A 50 m-long radiocarbon dated core was studied through sediment and pollen analysis to reconstruct the Holocene mangrove and environmental changes at a coastal site Pakhiralaya in the Sundarban Biosphere Reserve in the western Ganga-Brahmaputra Delta, India. This biosphere reserve harbours a diverse mangrove ecosystem and supports a large number of people living in the area. Pollen and stratigraphic data indicate the existence of a brackish water estuarine mangrove swamp forest in this area during the last 9880 cal yr B.P. The development of the mangrove forest is not shown continuously in the Holocene record. Rapid transgression of the sea (9240 cal yr B.P.) halted the development of the mangrove. After about 8420 cal yr B.P. mangrove recolonised the area and persisted until 7560 cal yr B.P. as a result of a balance between the sedimentation and sea level fluctuation. The mangrove disappeared again from the site until 4800 cal yr B.P. because of a high sedimentation rate and possible delta progradation with loss of habitats. The reappearance of mangrove at the study site occurred with a return of a brackish water estuarine environment and the site then gradually became supra tidal during the mid-late Holocene. The continuity of the mangrove development and dynamics was interrupted by the fluctuating sea levels. Climatic fluctuations were viewed as an indirect factor influencing the mangrove ecosystem.
The Sundarban Mangrove Forest in the Sundarban Biosphere Reserve, located at the mouth of the Ganga–Brahmaputra Delta in India, is the most diverse mangrove ecosystem in the world. Sediment cores were taken from two widely separated islands in that reserve: Chamta (CMT) and Sudhyanyakhali (SDK). Pollen analysis and radiocarbon dating were used to study the Holocene development and dynamics of this unique ecosystem. Modern pollen rain study reveals a strong relation between modern pollen rain and the present vegetation, as well as a high rate of Phoenix palludosa pollen production.The pollen records indicate that man-grove existed at CMT from ~5960 and at SDK from ~1520 cal yr BP. Changes in relative sea level, including the frequency and intensity of inundation as well as fluctuating precipitation, have been the major factors along with geomorphic processes that control the development and dynamics of the mangrove in the area during the Holocene. The mid Holocene mangrove at CMT declined, to be progressively replaced by successive communities, and eventually reached climax stage, while the SDK site is transitional in nature. The mangrove responds rapidly to changes in environmental conditions at both locations. Because of large-scale anthropogenic interventions, it is unlikely that similar rapid responses will occur in the future.
Available information, including the latest ones, on palynological investigations of Late Quaternary deposits of Lower Bengal Basin revealed the existence of brackish water swamp forest similar to the present day mangrove forest of Sundarbans in four different periods, viz., ca. 32,000 ca. 22,000, ca. 14,500 and ca. 7000 yrs B.P, established by radiocarbon dating. Consideration of present day environment of mangroves of the Sundarbans reflects their intertidal habitat. Sea-level changes in the Lower Bengal Basin demonstrates recurrent mangrove forest through the Late Quaternary using pollen analytical method and study of logs of woods in semi-decomposed state. Attempts have been made here to collocate the fossil data with Carbon 14 dating.
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