appearance species that characterize the lowermost Dakhla Formation of the G. aegyptiaca (CF8a) and R. hexacamerata (CF8b) are small forms of the planktic species.
Periods of shallowing with organic shales indicate environmental stress conditions . Most of the Globotruncana aegyptiaca CF8a zone probably migrated to the basin during suitable depths of warm intervals and nutrients. More than 60% of the recorded species in CF8a are present in CF8b and continued to CF5 at least.
5.3 Early Maastrichtian (CF7–CF4)
Through the event of the CF6 zone, a gradual increase in the relative abundance of Gublerina rajagopalani Govindan ranging between 0.03 and 6.7%, Planoheterohelix planata (Cushman) ranging between 2.7 and 9.3%, and other heterohelicids to 30% (Figure 8) have been shown the tolerate and thrive in a wide range of environmental conditions as in high-stress environments affected by temperate, salinity, nutrients, and oxygen variations due to sea-level fluctuations and receiving more meteoric water. These conditions may increase the population, abundance, and stability of life of different organisms [56,57].
CF6 zone is characterized by the last appearance of Globotruncana linneiana (d’Orbigny) at the top marking the base of CF5 with the first appearance of Racemiguembelina fructicosa (Egger) coincident with another sea-level fall at CF6 zone and the overlying CF5 boundary marking a major sequence boundary dated 69.4 Ma [52,58] with minor extinctions at the end of warming climate which is called Middle Maastrichtian Event (MME).
The first occurrence of Pseudotextularia intermedia (De Klasz) (CF5) must be recorded at 69.83 Ma level , meanwhile, it is only recorded partially through the Pseudoguembelina hariaensis (CF3). We think that the disappearance of Pseudotextularia intermedia (De Klasz) may be interpreted as due to ecological reason. The same observation was noticed by refs [7,29,55,59,67,73,80]. Authors of ref.  in Tunisia considered CF5 as a gap or partially representing a hiatus interval.
The latest zone CF4 records a sharp decrease in P/B, globotruncanids, rugoglobigerinids, and heterohelicids implying a fall in sea level and the development of dark grey shales in a regressive regime. Authors of ref.  recognized that ocean acidification and reduced water mass stratification are contributors to the high-stress environments of the planktic foraminiferal assemblages except for the opportunistic species during the Late Maastrichtian CF4–CF2 zones.
The high-stress environments induced by the Deccan eruption prevented the normal growth in some Late Maastrichtian planktic foraminifera such as Guembelitria [56,63,80,81]. The Deccan eruption occurred in two phases: the first phase happened during CF4 (normal chron C30N ∼ 66.5–67 Ma), and the authors of ref.  aged that to be during 66.6 Ma. The most vigorous second eruption occurred during CF2 (65.4–65.2 Ma) [83,84,85].
The sediment deposition began in the Late Maastrichtian with global cooling and a sea-level fall at ∼66.8 Ma (CF4 and CF3) showing warm temperatures and rapid cooling before the CF4/CF3 hiatus. The climate warmed to 2–3°C before and after this cooling and appears correlative with the onset of Deccan volcanism (phase 1) . During the CF4 and CF3 zones, the climate warm events are accompanied by decreased planktic foraminiferal diversity and increased dissolution effects [57,71,72,73].
Pseudotextularia elegans (Rzehak) flourished in the unusual environment and developed morphological diversities implying that the environment had influenced this species, and other planktic foraminifera reduced test size to avoid the strike of the Deccan volcanism. Some other associated planktic foraminifera also shows abnormalities in these environments . Thus, the reduction in the size of the planktic foraminifera is produced by major environmental factors including warming of climate, volcanogenic factors such as aerosols (acid rain), CO2 emissions related to Deccan volcanism, and/or sea-level changes provoked by local tectonic changes. Zone CF1 largely represents a regressive regime and the general absence of P. elegans (Rzehak) in the zone indicates that the warm water of the sea could not sustain the species .
Many authors noted test irregularity, dwarfing, and unusual variation in a species of Late Maastrichtian planktic foraminifera as an effect of environmental stress [72,88,89,90]. While authors of ref.  mentioned that the biotic effects of Chicxulub impact and “Lilliput Effect”  in some Late Maastrichtian planktic foraminifera were accounted for environmental stress [57,91,93]. Environmental factors caused a change in test size and population density in Pseudotextularia elegans (Rzehak) during the Late Maastrichtian [7,72].
5.4 Late Maastrichtian interval (CF3–CF1)
The globotruncanids appear in low diversity (only seven species) and are detected throughout the Duwi section. Due to more environmental changes and shallowing basin than in west-central Sinai, the most tolerant species such as Globotruncana aegyptiaca Nakkady, G. bulloides Vogler, G. rosetta (Carsey), and G. dupeublei have thrived under shallowing conditions at different levels of gray shales which reduced water mass stratifications .
The foraminiferal species richness began to decline dramatically, and diversity decreased gradually from the interval of the P. hariaensis zone (CF3) affecting the complex subsurface and thermal intermediate dwellers (Figure 8) leading to marked richness of dwarfed heterhelicids as Planoheterohelix globulosa (Ehrenberg), Planoheterohelix planata (Cushman), Gublerina rajagopalani Govindan, and Planoglobulina riograndensis. Pseudoguembelina palpebra zone (CF2) is associated with the development of “photosymbiotic” species Racemiguembelina powelli Smith and Pessagno and Plummerita reicheli, as reported by refs [66,94].
The cooler ecosystem at the Late Maastrichtian time (CF3) and a new sequence of high sea levels between 68.33 Ma and slightly to 66.0 Ma associated with new species of Racemiguembelina fructicosa (Egger), R. powelli Smith and Pessagno, Globotruncana esnehensis Nakkady, and Abathomphalus mayaroensis (Bolli) are originated through this interval. The cosmopolitan Abathomphalus mayaroensis (Bolli) is very rare in tropical, subtropical, and Tethyan paleogeographic provinces. The Globigerinelloides, Rugoglobigerina, and Heterohelicids reached maximum relative abundance through this interval, while the globotruncanids gradually decreased up-section (Figure 7). Minor extinction event is marked by the last occurrence of Contusotruncana contusa (Cushman), Globotruncana bulloides Vogler, and Globotruncanita pettersi and the disappearance of Archaeoglobigerina cretacea (d’Orbigny).
The latest biozones CF2 and CF1 spanning from 67.87 Ma to 66.0 Ma are generally characterized by heterohelicids blooms specially Planoheterohelix globulosa (Ehrenberg), gradual decreasing of diversity associated with the warming phase before the K/Pg boundary event, indicating highly biotic stress [53,54,56,63,89,95].
The Guembelitria cretacea is not registered possibly because of eutrophic conditions  or may result from high terrigenous runoff, upwelling of nutrient-rich water, phosphorus input of pre-existing phosphatic Duwi Formation, indicating environmental instability as recorded by refs. [97,98].
5.5 Dwarfism of species
Authors of ref.  recorded the foraminiferal size reduction and termed it as the “Lilliput effect.” This effect may report the aftermath of mass extinctions and rarely pre-extinction due to a highly stressful environment. The dwarfism of the planktic foraminifera species was observed and documented from the uppermost Maastrichtian sediments in Egypt, Israel, Madagascar, and South Atlantic [53,54,55,59,79,80].
In the Duwi section, dwarfed planktic foraminiferal species were observed at CF8b with some small lower levels of relatively high organic materials. The CF5/CF4 boundary or so-called MME includes dwarfed species of Rugoglobigerina, Heterohelix, Globotruncana bulloides Vogler, and Globotruncanella compressiformis.
Another example is a very small (dwarfed) stress-tolerant species of some Pseudoguembelina which may be adapted to warm temperatures at low latitudes as observed at CF4/CF3 boundary and even through CF3. The dwarfed specimens amounting to an average of 70% in CF2, while 40% in CF1 may be due to a sharp diversity drop. These indicate an upward Maastrichtian interval. There are more stressed environmental conditions globally or locally due to sea-level regression, which may be associated with increased competition and possibly carbonate dissolution pointing to high biotic stress [55,73,94].
5.6 Depth ranking
The recent study by authors of refs [55,94] for determining the depth ranking enables us to recognize the following concepts in the Duwi section.
5.6.1 Deep dwellers
The deep dwellers contain Abathomphalus mayaroensis (Bolli), Globotruncanita stuartiformis (Dalbeiz), Gublerina cuvillieri Kikoine, and Gublerina rajogopalani compared with that cited by refs [55,94], the following points can be noted (Figure 9).
Duwi material contains rare to very rare deep dwellers faunal group, it ranges between 0 and less than 5% maximum.
The Gublerina rajogopalani rarely reaches 5% of all the intervals, it seems to be endemic species to the deepest and coldest habitat .
Despite the “cosmopolitan Late Maastrichtian,” Abathomphalus mayaroensis (Bolli) is very rare or even absent through the present material indicating its endemism to the deepest seas.
5.6.2 Thermocline dwellers
There are thermocline planktic dwellers living in the thermocline layer represented by keeled globotruncanids (Figure 9). Keeled globotruncanids range, in the Duwi section, from the Latest Campanian as more than 35% to less than 25% in the cold CF3 zone and decrease gradually to reach less than 2% at the CF1 zone. This indicates that the globotruncanids are considered cooler group habitats as suggested by authors of ref. .
Both species, Racemiguembelina fructicosa (Egger) and R. powelli Smith and Pessagno are very low in abundance (less than 1%) through the Late Maastrichtian (CF2) confirming the absence of a distinct thermocline layer during the warm interval of the Duwi Basin.
Globigerinelloides species range in abundance between 10 and 25% through the Latest Campanian to an Early Maastrichtian interval of the warmer climate and shallower depth. Meanwhile, Globigerinelloides species increase in abundance to 35% through the Late Campanian to Maastrichtian cold zones (CF8a, CF6, and CF3) (Figure 7) which is considered as a cooler interval. The Globigerinelloides suggest that they inhabited depths shallower than most keeled globotruncanids in the Tethyan water .
5.6.3 Subsurface dwellers
It is considered the biggest and more abundant planktic group in the material of the Duwi basin. It includes a lot of globotruncanids groups, besides all the heterohelicids and rugoglobigerinellids. The subsurface dwellers range in abundance between 45 and less than 70% through the Latest Campanian to Early Maastrichtian, and decrease gradually between 40 and 20% through the CF4 zone and increase in abundance to 60% in the CF3–CF1 zones because of the large increase in abundance of heterohelicids specially Planoheterohelix globulosa (Ehrenberg) (Figure 8). These groups changed habitats, as more opportunistic fauna, from localities and different climates. Planoheterohelix globulosa (Ehrenberg) is more enriched and most abundant through the Latest Campanian (CF8a and CF8b) to the Latest Maastrichtian (CF2 and CF1).
5.6.4 Surface dwellers
This is a group of Pseudoguembelina spp. of the Maastrichtian age that reflects the warm temperatures of the oceans as a “mixed layer” as reported in ref. . It reaches 12% as the maximum abundance of the Duwi section planktic species represented by Pseudoguembelina costillifera, P. costulata (Cushman), P. excolata (Cushman), P. kempensis Esker, P. palpebra Brönnimann and Brown, and P. hariaensis Nederbragt. Authors of ref.  suggested that they inhabited the warmest or shallowest parts of the mixed layer (Figure 8). Rugoglobigerina spp. are observed in more common abundance than the Pseudotextularia spp. The Rugoglobigerina spp. range between 12 and 30% and returns to 20% through the lower Maastrichtian and exceeds the maximum through the CF4 zone (Figure 10).
During the Maastrichtian age, surface dwellers, subsurface dwellers, thermocline dwellers, and a low abundance of deep dwellers are populated by Planoheterohelix globulosa (Ehrenberg) and even Planoheterohelix planata (Cushman). This indicated that low water ranking through different climates with high action of upwelling currents which implies the majority of planktic foraminifera inhabited through different depth levels and sea-level fluctuation with a relatively warm climate period. This conclusion in the Duwi section is greatly in agreement with refs [2,3,55,73,94,100] across different localities of the Tethys, Indian and Atlantic oceans.
5.7 Opportunistic species
The low oxygen tolerant Planoheterohelix globulosa (Ehrenberg) and its associated Planoheterohelix planata (Cushman) significantly increase in relative abundance from the Latest Campanian to the Latest Maastrichtian. P. globulosa (Ehrenberg) ranged between 10 and 18% in abundance through zones CF8a to CF4 and abruptly increase in abundance by about 20% in zone CF3 to more than 45% in both CF2 and CF1 zones. Planoheterohelix planata (Cushman) started its abundance from CF4 to CF1 zones to reach a 10% maximum at different levels. The global warming documented in CF2 and CF1 zones worldwide is recorded in the Duwi Basin by a high decrease or even absence of the global Guembelitria cretacea and high relative abundance of Planoheterohelix globulosa (Ehrenberg) and robust index species Globotruncana esnehensis Nakkady and Globotruncana aegyptiaca Nakkady.
The heterohelicids reach abundance abruptly between 35 and 60% through the CF3 zone and exceed more than 60% in CF2 and CF1 zones. The dominance of heterohelicids has also been recorded in Tunisia, Israel, Italy, Spain, Texas, Denmark , and others indicating decreased water mass stratification and expanded low oxygen environments as ecological stress.
The Planoheterohelix globulosa (Ehrenberg) only shared 45% of this high abundance. The robust index species Globotruncana esnehensis Nakkady increased to 15% in the CF3 zone and returned to 5% at both CF2 and CF1 zones, while the other robust Globotruncana aegyptiaca Nakkady species was 2% in CF3 and returned to less than 1% in CF2 and CF1 zones.
Not only the heterohelicids are considered opportunistic groups but also, Globotruncana aegyptiaca Nakkady and Globotruncana esnehensis Nakkady must be grouped as the most tolerant species. The Duwi Basin in CF2 and CF1 zones became shallower with some organic material, warm humid climate, high rainfall, low salinity and oxygen, and low primary production in a basin of very shallow marginal environment.
6 Summary and conclusion
From the Latest Campanian through the Maastrichtian, four major climate and faunal events are identified that ultimately ended with the K/Pg mass extinction.
Event 1: In the Late Campanian/Early Maastrichtian, maximum cooling included zone CF8a followed by warm zone CF8b of Late Campanian and return to cooling zones CF7 and CF6 of Early Maastrichtian. This event is associated with rapid planktic diversity as a result of increased nutrient input due to enhanced upwelling, coastal erosion, and/or volcanic activity.
Event 2: In the Early/Late Maastrichtian transition, warming occurred during MME (CF5 to lower CF4). Warming led to increased water mass stratification that sustained maximum diversity. A minor extinction of some planktic foraminifera ends Event 2 and marks more stressful marine conditions for foraminifera species living in subsurface and thermocline depths.
Event 3: Return to maximum cooling in zone CF3 increased stress conditions for marine calcifiers leading to reduced species populations and dwarfing. The global cooling and faunal turnover are due to increased CO2 uptake by the oceans as a result of Deccan Trap volcanism.
Event 4: Massive Deccan volcanic eruptions (phase 2) in zones CF2 and CF1 coincident with increasingly high-stress environments, decreasing abundance of large specialized species, and dominance of the disaster opportunist Guembelitria cretacea. Positive effects correlate with increased nutrient input, increased water mass stratification, and increased ecological niches during climate warming. Negative effects can be linked to increased tempo and rate of volcanism resulting in ocean acidification, carbonate crisis, and extinction.
The low oxygen tolerant Planoheterohelix globulosa (Ehrenberg) significantly increase in relative abundance from the Latest Campanian to the Latest Maastrichtian and ranged between 10 and 18% in abundance through zones CF8a–CF4 and abruptly increase in abundance from about 20% in zone CF3 to more than 45% in both zones CF2 and CF1. The global warming documented in CF2 and CF1 zones worldwide is recorded in the Duwi Basin by the absence of the global Guembelitria cretacea and the high relative abundance of Planoheterohelix globulosa (Ehrenberg).
Globotruncana esnehensis Nakkady increased to 15% in the CF3 zone and returned to 5% at both CF2 and CF1 zones, while the other robust Globotruncana aegyptiaca Nakkady species was 2% in CF3 and returned to less than 1% in CF2 and CF1 zones. Globotruncana aegyptiaca Nakkady and Globotruncana esnehensis Nakkady must be grouped as the most tolerant species. The Duwi Basin in CF2 and CF1 zones became shallower with some organic material, warm humid climate, high rainfall, low salinity, and oxygen.
Two chert bands and topmost conglomeratic phosphate beds (zone CF8a) separate the uppermost Duwi Formation and the lowermost of the Dakhla Formation (Hamama Member). The Latest Campanian zones CF8a and CF8b belong to the lower stratigraphic interval of the Hamama Member. The keeled species of Globotruncana, Contusotruncana, and Globotruncanita show strong fluctuations through all the Campanian intervals adopting typical Tethyan assemblages. More than 60% of the recorded species in CF8a are present in CF8b and continued to CF5 at least.
Sea-level fall hiatus coinciding with the CF4/CF3 boundary interval may cause common dwarfed specimens in several horizons and extinctions of Globotruncana bulloides Vogler at level 67.87 Ma and Planoglobulina carseyae (Plummer). The observed low abundance of planktic specimens between zones CF4 and CF3 may be due to the presence of pyrite with black shale interval suggesting low oxygen condition. This event coincides with global environmental perturbations of sea-level fall in the Indian and Tethyan Oceans. Authors of ref.  illustrated that the presence of Pseudotextularia elegans (Rzehak) in zone CF4 indicates a connection of the basin with the warm Tethys. The dark gray shales towards the latest zone CF4 record a sharp decrease in P/B, globotruncanids, rugoglobigerinids, and heterohelicids implying a fall in sea level and the development of black shale in a regressive regime.
Species diversification of the planktic foraminifera during the Late Maastrichtian (CF4), also in the interval C8a and C8b, is very high and nearly equal to those in CF4, so there are two important intervals of highest diversification.
Sincere thanks and gratitude to the Researchers Supporting Project number (RSP2023R249), King Saud University, Riyadh, Saudi Arabia for funding this research article.
Author contributions: This work was carried out by notable contributions from all authors. All authors contributed critically to the drafts and gave final approval for publication. All authors have read and agreed to the published version of the manuscript.
Conflict of interest: The authors declare no conflict of interest.
Taxonomic list of cited planktonic foraminifera with author attributions and dates mentioned in the text and noted in Table 1
Abathomphalus mayaroensis (Bolli, 1951)
Archaeoglobigerina cretacea (d’Orbigny, 1840)
Archaeoglobigerina australis (Huber, 1990)
Contusotruncana contusa (Cushman, 1926)
Contusotruncana morozovae (Vasilenko, 1961)
Contusotruncana fornicata (Plummer, 1931)
Gansserina gansseri (Bolli, 1951)
Globigerinelloides prairiehillensis (Pessagno, 1967)
Globigerinelloides alvarezi (Eternod Olvera, 1959)
Globigerinelloides bollii (Pessagno 1967)
Globigerinelloides escheri (Kaufmann, 1865)
Globigerinelloides messinae (Brönnimann, 1952)
Globigerinelloides multispinus (Lalicker, 1948)
Globigerinelloides subcarinatus (Bronnimann, 1952)
Globigerinelloides volutus (White, 1928)
Globotruncana aegyptiaca (Nakkady, 1950)
Globotruncana bulloides (Vogler, 1941)
Globotruncana linneiana (d’Orbigny, 1839)
Globotruncana mariei (Banner and Blow, 1960)
Globotruncana orientalis (El Naggar, 1966)
Globotruncana rosetta (Carsey, 1926)
Globotruncana dupeublei Caron
Globotruncana esnehensis (Nakkady, 1950)
Globotruncanella petaloidea (Gandolfi, 1955)
Globotruncanella compressiformis (Pessagno, 1962)
Globotruncanita conica (White, 1928)
Globotruncanita insignis (Gandolfi, 1955)
Globotruncanita pettersi (Gandolfi, 1955)
Globotruncanita stuarti (de Lapparent, 1918)
Globotruncanita stuartiformis (Dalbiez, 1955)
Gublerina rajagopalani (Govindan, 1972)
Gublerina cuvillieri (Kikoine, 1948)
Guembelitria cretacea (Cushman, 1933)
Plummerita hantkeninoides (Brönnimann, 1952)
Plummerita reicheli (Bronnimann, 1952)
Muricohedbergella monmouthensis (Olsson, 1960)
Laeviheterohelix dentata (Stenestad, 1968)
Laeviheterohelix glabrans (Cushman, 1938)
Laeviheterohelix pulchra (Brotzen, 1936)
Laeviheterohelix turgida (Nederbragt, 1991)
Planoheterohelix globulosa (Ehrenberg, 1840)
Planoheterohelix planata (Cushman, 1938)
Planoheterohelix labellosa (Nederbragt, 1991)
Planoheterohelix sphenoides (Masters, 1976)
Planoheterohelix stenopos (Masters 1976)
Planoglobulina brazoensis (Martin, 1972)
Planoglobulina acervulinoides (Egger, 1900)
Planoglobulina carseyae (Plummer, 1931)
Planoglobulina multicamerata (De Klasz, 1953)
Planoglobulina riograndensis (Martin, 1972)
Pseudoguembelina costellifera (Masters, 1976)
Pseudoguembelina costulata (Cushman, 1938)
Pseudoguembelina excolata (Cushman, 1926)
Pseudogumbelina hariaensis (Nederbragt, 1991)
Pseudoguembelina kempensis (Esker, 1968)
Pseudoguembelina palpebra (Bronnimann and Brown, 1953)
Pseudotextularia intermedia (de Klasz, 1953)
Pseudotextularia nuttalli (Voorwijk, 1937)
Pseudotextularia elegans (Rzehak, 1891)
Pseudotextularia deformis (Kikoine, 1948)
Racemiguembelina powelli (Smith and Pessagno, 1973)
Racemiguembelina fructicosa (Egger, 1900)
Rugoglobigerina hexacamerata (Bronnimann, 1952)
Rugoglobigerina macrocephala (Bronnimann, 1952)
Rugoglobigerina milamensis (Smith and Pessagno 1973)
Rugoglobigerina pennyi (Bronnimann, 1952)
Rugoglobigerina reicheli (Bronnimann, 1952)
Rugoglobigerina rugosa (Plummer, 1926)
Rugoglobigerina scotti (Bronnimann, 1952)
Rugotruncana subcircumnodifer (Gandolfi)
Rugotruncana circumnodifer (Finlay, 1940)
Spiroplecta americana (Ehrenberg, 1844)
Ventilabrella ornatissima (Cushman and Church, 1929)
Ventilabrella austinana (Cushman, 1938)
Heterohelix vistulaensis (Peryt, 1980)
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