부산대학교 도서관

The Bay of Bengal and the Andaman Sea are sensitive to Indian monsoon variation in terms of rainfall, runoff, and terrigenous sediment discharge. These regions provide the great opportunity to explore the history of Indian monsoon and its influence on the land/marine environment. Given that the origin and evolution of long-term (tectonic timescale) Indian monsoon are still a topic of debate, study on tectonic timescales could help to improve our understanding of the Indian monsoon variability history. In this dissertation, samples of the International Ocean Discovery Program (IODP) Expedition 353 Site U1445 (the Bay of Bengal) and Site U1447 (the Andaman Sea) were used to reveal the long-term evolution of the Indian monsoon since the late Miocene. At IODP Site U1445, the concentrations and mass accumulation rates (MARs) of biogenic opal, CaCO3, total organic carbon (TOC), and total nitrogen were measured over the Mid-Pleistocene Transition (MPT) to identify the links between surface water marine biogenic production and the Indian summer monsoon. TOC MARs were found to reflect both surface water marine productivity and terrestrial organic matter through the measurements of sediment C/N ratios and δ13CSOM values. Nonetheless, it was identified a shift in the paleoproductivity regime from a dominance of biogenic opal deposition prior to the MPT to the dominance of CaCO3 deposition following the MPT. The shift in biogenic marine productivity across the MPT was closely related to riverine discharge, which was primarily controlled by the intensity of the Indian monsoon. These results, therefore, infer a decrease in riverine discharge to the Bay of Bengal across the MPT in response to a weakened Indian summer monsoon (and/or strengthened Indian winter monsoon). In addition, changes in the intensity of the Indian monsoon across the MPT were more closely linked to the global climate cooling rather than the gradual uplift of the Himalaya and Tibetan Plateau.At IODP Site U1447, Nd-Sr isotopes (εNd, 87Sr/86Sr) of detrital particles, clay mineral compositions, and δ13C of sediment organic matter (δ13CSOM) were measured to reveal the sediment provenance changes and/or weathering intensity variations in association with the Indian monsoon intensity change. The shipboard age model based on biostratigraphic data and paleomagnetic reversals shows that IODP Site U1447 preserves late Miocene (~10 Ma) sediments. Nd/Sr isotope systematics demonstrates that the sediments originated mainly from the Myanmar region, including the Irrawaddy River, Salween River, Sittang River and Indo-Burman-Arakan Ranges without a significant change of sediment provenance since the late Miocene. Thus, temporal variations of clay mineral compositions, represented as smectite/(illite+chlorite) [S/(I+C)], indicate the long-term variations of physical/chemical weathering intensity attributable to intensity changes of Indian winter/summer monsoon. A gradual decreasing trend of S/(I+C) ratios indicates stronger physical and/or weaker chemical weathering since the late Miocene, as a result of strengthening of Indian winter monsoon (and/or weakening of Indian summer monsoon), which seems be closely related to global cooling since the late Miocene. Distinct decrease of S/(I+C) ratios occurred at ~9.2-8.5 Ma, ~3.6 Ma, ~2.4 Ma, and ~1.2 Ma, which may be attributed to the combined effect of both global cooling and Tibetan Plateau Uplift as a local response. In addition, δ13CSOM values at IODP Site U1447 were higher at ~3.5 Ma and after 1.5 Ma when S/(I+C) ratio was minima, which may imply an increase of C4 plant in Myanmar region as a result the strong Indian winter monsoon (or weak Indian summer monsoon).