2:45 PM - 3:00 PM
[HCG21-05] Chemosedimentary sequence analysis using biomarker profiles of the sediments in Bengal Fan
Keywords:Bengal Fan, Terrestrial plant-derived terpenoid, Chemosedimentary sequence, Turbidite, IODP
The Bengal Fan in northeastern Indian Ocean is a submarine fan formed by transport of huge clastic sediments by the confluent Ganges and Brahmaputra rivers as a direct result of the India–Asia collision and uplift of the Himalayas and the Tibetan Plateau (Curray et al., 2002). Turbidite in submarine fan is basically formed by turbidity currents caused by flood and earthquake, and terrigenous organic matter (OM) by delivering from land area through the turbidity current is abundant (Baudin et al., 2010; Biscara et al., 2011). Thus, the sedimentary OM analysis can be applied to evaluating transport and depositional processes in the turbiditic sediments. In the present study, we analyzed biomarkers in sediment core samples from the Bay of Bengal to reconstruct sedimentary systems of the Bengal Fan.
We used the sediment cores recovered at the Bengal Fan, Site U1444, by Integrated Ocean Drilling Program (IODP) Expedition 353. The ages were determined by microfossil stratigraphy, and the lowermost horizon was estimated to be about 6Ma. Core U1444A is composed of 4 sedimentary facies. Unit 1 comprises silty sand and silty clay with various thick turbidites. Unit 2 consists of clay with thin turbidites. Unit 3 contains silty sand and clayey silt, although the core recovery was poor. Unit 4 is composed of silty clay with thin turbidites. In this study, we sampled and analyzed separately the coarse-grained layer and the fine-grained layers just above and below the coarse-grained layer (just above: basically Tmud, just below: hemipelagite) in turbiditic sequences. Freeze-dried sediments were extracted by solvents and then separated into fractions by silica-gel column. These fractions were analyzed by GC-MS and GC-FID.
In the U1444A sediments, steroids and terrestrial plant-derived terpenoids were detected. We analyzed C29 /C27 steroid ratios as the proxy of terrigenous input. In the turbiditic sequence of Unit 1, the plant-derived terpenoid concentrations hardly varied, while the C29/C27 steroid ratios were higher only in the coarse-grained layer. In the turbiditic sequences of Unit 2 and Unit 4, the plant-derived terpenoid concentrations and C29/C27 steroid ratios of the coarse-grained layers and the T-mud layers just above the coarse-grained layers tended to be higher than those of the hemipelagite layers just below the coarse-grained layers. These results indicate that the terrigenous input in the coarse-grained layer and Tmud were higher and the terrigenous matter were directly and efficiently transported from land and coastal areas by turbidity currents in the Bengal Fan. On the other hand, some sequences showed lower plant-derived terpenoid concentrations and C29/C27 steroid ratios in the coarse-grained layer than in the fine-grained layer, and a higher ratio of dehydroabietic acid (DAA), a conifer-derived terpenoid. These results suggest that the DAA contained in more refractory resins and wood might be redeposited by submarine landslides. The trends of the biomarker profiles in the turbiditic sequences changed with long time scale from the Miocene to Pleistocene, and the long-term variations were found to be roughly synchronous to the historical changes of the uplift of the Himalayas shown by Clift et al (2008). Thus, the sedimentary systems of the Bengal Fan recorded by the biomarker profiles were broadly related to the uplift of the Himalayas.
We used the sediment cores recovered at the Bengal Fan, Site U1444, by Integrated Ocean Drilling Program (IODP) Expedition 353. The ages were determined by microfossil stratigraphy, and the lowermost horizon was estimated to be about 6Ma. Core U1444A is composed of 4 sedimentary facies. Unit 1 comprises silty sand and silty clay with various thick turbidites. Unit 2 consists of clay with thin turbidites. Unit 3 contains silty sand and clayey silt, although the core recovery was poor. Unit 4 is composed of silty clay with thin turbidites. In this study, we sampled and analyzed separately the coarse-grained layer and the fine-grained layers just above and below the coarse-grained layer (just above: basically Tmud, just below: hemipelagite) in turbiditic sequences. Freeze-dried sediments were extracted by solvents and then separated into fractions by silica-gel column. These fractions were analyzed by GC-MS and GC-FID.
In the U1444A sediments, steroids and terrestrial plant-derived terpenoids were detected. We analyzed C29 /C27 steroid ratios as the proxy of terrigenous input. In the turbiditic sequence of Unit 1, the plant-derived terpenoid concentrations hardly varied, while the C29/C27 steroid ratios were higher only in the coarse-grained layer. In the turbiditic sequences of Unit 2 and Unit 4, the plant-derived terpenoid concentrations and C29/C27 steroid ratios of the coarse-grained layers and the T-mud layers just above the coarse-grained layers tended to be higher than those of the hemipelagite layers just below the coarse-grained layers. These results indicate that the terrigenous input in the coarse-grained layer and Tmud were higher and the terrigenous matter were directly and efficiently transported from land and coastal areas by turbidity currents in the Bengal Fan. On the other hand, some sequences showed lower plant-derived terpenoid concentrations and C29/C27 steroid ratios in the coarse-grained layer than in the fine-grained layer, and a higher ratio of dehydroabietic acid (DAA), a conifer-derived terpenoid. These results suggest that the DAA contained in more refractory resins and wood might be redeposited by submarine landslides. The trends of the biomarker profiles in the turbiditic sequences changed with long time scale from the Miocene to Pleistocene, and the long-term variations were found to be roughly synchronous to the historical changes of the uplift of the Himalayas shown by Clift et al (2008). Thus, the sedimentary systems of the Bengal Fan recorded by the biomarker profiles were broadly related to the uplift of the Himalayas.