Rapid Holocene sea-level changes along the Iranian Caspian coast

摘要

The Caspian Sea is well-known for its rapid sea-level change. During 1929-1995, a full sea-level cycle was observed. First, the sea level dropped ~3 m with a lowstand in 1977, followed by a 3 m rise to 1995, after which the sea level has been relatively stable. These oscillations are a specific feature of the Caspian Sea and its sedimentary record. The main purpose of this study is to reconstruct the sea-level curve in the Holocene by using sedimentological and biostratigraphic analysis and radiocarbon dating along the Iranian part of the Caspian shore. Remote sensing images and historical maps show that two lagoons totally emerged, and the Gorgan delta prograded rapidly at a rate of around 160 m y~(-1) until the 1975 lowstand. Gorgan Bay was reduced in size considerably and the connection to the sea was blocked due to growth of a spit and change in base level. When sea level started to rise again, the coastal morphology rapidly changed and the Gorgan delta retrograded at the rate of around 140 m y~(-1). These sedimentary dynamics can be recognized in the preserved deposits. In addition to the recent dynamics, core data from the southeastern lowlands show four earlier highstands. Using characteristic barrier-lagoon deposits, early Holocene sea level rose until a high-stand was reached of ca. -34 m. This phase was followed by fluvial deposition in the Gorgan delta associated with a base level fall. There is also an evidence of sea-level rise between 5000 and 2300 BP at ca. -27.7 m. On top of these deposits there is evidence of a highstand between 2700 and 2300 BP at ca. -23.5 m. The fourth highstand from the core data is dated to the Little Ice Age at ca. -24 m. Data from these last two highstands correspond well with other observations from the Caspian region.