4:15 PM - 4:30 PM
[MIS01-09] Paleoecological variations in marine producers of the Andaman Sea during the late Miocene reconstructed by algal biomarkers in sediments from site U1447 (Exp. 353)
Keywords:Andaman Sea, Indian Ocean, IODP, biomarker, marin primary production
The sediments from site U1447 deposited during the late Miocene (Unit IV and the lower part of Unit III) are mainly composed of nannofossil-rich clay, clayey nannofossil ooze and biosilica-rich clay and percentage of biosilica increase from the lower part of Unit IV (Subunit IVb). In addition, the sedimentation rates of these deposits are constant (~6.5 cm/ky.). The extraction of freeze-dried sediments was fractionated using silica-gel column and analyzed by GC-MS.
Haptophyte species Reticulofenestra minuta are main component among the genus Reticulofenestra in the samples characterized by higher alkenone concentrations. We suggested R. minuta was main alkenone producer in the Andaman Sea during late Miocene. The C38 methyl alkenone ratios among alkenones increase with increase in R. pseudoumbilicus abundance and decrease in alkenone concentrations after 7 Ma. The alkenone composition in the Baltic Sea (lower SSS region) is characterized by lower relative abundance of C38 methyl alkenones (Schulz et al., 2000). The C38 methyl alkenone ratio consistently increase with the decreasing trend of C32 diols ratios (fresh water indicator; Lattaud et al., 2017), which indicates diminishment of freshwater input after 7 Ma. The diol Index-2 (DI-2) indicate productivity of diatom genus Proboscia (Rampen et al., 2014). The DI-2 values are higher from 7.5 to 6 Ma, although diatom tests rapidly decrease after 7 Ma. From our biomarker results, the diatom productivity was consistently higher after 7 Ma. We presume that such diatoms were degraded by dissolution during/after deposition after 7 Ma. Variations in dinosteroid ratios among steroids (dinoflagellate productivity indicator), concentrations of steroids and diols, stanol/sterol ratios (paleoredox indicator) and taraxerol ratios among triterpenoids (mangrove indicator) synchronously increase after 7Ma. Indian summer monsoon enhanced after 7 Ma due to the Himalayan uplift (Gupta et al., 2015). These evidences are inconsistent with result of fresh water diminishment after 7 Ma evaluated from C32 diols and C38 methyl alkenone ratio. In this region, the higher C38 alkenones and lower C32 diols ratios merely indicate higher contribution of marine coccolithophorid (R. pseudoumbilicus) and eustigmatophyta. Thus, we conclude that productivity of diatom, dinoflagellate and marine eustigmatophyta against R. minuta elevated in the Andaman Sea, and mangrove vegetation expanded in the hinterland due to enhancement of monsoonal activity after 7 Ma.