Invited Research PapersThe Campanian paleoenvironment: Inferences based on benthic foraminifera from northeastern Iran
Introduction
The benthic foraminifera are robust proxies for estimating past bathymetry, productivity, oxygen availability, ventilation, and changes in sea level, across time (Sliter and Baker, 1972; van Morkhoven et al., 1986; Widmark and Malmgren, 1988; Berggren and Miller, 1989; Koutsoukos and Hart, 1990; Widmark, 1995; Speijer and Van der Zwaan, 1996; Kouwenhoven et al., 1997; van der Zwaan et al., 1999; Holbourn et al., 1999, Holbourn et al., 2001; Alegret and Thomas, 2001, Alegret and Thomas, 2013; Cetean et al., 2011; Farouk and Jain, 2016; Wolfgring et al., 2016; Jain and Farouk, 2017; Jain et al., 2019; Farouk et al., 2020).
The present study quantitatively analyses the benthic foraminiferal distributional patterns for the Campanian Abtalkh Formation exposed in the Kopeh-Dagh Basin (northeastern Iran (see Fig. 1). The Abtalkh Formation is exposed throughout the basin and has largely been studied for its age assignment and stratigraphic position, using either planktic foraminifera or calcareous nannofossils (Table 1). Recently, however, Ghourchaei et al. (2015) from the western part of the Kopeh-Dagh Basin (Aitamir syncline; 37°20′00″ N; 57°40′21″ E) attempted to reconstruct late Campanian-Maastrichtian surface water productivity and seafloor oxygenation based on the occurrence patterns of generic-level planktic and benthic foraminifera. Based on their low-resolution generic-level identifications, they noted a strong linkage between changes in surface-water productivity and bottom-water aeration. In their study, the rotaliids (trochospiral forms) formed the most dominant fraction, making up 70% of total benthic foraminiferal population. Ghourchaei et al. (2015) further noted that moderately mesotrophic conditions prevailed throughout the Abtalkh Formation, and that such conditions “lasted until the latest part of the Maastrichtian”.
An attempt is made here to expand (Early to Middle Campanian; sensu Gradstein et al., 2012), and refine this broad analysis with inferences from sampling of benthic foraminifera vis-à-vis paleobathymetry, paleoproductivity, paleoxygenation, and sea level changes, throughout the Campanian. We aim to answer the following research questions: 1) Did the benthic foraminiferal assemblages significantly changed throughout the Campanian? 2) What are the factors controlling the distribution patterns of benthic foraminifera? and, 3) Can specific abiotic factors (e.g., sea-level, sediment type) be correlated to specific biotic traits of benthic foraminifera (e.g., percentage of infauna/agglutinated/high organic-flux species)?
Section snippets
Geological setting
The Abtalkh Formation is exposed along the Kopeh-Dagh sedimentary basin (northeast Iran) and stretches from the Caspian Sea to Afghanistan (Figs. 1A-B) and covers an estimated area of about 55,000 km2 (Afshar Harb, 1994). The ~6000 m thick sedimentary sequences are bracketed to age between Devonian to Oligocene with the most complete ~2000 m thick Cretaceous successions in Iran (Fig. 1C), consisting of marine shales, marly limestones, and sandstones (Stocklin, 1968). The studied section is
Sampling
Sixty-two samples (12,961 foraminiferal specimens; see Figure 2 for sampling intervals; see also Table 2) were studied and 90 benthic foraminiferal species were identified (Appendix 1). After separating 350 g of sediment for each sample, they were soaked in the 10% H2O2 solution. Samples were then washed through >63 μm sieve size. Approximately 200–300 benthic foraminifera specimens were picked for analyses, following the procedure enumerated in Keller and MacLeod (1995). The planktic
Planktic foraminiferal biostratigraphy
The Abtalkh village section (Figs. 1B-C) records 40 planktic foraminifera species belonging to 15 genera, forming five zones of Campanian age (Fig. 2). These zones are: Globotruncanita elevata Partial Range Zone, Contusotruncana plummerae Interval Zone (IZ), Radotruncana calcarata Total Range Zone (TRZ), Globotruncanella havanensis IZ, and Globotruncana aegyptiaca IZ. The zones are based on the occurrences of Globotruncanidae (following the works of Caron, 1985; Robaszynski and Caron, 1995;
Bathymetry
For the lower Paralabamina lunata Assemblage (samples 1–29) the eight most abundant species (constituting ~50% of the total assemblage) are characteristic Velasco-type species of Cushman (1925) - Coryphostoma incrassata, Cibicidoides voltzianus, Stensioeina excolata, Paralabamina lunata, Praebulimina reussi, Spiroplectammina spectabilis, Cibicidoides pseudoacutus, and Gaudryina pyramidata (see also Cushman, 1926; White, 1928a, White, 1928b; Berggren, 1974a; Berggren and Aubert, 1975; van
Conclusions
Comparable Campanian benthic foraminiferal studies are rare. Hence, the present quantitative analyses of 90 benthic foraminifera species from 62 samples from the Abtalkh Formation, exposed along the Kopeh-Dagh sedimentary basin (northeastern Iran), assumes importance. The present study attempts to reconstruct the Campanian paleoenvironment spanning from the Early Campanian Globotruncanita elevata Partial Range Zone to the Upper Campanian Globotruncana aegyptiaca IZ. Benthic foraminifera were
Declaration of Competing Interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors profusely thank Dr. Chris Lowery (University of Texas Institute for Geophysics, Texas, USA) for his time, patience and excellent suggestions that immensely improved the manuscript and to the Editor-in-Chief Dr. Thomas Algeo for his constructive suggestions and his cooperation.
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