Mid Holocene Marine Transgression at Eastern Coastal Margin of Bangladesh—Implications for Past Sea Level Change

Abstract

Sedimentological, palynological data indicate that mangrove community developed under transgressive condition in and around Maheskhali and Kutubdia Island areas during Mid Holocene time (7000 to 5500 years BP) leading to the locally wide spread deposition of organic-rich sediments. During Holocene time global rise and fall of eustatic sea level played an important role not only on the depositional environment but in creating a geomorphic feature on the island. Recurrent occurence of freshwater and mangrove pollen in Maheskhali and Kutubdia Island area indicate that these areas have undergone cyclic marine and non-marine influence. Since the Last Glacial Maximum (about 20,000 years ago), sea level has risen by over 120 m at locations far from present and former ice sheets, as a result of loss of mass from these ice sheets. There was a rapid rise between 15,000 and 6,000 years ago at an average rate of 10 mm/yr. Based on geological data, global average sea level may have risen at an average rate of about 0.5 mm/yr over the last 6000 years and at an average rate of 0.1 to 0.2 mm/yr over the last 3000 years. Vertical land movements are still occurring today as a result of these large transfers of mass from the ice sheets to the ocean. During the last 6000 years, global average sea level variations on time-scales of a few hundred years and longer are likely to have been less than 0.3 to 0.5 m. First transgression was noticed around 6000-5500 cal BP and then a subsequent regression of the bay had been observed from around 5500 cal BP. This was again followed by another small scale transgression episode occurred around 2500—2000 cal BP. So the palynomorph assemblages from the Holocene sediment sample indicate that Maheskhali and Kutubdia Island and its surrounding area was an intertidal environment occupied by mangrove community.