Oxygen level, primary productivity, and water turbulence during the OAE2 interval of Zagros Basin (SW Iran): Benthic foraminiferal variations in the carbonate microfacies

Journal Pre-proof Oxygen level, primary productivity, and water turbulence during the OAE2 interval of Zagros Basin (SW Iran): Benthic foraminiferal variations in the carbonate microfacies

Hossein Vaziri-Moghaddam, Behnaz Kalanat PII:

S1342-937X(20)30062-9

DOI:

https://doi.org/10.1016/j.gr.2020.01.010

Reference:

GR 2296

To appear in:

Gondwana Research

Received date:

2 August 2019

Revised date:

13 January 2020

Accepted date:

14 January 2020

Please cite this article as: H. Vaziri-Moghaddam and B. Kalanat, Oxygen level, primary productivity, and water turbulence during the OAE2 interval of Zagros Basin (SW Iran): Benthic foraminiferal variations in the carbonate microfacies, Gondwana Research(2020), https://doi.org/10.1016/j.gr.2020.01.010

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© 2020 Published by Elsevier.

Journal Pre-proof Oxygen level, primary productivity, and water turbulence during the OAE2 interval of Zagros Basin (SW Iran): benthic foraminiferal variations in the carbonate microfacies

Hossein Vaziri-Moghaddama, Behnaz Kalanat b, * a- Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran b- Department of Botany, Research Institute of Forests and Rangelands, Agricultural Research Education

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and Extension Organization (AREEO), Tehran, Iran

Abstract boundary

(upper

Sarvak

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Cenomanian/Turonian

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*[email protected]

Formation)

benthic

foraminiferal

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assemblages were analyzed to reconstruct oxygen level, primary productivity, and water turbulence in the Izeh Zone, Zagros Basin. The interplay between environmental perturbations

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during the Oceanic Anoxic Event 2 (OAE2) and regional tectonic activities in the Zagros Basin

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resulted in formation of various benthic foraminiferal assemblages in the study section. The OAE2 interval at the region of study starts with extinction of rotaliporids at the onset of δ13C

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positive excursion (peak “a”), which is associated with population of infaunal benthic foraminifera (especially Bolivina alata). The following interval at the onset of Whiteinella archaeocretacea Biozone is characterized by the total absence of benthic taxa and dominance of planoheterohelicids (“Heterohelix shift”) in the black shale strata, indicating expansion of oxygen minimum zone and unhospitable conditions for both benthic and planktic foraminifera. The upper part of OAE2 interval (including δ13C peaks “b” and “c”) coincides with harbinger of Neo-Tethys closure in the Arabian plate, causing a compressional tectonic regime, and creation of uplifted terrains in the basin. The relative sea level started to locally fall in this succession, 1

Journal Pre-proof which was accompanied by a better ventilation of seafloor, lower TOC contents, and reappearance of benthic foraminifera.

Keywords: Benthic foraminifera; Cenomanian–Turonian boundary; OAE2; Sarvak Formation

1. Introduction

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The Cenomanian/Turonian (C/T) boundary event, known as Oceanic Anoxic Event 2

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(OAE2) has been associated with a major volcanism in the large igneous provinces (Bralower et

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al., 1997; Turgeon and Creaser, 2008; Du Vivier et al., 2014; Kingsbury et al., 2018), causing a

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significant shift in pCO2, primary production, marine oxygenation, and biotic assemblages (e.g., Jarvis et al., 1988; Leckie et al., 2002; Adams et al., 2010; Barclay et al., 2010; Owens et al.,

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2013). Among the other marine organisms, benthic foraminiferal assemblages show significant

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changes during the environmental perturbations of OAE2 (Leckie et al., 1998; Holbourn and Kuhnt, 2002; Gebhardt et al., 2004; Kuhnt et al., 2005; Friedrich et al., 2006; Friedrich, 2010;

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Elderbak et al., 2014; Kalanat et al., 2017, 2018b). The dependance of benthic foraminifera to

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water masses has made them useful for reconstructions of productivity and bottom water oxygenation as two main environmental factors controlling the foraminiferal assemblages at the seafloor (e.g., Jorissen et al., 1995, 2007; Van der Zwaan et al., 1999). The trophic oxygenmodel (TROX-model; Jorissen et al., 1995) has been proposed based on the observed relationship among shell morphology of benthic foraminifera, benthic microhabitats, and interplay between food and oxygen concentrations in the deep-water palaeoenvironments. This model demonstrated that under oligotrophic-oxygenated conditions, low food supply is the limiting factor and vertical distribution of benthic foraminifera is restricted to the sediment-water

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Journal Pre-proof interface and shallow sediment depths. Epifaunal and shallow infaunal microhabitats are advantageous in these environments. Under the eutrophic conditions, on the other hand, oxygen availability is the limiting parameter and low oxygen-tolerant infaunal taxa dominate the benthic assemblages. The benthic foraminiferal microhabitat interpretation is often based on their δ13C and δ18O or shell morphology. The carbon and oxygen isotopes analysis indicates that related taxa with the

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same mineralogy, life style or habitat tend to have similar isotopic values. For example, calcitic

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epifaunal forms usually show higher δ18O and lower δ13C values related to infaunal taxa (Fisher

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and Arthur, 2002; Wendler et al. 2013). There are rare stable isotope studies for the assignment

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of benthic foraminiferal life style, which caused the use of shell morphology instead (Corliss, 1985; Corliss and Chen, 1988; Koutsoukos and Hart, 1990), however this factor has just 75%

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accuracy (Buzas et al., 1993). This reveals the complication of using benthic foraminifera as

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palaeoenvironmental markers, however, the study of benthic foraminifera alongside geochemical and other biotic data provide a useful basis for interpretation of environmental perturbations.

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This study considers the variations of benthic foraminiferal assemblages in the

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Cenomanian–Turonian microfacies of the Izeh Zone (Zagros Basin) .The combination of benthic foraminiferal data (life strategy, benthic foraminiferal oxygen index, abundance and diversity) with the other biotic, sedimentological, and geochemical analysis (e.g. TOC contents) (Kalanat and Vaziri-Moghaddam, 2019 a, 2019b) are here applied to reconstruct oxygenation, primary productivity, and water turbulence in the study section.

2. Geological setting and study area 2.1. Zagros Basin

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Journal Pre-proof The Zagros Fold and Thrust Belt, formed in the northeastern of Arabian Plate (Figs. 1A, 1B), is located across the Alpine–Himalayan belts (Agard et al., 2011). It comprises a thick sedimentary sequences from Precambrian to Recent (James and Wynd, 1965; Berberian and King, 1981; Motiei, 1993). The mid-cretaceous deposits throughout the Arabian plate are marked by significant variations in the sedimentary facies and thickness. Several studies in this area indicate that deep pelagic/hemiplegic intra-shelf basins in this time were surrounded by a

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widespread shallow water carbonate platform (e.g., van Buchem et al., 2002; Razin et al., 2010;

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Piryaei et al., 2010, 2011; Navidtalab et al., 2016, 2019; Vincent et al., 2015; Wohlwend et al.,

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2016; Hennhoefer et al., 2019). This is attributed to the development of the horst-graben

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architecture in an extensional tectonic regime and rifting along the N/S oriented basement faults in the basin (Wrobel-Daveau et al., 2010; Navidtalab et al., 2019). Similar rifting phases were

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also reported from Jurassic and Early Cretaceous deposits in the Zagros Basin (Tavani et al.,

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2018; Noori et al., 2019). The Cenomanin extensional regime is proposed to be before the harbinger of the Neo-Tethys closure and tectonic inversion around the Cenomanin/Turonian

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boundary (between 93.7–94.1 Ma) (Navidtalab et al., 2016), which is associated with ophiolite

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obduction (94 Ma) in the region (e.g., Neyriz and Oman ophiolits) (Warren et al., 2005; Babaie et al., 2006). This compressional tectonic regime in the Zagros Basin is characterized by sealevel fall and widespread exposure of carbonate platforms in the shallower horst environment (e.g., Piryaei et al., 2010, 2011; Hajikazemi et al., 2010, 2012; Rahimpour-Bonab et al., 2013; Mehrabi et al., 2014; Vincent et al., 2015; Navidtalab et al 2016) (Fig. 1c).

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Fig. 1. A. Late Cenomanian paleogeographic reconstruction of Tethyan realm (Modified after Barrier and Vrielynck, 2008). B. Present day map of Iran and surrounded area (Sharland et al.,

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2001). The location of Zagros Basin is marked by brown color. Main faults and structural

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subdivisions of Zagros Basin are indicated (after Motiei, 1993). Abbreviations and Acronyms: L (Lurestan zone); Iz (Izeh zone); DE (Dezful Embayment); F (Fars Zone); HZ (High Zagros Fault); ZFF (Zagros Front Fault); BF (Balarud Fault); KMF (Kharg Mish Fault); KF (Kazerun Fault); MFF (Mountain Front Fault). C. Upper Cretaceous rock units in the Zagros Basin (SW Iran) (adopted from James and Wynd, 1965).

2.2. Lar Anticline section This study focuses on an outcrop section located in the south flank of Lar Anticline in the south of Izeh Zone (Fig. 1). The section spans the upper part of Sarvak Formation and contains three 5

Journal Pre-proof Cenomanian–Turonian planktic foraminiferal biozones, Rotalipora cushmani, Whiteinella archaeocretacea, and Helvetoglobotruncana helvetica (Kalanat and Vaziri-Moghaddam, 2019b). The dominance of deep pelagic facies as well as absence of evidence for subearial exposure during the harbinger of Neo-Tethys closure around the C/T boundary propose that the Lar Anticline section has been deposited in a graben-like setting (intra-shelf basin) in the Zagros Basin. The influence of basement faults on the depositional history of Lar Anticline section due

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to extensional tectonic regime has been proved during the Jurassic-Early Cretaceous Fahliyan

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Formation by considerable facies variation of equivalent depositional sequences in a relatively

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short distance (13 Km) (Noori et al., 2019). From the Early Cretaceous upward this section has

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experienced a significant deepening, indicated by replacement of inner platform and platform margin facies with deep marine radiolarian and sponge spicule-bearing strata (Noori et al., 2019).

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The microfacies analyses of study area (Kalanat and Vaziri-Moghaddam, 2019a) propose that

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during the late Cenomanian, this section was deposited in an intra-shelf basin. By the end of Cenomanian the compressional tectonic regime due to closure of Neo-Tethys Ocean led to local

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sea-level fall and development of platform-top facies in the Lar Anticline section. The early

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Turonian strata are associated with a sea-level rise and development of oligosteginid-dominated mid, outer ramp, and intra-shelf basin (Kalanat and Vaziri-Moghaddam, 2019a). Three third-order depositional sequences were recognized in the study section. Global correlation of these sequences suggests influence of local tectonic activities as controlling factor for sea-level fluctuations from the latest Cenomanian upward (Kalanat and Vaziri-Moghaddam, 2019a). The OAE2 interval in the Lar Anticline section is characterized by δ13C positive excursion (Kalanat and Vaziri-Moghaddam, 2019b). Re-interpretation of this record indicated that three

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Journal Pre-proof correlatable peak points (a-c) described by Jarvis et al. (2011) are well displayed by the δ13C curve (Fig. 2). The δ13C peak “a” occurs at the extinction level of R. cushmani (sample 32) and below the "Heterohelix shift". Peak “b” (sample 47) fallows an ephemeral decrease in the δ13C values in the middle part of W. archaeocretacea Biozone. Peak “c” occurs at the sample 64 before the carbon-isotope profile gradually decreases to the pre-excursion values and OAE2 ends

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around the first appearance of H. helvetica at sample 66.

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3. Material and methods

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The differentiation of benthic foraminiferal assemblages was based on their relationship

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with identified microfacies through the Lar Anticline section (Kalanat and Vaziri-Moghaddam, 2019 a). Study of matrix, skeletal (e.g., foraminifera, calcispheres, radiolarian, red algae) and

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non-skeletal (e.g., peloid and intraclast) components in the thin sections were used for

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microfacies analysis (Kalanat and Vaziri-Moghaddam, 2019 a). Benthic foraminifera in the most samples are the subordinate components and do not reveal in the thin sections. For this reason

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these taxa were studied after extraction from their host deposits. All samples washed using “cold

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acetolysis” technique of Kariminia (2004), except for samples 56-58, which the high degree of induration prevented from extraction of foraminifera and they studied using the thin sections. The study of benthic foraminifera was based on count of 100 specimens (supplementary Table S1), because the number of benthic taxa in samples is much less than those for planktic foraminifera. Fatela and Taborda (2002) believed that 100 specimens would be sufficient for reliable results, if only the dominant taxa with abundance more than 5% were interpreted. The benthic taxa that display abundances >5% were classified in the Table 1. The absolute abundance of benthic foraminifera (number per gram of bulk dried sediments; BFN) was calculated for

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Journal Pre-proof >350, >125 and >44 μm mesh size. The >44 fraction was used instead of the >63 μm fraction to avoid the loss of dwarfed taxa, which are abundant in the study section. The ratio of planktic to benthic foraminifera (% benthic), simple diversity (number of species) and Shannon-Wiener diversity index (H(S)) (Murray, 1991, 2006) were also determined in the study section. The benthic specimens were assigned to epifaunal and infaunal morphogroups by their microhabitat interpretation based on isotopic data and also by comparing their test morphology in the absence

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of isotopic data (Table 1). The oxygenation of bottom water at the study section was estimated

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by using benthic foraminifera oxygen index: BFOI= [{I/(I+D)}-1]*50 (where I and D are

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number of intermediate and dysoxic indices, respectively; Kaiho, 1994). This equation was used

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since the oxic indices (≥ 350; thick-walled epifauna) are totally absent in this interval. Based on BFOI equations, Kaiho (1994) recognized five dissolved oxygen conditions including high oxic

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-40), and anoxic (BFOI = -55).

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(BFOI = 50 – 100), low oxic (BFOI = 0 – 50), suboxic (BFOI = -40 – 0), dysoxic (BFOI = -50 –

The characteristics of benthic foraminiferal assemblages (BFN, BFOI, benthic foraminiferal

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diversity, %benthic) and their relationship with identified microfacies in the region of study are

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summarized in the table 2.

The geochemical data (δ13C and δ18O for bulk carbonate, TOC and CaCO3 values) and quantitative values of planktonic foraminifera, calcisphere, and radiolarian are based on Kalanat and Vaziri-Moghaddam (2019 b) (see page 3 for the methods and Supplementary data located at “https://doi.org/10.1016/j.palaeo.2019.109238” for geochemical values).

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Journal Pre-proof Table 1. Microhabitat classification of dominant calcareous benthic foraminifera (>5%) identified in the study section. Fisher and Arthur (2002) and Wendler et al. (2013) display data based on δ13C and δ18O analysis. Test form

Habitat

Reference(s)

Bolivina

Tapered elongated biserial

Infauna

Koutsoukos et al. (1990); Holbourn et al. (1999a); Gebhardt et al. (2004); Gebhardt (2006); Kuhnt et al. (2005); Friedrich et al. (2006, 2009)

Coryphostoma

Tapered elongated biserial

Infauna

Koutsoukos et al. (1990)

Gabonita

Tapered elongated biserial

Infauna

Koutsoukos et al. (1990); Kuhnt and Wiedmann (1995); Holbourn et al. (1999a); Gebhardt et al. (2004); Kuhnt et al. (2005); Friedrich et al. (2006, 2009)

Gavelinella

low trochspiral

Epifauna

Leckie et al. (1998); Wendler et al. (2013); Frenzel (2000); Herrle et al. (2003a, 2003b)

Globorotalites

Trochospiral, conical

Epifauna

Gyroidinoides

Trochospiral

Lenticulina

Lenticular

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Main genera (>5%)

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Thomas (1990); Frenzel (2000)

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Epifauna

Erbacher et al. (1998); Frenzel (2000); Herrle et al. (2003a, 2003b); Friedrich et al. (2005); Friedrich and Erbacher (2006) Wendler et al. (2013); Thomas (1990)

Epifauna

Koutsoukos et al. (1990)

Infauna

Leckie et al. (1998); Holbourn et al. (1999); Fisher and Arthur (2002); Kuhnt et al. (2005)

PlanoconvexTrochospiral

Epifauna

No reference found

Praebulimina

Tapered elongated triserial

Infauna

Wendler et al. (2013); Thomas (1990) Koutsoukos et al. (1990); Coccioni et al. (1993); Widmark (1997); Holbourn et al. (2001); Friedrich et al. (2006, 2009)

Valvulineria

Trochospiral

Epifauna

Fisher and Arthur (2002); Thomas (1990)

Neoeponides

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Neobulimina

BiconvexTrochospiral

Tapered elongated triserial to biserial

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Lingulogavelinella

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Infauna

4. Benthic foraminiferal results Benthic foraminifera are the subordinate components in the most samples of the study section, except for samples 25 and 56-63, where their abundance reaches >50% (Fig. 2). 9

Journal Pre-proof Calcareous hyaline forms dominate the benthic assemblages (Fig. 3, supplementary Table S1) but agglutinated (Cuneolina), microgranular (Nezzazata), and porcelaneous (Quinqueloculina, Nezzazatinella) tests (Fig. 4) become the only benthic foraminiferal components in samples 56-58, where planktic foraminifera are absent (Fig. 2). The most important benthic foraminiferal

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species are illustrated in the Figure 4.

Fig. 2. Biotic and geochemical fluctuations during the late Cenomanian–early Turonian sea-level changes in the Lar Anticline section. The benthic foraminiferal assemblages and intervals of environmental changes are demonstrated in the figure. The gray bands are the black shale intervals with lack of benthic foraminifera. The geochemical data, Planktic foraminiferal morphotypes, % radiolarian and % calcisphere are based on Kalanat and Vaziri-Moghaddam (2019b). The δ13C peaks “a-c” are re-interpreted based on Jarvis et al., (2011). Depositional environment and sequence stratigraphy are from Kalanat and Vaziri-Moghaddam (2019a).

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Journal Pre-proof Study of benthic foraminifera indicates large variations in simple diversity and the ShannonWiener diversity index from 0 to 12 and 0 to 2, respectively. BFN changes from 0 to 4000 individuals per gram and BFOI ranges from -2 to -55 (Fig. 2, supplementary Table S1). The benthic foraminiferal characteristics change in relationship with identified microfacies in the Lar Anticline. This used to differentiate thirteen benthic assemblages, described below. These assemblages were deposited in the outer platform and intra-shelf basin (lower 39 m of the study

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section), platform-top (39-41 m) and calcisphere (oligosteginid)-dominated ramp (upper 13 m of

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the study section) (Figs. 2, 3, 5, 6, table 2).

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4.1.1. Benthic foraminiferal assemblages in the microfacies of outer platform and intra-shelf basin

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Assemblage 1 is related to microfacies 1 (Radiolarian packstone) (Table 2), which is

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dominant in the light-colord indurated marlstones with TOC contents <1%. It is characterized by high abundance of radiolarians and low abundance (few specimens) and diversity of planktic

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foraminifera. Dwarfed and scattered Planoheterohelix is the most common planktic foraminifera

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in this microfacies. Benthic foraminifera are totally absent in this assemblage (Figs. 2, 3, 5). Assemblages 2 and 3 occur in the organic carbon-poor (TOC<1%) marlstones of microfacies 2 (Planktic foraminifera radiolarian wackestone) (Table 2). This microfacies represents low abundance and diversity of biserial and trochospiral planktic foraminifera (Planoheterohelix, Whiteinella, Clavihedbergella, and Muricohedbergella). Radiolarians are also present in these intervals. Benthic foraminifera in the assemblage 2 are subordinate components (% benthic <10), showing low abundance (BFN = 100-200) and diversity. Infaunal taxa comprise about 50% of

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Journal Pre-proof benthic assemblage. BFOI indicates <-20 values (Fig. 2). Gyroidinoides depresus and Bolivina alata are the most abundant taxa in this assemblage (Fig. 3).

Table 2. Benthic foraminiferal assemblages and their characteristics in the identified

platform-top

5-12

biserial trochospiral digitate

501000

4-10

0.41.6

9-24

biserial trochospiral planispiral keeled biserial trochospiral digitate biserial trochospiral

Bolivina alata Praebulimina prolixa Neobulimina albertensis Bolivina alata Neobulimina albertensis

-

<-20

100200

mediu m

<1

<50

0

≤10

-5 to -20

As3

low

>1

0

0

≤10

-20 to 48

low

>2

0

low

>2

0

low

>2

0

low

<1

low

>2

As9 low As10

~1

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BFN

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-

-

≤20

-38 to 50

>1000

6-12

1.62

5-6

0

≤10

-45 to 50

>100

4-6

0.41

6-11

0

0

-55

0

0

0

1-2

biserial

-

-

<50

0

≤10

-20

100200

4-8

1.82

4-6

biserial trochospiral

intraclast, sponge spicule

0

0

≤10

-50

>1000

6

1.2

10

biserial trochospiral

na

As8

301000

0

As5

As7

4-8

Dominant benthic species

≤10

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0

BFOI

<50

Energy level

<1

As2

Other components

1.82

0

As4

Planktic morphotypes

H(S) benthic foram.

-

10-12

Gyroidinoides depresus Valvulineria sp. Neoeponides auberii Praebulimina spp. Gyroidinoides depresus G. angustiumbilicatus Valvulineria sp. Lingulogavelinella sp. Bolivina alata Praebulimina prolixa P. kickapooensis

-

Simple diversity (planktic foram.)

Simple diversity (benthic foram.)

biserial trochospiral

% Benthic

5-12

0

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11.8

Calcisphere (%)

sponge spicule

Radiolarian (%)

-

0

0

>20

1030

-10 to 30

50500

6-12

1-2

9-15

biserial trochospiral

Praebulimina spp. Gyroidinoides depresus Valvulineria sp. Neobulimina albertensis Gabonita laevis P. kickapooensis Valvulineria sp. Praebulimina prolixa

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7- Planktic foraminifera calcisphere wackestone/packstone 8- Laminated calcisphere wackestone/packstone 9- Planktic foraminifera bioclast intraclast wackestone/packstone 10- Peloidal grainstone

biserial

>50

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mid-ramp

outer ramp and basin

steep-side of intra-shelf basin oligosteginid-dominated ramp

6- Planktic foraminifera bioclast intraclast wackestone/packstone

1-6

<1

As6 5- Heterohelix wackestone/packstone

0

mediu m to high mediu m

As1

2- planktic foraminifera radiolarian wackestone

3- Diverse keeled and non-keeled planktic foraminifera wackestone 4- Low diversity nonkeeled planktic foraminifera wackestone/packstone

0

TOC (%)

outer platform and intra-shelf basin

1- Radiolarian packstone

Benthic assemblages

Microfacies

Depositional environment

maicrofacies of the Lar Anticline section.

-

intraclast, sponge spicule peloid

high

<1

<30

7090

0

-

0

0

0

2-3

biserial

-

peloid

high

<1

<10

<20

3060

~0

5003000

5-12

0.41.7

9-12

Neoeponides auberii

intraclast, red algae, bivalve fragments

high

<1

0

0

100

-

-

-

-

0

biserial trochospiral digitate triserial -

As11

As12

Nezzazata miliolids

peloid red algae, bivalve and echinoid fragments, cortoid, oncoid,

As13 11- Cortoidal bioclast grainstone

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Fig. 3. Distribution and abundance of calcareous hyaline benthic foraminifera in the Lar

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Anticline section. The benthic foraminiferal assemblages are indicated in the figure.

Assemblage 3 is also observed in the radiolarian-bearing marlstones of microfacies 2 (Figs.

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3, 4, Table 2). Benthic foraminifera represent low abundance (BFN = 30-1000; %benthic <20) but diverse components. Simple benthic diversity and H(S) in this assemblage reach up to 12 and 2, respectively. BFOI varies from -5 to -20 (Fig. 2). Epifaunal foraminifera (Gyroidinoides depresus, Lingulogavelinella sp., Valvulineria sp., and Globorotalites subconicus) are the most abundant benthic taxa, whereas the infaunal taxa such as Coryphostoma midwayensis are the subordinate benthic components (Fig. 3).

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Fig.4. Benthic foraminiferal species from the Lar Anticline section. 1a, b, c. Gyroidinoides depresus

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(Alth), sample 358 (scale bar = 50 μm). 2. Gabonita laevis (Roemer), sample 377 (scale bar = 20 μm). 3. Neobulimina albertensis (Stelck and Wallr), sample 377 (scale bar = 50 μm). 4a, b, c. Neoeponides auberii (d’Orbigny), sample 383 (scale bar = 50 μm).5. Bolivina alata (Seguenza), sample 336.1 (scale bar = 50 μm). 6. Coryphostoma midwayensis (Cushman), sample 336.9 (scale bar = 50 μm). 7a, b, c. Lingulogavelinella globosa (Brotzen), sample 350.8 (scale bar = 50 μm). 8. Praebulimina prolixa (Cushman and Parker), sample 352.7 (scale bar = 50 μm). 9. Laevidentalina reussi (Neugeboren), sample 382 (scale bar = 200 μm). 10a, b, c. Lingulogavelinella sp., sample 350.8 (scale bar = 50 μm). 11. Cuneolina sp. sample 380 (scale bar = 200 μm). 12. Cuneolina laurentii (Sartoni and Crescenti), sample 380 (scale bar 200 μm). 13. Nezzazatinella picardi (Henson), sample 370b (scale bar = 200 μm). 14. Quinqueloculina sp. sample 380.5 (scale bar = 200 μm). 15. Nezzazata concava (Smout) sample 380.5 (scale bar = 200 μm). 16. Nezzazata conica (Smout) sample 380.5 (scale bar = 200 μm).

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Journal Pre-proof Assemblage 4 occurs in the marlstones of microfacies 3 (Diverse keeled and non-keeled planktic foraminifera wackestone) with TOC contents >1% (Table 2). Planktic foraminifera are characterized by high diversity and abundance of keeled (Thalmanninella, Rotalipora), planispiral (“Globigerinelloides”), trochospiral (Whiteinella, Muricohedbergella), and biserial (Planoheterohelix) morphotypes. Benthic foraminifera represent low abundance (%benthic <20, BFN = 50-1000). Simple diversity and H(S) vary between 4-10 and 0.4-1.6, respectively. BFOI

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fluctuates between -20 to -48 in this assemblage (Fig. 2). Generally, infaunal taxa dominated this

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benthic assemblage (>50 %) with Bolivina alata, Coryphostoma midwayensis, Praebulimina

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prolixa, and Praebulimina kickapooensis. Epifaunal Gyroidinoides depresus is also present in

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some parts (Fig. 3).

Assemblages 5 and 6 correspond to the microfacies 4 (Low diversity non-keeled planktic

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foraminifera wackestone/packstone) with TOC >2% (Table 2). Planktic foraminifera in this

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microfacies are characterized by relatively low abundance of trochospiral, biserial, and diagitate morphogroups (Whiteinella, Muricohedbergella, Planoheterohelix, and Clavihedbergella).

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Benthic foraminifera in the assemblage 5 are relatively abundant (%benthic >20%, BFN

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>1000) and diverse (simple diversity = 6-12; H(S) = 1.6-2). BFOI oscillations between -38 and 50 (Fig. 2). Bolivina alata, Neobulimina albertensis, Praebulimina kickapooensis, and Praebulimina prolixa are the most abundant benthic foraminiferal species in this assemblage. Gyroidinoides depresus and Lingulogavelinella globosa are the subordinate benthic components (Fig. 3). Benthic foraminifera in the assemblage 6 are abundant (BFN >1000), but their number is much less than planktic foraminifera (%benthic <10). Low diversity, small and thin-walled

15

Journal Pre-proof infaunal taxa such as Bolivina alata are common in these intervals. BFOI indicates the values >40 in this assemblage (Figs. 2, 3). Assemblage 7 corresponds to the black shale strata of microfacies 5 (Heterohelix wackestone/packstone) with TOC >2% and CaCO3 <40% (Table 2). This microfacies is dominated by biserial Planoheterohelix genus. Benthic foraminifera are totally absent in this assemblage, indicating BFOI = -55 (Figs. 2, 3, 5).

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Assemblages 8 and 9 occur in the microfacies 6 (Planktic foraminifera bioclast intraclast

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wackestone/packstone) with TOC contents between 0.9-2.3 % (Table 2). Benthic foraminifera in

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the assemblage 8 are characterized by low aboundance (% benthic <10; BFN = 100-200) and

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relatively high diversity (simple diversity = 4-10; H(S) = 1.8-2). Infaunal taxa comprise 50% of benthic assemblage with Praebulimina spp. and Coryphostoma midwayensis. Epifaunal

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Gyroidinoides depresus and Valvulineria sp. are also present in this assemblage.

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Assemblage 9 is characterized by absence of epifaunal benthic taxa. Abundant (BFN > 1000) infaunal benthic foraminifera with Neobulimina albertensis, Gabonita Laevis, and

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Praebulimina kickapooensis dominated this assemblage. BFOI <-40 is other characteristics of

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this assemblage (Figs. 2, 3).

4.1.2. Benthic foraminiferal assemblages in the microfacies of oligosteginid-dominated mid, outer ramp, and intra-shelf basin Assemblage 10 occurs in thin bedded marlstone and shale strata correspond to microfacies 7 (planktic foraminifera calcisphere wackestone/packstone) (Table 2). TOC contents sometimes reach up to 1%. Trochospiral and biserial morphotypes are the main planktic foraminifera, which occur in low abundance in this microfacies. Benthic foraminifera also indicate low abundance

16

Journal Pre-proof (BFN = 50-500, %benthic <30). Simple diversity and H(S) show relatively high values, about 10 and 1.6, respectively. Infaunal and epifaunal benthic foraminifera are both present. BFOI indicates the values >-40 (Fig. 2). The benthic assemblage is mainly represented by Valvulineria sp. and Praebulimina prolixa (more than 50%). Neobulimina albertensis, Gabonita laevis, Praebulimina kickapooensis, and Gyroidinoides depresus are also present (Fig. 3). Assemblage 11 corresponds to the laminated marlstones of microfacies 8 (laminated

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calcisphere wachstone/packstone) with TOC values <1% (Table 2). This microfacies is

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characterized by high abundance of calcispheres and low abundance and diversity of planktic

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foraminifera. Absence of benthic foraminifera is other characteristic of this assemblage (Figs. 2,

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3, 6).

Assemblage 12 is observed in intraclast-bearing limestone and marlstone beds of

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microfacies 9 (Planktic foraminifera bioclast intraclast wackestone/packstone) with TOC <1%

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(Table 2). Planktic foraminifera, including biserial (Planoheterohelix), trochospiral (Whiteinella, Muricohedbergella), digitate (Clavihedbergella), and triserial (Guembelitria) morphotypes are

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fairly diverse and abundant in this microfacies. Red algae, bivalves, and calcispheres are the

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subordinate components. Benthic foraminifera are relatively abundant (BFN = 500-3000) and comprise more than 50% of foraminiferal assemblage. This assemblage is dominated by large epifaunal benthic foraminifera. BFOI indicates near zero values in this assemblage (Fig. 2). Epifaunal Neoeponides auberii is the most abundant benthic taxa in this interval (>70%). Gyroidinoides depresus, Valvulineria sp., Praebulimina kickapooensis, Praebulimina prolixa, Neobulimina albertensis are also present (Fig. 3).

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Journal Pre-proof 4.1.2. Benthic foraminiferal assemblage in the platform-top (inner platform) microfacies Microfacies 10 and 11 (Peloidal grainstone and Bioclast cortoidal grainstone) represent similar benthic components named assemblage 13 (Table 2). These deposits are composed of indurated marlstones with TOC contents <1%. The biotic assemblage is characterized by lack of planktic foraminifera and dominance of agglutinated, microgranular, and porcelaneous benthic foraminifera in a grainstone texture. Bioclast debris, cortoids, peloids, red algae, and oncoids are

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the other components (Figs. 6, 7). Quinqueloculina sp., Nezzazata conica, Nezzazata concava,

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abundant benthic foraminifera in this assemblage.

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Nezzazata sp., Cuneolina laurentii, Cuneolina sp., and Nezzazatinella picardi are the most

5. Discussion

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5.1. Primary productivity, bottom water oxygenation, and water turbulence in the biotic

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assemblages

Radiolarian-bearing strata related to assemblages 1, 2, and 3 are interpreted to be deposited

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under effect of upwelling currents. The radiolarians in the assemblage 1 show oriented fabric in

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the thin sections, suggesting they were transported by bottom currents. The intensity of upwelling likely decreases toward the assemblage 3 due to decrease of radiolarians abundance and absence of orientated texture (Fig. 5). In contrast with many other upwelling areas, which are characterized by the development of an intense oxygen minimum zone (e.g., Tarfaya Basin, Morocco; Casamance Basin, Senegal; Holbourn et al., 1999b; Gebhardt et al., 2004), these benthic assemblages indicate relatively well oxygenated condition. High oxygen concentrations are indicated by low TOC values in these

18

Journal Pre-proof eutrophic environments. Similar oxygenated upwelling area is reported from Cape Blanc off NW Africa (Jorissen et al., 1995; Morigi et al., 2001). Benthic foraminifera are absent in the assemblage 1 (Figs. 2, 3, 5). It may be related to high water velocities in the strong upwelling regime. High energy environments leave a certain impact on the benthic foraminifera, however little is known about foraminiferal assemblage in deep high energy environments (e.g., Schönfeld, 1997, 1998, 2002a, 2002b). It has been suggested that in

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the high water turbulence, epibenthic foraminifera use large and heavy objects (such as coarse

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shell debris, sponges, crinoids, and pebbles) to provide microenvironmental stability and achieve

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maximum access to food particles in the water column (Schönfeld, 2002a, 2002b; Jorissen et al.,

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2007). Therefore, the lack of benthic foraminifera in the assemblage 1 may be explained by absence of coarse sediments in these intervals. One alternative possibility for absence of benthic

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foraminifera could be over-eutrophication of environment, which exceeds the optimal levels of

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benthic foraminifera. Assemblage 2 is associated with the appearance of benthic foraminifera. Infaunal and epifaunal groups are both present due to high nutrient availability and oxygen

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contents (e.g., Van der Zwaan et al., 1999; Jorissen et al., 2007). Assemblage 3 is marked by

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higher abundance and diversity of benthic foraminifera. Epifaunal groups dominated this oxygenated environment. We conclude that the abundance and diversity of benthic foraminifera increase during decline of upwelling strength (decline of water turbulence and/or high eutrophication) in the radiolarian-bearing assemblages (Fig. 5). Assemblages 4, 5, 6, and 7 are characteristics of organic-enriched deposits, which propose better preservation of organic matter in a low-oxygenated environment. Dominance of infaunal benthic foraminifera (low BFOI) in these assemblages also indicates low oxygenation in the bottom water (e.g., Van der Zwaan et al., 1999; Friedrich et al., 2006; Jorissen et al., 2007;

19

Journal Pre-proof Friedrich, 2010). Abundance of the infauanl taxa in the assemblage 4 is low, probably because of relatively lower food supply. Low productivity in the assemblage 4 is also indicated by high abundance of large keeled rotaliporids, which preferred oligotrophic conditions (e.g., Hart et al., 1999; Coccioni and Luciani, 2004). Increased abundance of infaunal benthic foraminifera in the assemblage 5 proposes more primary productivity and organic matter flux to the seafloor (e.g., jorissenet al., 2007, Friedrich et al., 2006). Density of infaunal taxa in assemblage 6 is greater

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than assemblage 5, but the diversity shows a sharp decrease. This suggests that oxygen

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concentrations fall below and/or organic matter flux exceeds the optimal levels of some benthic

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foraminifera, which resulted in their replacement by more resistant species (Jorissen et al., 2007).

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The lack of benthic taxa in the black shales of assemblage 7 indicates expansion of anoxic conditions in the sediments (Kaiho, 1994). The benthic foraminiferal changes in assemblages 4-7

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propose that in the low oxygen levels (even dysoxic conditions), benthic foraminiferal densities

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may be very high and oxygen concentration is a critical factor only below a certain threshold value. This value seems to be low for some specimen such as Bolivina alata in the Lar Anticline

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section. Modern benthic foraminiferal data also support this conjecture (Bernhard et al., 2001).

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Assemblages 8 and 9 are characterized by the presence of inner platform derived taxa (such as miliolids) and intraclasts (containing inner platform taxa in a grainstone texture) in a relatively deep environment with sponge spicules and planktic foraminifera. This may suggest the steepside of an intra-shelf basin with slumping of adjacent shallow high-energy deposits. Intercalation of radiolarian-bearing and organic-rich strata proposes that periodic upwelling may have occurred in this environment, too. Presence of both infaunal and epifaunal groups in the assemblage 8 proposes well nutrient and oxygen availability (e.g., Van der Zwaan et al., 1999;

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Journal Pre-proof Jorissen et al., 2007). High TOC values and lack of epifaunal group in the assemblage 9 suggest

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expansion of oxygen minimum zone in this environment.

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Fig. 5. Reconstruction of biotic assemblages and environmental changes in the lower 39 m of

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Lar Anticline section. Planktic foraminiferal data and microfacies are from Kalanat and VaziriMoghaddam (2019 a, 2019b).

Assemblage 10 represents both epifaunal and infaunal benthic foraminifera, which may reflect a mesotrophic conditions with relatively well oxygenation of bottom water (e.g., Van der Zwaan et al., 1999; Jorissen et al., 2007). Assemblage 11 is marked by dominance of calcispheres and very low abundance of planktic foraminifera in a laminated marlstone. Irregular and discontinuous laminas are interpreted to be deposited in a high energy mid-ramp under effect of storm waves (Kalanat and Vaziri21

Journal Pre-proof Moghaddam, 2019a). The presence of opportunistic thermophile calcispheres suggests high levels of nutrient (Dias-Brito, 2000). Benthic foraminifera are absent in this eutrophic high energy environment, which suggest unfavorable conditions (high energy and/or over-

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eutrophication) for benthic foraminifera similar to those in assemblage 1.

Fig. 6. Reconstruction of biotic assemblages and environmental changes in the upper 14 m of

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Lar Anticline section. Planktic foraminiferal data and depositional environments are from Kalanat and Vaziri-Moghaddam (2019 a, 2019b).

Assemblage 12 is marked by presence of Guembelitria as a marginal marine index (Leckie et al., 1998; Coccioni and Luciani, 2004), r-strategists surface dweller planktic foraminifera, as well as calcispheres. These components suggest a relatively shallow marine environment with eutrophic conditions (e.g., Dias-Brito, 2000). The nutrients may have been supplied by rapid transgression of marine and leaching of surrounding exposed areas. This assemblage is also 22

Journal Pre-proof characterized by presence of inraclasts with the same texture and components of their host deposits, which may indicate effect of storm waves (Kalanat and Vaziri-Moghaddam, 2019 a). The high abundance and diversity of benthic foraminifera, which are dominated by epifaunal taxa, may reflect high bottom water oxygenation. The intraclasts and coarse bioclasts debris in this high energy environment may have been used as elevated substrates with better fixation strength, which probably provide a favorable ecological niches for epifaunal taxa (Schönfeld,

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2002a, 2002b) (Fig. 6).

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Quantitative data for assemblage 13 do not exist due to the highly indurated nature of these

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deposits, but the components and texture in the thin sections suggest a high-energy shallow

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environment in the inner platform (Fig. 6). Presence of miliolids as sensitive oxygen markers (Jorissen, 1999; den Dulk et al., 2000) indicates high oxygen levels in the inner platform. The

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dominance of cortoids, oncoids, and high levels of bioerosion (Fig. 7) may reflect that this

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assemblage was deposited in the eutrophic conditions (Flügel, 2010).

Fig. 7. The presence of cortoids, oncoids, and high levels of bioerosion suggests a relatively high nutrient supply in the assemblage 13.

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Journal Pre-proof 5.2. OAE2 structure in the Lar Anticline section 5.2.1. Before the carbon-isotope positive excursion The interval before the carbon-isotope positive excursion (late Cenomanian, R. cushmani Biozone) has been deposited in a deep intra-shelf basin environment (Kalanat and VaziriMoghaddam, 2019a). This interval starts with the organic matter-rich strata (0-11 m) dominated by benthic assemblage 4 and 6 (Fig. 2), which indicates dysoxic conditions at the base of section.

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The uppermost part of R. cushmani Biozone (11-23 m) is associated with sea-level transgression

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and radiolarian-rich strata, which propose development of upwelling currents in this region

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(Kalanat and Vaziri-Moghaddam., 2019a, 2019b). Transition to the lower TOC values and high

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abundance of epifaunal benthic foraminifera in this succession has been probably caused by better ventilation of seafloor. This interval is dominated by alternation of benthic assemblage 3

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and 4 (Fig. 2).

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In a global scale, the interval before the carbon-isotope excursion indicates a shift in strontium and osmium isotopes to more unradiogenic values suggesting not only construction but

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also destruction of large igneous provinces. This resulted in elevation of nutrient levels and

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primary productivity (Jenkyns et al., 2017). This interval also reveals high volcanic degasing (Barclay et al., 2010; Turgeon and Creaser, 2008) and deposition of organic carbon-rich strata in some basins (e.g., Jarvis et al., 2011; Gale et al., 2019) and can be correlated with organic matter deposition and dysoxic conditions at the base of Lar Anticline section.

5.2.3. Carbon-isotope excursion The carbon-isotope peak “a” in the study area is accompanied with stepwise extinctions of rotaliporids and “Gobigerinelloides” bentonensis and also dominance of infaunal benthic

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Journal Pre-proof foraminifera (benthic assemblages 5 and 6). This interval followed by “Heterohelix shift” (Kalanat and Vaziri-Moghaddam, 2019b) and total absence of benthic foraminifera (benthic assemblage 7), which indicate the stressful conditions for both benthic and planktic assemblages. It proposes creation of a strong dysoxic and anoxic conditions in this interval (23-32 m) except for a few low TOC radiolarian-bearing samples reflecting short-term increases in the oxygenation of basin.

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Similar to the Lar Anticline section, the disappearance of rotaliporids in many Cenomanian–

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Turonian sections is associated with initiation of δ13C positive shift (the first positive peak)

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(Falzoni et al., 2018 and the references therein). This interval in the low-mid latitude sections is

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characterized by cool well-oxygenated environment with low-TOC strata in the Plenus Cold Event (e.g., Jarvis et al., 2011; Jenkyns et al., 2017; Gale et al., 2019). However, there are

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evidences for widespread environmental changes during the Plenus Cold Event across the

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northern hemisphere, but O'Connor et al (2019) suggested that the local effects played a modifying role on the local timing and expression of this event. In the Lar Anticline section,

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development of organic matter-rich strata (assemblage 5 and 6) in this interval reflects the local

changes.

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expansion of OMZ likely because of local oceanographic conditions independent of global

From 32 to 41 m (δ13C peak “b”) the Lar Anticline section experience a sea-level fall, probably due to harbinger of Neo-Tethys Ocean compressional event around the C/T boundary (e.g., Navidtalab et al., 2016). Transition to the lower TOC values and reappearance of benthic foraminifera are characteristics of these strata. This interval starts with the intraclast-bearing strata and dominance of benthic assemblages 8 and 9. In the 39-41 m, the peak of tectonic activity resulted in establishment of a platform-top (inner platform) environment in the Lar

25

Journal Pre-proof Anticline section and deposition of benthic assemblage 13 in an oxygenated setting. The end of carbon-isotope positive excursion (peak “c”; 42-45 m) is characterized by a sea-level rise, dominance of calcisphere-bearing facies, and presence of benthic assemblage 12 in a relatively well-oxygenated environment with high abundance of large epifaunal benthic foraminifera. Whereas, the global Cenomanian/Turonian boundary sea-level transgression does not reach to its maxima until the early Turonian (Haq, 2014) and carbon-isotope peaks “b” and “c” are

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associated with organic matter-rich layers in many sections around the world (e.g., Friedrich et

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al., 2006; Sinninghe Damsté et al., 2010; Jarvis et al., 2011; Gale et al., 2019), the Lar Anticline

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section reveals sea-level fall and low TOC values in the late Cenomanian interval. This proposes

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the role of local tectonic activities around the C/T boundary of study area. The sea-level fall and extensive exposure of carbonate platforms were recorded throughout the Arabian Plate in this

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time (e.g., van Buchem et al., 2002; Hsjikazemi et al., 2010, 2012; Rahimpour-Bonab et al.,

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2013; Vincent et al., 2015).

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5.2.4. Above the carbon-isotope positive excursion

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The interval after OAE2 is characterized by return of δ13C curve to the pre-excursion values at the lower part of Turonian succession. It is associated with sea-level transgression, development of a calcisphere-dominated ramp, and alternation of benthic assemblages 10 and 11. While the lower Turonian strata are well preserved in the studied region, they vary from completely absent or preservation above an exposure surface in some sections of Zagros Basin (e.g., Mehrabi and Rahimpour-Bonab, 2014; Vincent at al., 2015; Navidtalab et al., 2019), which confirm the position of Lar Anticline in a graben-like setting in the Cenomanian–Turonian time.

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Journal Pre-proof 5.3. Distribution of index benthic foraminifera in the Lar Anticline section Bolivina alata is a dominant species in the organic–enriched strata of the study section (assemblage 4, 5, and 6) (Fig. 3). It is the first report of B. alata from organic carbon-rich deposits of OAE2, however another species of this genera (B. anambra) is described as characteristics of Cretaceous low-oxygen assemblages (e.g., Holbourn et al., 1999a; Gebhardt et al., 2004; Gebhardt, 2006; Kuhnt et al., 2005; Friedrich et al., 2006, 2009). This species is almost

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the only benthic foraminifera found in some low-diversity assemblages and the last taxon that

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becomes absent before anoxic condition, which proposes high tolerance of this elongated thin-

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walled taxon in the high organic matter flux and dysoxic bottom water conditions.

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Praebuliminids have been described as typical taxa found in the eutrophic low-oxygenated environments of mid-Cretaceous black shale strata and Upper Cretaceous deposits (Koutsoukos

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et al., 1990; Coccioni et al., 1993; Widmark, 1997; Holbourn et al., 2001; Friedrich et al., 2006,

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2009). In the study section the highest abundance of these taxa occurs in the organic-rich strata of assemblage 4 accompanied by decrease of B. alata. They are also present in the lower

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abundance in assemblages 3, 5, 8, 9, 10 and 11. Distribution of praebuliminids in the study

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section may reflect that they are less tolerant to oxygen deficiency in competition with B. alata. Similarly, this trend is observed for Coryphostoma midwayensis, which suggests the same favorable environmental conditions for this taxon. Neobulimuna albertensis is one of the most tolerant species to low oxygen conditions that is typical of marginal basins such as Western Interior Seaway (WIS) (Leckie et al, 1998; Elderbak et al., 2016, 2014) and northern and northwestern African margin (e.g., Gebhardt et al., 2004). It is proposed that this taxon can even survive in the anoxic conditions because of its association with chemoautotrophic bacteria (West et al., 1998). N. albertensis is absent at the base of our

27

Journal Pre-proof study section in the intra-shelf basin facies, but it is more abundant in the shallower environment at the top of section. It is the dominant taxon in assemblage 9 that may reflect that shallower water depth with different substrata and/or organic matter quality was more favorable for N. albertensis. Gabonita is described as a special indicator of high organic matter flux and oxygen deficiency (Kuhnt and Wiedmann, 1995; Holbourn et al., 1999a) in the strong coastal upwelling

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cells of marginal seas in the western and northwestern Africa (Kuhnt and Wiedmann, 1995;

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Gebhardt et al., 2004; Kuhnt et al., 2005). This taxon is absent at the base of study interval, but

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appear at the top of section in low abundance accompanied by N. albertensis. Absence or low

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abundance of Gabonita may be related to the lack of eutrophic low-oxygenated marginal seas in this succession.

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Neoeponides auberii is frequent in the assemblage 12; the interval with high nutrient supply

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which are under effect of storm waves. It can be concluded that the high-energy oxygenated environments are favorable for N. auberii.

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Gyroidinoides is interpreted as an indicator for enhanced productivity in the Cretaceous

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OAEs (Erbacher et al., 1998; Herrle et al., 2003a, 2003b; Friedrich et al., 2005; Friedrich and Erbacher, 2006). This genus is the most abundant epifaunal taxa in the study section. Its abundance increases sharply in the assemblage 3, which suggests that this taxon prefers eutrophic and oxygenated environments. Valvulineria, Lenticulina, and Lingolugavelinella also indicate a similar trend with Gyroidinoides. Valvulineria becomes more abundant in the assemblage 11, which suggests mesotrophic conditions are more favorable for this genus. The Lar Anticline benthic foraminiferal genera show affinity to taxa of the tropical Atlantic Ocean (Demerara Rise Basin; Friedrich et al., 2006), which is characterized by Bolivina,

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Journal Pre-proof Praebulimina, Gavellinella, and Tappanina. This may reflect the similar regime of productivity and oxygen deficiency at the same tropical environment. However, larger amounts of organic matter production in the Demerara Rise Basin resulted in differences in the presence of some genera. The similar assemblages are also reported in the marginal seas of western and northwestern Africa (Nyong and Ramanathan, 1985; Holbourn et al., 1999a, 1999b; Gebhardt et al., 2004; Kuhnt et al., 2005). However, different species of Gabonita occur sometimes in very

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high abundance in these basins, which probably is related to strong coastal upwelling cells of

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these area and shallower water conditions (Gebhardt et al., 2004).

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6. Conclusions

Our study on the late Cenomanian–early Turonian benthic foraminiferal assemblages

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accompanied by other biotic (planktic foraminifera, radiolarians, calcispheres), sedimentological,

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and geochemical data provides the following results: 1- Three general intervals for deposition of benthic foraminifera during carbon-isotope positive

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excursion have been recorded in the Lar Anticline section including: a- High Population of

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infaunal benthic foraminifera, which is associated with extinction of rotaliporids and “Globigerinelloides” bentonensis (δ13C peak “a”). b- Expansion of OMZ and absence of benthic foraminifera at the black shale strata of W. achaeocretacea Biozone. c- Better ventilation of seafloor and reappearance of benthic foraminifera due to local tectonic activities and sea-level fall around the C/T boundary (δ13C peaks “b” and “c”). 2- Bolivina alata-dominated assemblages were characteristic of organic matter-enriched intervals. This taxon is the last species, which disappears before anoxic conditions in the study section. This indicates that B. alata is a low-oxygen, high-productivity indicator. Praebulimina,

29

Journal Pre-proof Coryphostoma, Lingulogavellinella, Lenticulina, Gyroidinoides, and Valvulineria are the genera with higher abundance in the lower-TOC intervals, suggesting their less resistance to low oxygen and/or eutrophic conditions in comparison with B. alata. 3- The intensification of OMZ resulted in absence of benthic foraminifera in the assemblage 7. This is associated with “Heterohelix shift”, which proposes unhospitable conditions for both benthic and planktic foraminifera. Benthic foraminifera are also absent in the oxygenated high-

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energy assemblages 1 and 11. This indicates that the benthic foraminifera as proxies for

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palaeoecological reconstructions must be used with other sedimentological, geochemical, and

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biological data.

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6- The TROX model (with these patterns: high productivity, low oxygenation, dominance of infaunal group versus low productivity, higher oxygenation, dominance of epifaunal groups)

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does not fit very well with Lar Anticline benthic foraminiferal assemblages because the

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ventilation of seafloor has not been considered in this model. For example, the oxygenated seafloor can occur during high levels of organic matter supply in the environments that are under

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effect of upwelling or storm waves. Under these well oxygenated, high productivity conditions

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(i.e., assemblages 3, 12, and 13), epifaunal morphogroups are dominant. Another example is related to low productivity and low levels of food supply in a low-oxygen environment (i.e. assemblage 4), which provide more hospitable conditions for infaunal groups.

Acknowledgements We gratefully acknowledge Professor Nance for his editorial guidance and Professor R. Mark Leckie, Dr. Chris Lowery, and an anonymous reviewer for their fruitful and detailed reviews that significantly improved the quality of this manuscript.

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Journal Pre-proof Declaration of interest statements

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The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Graphical abstract

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Journal Pre-proof Highlights The study interval was deposited during OAE2 across the C/T boundary OAE2 starts with rotaliporid extinction and population of infaunal benthic foraminifera Benthic foraminifera became absent in the black shale strata of W. arch. zone

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From the middle of OAE2 upward local tectonic inversion controls benthic assemblage

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