Sedimentsdeposited beneath mangrove or salt marsh vegetation provide useful indicationsof present and past sea levels and paleoclimate. During Holocene time, globalrise and fall of Eustatic Sea level played an important role isn thedepositional environment. Environmental changes due to the Quaternary sea-levelfluctuations had a profound impact on the distribution of mangrove habitats atboth local and regional scales that kept their records on peat deposit. This research is focused to identify the climaticcondition that persisted and its changes in Quaternary Period studying the peatdeposits around Bangladesh and correlate them.
Itexquisitely preserves pollen and other microfossils that has been used tointerpret the depositional environments, climatic conditions and sea levelchanges in Holocene period around Bangladesh. For this, polen and otherconstitute microfossils in peat deposits of four different parts of Bangladeshhas been analysied. In addition, C14 age dating has been performedto fit them in proper age boundary. Polenanalysis along with age dating indicats that mangrove community developedperdominantly under saline with minor brakish condition around Bangladesh inHolocene time leading to the locally wide spread deposition of organic richsediments. Recurrent occurance of salineand brakish water mangrove in Bangladesh indicates that these area undergonecyclic marine and non marine influence. This verifies the trgression andregression pattern that was persited during the delta propagession towards thesouth. Lastly, paleo coasline reconstruction shows the continued southwardregression trend of the sea level in Holocene which results the present conditon. 1.
1: BackgroundPaleoclimatologyis the study of climate prior to the period of instrumental measurement. Itsobjective is to reconstruct the earths past climate history especially inQuaternary Period. On a global scale many different types of indicator are usedin paleoclimatic studies such as raised beaches, shells, pollens, microfossilsetc. Palynology is one of these powerful and useful tools to reconstruct theQuaternary environment.
Because there is a bread and butter relationshipbetween the growth of plant species and the status of climatic variables aswell as the physiographic conditions for a specific time period reflecting thethen climatic status. Sediments deposited beneath mangrove or salt marshvegetation provide useful indications of present and past sea levels andpaleoclimate. Litho-bio-chrono-stratigraphic techniques to reconstruct theHolocene sea-level changes are well established and have been appliedsuccessfully in many coastal areas of the world including in Bangladesh (Alam,1972; Tooley, 1978; Shennan 1982; Islam 2001).Environmentalchanges associated with Quaternary sea-level fluctuations have had a profoundimpact on the distribution of mangrove habitats at both local and regionalscales that kept their records on peat deposit (Woodroffe and Grindrod, 1991;Grindrod et al., 1999).
Thisresearch is focused to identify the climatic condition that persisted and itschanges in Quaternary Period studying the peat deposits around Bangladesh andcorrelate them. Peat, an unconsolidated deposit of semi-carbonized plantremains in a water saturated environments such as a bog or fen and ofpersistently high moisture content. It exquisitely preserves pollen and othermicrofossils that will be used to interpret the climatic and sea level changes.Considerable quantities of peat deposits have been discovered in variouslocalities of Bangladesh.
Among them this research will include the peatdeposits of Dhaka and surrounding’s, Baghia-Chanda beel of Madaripur andGopalgonj district, and Sundarbans of Khluna district. 1.2: Rational of the StudyTheshoreline of Bay of Bengal has been observed not to be static in relation toprevious geological events (Umitsu 1987, 1993; Kudrass et al.
, 1999;Goodbred and Kuehl 2000; Islam 2001).During the peak of the last glaciations (18 kyr BP) the Bengal Lowlandexperienced dry climatic conditions and sea-level was 100 meters or more lowerthan the present sea level (Umitsu, 1987). At about 12 kyr BP, south-westmonsoon became prominent which caused heavy rainfall and sea-level started risingvery rapidly (Monsur, 1995).
This amplified monsoon water plus deglaciated meltwater from the Himalayas flowed over the Bengal Lowland and the initialMadhupur and Barind surfaces were highly dissected, creating some local poolsand depressions (Monsur, 1995). It seems that the coastal processes and climatehad a significant control on topography of this region during Late Pleistocenetime. TheIndian Summer Monsoon (ISM) during early to middle Holocene was generallystronger than today, with peaks identified at 8.5, 6.
4, and 2.7 k.y. B.P.detected in numerous ISM records (e.g., Fleitmann etal.
,2003; Gasse et al., 1991; Van Campo et al., 1996; Wang et al.
,2005), but weaker than today between 5000- 1200 yrs BP (Naidu, 2004). (Umitsu,1993), Kudrass et al., 1999; and Goodbred and Kuehl (2000) stated thatduring the mid-Holocene sea level of the bay was slightly higher, the climatewas warmer, and rivers of this region discharged up to two and half times morethan in present times. It has generally been accepted that around 6000 yrs BP,eustatic sea-level was higher than the present sea level. As a coastal region,it was thought that there should be some evidences of Mid-Holocene marinetransgression in and around Gopalgonj and Madaripur district.A limited number of detailed studies havebeen made in the last few decades to reconstruct the Holocene sea-level changeof the Bengal Lowland, including those of Umitsu (1987, 1993), Banerjee and Sen(1987), and Islam (2001). These past attempts to reconstruct Holocene sea-levelhistory have been based on borehole samples.
The past studies did not determinethe paleo-MSL and timing, magnitude of mid-Holocene highstand, either. The RSLcurves constructed by the authors did not show any specific regression phase duringHolocene and showed continuously rising RSL through the Holocene. The paststudy areas are on low-lying deltaic deposits which are susceptible tolong-term subsidence due to compaction of sediments by anthropogenic activitieswhich may miscalculate the altitude of paleo-MSL. Our present study considersthree large outcrops for facies analysis. Our study, the first of its type inthe Bengal Lowland, presents a detailed description of the sedimentary facies,and discusses the relationship between these facies successions along withfaunal analyses of pollen which yield the maximum information about thepaleoenvironmental changes in this region. 1.3: Aims and ObjectivesThemajor objectives of this study are:Ø Reconstruction of Quaternary paleo-depositionalenvironment of the regions.
Ø Correlatethe environmental changes all over the study area. Ø Toevaluate the paleo-floral diversification in the study area. 1.4: Review of LiteraturePeat is an unconsolidated deposit of semi-carbonized plant remains in awater saturated environments such as a bog or fen and of persistently highmoisture content. In the process of coal development, peat is an early stage orrank. Carbon content is about 60% and oxygen content is about 30%. Itexquisitely preserves pollen and other microfossils that will be used tointerpret the climatic and sea level changes.
Geological Survey ofPakistan (GSP) and Geological Survey of Bangladesh (GSB) discoveredconsiderable quantities of peat deposits in various localities of Bangladesh.Amongst the deposits Baghia-Chanda beel of Madaripur and Gopalgonj district, Kola Mauza of Khulna district, ChatalBeel of Maulvi Bazar district, Pagla and Salla of Sunamganj district are worthmentioning. GSP started to investigate peat since 1949 and in 1953 the firstpeat field at Baghia-Chanda Beel of Madaripur district was discovered. Thesurvey continued in the years 1954 and 1955 and peat was found to extend in anarea of 500 sq km.
Further detail investigation on reserves continued in theyears 1955-57 and 0.6m to 3.3m thick (average 2m) peat was found from thesurface to near surface (up to 4m depth).
The reserve of the dry peat iscalculated as 150 million tons. Fried Krupp Rhostoff, a German Company workedon this peat for its utilization. TheQuaternary evolution of the Bengal Basin was first described by Morgan andMcIntire in 1959. They concluded that structural activity, primarily faultinghas significantly influenced Quaternary geology of the region.
Withregards to ancient topography of Bangladesh, particularly the origin andevolution of the Pleistocene tracts (Faridpur and Barisal tracts), Geologistbelieved that the Padma river resulted directly from a major increase in watervolume of the river. During the glacial and interglacial periods the combinedeffects of seaward subsidence and landward uplift have caused a warping of thealluvial land, which are called the Pleistocene terraces. Afterwards thedissected valleys were filled up with alluvial sediments, generating a recentfloodplain surface at lower position than the initial Pleistocene Terraces. Chowdhury et al. (1985) put ahypothesis about the origin of the “Swatch of No Ground” a submarine canyon asan estuary of the last glacial maximum and if the hypothesis is accepted thenpossibly most of the northern parts of the Bay of Bengal, during that time,were very dry land. And during that time the eustatic sea level was about 100to 130 m below the present mean sea level.Umitsu,M.
(1987 and 1993) conducted two intensive researches to describe the landformevolution and sea level movement in the Bengal Lowland. The author attempts todiscuss the general framework of landforms and late Quaternary geology in theBengal Lowland. The author tried to make clear the stratigraphy of the lateQuaternary sediments in the Bengal Lowland based on the detail analysis of therecent sediments and radio-carbon ages.
In his papers the author showed theQuaternary formation of the landform and subdivided the landform into fivemembers based on the characteristics of the sediments, specially their faciesand grain size sequences. He constructed a sea level curve for this regionshowing a regression between ca. 12,000 and 10,000 yr BP. He showed that duringthe maximum epoch of the last glacial age, the rivers flowing in the BengalLowland deposited gravels on the valley floors. By 12,000 y BP, the sea levelrose up to about 45 m below present level, and the lower member deposited. Theauthor also showed that during ca. 12,000 and 10,000 yr BP, the delta andfloodplain surface was slightly dissected according to the regression of thesea level. After the regression, the middle member characterized by finesediments in the deltaic condition deposited according to the re-transgression.
The upper member deposited during 10,000 (or 8000) yr BP and 6000 (5000) yr BP,he stated. He also added that after ca.5000 yr BP, broad peat land developed widely in the Bengal Lowland. He stated that the coastline in the earlyHolocene retreated towards the central part of the present Ganges Delta. In themiddle and late Holocene, the silt and clay with occasional peat layers of theuppermost unit indicate that the lowland gradually became marshy and poorlydrained as the rate of transgression became slower.
Islam(2001) has conducted a research on sea level changes in Bangladesh and appliedlitho-bio-chrono-stratigraphic techniques to reconstruct the paleoenvironmentof Bangladesh. He used this approach as it has been applied successfully in manycoastal area of the world (Tooley, 1978; Shennan, 1982; Ireland, 1987).