Sediments
deposited beneath mangrove or salt marsh vegetation provide useful indications
of present and past sea levels and paleoclimate. During Holocene time, global
rise and fall of Eustatic Sea level played an important role isn the
depositional environment. Environmental changes due to the Quaternary sea-level
fluctuations had a profound impact on the distribution of mangrove habitats at
both local and regional scales that kept their records on peat deposit.

 

This research is focused to identify the climatic
condition that persisted and its changes in Quaternary Period studying the peat
deposits around Bangladesh and correlate them. It
exquisitely preserves pollen and other microfossils that has been used to
interpret the depositional environments, climatic conditions and sea level
changes in Holocene period around Bangladesh. For this, polen and other
constitute microfossils in peat deposits of four different parts of Bangladesh
has been analysied. In addition, C14 age dating has been performed
to fit them in proper age boundary.

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Polen
analysis along with age dating indicats that mangrove community developed
perdominantly under saline with minor brakish condition around Bangladesh in
Holocene time leading to the locally wide spread deposition of organic rich
sediments. Recurrent occurance of  saline
and brakish water mangrove in Bangladesh indicates that these area undergone
cyclic marine and non marine influence. This verifies the trgression and
regression pattern that was persited during the delta propagession towards the
south. Lastly, paleo coasline reconstruction shows the continued southward
regression trend of the sea level in Holocene which results the present  conditon. 

1.1: Background

Paleoclimatology
is the study of climate prior to the period of instrumental measurement. Its
objective is to reconstruct the earths past climate history especially in
Quaternary Period. On a global scale many different types of indicator are used
in paleoclimatic studies such as raised beaches, shells, pollens, microfossils
etc. Palynology is one of these powerful and useful tools to reconstruct the
Quaternary environment. Because there is a bread and butter relationship
between the growth of plant species and the status of climatic variables as
well as the physiographic conditions for a specific time period reflecting the
then climatic status. Sediments deposited beneath mangrove or salt marsh
vegetation provide useful indications of present and past sea levels and
paleoclimate. Litho-bio-chrono-stratigraphic techniques to reconstruct the
Holocene sea-level changes are well established and have been applied
successfully in many coastal areas of the world including in Bangladesh (Alam,
1972; Tooley, 1978; Shennan 1982; Islam 2001).

Environmental
changes associated with Quaternary sea-level fluctuations have had a profound
impact on the distribution of mangrove habitats at both local and regional
scales that kept their records on peat deposit (Woodroffe and Grindrod, 1991;
Grindrod et al., 1999). This
research is focused to identify the climatic condition that persisted and its
changes in Quaternary Period studying the peat deposits around Bangladesh and
correlate them. Peat, an unconsolidated deposit of semi-carbonized plant
remains in a water saturated environments such as a bog or fen and of
persistently high moisture content. It exquisitely preserves pollen and other
microfossils that will be used to interpret the climatic and sea level changes.
Considerable quantities of peat deposits have been discovered in various
localities of Bangladesh. Among them this research will include the peat
deposits of Dhaka and surrounding’s, Baghia-Chanda beel of Madaripur and
Gopalgonj district, and Sundarbans of Khluna district.

 

1.2: Rational of the Study

The
shoreline of Bay of Bengal has been observed not to be static in relation to
previous 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 Lowland
experienced dry climatic conditions and sea-level was 100 meters or more lower
than the present sea level (Umitsu, 1987). At about 12 kyr BP, south-west
monsoon became prominent which caused heavy rainfall and sea-level started rising
very rapidly (Monsur, 1995). This amplified monsoon water plus deglaciated melt
water from the Himalayas flowed over the Bengal Lowland and the initial
Madhupur and Barind surfaces were highly dissected, creating some local pools
and depressions (Monsur, 1995). It seems that the coastal processes and climate
had a significant control on topography of this region during Late Pleistocene
time.

The
Indian Summer Monsoon (ISM) during early to middle Holocene was generally
stronger 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 et

al.,
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 that
during the mid-Holocene sea level of the bay was slightly higher, the climate
was warmer, and rivers of this region discharged up to two and half times more
than 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 marine
transgression in and around Gopalgonj and Madaripur district.

A limited number of detailed studies have
been made in the last few decades to reconstruct the Holocene sea-level change
of the Bengal Lowland, including those of Umitsu (1987, 1993), Banerjee and Sen
(1987), and Islam (2001). These past attempts to reconstruct Holocene sea-level
history have been based on borehole samples. The past studies did not determine
the paleo-MSL and timing, magnitude of mid-Holocene highstand, either. The RSL
curves constructed by the authors did not show any specific regression phase during
Holocene and showed continuously rising RSL through the Holocene. The past
study areas are on low-lying deltaic deposits which are susceptible to
long-term subsidence due to compaction of sediments by anthropogenic activities
which may miscalculate the altitude of paleo-MSL. Our present study considers
three large outcrops for facies analysis. Our study, the first of its type in
the Bengal Lowland, presents a detailed description of the sedimentary facies,
and discusses the relationship between these facies successions along with
faunal analyses of pollen which yield the maximum information about the
paleoenvironmental changes in this region.

 

 

1.3: Aims and Objectives

The
major objectives of this study are:

Ø 
Reconstruction of Quaternary paleo-depositional
environment of the regions.

Ø  Correlate
the environmental changes all over the study area.

Ø  To
evaluate the paleo-floral diversification in the study area.

 

1.4: Review of Literature

Peat is an unconsolidated deposit of semi-carbonized plant remains in a
water saturated environments such as a bog or fen and of persistently high
moisture content. In the process of coal development, peat is an early stage or
rank. Carbon content is about 60% and oxygen content is about 30%. It
exquisitely preserves pollen and other microfossils that will be used to
interpret the climatic and sea level changes.

Geological Survey of
Pakistan (GSP) and Geological Survey of Bangladesh (GSB) discovered
considerable 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, Chatal
Beel of Maulvi Bazar district, Pagla and Salla of Sunamganj district are worth
mentioning. GSP started to investigate peat since 1949 and in 1953 the first
peat field at Baghia-Chanda Beel of Madaripur district was discovered. The
survey continued in the years 1954 and 1955 and peat was found to extend in an
area of 500 sq km. Further detail investigation on reserves continued in the
years 1955-57 and 0.6m to 3.3m thick (average 2m) peat was found from the
surface to near surface (up to 4m depth). The reserve of the dry peat is
calculated as 150 million tons. Fried Krupp Rhostoff, a German Company worked
on this peat for its utilization.

The
Quaternary evolution of the Bengal Basin was first described by Morgan and
McIntire in 1959. They concluded that structural activity, primarily faulting
has significantly influenced Quaternary geology of the region.      

With
regards to ancient topography of Bangladesh, particularly the origin and
evolution of the Pleistocene tracts (Faridpur and Barisal tracts), Geologist
believed that the Padma river resulted directly from a major increase in water
volume of the river. During the glacial and interglacial periods the combined
effects of seaward subsidence and landward uplift have caused a warping of the
alluvial land, which are called the Pleistocene terraces. Afterwards the
dissected valleys were filled up with alluvial sediments, generating a recent
floodplain surface at lower position than the initial Pleistocene Terraces. Chowdhury et al. (1985) put a
hypothesis about the origin of the “Swatch of No Ground” a submarine canyon as
an estuary of the last glacial maximum and if the hypothesis is accepted then
possibly 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 100
to 130 m below the present mean sea level.

Umitsu,
M. (1987 and 1993) conducted two intensive researches to describe the landform
evolution and sea level movement in the Bengal Lowland. The author attempts to
discuss the general framework of landforms and late Quaternary geology in the
Bengal Lowland. The author tried to make clear the stratigraphy of the late
Quaternary sediments in the Bengal Lowland based on the detail analysis of the
recent sediments and radio-carbon ages. In his papers the author showed the
Quaternary formation of the landform and subdivided the landform into five
members based on the characteristics of the sediments, specially their facies
and grain size sequences. He constructed a sea level curve for this region
showing a regression between ca. 12,000 and 10,000 yr BP. He showed that during
the maximum epoch of the last glacial age, the rivers flowing in the Bengal
Lowland deposited gravels on the valley floors. By 12,000 y BP, the sea level
rose up to about 45 m below present level, and the lower member deposited. The
author also showed that during ca. 12,000 and 10,000 yr BP, the delta and
floodplain surface was slightly dissected according to the regression of the
sea level. After the regression, the middle member characterized by fine
sediments 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 early
Holocene retreated towards the central part of the present Ganges Delta. In the
middle and late Holocene, the silt and clay with occasional peat layers of the
uppermost unit indicate that the lowland gradually became marshy and poorly
drained as the rate of transgression became slower.

Islam
(2001) has conducted a research on sea level changes in Bangladesh and applied
litho-bio-chrono-stratigraphic techniques to reconstruct the paleoenvironment
of Bangladesh. He used this approach as it has been applied successfully in many
coastal area of the world (Tooley, 1978; Shennan, 1982; Ireland, 1987).