| In this work, surface sediment samples from a matrix of ninety-five sites and one gravity sediment core (ADM-9) that extended from the southeastern Andaman Sea were measured for their grain size, clay mineral, geochemistry and isotopes indices to provide the characteristics of the sediment distribution and provenance in this area, and together with the 14C dating profile to examine the sedimentary responses to the Indian Summer Monsoon variations since the Last Glacial Maximum.The results showed that the sediments from the southeastern Andaman Sea are mainly composed of silty sand, sandy silt and silt. R-mode factors analyses towards the elements shows that the sediments from the southeastern Andaman Sea are mainly composed of fine-grained terrigenous detrital component, the biogenic material and authigenic materials from the water column were less influenced on the sediment elment variation. The samples show enrichments in the light REEs with flat heavy REEs patterns with less Ce anomalies and moderate negative Eu anomalie. This result of REE showed that the sediment component was mainly derived from the terrigenous origins. REEs compositions of the sediments were constrained by the sediment provenance, sediment types and biogenic carbonate as well as the occurrence of heavy mineral. The clay mineral assemblage consists of illite (45%), kaolinite (31%), chlorite (18%) and smectites (6%). The clay mineral compositions of the sediments were mainly constrained by sediment provenance and transport path.ΣLREE/ΣHREE,δEu, (La/Yb)N and clay mineral assemblage are selected to identify the different sediment origins. The results showed that the sediments in northwestern study area are mainly originated from the Irrawaddy River, the sediments of the inner continental shelf of Malay peninsula mainly originate from the rivers in the Malay peninsula and coastal erosion. The northern study area is covered by the relict sands which formed probably during the Holocene transgression. The sedimentary regime in the central and southern area is characterized of a mixed pattern where the sediments are mainly from the rivers of Sumatera and Malay peninsula and Irrawaddy River driven by the by monsoon-induced currents.Based on the comprehensive elemental geochemistry analysis and a comparative study of the gravity sediment core (ADM-9) in the study area, the evolution of the sedimentary environment, elemental records and its responses to the Indian Summer Monsoon variations since Last Glacial Maximum were discussed. The 14C chronology results show that the sedimentation rate ranged from 5.3 to 20.1 cm/ka. Sensitive grain size, terrigenous mass accumulation rates, elements ratios, clay mineral assemblage, CaCO3 and Babio mass accumulation rates were applied to reconstruct the intensity of the terrigenous input, southwest monsoon current, the weathering condition in the source area and paleo-productivity since 26ka BP. The results show that, the intensity of the terrigenous input, Indian monsoon-induced current, paleo-productivity and the weathering intensity in the source area were enhanced during～11.5-6ka BP, and shifted between ～15ka BP and ～11.5ka BP and weakened during ～26-15ka BP and since ～6ka BP.Proxies of chemical weathering (K/Al and smectite/(illite+kaolinite)), terrigenous input (terrigenous mass accumulation rates and 16.8-21.2 μm sensitive grain size) and coupled with the δ18O of G.ruber together revealed that the intensity of Indian Summer Monsoon weakened during ～26-15ka BP, shifted between ～15ka BP and ～11.5ka BP, and then gradually strengthened to the maximum during ～7-9 ka BP, with a subsequent general weakened pattern since 6ka BP. Based on the power spectrum analysis of the 16.8-21.2μm grain size population since 26ka BP, the results showed that 1619yr periodicity in the Indian Summer Monsoon was identified, which could be evidenced for the shor-term changes in ISM intensity that maybe influenced by the solar activity. |
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