Monthly-resolved Pb isotopic determination in coral skeletons and its application
Date Issued
2012
Date
2012
Author(s)
Chou, Yu-Chen
Abstract
Pb concentrations and isotopic compositions in coral skeletons have been used to understand the annual to decadal trends of natural hydrological evolution, ocean circulation, and anthropogenic pollution. However, the low Pb content in coral skeletons, commonly of 10s-100s ng/g, has hindered determination of monthly variation of the coral Pb isotopic compositions. Consequently, the applications of Pb isotopic study have been limited.
In this study, the chemical separation of Pb and instrumental technique were improved. A reliable procedure was developed for measuring the isotopic compositions (204Pb, 206Pb, 207Pb, and 208Pb) of pictogram Pb in coral skeletons. The isotopic analysis was conducted on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Electron NEPTUNE. The overall procedural Pb blank was only 2 ± 3 pg. Replicate measurements on a standard NIST SRM 981 showed that a precision of ± 2‰ (2σ) with 300-1000s pg of Pb could be achieved. This quantity is equivalent to 5-10 mg of coral sample. A sample size of 1.5 ng of Pb is recommended for routine analysis with acceptable external precision.
The monthly-resolution Pb isotopic records during 1978-1982 of a coral Porites from the Son Tra Island, central Vietnam (16°12''59.4”, 108°1''57.1”) were reconstructed. The Pb concentrations vary from 35 to 70 ppb. The Pb isotopic ratios are 2.4262-2.4813 for 208Pb/207Pb, and 1.1226-1.1745 for 206Pb/207Pb. The results yield the following important points: (1) A clear seasonal-to-annual variation in monthly Pb isotopic composition was observed; which had never been reported in the literature. (2) The isotopic compositions 208Pb/207Pb vs. 206Pb/207Pb ratios suggest a mixing between fluvial sediments and adjacent open-ocean seawater from the South China Sea. (3) The Pb concentrations time series shows an interannual gradual increasing trend from 1978 to 1982. (4) The anomalously low 208Pb/207Pb (2.426-2.433) and 206Pb/207Pb (1.122-1.128) ratios in the summer and autumn of 1980 could be due to the contribution of aerosols from Malaysia, Indonesia and Thailand. The aerosols were likely delivered by the Asian summer monsoon and southwesterly flow after typhoon. The present study indicates that monthly resolved coral Pb isotopic sequence can be used as a proxy and tracer for the monthly-seasonal environmental change and the impact of anthropogenic perturbation. The coral Pb isotopic records may help understand the nature of regional hydrological dynamics, planning of resource management, and making of sustainability policy.
Subjects
Pb
lead isotopes
MC-ICP-MS
coral
central Vietnam
Type
thesis
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