چكيده لاتين :
Introduction
The Zagros Mountains belong to the West-Asiatic Fertile Crescent, where the transition from the economy based on hunting-gathering to food production took place at the decline of the last glaciations, giving birth to agriculture, which later spread over the whole of Europe. Pollen assemblage (Leopold et al;1982; Kutzbach et al; 1996) and Grain-size measurements (Halfman and Thomas, 1984; Noren et al;2002) are widely used in global lake sediment studies. Grain-size of the lake sediments is mainly controlled by the distance of the core site from the shoreline, the kinetic energy of the lake circulation and the source of the sediments (Lerman, 1978). The sediments sorting principle states that the grain size of lake sediments becomes finer and finer from the shore to the center, and sediment belts of different grain-size can be distinguished. Lake Zeribar sediments, providing a record of climatic variations more than 40,000 years long, have been the subject of multidisciplinary investigations reported in several publications (among others: plant macrofossils by Wasylikowa, 1967, 2005, diatoms by Snyder et al; 2001; stable isotopes by Stevens et al; 2001). However, sediments of the lake have not yet been analyzed for the relationship between pollen assemblages and grain-size, whereas it could reveal important information about the lake history and sedimentary process-geomorphology. This study uses relationship between pollen assemblages and grain-size analysis to characterize and describe grain –size fraction of samples collected from lake Zeribar. Interpretation of water-lake level changes, depositional environments and paleoclimatic significance of the resulting data are presented.
Methodology
Fieldwork and sampling
A 6.88 m long core was extracted from the west part of the lake by a standard chamber corer, the Russian corer, 50 cm in length and 5 cm in diameter .
Grain-Size Analysis
Sediments were sampled at an intervals of 1-10 cm. All samples were split into halves and weighed. One half was wet-sieved using a 63 ?m diameter sieve. The > 63 ?m fraction (sand and granule) was dried and weighed for sand and granule content. The other half was analyzed for mineral type. The < 63 ?m fraction was analyzed using a laser diffraction particle size analyzer (Micro tec A-22, Analysette 22 ) which utilizes grain-size range, 0.001-2 mm. Samples were treated with 30% H2O2 to remove organic matters. The samples were further dispersed via 10 minutes of exposure in an ultrasonic bath just before size analysis. For the purpose of particle-size specification , the following scale used by Folk and Ward (1957) was adopted; granule: > 2mm, sand: 2000-63 ?m (-1 to 4? ), silt: 63-3.9 ?m (4-8?), clay: 3.9-0.24 ?m (8-12?).
Pollen analysis and pollen diagram
The results of pollen analysis were discussed at length by Van Zeist and Bottema (1977).
Chronology
Radiocarbon dating of the sediments was performed for three bulk sediments using a standard Accelerator Mass Spectrometer (AMS) method at the Institute of Accelerator Analysis Ltd, Japan. The 14C AMS dates were calibrated to years AD and calendar years BP using OxCal v.4.1 (Bronk Ramsey, 2009) and IntCal09) Reimer et al, 2009).
Results and Discussion
Environmental and climate change implied by relationship between pollen assemblages and grain-size at Zeribar lake.
Based on the percentages of the clay: ( < 2 ?m), silt: (2–63 ?m) and sand: ( > 63 ?m) size fractions, median, mean and mode sample diameters, frequency curves and lithology, we divided the core into 4 phases A (688-528 cm, 8950-6870 cal yr BP), B (528–423cm, 6870-5500 cal yr BP), C (423–244 cm, 5500–3170 cal yr BP), D (244–100 cm, 3170–1300 cal yr BP). Then we discuss the relation of the pollen assemblages and grain-size in each stage.
During phase A (688-528 cm, 8950-6870 cal BP). The percentage content of silt increases to ~74.8%, while the content of sand decreases to ~6.33%. The percentage of Atriplex-type and Artemisia pollen decreases in this section. The Poaceae and tree-pollen concentration increases.
During phase B (528–423 cm, 6870–5500 cal yr BP), the percentage of sand (average=14%) increases sharply while the percentage of silt (average=67.18%) decreases. The percentage of Salix pollen increases in this section, reaching its highest value (average=35%). The Quercus pollen concentration decreases.The relatively high content of sand likewise implies a low lake level, which reflects effective moisture in the whole drainage, is low.
During phase C (423–244 cm, 5500–3170 cal yr BP). The percentage content of silt increases to ~77.4%, while the content of sand decreases to ~5.4%. The Quercus concentration (average
50%) markedly increases but the percentages of Salix and Fraxinus pollen decrease sharply. indicating high effective humidity and moisture in lake Zeribar. The high and stable content of silt, fine components and Quercus concentration in the sediments indicates that lake-level reaches its highest value in the Holocene at this time.
During phase D (244–100 cm, 3170-1300 cal yr BP), the content of sand (average=10.5%) increases while the content of silt (average=69.86%) decreases. Several cycles in grain-size may be related to centennial climate cycles. The content of Salix and Fraxinus Pistacia (average=5%) Poacae (average=50%) increases while Quercus decrease. The high content of the coarse component and an increase of Salix, decrease of Quercus suggest lake-level lowering.
Conclusion
During 8950-6870 and 5500-3170 cal yr BP, the high and stable content of silt, fine components with high Quercus pollen concentration in the sediments and the low percentages of Salix and Fraxinus pollen suggests the existence of warm and wetter climate, increased spring rains, episode of higher lake water level, existence of fresh-water conditions, prevailing high-energy condition, dominance of erosional processes, seasonal supply of detritus, inflows strength and dominance of chemical weathering. The high coarse component with low Quercus pollen concentration in the sediments and the high percentages of Salix and Fraxinus pollen suggests the existence of dry climate, reduced rainfall, occurrence drought, lake-level lowering, , prevailing low-energy condition, absent of seasonal supply of detritus, down conditions tidal changes and dominance of physical weathering about 6870-5500 and 3170-1300 cal yr BP. During the late Holocene variations of water-level occurred irregularly, as effects of precipitation changes, occasional lake overflows and perhaps also human activities reflected by the continuous curve of Plantago lanceolata, an indicator species of pasturage.
