Foraminifera dissolution phases in the upper cretaceous succession of Jebel Duwi, Egypt

The present work examines the function of differential dissolution in planktic and benthic foraminifera and introduces the major factors that affect dissolution in early Mid-Maastrichtian Event MME and Late Maastrichtian Event LME paleoenvironmental reconstructions. Two observations on foraminifera dissolution in the Dakhla Formation, where planktic foraminifera are more susceptible to dissolution than benthic foraminifera in the Hamama Member of the Jebel Duwi section, Eastern Desert (Egypt). They are characterized by a decline in the planktic/benthic (P/B) ratio and a rise in the agglutinated percentage. The results of those observations are used for revealing foraminiferal dissolution in these two zones (Racemiguembelina fructicosa CF4a and Pseudoguembelina palpebra CF2). During these two zones, there were excessive relative abundances of agglutinated foraminifera, indicating that these two intervals witnessed severe carbonate dissolution. The high total organic carbon (TOC) and low pH due to the presence of a lot of organic matter in the black shale’s of the Dakhla Formation of the upper Maastrichtian age is the explanation for the observed dissolution that has nothing to do with oceanographic or volcanic processes. The presence of pyrite within the black shale interval suggests low oxygen conditions and the potential for meteoric water to react with the pyrite and form sulfuric acid. This acid could dissolve any carbonate material well after the original deposition.


Introduction
The study of the dissolution of foraminifer tests of the Jebel Duwi section ( Fig. 1) has been noticeably neglected in stratigraphic succession in the Eastern Desert. The planktic and calcareous benthic foraminifera are susceptible to dissolution, with planktic foraminifera being more soluble than benthic ones due to the porous chamber walls of planktic forms that evolved to maintain buoyancy in surface waters. The dissolution of planktic foraminifera results in depressed planktic/benthic (P/B) ratios [1][2][3][4][5]. It is widely known that benthics foraminifera tests are much less susceptible to dissolution than planktic [6], and [4][5][6][7][8][9].
The basic assumption in paleoenvironmental reconstructions is that fossil assemblages truthfully mirror the original biocoenosis and underlying environmental signals [10]. However, fossil communities and assemblages contain time-averaged signals, which actually could mainly be driven by contrasts in (temporary) environmental variations [11,12]. Taphonomic factors, especially selective dissolution, can severely alter the composition of foraminiferal fossil assemblages [2,[13][14][15][16]. Although preferential 1 3 dissolution in foraminiferal assemblages is widely recognized in modern and quaternary deep-sea sediments [2,4,17,18], the phenomenon is often neglected in studies dealing with older sediments.
For instance, planktic/benthic (P/B) ratios are often employed for paleodepth and sea-level reconstructions [10,19,20]. However, since planktic foraminifera are generally more susceptible to dissolution than benthic [21], partial dissolution leads to depressed P/B ratios and potentially underestimated paleodepths.
The Authors study the uppermost Maastrichtian interval hemipelagic foraminiferal assemblage from the Jebel Duwi section of Egypt, to reveal the effects of differential dissolution on the composition of foraminiferal assemblages. This time slice was chosen because dissolution phenomena are a recurrent problem for upper Maastrichtian foraminiferal assemblages, especially in connection with the global warming event known as The Mid-Maastrichtian Event (MME), which began by the end of the early Maastrichtian and persisted till upper zones CF6 to middle CF4a (C31n), and The Late Maastrichtian Event LME, which referred to that the faunal changes began in the latest Maastrichtian CF3-CF1 interval and ended at the Cretaceous/Paleogene K/Pg boundary and therefore spanned the last 450 kyr of the Maastrichtian [22].
However, this warming was attributed to the abrupt reorganization of intermediate oceanic circulation [23], to Ninety East Ridge volcanism [24]. A severe phase of volcanic activity is recorded at DSDP Site 216 beginning ~ 69.5 Ma and spanning zones CF5-CF3. Deccan volcanism started close to the end of the mid-Maastrichtian event at the base of C30n (~ 67.1 Ma, [25]).
The Mid-Maastrichtian Event (MME) marked the early Maastrichtian cooling trend and was related to acme occurrences of inoceramids in intermediate water depths earlier than the extinction of this organism [26]. During the mid-Maastrichtian numerous events provoke the most important oceanographic, climatic, and biotic modifications [23,[26][27][28][29]. The brief reversal in ocean circulation manifested by the shift of intermediate to deep water sources toward high southern latitude sites possibly occurred by tectonic processes [23,29]. The changes in benthic foraminiferal stable oxygen isotope signatures caused eustatic sea-level falls [28,30,31].
The Mid-Maastrichtian Event (MME) was observed in the sedimentary report at the Dakhla Formation (Hamama Member) of the Jebel Duwi section, Eastern Desert (Egypt) during the Racemiguembelina fructicosa CF4a Subzone. The MME in the study area lies within the lower part of the planktic foraminiferal Racemiguembelina fructicosa Subzone (CF4a).
Close to the MME, the more specialized planktic foraminifers start to decrease in diversity and more generalized groups increase, indicating a shift to less oligotrophic conditions, a trend that strongly accelerated near the end of the Maastrichtian [32].
Racemiguembelina fructicosa Subzone marks the Late Maastrichtian Event (LME). Keller et al. [24] suggested that the faunal responses include species dwarfing, reduced populations of specialized species, temporary exclusions, dominance through generalist and/or disaster opportunist species, and general diversity decrease, as observed in the Indian Ocean, Tethys, and South Atlantic Oceans. These authors attributed the reason for this warming to Ninety East Ridge volcanism.
The late Maastrichtian warming event was defined by a global temperature increase of ~ 2.5-5 °C that occurred ~ 150-300 ky before the Cretaceous/Paleogene (K/Pg) mass extinction. This transient warming event has traditionally been associated with a major pulse of Deccan Traps (west-central India) volcanism; however, large uncertainties associated with radiogenic dating methods have long hampered a definitive correlation. The persistent warming trend began near the base of C29r (cycle 18, 355 ky pre-K/Pg) and reached maximum temperatures of 280 ky (an increase of 3.2-4 °C). In deep-sea sections, the onset of Deccan-related climate warming is generally placed at ~ 300 ky pre-K/Pg. High temperatures persisted across the CF2/CF1 boundary (229 ky), then gradually decreased until 70 ky (cycle 3.5), followed by rapid 3-4 °C cooling by 45 ky (cycle 2.5) persisting until 25 ky (cycle 1.5) [33].
The study of the dissolution of foraminifer's tests of the Jebel Duwi section (Fig. 1) has been noticeably neglected in stratigraphic succession in the Eastern Desert. The planktic and calcareous benthic foraminifera are susceptible to dissolution, with planktic foraminifera being more soluble than benthic ones due to the porous chamber walls of planktic forms that evolved to maintain buoyancy in surface waters. The dissolution of planktic foraminifera results in depressed P/B ratios [1][2][3][4][5]. It is widely known that benthic foraminifera tests are much less susceptible to dissolution than planktic [4][5][6][7][8][9].
Nguyen and Speijer [34] studied benthic foraminifera dissolution susceptibility to architectural types. It was found that unilocular, uniserial, biserial, triserial, and milioline groups are more dissolvable than trochospirals and planispirals, which can be extra resistant.

(Anomalinoides and Cibicidoides) have an intermediate dissolution susceptibility.
This study aims to present the first study of the dissolution of planktic and benthic tests in the composition of foraminiferal assemblages in zones CF4a and CF2 of the Hamama Member (uppermost Maastrichtian interval) of the Duwi section. The study examines the function of differential dissolution in a wide variety of planktic and benthic foraminifera and introduces the major factors that affect dissolution in early Mid-Maastrichtian Event MME and Late Maastrichtian Event LME paleoenvironmental reconstructions.

Geological setting and stratigraphy
The Jebel Duwi is located in the eastern part of the Central Eastern Desert of Egypt. This study area is divided into Jebel Duwi and the coastal plain. The former is an elongate NW-trending ridge that drops precipitously to the southwest and slopes slightly to the northeast (Fig. 2). The Duwi area can be seen as tilted faulted blocks dissected by major and minor faults in the Red Sea tectonics (Fig. 2). A major phosphogenic episode occurred during the Late Cretaceous of shallow to the epicontinental shelf of the southern Neo-Tethys Ocean [36]. There is a common association between phosphatic strata and chert and organicrich sediments (black shales in Duwi) in the Middle East and Egypt. This is interpreted to indicate that phosphorite accumulation is associated with highly productive surface waters, likely caused by upwelling currents [37]. During the Upper Cretaceous, the southern platform of Egypt had epicontinental basins (not deeper than 100-150 m.) consisting of siliciclastic carbonate, marl, limestones, phosphate granules, and chert. The Duwi Formation was laid down in extraordinarily shallow epicontinental-neritic seas that flaked the southern margin of the Tethys Ocean in Egypt [38][39][40].
The marine deposition of Jebel Duwi led to the accumulation and preservation of organic-rich sediment contributing to the dominant formations. The black shales of the Dakhla Formation are assigned to the upper Maastrichtian age [40], presenting an impressive example of a major marine transgression event. Marine transgression during the Late Cretaceous-Early Tertiary times inundated Egyptian territory north to the latitude of Aswan (24N). It became punctuated by minor regressions due to tectonic pulses [41][42][43].
The Dakhla Formation (Maastrichtian-Early Paleocene) was deposited in environments ranging from shore to deep marine [44][45][46][47][48] and reached its maximum thickness at Jebel Duwi (175 m). It consists specifically of organicrich shale and argillaceous limestone. According to [49], the Dakhla Formation is composed of the lower Hamama Member (Maastrichtian) and the upper Beidaa Shale Member (Lower Paleocene), which are unconformably overlying each other [50]. The lowest beds of the Hamama Member (2-4 m thick) are darkish grey to black organic-rich calcareous clays, a few thin limestone ledges in the mid-section, monotonous greenish to grey marls, and shales with gypsum veins in the upper beds (Fig. 3). The top of the Hamama Member is an erosional surface (hiatus),  marked by reworked fossils in a clay matrix, due to the regression phase [51,52].
Orabi and El Gammal [53] recognized that the boundary between Racemiguembelina fructicosa Subzone (CF4a) and Pseudoguembelina hariaensis zone (CF3) is marked by abrupt thin layers of dark grey shales represented by very small (dwarfed) planktic species of compressed tests and impregnated by reddish colors such as Globigerinelloides spp.
Pseudoguembelina palpebra (CF2) of the late Maastrichtian zone of the Hamama Member is characterized by planktic assemblages, composed of heterohelicids, especially Planoheterohelix globulosa (Ehrenberg). This fauna represents a low diversity species population linked to high-stress environments, that's pretty high TOC, and low pH due to the presence of a lot of organic matter is likely an explanation for the observed dissolution that has nothing to do with oceanographic or volcanic processes. Additionally, the Plummerita hantkeninoides (CF1) zone has dominant planktic foraminifera with very few keeled globotruncanids and is composed of heterohelicids (Fig. 3).

Foraminiferal study
Quantitatively describe the resolution sampling used to examine foraminifera (planktic and benthic) from the Hamama Member (uppermost Maastrichtian interval). About forty-five rock samples constitute a 50 m thick with appropriate spacing. For this study, we selected only 19 rock samples of an interval representing CF4a-CF1 (22.8 m thick of upper Maastrichtian). In the laboratory, 100 g from each sample was washed through a 63 µm sieve and dried in an oven at a temperature of 50 °C. A > 63 m fraction was used after disaggregation in quantitative foraminiferal studies. Accordingly, quantitative studies were based on representative splits (using a modified Otto microsplitter) of approximately 300 specimens selected from each sample to calculate the planktic/benthic (P/B ratio). Preservation of foraminifera assemblages for the Duwi section is usually favorable, though the dissolution of calcite shell is obvious in a few samples. The numbers of dissolved planktic specimens in all samples have been counted, together with the quantity of dissolved benthic specimens (tests Table 1 Planktic and benthic foraminiferal species counts, different morphogroups, total dissolved species, P/P + B ratio, Fragmentation index (FI) and Total Organic Carbon (TOC) in the studied samples collected from Gebel Duwi   Table 1). The foraminifera tests were mounted on standard 60-square micropaleontological slides and scanned using a Scanning Electron Microscope of Alexandria University (JSM-IT200, Series Jeol) (Figs. 4,5,6,7,8,9). The preservation of planktic foraminiferal tests is usually good to moderate in the study area. We picked fragments of planktic foraminiferal tests over the entire 19 sample set and > 63 mm splits for the taphonomic analysis ( Table 1). The fragmentation index is a well-known taphonomic proxy of the carbonate saturation state or dissolution of calcium carbonate [54][55][56]. The fragmentation index (FI) measures the percentage of planktic foraminiferal fragments (specimens that include much less than two-thirds of an entire test) based on the total number of whole tests [54]. The dissolution effects upon the composition of planktic foraminifera assemblages have been considerably higher in samples with FI > 40%.

Determination of total organic carbon (TOC)
Samples representing CF4a-CF1 were chosen to determine (TOC) by using a Hochtemperatur-TOC/TNb-Analysator (LiquiTOC) ( Table 1), where about 200 mg of pulverized sample was used. Carbonate was eliminated via treatment with 10% aqueous hydrochloric acid. The residue was used for the determination of TOC via combustion analysis at temperatures attaining 850 °C. The evolved gas (CO 2 ) was recorded as a carbon percentage and was measured quantitatively and concurrently by infrared detectors.

Planktic foraminiferal turnover
The studied planktic foraminiferal assemblages are characterized by a high diversity and a wide range of morphological variability of the tests. The species richness remains very high and ranges between 89 and 104 species per sample. About 61 species belonging to 20 genera were identified in this section ( Table 2). The P/B ratio ranges from 38.8 to 20.6% in Zone CF4a and 30.7-15.1% in Zone CF2 (Table 1), suggesting a relatively stable paleobathymetry throughout the Duwi section. The Maastrichtian planktic foraminiferal ecogroups are well represented by heterohelicids, pseudoguembelinids, globotruncanids, rugoglobigerinids, hedbergellids, and Globigerinelloids (Fig. 3).

Planktic foraminiferal fragmentation index (FI)
We have identified FI values higher than 40% in four samples (28,31,40, and 41 respectively within the two interval zones CF4a Subzone and CF2 Zone) underneath the Cretaceous/Paleogene boundary K/Pg (Table 1). Large oscillations in the FI are recognized within both these intervals (samples [24][25][26][27], suggesting relatively rapid changes in carbonate dissolution intensity. Obviously, the majority of the studied section is below a moderate/strong dissolution restriction where the FI < 40%, particularly in the uppermost Maastrichtian (samples [43][44][45]. The fragmentation index shows high values of planktic foraminiferal tests at CF4a Subzone and CF2 Zone (Table 1), these episodes are characterized by a very high abundance of the low oxygen tolerant genus Heterohelix, and an increase in the biserial morphotype (intermediate) with elongated terminal chambers (Table 3).

Distribution of benthic morphogroups
Calcareous benthics foraminifera tests are sensitive to environmental parameters change such as depth, the flux of organic nutrients to the seafloor, oxygen levels of the seafloor, salinity, food quality and quantity, substrate, temperature, etc. [57][58][59][60]. The authors have observed the morphogroup category of [60] in the area under consideration. The morphogroups distribution inside this section is illustrated in Fig. 10.

Phase of dissolution
Two inferred foraminiferal dissolution phases were observed in the Dakhla Formation (Hamama Member) (Fig. 11). A dissolution examination was performed to evaluate factors that affect the foraminiferal assemblage of the Maastrichtian zones CF4a and CF2. The results confirm that planktic foraminifera are much more susceptible to dissolution than benthic foraminifera. Within Zone CF4a, dissolved planktic constituent range between 4 and 40, while dissolved benthic constituent range between 1 and 21. The total planktic dissolved in zone CF2 ranged between 29 and 40, and the total benthic dissolved ranged between 19 and 25 ( Table 1).

Dissolved planktic foraminifera
In the studied section of Jebel Duwi, there was mainly a decrease in the number of globotruncanids and the dominance of heterohelicids, which are considered opportunistic taxa. The observed low abundance of planktic specimens between zones CF4a and CF3 (Fig. 12) may be due to the presence of pyrite (sample 31) within the black shales' interval, suggesting low oxygen conditions [61]. Pyrite also suggests the potential for meteoric water to react with the pyrite and form sulfuric acid, which could then dissolve any carbonate material well after the original deposition. The planktic and benthic change indicate a continuous and even increasingly high-stress environment during the CF4a zone, which agrees with many authors [22, 62, (Table 1). That's pretty high TOC and low pH due to the presence of a lot of organic matter is another likely explanation for the observed dissolution that has nothing to do with oceanographic or volcanic processes (Fig. 12).

Dissolved benthic foraminifera
The depletion of oxic conditions may be a premonition of an even more significant future change in global climate, reduction of biodiversity, and accompanying extinction events, which proves the observation of dissolved oxygen (DO) values to be indispensable for modern science and environmental protection [67]. The taxa of the rounded trochospiral morphogroup (RT) are represented in dissolved zones CF4a (71-27 individuals) and CF2 (89-60 individuals) (Fig. 10). The presence of Valvulineria and abundant Gavelinella in this morphogroup is considered the strongest opportunistic fauna [68].
Several types of calcareous benthic foraminifera are identified, and some forms show dissolution holes in the early chambers, pitted surfaces, and abrasion at the test margins. The most dissolved benthics foraminifera in this phase is shown in Figs. 8 and 9.
The Upper Cretaceous black shales of the area under consideration are characterized by high organic matter, sulfide, and trace element contents [83]. The geochemical and petrographical research revealed that the Cretaceous black shales in the Duwi section were deposited in relatively euxinic to anoxic reducing marine environments.

P/B ratios as a dissolution proxy
These zones include, among other possible indicators of dissolution, low P/B ratios, low foraminiferal numbers, an excessive number of arenaceous agglutinated taxa, few numbers of benthic calcareous taxa (unilocular,   (Table 1). Parker and Berger [84] interpreted the reduced P/B foraminifera ratios as a reduction in planktic foraminifera because of their susceptibility to dissolution. Meanwhile, [34] interpreted the low P/B ratio as due to benthic foraminiferal abundances, which reflect the supply of organic carbon to the seafloor. The potential indicators of dissolution can be recognized by the decrease in P/B ratios, decrease in the number of globotruncanids, and dominance of heterohelicids. The increase in Lenticulina indicates that planktic forams are more likely to dissolve than benthic forams [4][5][6]34].
The heterohelicids abundances abruptly change from 35 to 60% through the CF3 Zone, and exceed 60% in the CF2 and CF1 zones (Fig. 7). The dominance of heterohelicids has also been recorded in Tunisia, Israel, Italy, Spain, Texas, Denmark [86] and others, suggesting reduced water mass stratification and accelerated low oxygen environments as ecological stress. The Duwi Basin during CF4a and CF2 zones became shallower, with low oxygen contents, low primary production, and low planktic content in the dissolution interval (Fig. 3).
The presence of Pseudotextularia elegans (Rzehak) in the zones CF4a and CF2 indicates a connection of the basin with the warm Tethys [87]. Moreover, the presence of dark grey shales (TOC content ranges between 2 and 12.2%, Table 1) in the upper part of these zones records a sharp decrease in P/B, globotruncanids, rugoglobigerinids, globigerinellids, and heterohelicids (Fig. 11).

Fragmentation index (FI)
The P/B ratio is unstable along the section (Table 1), indicating a decline in planktic foraminiferal flux to the seafloor as the result of a rapid rise of the lysocline. However, FI indicates relatively poor preservation and high amounts of test dissolution in intervals from CF4a and CF2 below the (K/Pg). This apparent contradiction between FI and P/B ratio could be explained by local surface water becoming more acidic, perhaps due to a short-lived increase in the atmospheric partial pressure of CO2 [56,88]. The fragmentation records discussed in the present work suggested widespread ocean acidification during the two zones CF4a and CF2 were probably linked to Deccan volcanism.
During CF4a and CF2, the increased fragmentation index (FI) indicates an increased dissolution intensity and/or poor preservation of CaCO 3 . [89] suggested that lysocline shuffling might be one explanation.

Conclusion
1. Two observations on foraminifera dissolution in the Dakhla Formation, where planktic foraminifera are more susceptible to dissolution than benthic foraminifera in the Hamama Member. They are characterized by a decline in the P/B ratio and a rise in the agglutinated percentage. Results of those observations are used for the first time revealing foraminiferal dissolution in these two zones (CF4a and CF2). 2. The Fragmentation index (FI) indicates relatively poor preservation and excessive quantities of test dissolution in Subzone CF4a and Zone CF2 below the K/Pg. This obvious contradiction among FI and P/B ratios could be explained by local surface water becoming more acidic, perhaps due to a short-lived increase in the atmospheric partial pressure of CO 2 . The fragmentation records in the present work suggested that widespread ocean acidification during the two zones CF4a and CF2 was probably linked to Deccan volcanism. 3. The observed low abundance of planktic specimens between zones CF4a and CF3 may be due to the presence of pyrite within the black shale interval, suggesting low oxygen conditions. Pyrite also suggests the potential for meteoric water to react with the pyrite and form sulfuric acid, which could then dissolve any carbonate material well after the original deposition. PlankƟc morphology key factor of dissoluƟon in CF2 40 41 43 43 Fig. 13 The planktic foraminifera morphology is a key factor governing dissolution susceptibility 4. The recorded dissolved calcareous benthic assemblage is diversified, moderately well-preserved, and represented by the following morphogroups: tapered and cylindrical morphogroup (T/C), plano-convex trochospiral morphogroup (PT), rounded trochospiral morphogroup (RT), lenticular morphogroup (L), and spherical morphogroup of genus Lagena (S), in decreasing order of abundance, and very rare flattened tapered morphogroup (FT). The rounded planispiral morphogroup (RP) does not show any dissolution feature. 5. The presence of black shales of the Dakhla Formation is assigned to the upper Maastrichtian age. The high total organic carbon (TOC) and low pH due to the presence of a lot of organic matter is another likely explanation for the observed dissolution that has nothing to do with oceanographic or volcanic processes.
Author contributions OHO collecting previous literature-reviewing and continuous follow-up. Of all parts of the research. AMS participation in determining the title of research-collecting. Previous studies-writing research content-identifying and organizing references-adjusting. Research formats in line with the journal.
Funding No funding was received to assist with the preparation of this manuscript.
Data availability Data will be made available on reasonable request.

Conflict of interest
The authors declare no conflict of interest.
Ethical approval Not applicable.
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