NEW INSIGHTS INTO LATE PLEISTOCENE-HOLOCENE CLIMATIC CHANGES FOR NE BANGLADESH DERIVED FROM SEDIMENT AND GEOCHEMICAL ANALYSES, AND FACIES INTERPRETATIONS

The Sylhet basin (NE Bangladesh) is a 3-6 mm/yr subsiding area of the northeast Bengal basin that is flooded to 3–4 m depth every monsoon season with sedimentation rates exceeding 7 mm/yr. The Ganges and Brahmaputra watersheds supplied the Sylhet basin with sediment originating from the Himalayas and Tibetan Plateau and deposited up to 20 km thick sediments since the Eocene. In the upper Pleistocene, long-term climate changes associated with the last glacial-interglacial cycles have affected the sediment formation. We studied sediments recovered from two wells from the Sylhet basin that were up to 183 m deep. One of our main goals is to understand climate variability and the sedimentary response to glacial and interglacial cycles in this heavily sedimented setting of the Sylhet basin at a distal location about 300 km from the shoreline. For this purpose, we conducted sediment analyses (grain size, X-ray fluorescence elemental chemistry, and radiocarbon dating). Based on these results, we defined the sediment facies as oxidized sandy mud, blue sandy mud, muddy coarse sand, interbedded muddy sand, and two paleosols associated with periods of exposure. The youngest paleosol occurs at a depth of 23-32 m, and it is C-14 dated at 8,890 ± 35 yr BP at 31 m using organic matter. It is composed of brown mud with a basal layer of coarse sand and contains carbonate micro-nodules. Al is the most abundant element. Fe and Mn are significant but less abundant. The oldest paleosol is recovered from a depth of 100-110 m. It is composed of brown mud interbedded with sand towards the top of the section. C-14 dated a charcoal layer above the oldest paleosol at 38,100 ± 1,300 yr BP. These ages are consistent with the early Holocene and Interglacial 3 (MIS-3, 25-60 ky BP), suggesting that the paleosols evolved through glacial stages 4 and 2 and into the interglacial. It is hypothesized that the climate was cold and dry due to the lack of organic matter in the sediments and that there was slow deposition and a lag in the formation of the paleosols relative to eustatic lowstands.