نوع مقاله : مقاله پژوهشی
نویسندگان
1 عضو هیات علمی. پژوهشگاه ملی اقیانوس شناسی و علوم جوی
2 استادیار پژوهشگاه ملی اقیانوسشناسی و علوم جوی. ایستگاه پژوهشی دریای خزر. مازندران. نوشهر.
کلیدواژهها
موضوعات
عنوان مقاله English
نویسندگان English
Introduction
Mangrove forests are dynamic coastal environments where climatic conditions and water levels lead to diverse chemical, physical and biological environmental changes in different time frames, which are recorded in their sediments. In this study, physicochemical (grain size, magnetic susceptibility, total organic carbon, colour, bulk density and porosity) and biological (diversity and abundance of benthic foraminifera) data from a radiocarbon-dated sedimentary core from Azini Creek were used to reconstruct palaeoenvironmental and the relative sea level change in Sirik mangrove during the last 2700 years.
Materials and Methods
The estuaries of the Sirik region are located between 26°15' to 26°25' N and 57°4' to 57°8' E with an area of 3500 km2 in Hormozgan province, at the boundary between the Oman Sea and the Strait of Hormuz. This area is the only estuary with two mangrove trees containing Avicennia marina (Hara) and Rhizophora mucronata (Chandal). Core Az (1.5 m long) was collected within the mangrove of the Azini Creek, using a 7 cm diameter Russian peat sampler. Physiochemical and biological analysis of sediments (magnetic susceptibility, grain size, total organic carbon %, colour, porosity bulk density and foraminiferal assemblage) were performed. One radiocarbon age was determined in the Poznan laboratory by the AMS radiocarbon method.
Results and discussion
The radiocarbon age of depth 144-146 cm shows that the base of the core ages about 2700 cal. yr BP. Based on physiochemical and biological parameters the core Az was subdivided into five units of A-E. In core Az, a total of 66 foraminifera species were identified, encompassing 24 genera and 20 families. Among these species, 44 are characterized by hyaline tests, 20 by porcelaneous tests, and 9 by agglutinated tests. Sediments of core AZ are not very diverse and are all composed of sandy silt and sandy mud. Unit A is composed of olive green (5Y-5/2) sandy silt with the highest magnetic susceptibility (6.3×10-5 SI) and foraminiferal diversity (mean 17 species in 10 cm3) belonging to three groups (shallow marine environment). In unit B, sand content decreases by 50% and sediments turn to sandy mud. In this unit frequency and diversity of foraminifera decrease and porcelaneous taxa disappear (playa-lagoon). In unit C sand content (mean: 38%) and frequency of foraminifera increases again (4600). This unit represents a low mangrove adjacent to the tidal channel. In units D and E sand content decreases gradually and organic carbon and plant remains increase dramatically. Concurrence of agglutinate and opportunistic hyaline taxa suggest the provenance of mangroves in the area.
The results showed that from 2700 to 1800 years ago, the relative sea level was about 1m higher than that of today and fluvial input to the area was higher than the present. At the end of this period, the decreasing trend in the relative sea level and humidity caused the relatively humid marginal coastal environment to become a shallow playa environment. From 1400 to 1800 years ago, stabilizing the relative sea level led to the expansion of tidal channels, which provided a suitable environment for the initiation of mangroves. At this time, the gradual decrease in temperature caused a relative increase in Mediterranean winter precipitation. This climatic optimum period is traceable in other parts of the Iranian Plateau and the Near East. During the last 1400 years, the development of the mangroves began in the region. This mangrove evolution matches to the mangrove growth in the Gowatr (SE extreme of Iran) and Khuran (west of Hormuz Strait). This period coincides with the relative dryness with high climatic fluctuations. In this period gradual increase in winter temperature caused the northward migration of winter westerlies and therefore reduction in winter precipitation. This climatic deterioration is observed in other parts of the Iranian Plateau.
Conclusion
With an age of about 4000 years, the Gowatr mangrove is the oldest in Iran. 2500 years later, mangroves entered the Strait of Hormoz in Sirik and Khuran. During the last 3000 years changes in the earth's orbit around the sun caused the mean annual sea surface temperature in the Oman Sea and the Persian Gulf has increase between 2-3 degrees Celsius, which is the result of an increase in the average winter air temperature. This study shows that during warmer periods, the northward movement of winter westerly winds causes a decrease in winter precipitation in the area.
Keywords: Azini, Mediterranean winter precipitation, Chandal, Harra, Foraminifera
Acknowledgements
This investigation evolved from a research project entitled: "The history of appearance and evolution of Iranian mangroves since the Mid-Holocene and its link to the climate and sea level change", being supported by a grant from the Iran National Science Foundation (INSF) (No. 99017678).
کلیدواژهها English
