Monsoon-influenced variation of clay mineral compositions and detrital Nd-Sr isotopes in the western Andaman Sea (IODP Site U1447) since the late Miocene

초록

Nd-Sr isotopes (epsilon(Nd), Sr-87/Sr-86) of detrital particles, clay mineral compositions, and delta C-13 of sediment organic matter (delta C-13(SOM)) at the International Ocean Discovery Program (IODP) Expedition 353 Site U1447 in the western Andaman Sea 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 (similar to 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 similar to 9.2-8.5 Ma, similar to 3.6 Ma, similar to 2.4 Ma, and similar to 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, delta C-13(SOM) values at IODP Site U1447 were higher at similar to 3.5 Ma and after 1.5 Ma when S/(I + C) ratio was minima, which may imply an increase of C-4 plant in Myanmar region as a result the strong Indian winter monsoon (or weak Indian summer monsoon).

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