1. Department of Geological Sciences, Brown University, Providence, Rhode Island 02912
2. National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
3. Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882
4. Departments of Plant Biology and Geology, University of Illinois, Urbana, Illinois 61801
5. Nanjing Institute of Geography and Limnology, Chinese Academy of Science, Nanjing, Jiangsu 210008, China