نوع مقاله : مقاله پژوهشی

نویسندگان

1 ژئومورفولوژی، دانشگاه اصفهان

2 فوق دکترای زمین شناسی، پالینولوژی دانشگاه مارسی فرانسه، پژوهشگاه ملی اقیانوس ّ شناسی و علوم جوی

چکیده

دریاچه ها به عنوان بایگانی تغییرات محیطی قاره ای در آخرین دوران زمین شناسی به حساب می آیند. در این پژوهش پس از مطالعات میدانی، یک مغزه ی رسوبی از شمال غرب دریاچه مهارلو برداشت و حساسیت مغناطیسی آن اندازه گیری شد. سپس به روش تلفیقی تعداد 150 نمونه انتخاب و تحت مطالعات؛ ژئوشیمی (LOI)، PH متری، مطالعات میکروسکوپی ذرات رسوبی و بقایای گیاهی،  (SEM)بقایای جانوری، آنالیز دانه بندی قرار گرفته، با استفاده از سن سنجی ایزوتوپی δ14C سن مغزه ی رسوبی تعیین گردید. شواهد نشان داد؛ سن کف مغزه ی رسوبیBP 60±10813 بوده، وجود لس های یخبندان، عدم وجود بقایای زیستی گیاهی و جانوری، مبنی بر حاکمیت شرایط سرد و خشک در فاز پایانی یانگر دریاس (اواخر دیریخبندان) در دریاچه مهارلو است. دوران هولوسن تحتانی، BP 60±10200 سال پیش ، ابتدا سرد و خشک بوده، سپس حاکمیت محیط فلوویال و دوران گرم و خشک نمایان می شود. هولوسن میانی از  BP60±4000 سال پیش با شروع فعالیت چشمه ها ظاهر شده، دوره ی بهینه اقلیم گرم و مرطوب همراه با تناوبهای کوتاه مدت خشک شدگی بر دریاچه تسلط می یابد. هولوسن فوقانی در BP 45±2000 سال پیش، روند رو به خشکی حوضه (پلایا) را نشان میدهد. تغییرات محیطی دریاچه مهارلو در این دوران؛ مشابه با آخرین دوره یخبندان دریاچه های؛ پریشان، هشیلان و نئور و هولوسن پیشین دریاچه ارومیه و منطقه اطلس شمالی می باشد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Reconstruction environmental changes of Maharlou Lake in Holocene

نویسندگان [English]

  • Fatima Sebek Khiz 1
  • Abdullah Saif 1
  • Mohammad Hossein Ramsht 1
  • Morteza Jamali 2

1 Geomorphology, Isfahan University

2 Post-doctorate in Geology, Palynology, University of Marseille, France, National Research Institute of Oceanography and Atmospheric Sciences

چکیده [English]

Extended Abstract:
1-Introduction
There is a direct relationship in evaporative deposit with the changes in the depth of the lake. Therefore?, valuable information is gained regarding the causation of changes in environment over time. Evaporite minerals formation is a natural phenomenon accruing in water-bodies containing deferent mineral Since sulfate is abundant in seawater and saline lakes, it is main component in evaporative minerals (Lewis and Mc Conchie,1994). Playa lake deposits even in small precipitation rates may cause major changes in lake-level, salinity and sedimentary records (Battarbee 1999). The sediments generated in present saline lakes and playas have been studied by numerous researchers in order to determine the paleoclimate (Spencer et al. 1984; Holliday et al. 1996; Crowley and Hook 1996; Li et al. 1996; Enzel and Wells 1997; Menking et al. 1997; Lowenstein et al. 1999; Sinha et al. 2006; Chao et al. 2009).
Playas are found in many parts of the world including Western US (Rosen 1991), Australia (Bowler 1986) and Iran (Krinsley 1970). In Iran, the ancient evaporites are present in sedimentary records of the Tertiary evaporites are found mainly in the Zagros Basin and in the Central Iran (Rahimpour-Bonab and Kalantarzadeh 2005, Rahimpour-Bonab et al. 2007a, b). In addition, Holocene evaporite deposits are widespread in numerous playas and lake/playas in various parts of Iran with different characteristics (e.g., Krinsley 1972; Shahrabi 1995; Farpoor and Krouse 2008; Farpoor et al. 2012). In his pioneering work, Krinsley (1970) studied morphology of surficial features of 60 playas within the interior of Iran. He determined characteristics, distribution, and development of the available playas. According to his findings, because the form and bedrock permeability of most of the Iranian basins have remained essentially stable since the late Pleistocene (Würm), subsequent hydrologic changes can be attributed to climatic changes.
Rahimpour-Bonab and Abdi (2012) with study on the lake/playa Meyghan Flowing water with low percentage of ions and Na–(Ca)–(Mg)–SO4–Cl–(CO3) types evolve, into a brine with high ions concentration during the geochemical evolution and mineral deposition. The geochemical evolution yields a Na–SO4–Cl type brine.
While clastic components (calcite, quartz etc.) show on increasing trend in relation to depth, evaporites abundances show opposite trends. Furthermore, the average grain size of sediments increases with depth indicating higher energy in the past. This study evaluated clastic sediments and evaporite deposits’ changes in the sedimentary core Maharlou lake by using physical and geochemical methods.
 
1-2- Study site (Physical setting)
The hypersaline Lake, Maharlou is located in SW Iran with an area of 24910 km2 and 1.5 m depth. The lake lies about 20 km east of Shiraz at 1,455 m above sea level (masl) [Fig. 1]. This lake is fed by direct precipitation and water inflow from surface run-offs, a few seasonal rivers and several karstic springs which compensate for the strong evaporation in a semi-arid climatic context (Dumas et al. 2003). The Maharlou catchment area consist of sedimentary rocks, mainly Precambrian diapiric evaporites associated with Mesozoic and Cenozoic limestone, sandstone, shale and dolomite formations. The formation of this lake goes back to the beginning of the Pleistocene because of its hydrogeological properties, past higher lake level stands and the survival of Pleistocene relict fish species (Krinsley 1970; Dumas et al. 2003).
2-Materials and methods
Lakes are accurate intercontinental ambiences to record past environmental changes. After theorical researches, vibracorer machine took 533cm core sample from the north west of the lake near Kaftarak village. Magnetic susceptibility tool measured the magnetic sensibilities. Sequences identified and analysed after cutting sedimental core. Then, 150 samples with high resolution checked to apply combination method, however only 84 items selected. SEM method and LOI analysing system used for geo-chemistry studies. After particle size analyses, sediments are studied to identify life remains of animal and plant as well as lithologic study by using polarizing and binocular microscopes.
 
3-Results and discussion
Lakes are accurate intercontinental ambiences to record past environmental changes. After theorical researches, vibracorer machine took 533cm core sample from the north west of the lake near Kaftarak village. Magnetic susceptibility tool measured the magnetic sensibilities. Sequences identified and analysed after cutting sedimental core. Then, 150 samples with high resolution checked to apply combination method, however only 84 items selected. SEM method and LOI analysing system used for geo-chemistry studies (Sabokkhiz et all 2015). After particle size analyses, sediments are studied to identify life remains of animal and plant as well as lithologic study by using polarizing and binocular microscopes (Sabokkhiz et al 2014). Finally, the age of the core measured by δ14c ageing technique and calculation methods. The age of the sedimental core bottom is 10813 ±60 years BP that presents the time period before Holocene. Existence of los like and lack of biological matters in 521 cm depth show the dominance of the last part the Younger drays (late Glacial). This event has the horizon of the fine grain sediments without life remains. Therefore younger dryas phase of the lake Maharlou is cold and dry. Early Holocene ages, 10200±60 years BP were cold and dry. There were evaporates with fine grains and without biological matters. Afterwards evaporate and siliciclastic particles increased, calcium carbonates decreased and fluvial environment dominance appeared followed by warm and dry age. Middle Holocene ages, 4000±60 years BP came to light by the activity of the springs when pleasant warm and wet weather controlled lakes. There were some fluctuating dry weather conditions until late Holocene age. A Hiatus and detached surface appeared in 3062±45 years BP that showed water offgoing from the lake along with surface weathering. Late Holocene starts from 2000±45 years BP. First demonstrated dry fluctuations then raining increase and then core surface which indicated drying trend of playa basin.
 
4- Conclusion,
4-1- The age of the sedimental core bottom is 10813 ±60 years BP
4-2- Therefore younger dryas phase of the lake Maharlou is cold and dry.
 Early Holocene ages, 10200±60 years BP were cold and dry weather and fluvial environment dominance appeared followed by warm and dry age.
4-3- Middle Holocene ages, 4000±60 years BP came to light by warm and wet weather controlled lakes. There were some fluctuating dry weather conditions until late Holocene age 3062±45 years BP that showed water offgoing from the lake along with surface weathering.
4-4- Late Holocene starts from 2000±45 years BP. First demonstrated dry fluctuations then raining increase and then core surface which indicated drying trend of playa basin.
4-5- These changes are similar to other studies for different lakes in North-West of Zagros mountains such as Parishan lake, Hashilan wetland, Neuor lake in Alborz mountains in late glacial. Recent researches about the time of the early Holocene on Oroumiyeh lake and north Atlantic sea showed lowering in monsoon currents and increase of extreme dryness in middle east, which have similar conclusions to our research on Maharlou lake.

کلیدواژه‌ها [English]

  • lake Maharlou
  • Holocene
  • invironmental changes
  • Younger drays