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Coralline Ba/Ca ratios and luminescent line records in Con Dao island, offshore Vietnam, as indicators of Mekong river runoff between 1948 and 1999

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This paper shows 52-year Ba/Ca ratios and bright luminescent lines recorded in coral Porites in the Con Dao Island area, offshore Vietnam, about 90 km from the mouth of the Mekong River. Bright luminescent lines and Ba/Ca ratios are cyclical and formed annually, where the brightluminescent lines and the highest Ba/Ca ratios are simultaneously formed. | Vietnam Journal of Earth Sciences 36 (2014) 432-438 Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences (VAST) Website: http://www.vjs.ac.vn/index.php/jse Coralline Ba/Ca ratios and luminescent line records in Con Dao Island, offshore Vietnam, as indicators of Mekong river runoff between 1948 and 1999 Dang Xuan Phong* Institute of Geography, Vietnam Academy of Science and Technology, Hanoi, Vietnam Accepted 25 December 2014 ABSTRACT The Ba/Ca ratios and luminescent lines recorded in coral skeleton in near-shore corals are reflected by fresh river flows. This paper shows 52-year Ba/Ca ratios and bright luminescent lines recorded in coral Porites in the Con Dao Island area, offshore Vietnam, about 90 km from the mouth of the Mekong River. Bright luminescent lines and Ba/Ca ratios are cyclical and formed annually, where the bright luminescent lines and the highest Ba/Ca ratios are simultaneously formed. The measured Mekong River discharge (MRD) and salinity data recorded during the 1985-1999 period show that the annual maximum MRD and minimum salinity in Con Dao Island were synchronous, and that the annual maximum Ba/Ca ratio was formed during maximum MRD and minimum salinity. When the annual minimum MRD and salinity were stable, the annual minimum Ba/Ca ratio was variable. Overall, the mean annual MRD and Ba/Ca ratio slightly decreased during the 1985-1999 period, suggesting that the Ba/Ca ratio is mainly controlled by MRD. The decreasing trend in Ba/Ca ratios over the period between 1948 and 1999 is probably related to the MRD reduction due to human activities and/or climate change. Keywords: Porites coral, Ba/Ca ratio, luminescent lines, Mekong River discharge. © 2014 Vietnam Academy of Science and Technology 1. Introduction The luminescent lines in coral skeletons potentially offer an important proxy relating to the contribution of organic matters in river runoff from land (Isdale, 1984; Isdale et al., 1998). Further investigation .