كليدواژه :
پيشروي و پسروي , تغييرات سريع درياي خزر , درياي خزر , دلتاي سفيدرود
چكيده فارسي :
از ويژگيهاي منحصربهفرد درياي خزر تغييرات سريع تراز آن نسبت به تراز درياهاي آزاد است. اين تغييرات سريع بين سالهاي 1929-1995 به وسيله ايستگاههاي اندازهگيري تراز آب و از سال 1992 به وسيله دادههاي آلتيمتري با دامنه در حدود سه متر به ثبت رسيده و اثرهاي درخور توجهي در مورفولوژي ساحل داشته است. با وجود اين، آثار تغييرات تراز پيش از ثبت تراز آب مبهم است و نتايج گوناگوني ارايه شده است. بنابراين، پيشبيني تراز بسيار پيچيده و اغلب با اشتباه توام است. بر اساس دادههاي راديو كربن به‌دست‌آمده از مناطق مختلف درياي خزر، دو دوره را ميتوان در اواخر هولوسن تفكيك كرد كه در مورفولوژي درياي خزر اثرهاي بسيار مهمي به‌جا گذاشته‌اند. اين دو دوره تقريباً منطبق بر دورههاي سردند و به عنوان دورههاي سرد 2600 سال پيش و دوره سرد كوتاه در اروپاي شمالي شناخته ميشوند. در اين مطالعه سعي شده است اثرهاي دو دوره با استفاده از دادههاي ميداني، عكس‌هاي هوايي، و نتايج راديو كربن از مناطق مختلف درياي خزر بررسي شود. نتايج نشان ميدهد سواحل شرقي به دليل شرايط توپوگرافي و مورفولوژي بيشترين تاثير را در برابر افزايش تراز در دو دوره سرد هولوسن داشته است.
چكيده لاتين :
Introduction:
The Caspian Sea (CS) can be subdivided into three parts: North, Middle and south parts (Figure 1). The CS experienced many cycles of Sea-level changes with vast emerged and submerged area. In the Khvalyn terraces in Dagestan two major phases have been distinguished. The Early Khvalyn transgression is represented by five successive marine terraces between +50 and 0 m at absolute height (Kroonenberg et al, 1997). Furthermore, at the boundary of Late Pleistocene and Holocene a -50 (Kakroodi et al., 2015) and by other researcher a -113 sea level record has been identified. This is why many researchers call it as a capricious level due to great sea level changes (Krooenebreg et al., 2007). Recent sea-level changes also got attention, experienced around 3 m sea-level changes between 1929 and 1977 and rose back in 1995. Five late Holocene have been identified which reached at least up to -22 absolute elevation. Present work aims at showing two phases of transgression and its impact on Iranian coast by Arial photograph and radiocarbon dating, a 2600 BP and Littlie Ice age.
Radiocarbon dating:
Radiocarbon dating results are available now from all over the CA along the Iranian coast. Further radiocarbon dating is also needed to achieve better understanding of sea level change. The crucial thing is the samples must be not reworked and then it is important the marine environment to take samples. Otherwise the results are not accurate and sometimes in a wrong way. The marine environment far away from the wave is the best target to take the samples. The absolute elevation is also important, unreported elevation of the samples might indicate different data and should consider the same coordinate systems for all data. In this work, nearly all sample elevations have been measured by DGPS device based on the mean sea –level of the Persian Gulf. Table 1 indicates the radiocarbon dating data along the three parts of the CA Sea, East, Middle and West parts.
The southeastern part of the CS:
The south-eastern part of the CS is one of the best targets to reconstruct the sea-level curve and coastal evolution. An overall study of the past and recent sea-level changes have been done in this part as a PhD thesis (Kakroodi et al., 2012). The dominated coastal features in this part are characterized by Barrier-Lagoon and delta deposits. This part is so sensitive to sea level changes due to low angle coast and any minor sea-level change will have its own impact on the coast.
Based on Arial photograph and filed data two highstands at -22 and -24 are distinguishable, Old Miankaleh spit and old delta deposits (Fig 2, 3). Linear old shoreline and old Gorgan delta is well preserved by aerial photograph and confirmed by field work. They are placed at absolute elevation of around -24 (Fig 2, 3).
In the southern part of Gorgan Bay, the Bagho outcrop presents marine deposits similar to present Minaklaeh spit characterized by crossbedding, medium to coarse sand and Caspian fauna. Eastward, it has been buried by Qareh Su and Gorgan Rivers and their deposits.
The middle part of the CS
Because of short distance of the Alborze Mountains and the sea and high energy environment, the coast is characterized by gravel and coarse sand which is known as erosional coast and also not sensitive to sea level changes as observed at the eastern part. However, there is a small old elongated lagoon indicating a Lagoon- barrier system in this part. Also, Results of a core study indicates a lagoonal deposits further south at the foot of Alborz mountains and overlaid now by fluvial and organic rich layer. This organic layer was dated around 500 BP which probably is an evidence of little ice age highstand (Ramezani et al., 2016). The lower part of the mentioned core, also Lagoonal deposits, at the depth of 3.6 m a dated sample around 2500 BP was reported. Totally two highstands at -24 and 19 absolute elevations were identified in this part.
The Western and East of Sefidrud delta part
The impact of sea-level change in this part is more studied than the other part of the CS. Sefidrud delta is the biggest river in the southern part of the CS originating from Zagrous Mountains and cross Alborz mountains providing around 70 sediment supply within the CS (Lahijani et al., 2009). Due to high sediment supply, the impact of sea-level change is different than the other parts.
Recent study of sea-level rise between 1977 and 1995 indicates even with 3 m sea level rise the coast has not been changed and in some part it prograded instead of landward shift (Kakroodi et al., 2014). This is because the coast will be developed in 3 dimensional not just a simple linear trend. Therefore accommodation space resulted by sea level rise is compensated by sediment supply and other process like longshore currents. Many papers have been published recently and most of them concentered on Little Ice Age impact on the coast (Leory et al., 2011; Naderi et al., 2013, Haghani et al., 2015. Present morphological features include a series of beach-barrier (Fig. 6, b), Lagoons, modern delta and old delta (Fig. 6).
Discussion and conclusion:
Rapid sea level change is unique in the Caspian Sea and therefore coast response against this sea- level change get attention for many researchers. A real observation of coastal response to sea-level change instead of using mathematical model enable us to predict coastal behavior. The CS experienced many cycles and in the late Holocene, two highstands had strong impact on the coast, a 2600 BP and Little Ice age respectively. These two highstands synchronized with cooling periods of Northern hemisphere. Now the level of the Caspian is situated around -28 below sea level and considering the elevation -24 and -22 at the periods of two mentioned highstands, large area of the southern Caspian was subemerged at those periods. Any model should consider this these highstands.
