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Paper Information

Journal:   QUATERNARY JOURNAL OF IRAN   FALL 2015 , Volume 1 , Number 3; Page(s) 181 To 189.



Coccolithophores are one of the main groups of marine phytoplankton playing key roles in the marine ecosystem as primary producers and in marine biogeochemistry. These organisms have gained considerable attention as they play a unique role in the global carbon cycle because of their combined effects on both the organic carbon and the carbonate pump. Although steady advances in research on coccoliths as biogeochemical agents and palaeontological proxies were obtained knowledge of the biology of these organisms has only progressed considerably in recent years. Calcareous nannoplanktons are one of the most important of phytoplankton for reconstruction of paleoinvironment The Persian Gulf Basin, is found between the Eurasian and the Arabian Plates. The Persian Gulf is described as a shallow marginal sea of the Indian Ocean that is located between the south western side of Iran and the Arabian Peninsula and south and southeastern side of Oman and the United Arab Emirates. Other countries that border the Persian Gulf basin include; Saudi Arabia, Qatar, Kuwait, Bahrain and Iraq. The Gulf extends a distance of 1000 km with an area of 240,000 square km. The Persian Gulf basin is a wedge-shaped foreland basin which lies beneath the western Zagros thrust and was created as a result of the collision between the Arabian and Eurasian plates. The Strait of Hormuz is the only link between the Gulf and the open water of Arabian Sea and Indian Ocean. It lies between Iran and a tiny separated territory of the Sultanate of Oman on the Arabian Peninsula. Measured along the median line, the Strait is about one hundred and four miles long but narrows to only twenty nautical miles at the northeastern end between Larak Island on the Iranian side, and the Quioins on the Omani side. The ability to reconstruct historical sea-surface salinity (SSS) patterns is essential to understanding past ocean circulation and climatic change. The calcareous nannoplankton is, as the most groups of marine organism, very sensitive to salinity fluctuations. The family Braarudosphaeraceae is one of the oldest extant coccolithophore families, it is distinguished by having a coccosphere formed of five-fold symmetric calcareous scales called pentaliths. The family has a long fossil record extending back to the Early Cretaceous. In the geological record, they show greater morphological variation in their pentaliths than in modern populations. Based on the morphology of the five segments forming a pentalith, six genera have been recognized in the Braarudosphaeraceae. Besides, although typical marine taxa, a few coccolithophores live in brackish waters and even in fresh ones. In the literature, Braarudosphaera bigelowii is often described as a species living in low-salinity coastal waters, since it is rarely recorded from the open oceanThe species Braarudosphaera bigelowii, a taxon confined mostly to shallow marine environment (first appearing in the Lower Cretaceous), is a good indicator of salinity fluctuations. Braarudosphaera bigelowii lives in the coastal areas, low salinity water and with entrancing of clastic materials. Low-salinity waters and unusual current conditions have also been put forward as a possible cause for past Braarudosphaerid blooms. The studies done on samples of the Persian Gulf, East of Strait of Hormuz and Oman Sea based on distribution of this species showed that in the study area frequency of this species very low and presence of this species in the western part of the Persian Gulf related to the entrance of fresh water to this area. the inflow of Arabian Sea waters occurs mainly in the northern part of the strait and forms a low-salinity wedge intruding westward into the gulf along the southern coast of Iran. Cyclonic circulation within the gulf carries these waters to the northwestern and shallow southeastern areas of the gulf where they are transformed to dense, saline waters which sink into the axial trough of the Persian Gulf and exit through the deepest part of the strait off northern Oman. A recent compilation of hydrographic data from the gulf indicates that the densest waters are formed in the northern gulf during winter, and that these waters propagate southward toward the Straits of Hormuz throughout the year. Slightly less dense, though saltier, waters are formed on the shallow southeastern shelves, which spill more intermittently into the deep gulf. While the general features of the exchange flow through the Straits of Hormuz are understood, the magnitude of the exchange is still poorly known because of large uncertainties in estimates of evaporation over the Persian Gulf. Evaporation and salinity in central part of the Persian Gulf are high and frequency of B.bigelowii is very low and finally in the East Stratit of Hormuz and Oman Sea the amount of salinity is high and in these parts B.bigelowii are rare.

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