Century scale variation of elemental Pb and Pb isotopes in the South China Sea region
Date of Issue2015
Asian School of the Environment
Singapore-MIT Alliance on Research and Technology
The contribution of Asian Pb to the global Pb cycle is increasingly important now that Asian countries have experienced intensive development and phased out leaded petrol decades later than Europe or North America. To assess the Asian Pb, this study investigates the spatial and temporal variation of Pb and Pb isotopes in the South China Sea (SCS) region. This study focuses on two sites: Nha Trang Bay represents an open ocean environment in western SCS and Singapore resembles a typical emerging Asian city. As seen from the 200-year-long coral off the Nha Trang Bay, central Vietnam, Pb was low in the western SCS until the late 1950s, and then increased ~4x from the late 1950s to 2000s. The peak Pb/Ca in the Vietnam coral lags peak Pb emission by at least 4-7 years, implying that upwelling in the western SCS might have transported older water masses to the coral. As suggested by Pb isotopes, Chinese emitted Pb accounts for ~46% of the Pb in the recent western SCS. Moving on to a typical emerging Asian city, the temporal variability of Pb and Pb isotopes in Singapore was reconstructed using: a) three corals located along an east-west transect of the Singapore Strait and b) a 40cm-long sediment core from MacRitchie Reservoir (Singapore’s central catchment reserve). The Pb content in the MacRitchie sediment has increased ~30x over the 20th century, and the Pb isotopes changes from natural composition to the composition found in Southeast Asian aerosols. 206Pb/207Pb in Singapore corals deviate from aerosols and I have not been able to find any direct source causing this deviation. I propose that the anomalous Pb isotope ratios are likely a result of isotope exchange between anthropogenic dissolved Pb and natural crustal particulates. The exchange process has been demonstrated through a closed-system isotope exchange experiment using waters from the Johor River mouth (the largest river discharging to Singapore Strait). The results presented in this dissertation provide new perspectives on Asian Pb emissions and improves our understanding of ocean Pb cycling.