Provenance of detrital sediments in Southern Andaman Sea since the last glaciation

Abstract

The Andaman Sea is a semi-enclosed system which offers such a good case for studying the source to sink transport process of sediment. This work has main objectives which are: 1) to reveal clay mineral assemblages in surface sediments, and understand present transport process; 2) to understand the clay mineral variation since the last glaciation; 3) to reconstruct the Indian monsoon evolution; 4) to explain the possible mechanism of marine environmental variation driven by climate change since the last glaciation. The materials used in this study include 61 surface sediment samples which were analyzed for clay minerals in order to characterize modern ocean transportation process, and 4 sediment cores (ADM 2, ADM 6, MASS-III-07, and MASS-III-10) in the southern Andaman Sea which were analyzed for clay minerals and element geochemistry, to reconstruct paleoenvironment over the last glaciation. The AMS-14C and oxygen isotopes (δ18O) dating from planktonic foraminifera were measured to establish a chronology of the cores.

The clay mineralogy of surface samples revealed four potential provenances, including Myanmar, Malay Peninsula, Sumatra, and Thailand. The major provenance is from Myanmar, which dominated by smectite (44%) from Irrawaddy and Salween river system. The Malay Peninsula, Sumatra, and Thailand are predominated with kaolinite (80%, 68%, and 47% respectively). From clay mineral assemblages, it can be identified that three provenances, Malay Peninsula, Sumatra, and Thailand, are controlled by climatological condition, while Myanmar (major provenance) is controlled by lithological setting. The modern transportation process of sediments in the Andaman Sea is controlled by two processes; surface circulations that are directly related to SW and NE monsoon, and differential setting of smectite.

Clay mineral variation in the sediment cores revealed no significant change in downcore variation. The clay mineral variations over the last glaciation of ADM2 and MASS-III-10 are related to the sea level and monsoon, ADM6 is under the monsoon influence during Holocene, while MASS-III-07 shows unclear controlling factor. As the results, it can indicate that the relative contributions of the sources did not vary through time since the last glaciation except MASS-III-10. It was found the different amount of sediment input between glacial and interglacial time, suggesting it may be due to another factor such as the river discharge. Core ADM2 is characterized by lower sedimentation rates in comparison to ADM6. The high sedimentation rate of ADM6 may occur due to high hydrodynamic condition because ADM6 was found the turbidity layer in the lower part of the core. The shallow water cores; MASS-III-07 shows high sedimentation rate (12.3 cm/ka) than MASS-III-10 (2.2 - 9.9 cm/ka). Although these two shallow water cores are located in the same area, their sedimentation rates were different. The high sedimentation rate of MASS-III-07 suggested that the core location may be existed in the disturbance zone, of which may be influenced by the current driven into the Andaman Sea from Malacca Strait by NE monsoon. The current may wash off some of the surface sediment that corresponded well with losing of the core top. Although it was found that the monsoon influence on the sediment deposition in the Andaman Sea, the results are not enough to explain the reconstruction of Indian monsoon evolution.