بررسی تغییرات پالئواکولوژیکی در رسوبات هولوسن دریای عمان (منطقه نگور) براساس نانوپلانکتون های آهکی

در این بررسی نمونه هایی از رسوبات بستر دریای عمان (منطقه نگور) انتخاب و پس از آماده سازی، مطالعه و عکسبرداری شد. تعداد ده جنس و پانزده گونه برای اولین بار از منطقه مورد مطالعه شناسایی و معرفی شد.در این منطقه نانوفسیلهایی مانند1943 gephyrocapsaoceanica kamptner، pseudoemilianialacunose (kamptner, 1963) ، gratner (1969c) و helicosphaeracarteri از نظر فراوانی گونه های غالب بود. برخی گونه ها به صورت نادر در نمونه ها حضور داشت که در این میان میتوان گونه braarudosphaerabigelowii braarud(1974)، deflandre (1935) را نام برد. همچنین، در نمونه های مورد مطالعه تعداد زیادی گونه های حمل شده و غالباً مربوط به بازه زمانی نئوژن گزارش میشود. بر مبنای فسیلهای یافت شده و تنوع گونه ای پایین آنها میتوان نتیجه گرفت که دریای عمان در منطقه نگور دریا یی حاشیه ای است که در عرض جغرافیایی پایین، کم عمق و نزدیک به ساحل است. حوضه مذکور برای رشد نانوفسیلهای آهکی بسیارمساعد و دارای شرایط یوتروفیک است.

Investigating paleoecological changes in the Oman Sea (Negour area) Holocene

IntroductionCoccolithophores are unicellular marine phytoplankton characterized by calcareous scales called coccoliths. Cocclithophores appeared in the fossil record during Late Triassic. They live at different depths in the photic zone owing to their requirement of sunlight for photosynthesis. Their distribution in the upper water column is influenced by surface oceanic circulation and different parameters such as temperature, salinity (Winter et al., 1994).The Gulf of Oman is located between 22 and 26°N and 56 and 62°E and opens on the Northwestern Indian Ocean and on the Arabian Sea. The general orientation of the gulf is northwest southeast. The basin narrows down and gets shallower towards the Strait of Hormuz the western most boundary of the Oman Sea. The Negour Area is located 25 and 20°N and 61 and 8°E.Materials and MethodsIn this study, a total of 52 surface sediments were chosen of the Oman Sea. The location of the surface sediments are presented in Figure 1. Sles were investigated using a Scanning Electron Microscope (SEM) (model: LEO 1450 VP with maximum 30v, Sputter coater (Au Pd) model: SC 7620) (Plate 1). In order to control the SEM preparation and to study general changes in the floral composition, a light microscope study was also undertaken using simple smear slides (Bown and Young, 1998). The sles for the SEM examination were prepared using the filtration technique of Andruleit (1996) . The sediment was weighed on a high precision balance (0.5 g) and filtered through polycarbonate filters (pore size 0.45γm) by means of a vacuum pump. A wedge shaped piece was cut of the dry filter, mounted on an aluminum stub and sputter coated with gold. The sles were examined with an Olympus BH 2 microscope polarizing light at a magnification of 2500X (Plate 2). For the distinction of some species such as Calcidiscusleptoporus (Murray and Blackman 1898) Leoblich and Tappan 1978 and Emilianiahuxleyi (Lohman 1902) Hay and Mohler, in Hay et al., we used gupsum plates. For the preparation of smear slides, at first the area around the sle was cleaned. A little amount of sle was cut completely and placed on a drop of distilled water over a glass slide. Next, the relatively concentrated solution was scattered using a tooth pick with two flat sides and dried in oven rapidly. Then, the slide was labelled and a lamella was placed over the dried sle and adhered with specific adhesive similar to the procedure used for preparing of marl and gypsum sles. It can be used for other lithologies except for hardest sediments. To ensure accuracy, 300 nannofossil specimens were counted per slide (relative abundancy). The determined species were photographed by a digital camera and then counted up to 300 numbers that its diagrams have drawn for paleoecology interpretations.DiscussionCalcareous nannofossils are generally well preserved in all the sles. In this study we have 3 groups of nannofossils. Some species have a high abundance such as Gephyrocapsaoceanica Ktner 1943, Helicosphaeracarteri (Wallich 1877) Kamtner 1954, Pseudoemiliania lacunose (Ktner, 1963) Gratner (1969c) and Emilianiahuxleyi (Lohman 1902) Hay and Mohler, in Hay et al. Some species are rarely found such as Umbilicosphaerasibogae (Weber van Bosse 1901) Gaarder 1968, Braarudosphaerabigelowii Gran Braarud, (1935) Deflandre (1947), Calcidiscusleptoporus (Murray Blackman 1898) Leoblich and Tappan 1978, Calcisoleniamurrayi Gran 1912, H. sellii Bukry Bramlette (1969b), Pontosphaeramutipora (Ktner, 1948) Roth (1970) (Fig. 2) and some species are reworked from the Cretaceous and Neogene such as Sphenolithusabies, Sphenolithusciperoensis, Sphenolithusmoriformis, Sphenolithus radians, Reticulofenestraminuta, Reticulofenestrapseudoumbilica, Reticulofenestraumbilica, Discoasterbrouweri and Discoasterdeflandrei.Rrelative abundances of the species are as follows (Negour area):1. Abundant taxa: The most abundant group is represented by Gephrocapsaoceanica Ktner 1943 (average 75.66%), Helicosphaeracarteri (Wallich 1877) Kamtner 1954 (average 4.3%), Gephyrocapsaericsonii McIntyre Be (1967) (average 5.9%) Pseudoemilianialacunosa (Ktner, 1963) Gratner (1969c) (average 5.19%) and Emilianiahuxleyi (Lohman 1902) Hay and Mohler, in Hay et al. (average 2.02%).2. Rare taxa: In this area, some species are rare, such as Helicosphaerasellii Bukry Bramlette (1969b) (average 0.16%), Calcidiscusleptoporus (Murray Blackman 1898) Leoblich and Tappan 1978 (average 0.95%), Umbilicosphaerasibogae (Weber van Bosse 1901) Gaarder 1968 (average 0.91%), Pontosphaeramultipora (Ktner, 1948) Roth (1970), (average 0.12%), Coccolithuspelagicus (Wallich, 1877) Schiller (1930) (average 0.86%), Gephyrocapsamuellera Breheret (1978a) (average 1.68%), Gephyrocapsaparallela Breheret 1978 (average 1%), and Braarudosphaerabigelowii Gran Braarud, (1935) Deflandre (1947) (average 0.2%).The distribution of G. oceanica Ktner 1943 mirrors warm water masses and shows a high Primary Productivity. High abundances of this taxon are located in marginal sea depositing in low latitude, shallow basin and near to coast. Coccolithuspelagicus (Wallich, 1877) Schiller (1930) is generally considered a cold water indicator in high latitude. Gephyrocapsamullerae Breheret 1978 is considered a cold water species (Kaemo and Sato, 2000). H. carteri (Wallich 1877) Kamtner 1954 consider it a warm water species (Roth, 1994). Bigelowii Gran Braarud, (1935) Deflandre (1947), which in the Mediterranean Sea seems to be related to turbidity of near shore waters and/or to low salinity in surface waters (Svábenická, 1999).In this study, high abundancy of G. oceanica Ktner 1943 suggests, Holocene sediments in the Negour areas were deposited in the warm waters and the Oman Sea were shallow basin and with high Primary Productivity. High abundances of this taxon are located in marginal sea deposited in low latitude, shallow basin and near to coast. G. muellerae Breheret 1978 and C.pelagicus (Wallich, 1877) Schiller (1930) species with very low abundances represent the sediments is deposited in low latitude and warm climate. Abundancy of H. carteri (Wallich 1877) Kamtner 1954 in surface sediments of Negouraera can be confirmed that this area is a warm basin with high productivity of calcareous nannoplanktons. Due to the low abundance of B. bigelowii Gran Braarud, (1935) Deflandre (1947) that is rare in the sles has been observed. It shows high salinity in the surface sediments of this basin.ResultsIn current study, sles of the Oman Sea (Negour area) surface sediments were selected and after their preparation were studied and photographed. For the first time, 10 genera and 15 species introduced in this area. Some nannofossils are the dominant taxa, such as Gephyrocapsaoceanica Ktner 1943, Pseudoemilianialacunosa (Ktner, 1963) Gratner (1969c) and Helicosphaeracarteri (Wallich 1877) Kamtner 1954. Some species are the rare taxa, such as Braarudosphaerabigelowii Gran Braarud, (1935) Deflandre (1947). In this study, we also observed many reworked species, specially mostly of them belong to Neogene. Based on the fossils founded and low diversity of them, we concluded Oman Sea (Negour area) is a marginal sea deposited in low latitude, shallow basin and near to coast. This basin was suitable for productivity of calcareous nannofossils with eutrophic conditions.