Palaeo-palynology of late Quaternary peat deposit from Lower Bengal Basin, India: A palaoecological approach

Abstract

Palynological evidence is recognized as a logical deduction of paleoclimatic extrapolation, as coastal vegetation is so specialized that any minor deviations in hydrological or tidal influences cause radical variations in the vegetational succession. Sedimentological and biological characteristics in terms of pollen/spore analysis of the peat along with dating of subsurface deposits are highly instructive in reconstructing the vegetational history, ecological status, and palaeoenvironment of the Quaternary. A comparative study of fossil pollen morphology was carried out with an objective towards the assessment of past vegetation and its similarity, if any, with the modern habitat. The present study involved investigation, accompanied by 14C dating of undisturbed bore-hole peat samples collected from Garia, a suburb of Calcutta (Lower Bengal Basin), near the world's largest mangrove forest, the Sundarbans. Pollen data indicates the existence of typical mangrove vegetation in and around Calcutta, presently confined to the Sundarbans forest, south of the studied area. The lowermost peat layer (ca. 7000–6500 BP) reveals that the vegetation sequence was comparatively uniform and mainly comprised of typical mangrove families such as Heritiera, Rhizophora, Sonneratia, Excoecaria, Phoenix and grasses, implying insignificant fluctuations in climatic conditions. The middle layer indicates a mixed vegetation of fern spores, mangroves and some terrestrial plants (e.g. Acanthus, Cynometra, Suaeda, and grass). The lower part of the middle peat layer (ca. 6450–6100 BP) consisted of marshy habitat as indicated by fern spores, whereas their absence in the upper part implies relatively dry environmental conditions. The upper peat layer (ca. 2600 BP) is dominated by grass, pollen of terrestrial habitats (Typha, Cassia, Eucalyptus, Desmodium, Salicornia, members of Fabaceae, Asteraceae and Rhamnaceae) and some typical mangroves (Heritiera and Excoecaria). Low amounts of fern spores indicate that the Lower Bengal changed towards terrestrial habitats. The pollen/spore assemblages in the three successive peat layers indicate the presence of typical mangrove vegetation, presently observed in the Sundarbans region. Frequent sea water inundation mixed with fresh water flow from the northern upstream at the low lying topography of Basin seems to have led to the formation of peat by deposition rather than by drifting.