Research Interests
My research applies geochemistry to problems in oceanography, paleoclimatology, and environmental concerns on a variety of timescales. To undertake these problems, I use naturally occurring isotopic variations due to radioactive decay or mass fractionation as time and process tracers and combine modeling techniques with radiochemical assay and mass spectrometry.
An important component of my research program has been to develop means of dating natural deposits. I have explored the use of decay-series isotopes and cosmogenic nuclides ( 10 Be and 26 Al) to determine ages and sedimentation rates of such deposits as marine sediments and ferromanganese nodules, fossil corals, and pedogenic carbonates, and the use of the fallout 137 Cs/ 135 Cs for recent sedimentation and erosion rates. In recent years, my colleagues/students and I designed an extraction technique which enables us to measure 26 Al in marine sediments thus paving the way of effectively using this cosmic-ray produced isotope for sediment chronology and other geophysical studies. Our exploratory research on the use of 226 Ra/Ba ratios for dating biogenic carbonates has yielded results showing the datability of coastal mollusk shells in Antarctica , which would aid in tackling the 14 C reservoir age problem in Southern Oceans. We have also developed the U-series isochron method for dating a variety of sedimentary materials ranging from travertine to soil carbonate to evaporite salts. Successful applications of this technique have led us to the establishment of late Quaternary chronologies of closed-basin lake salt deposits in the western U.S. , western China , and South America . Considerable efforts have also been spent to push the frontier in the use of 226 Ra and 226 Ra as water-mass time tracers.
Seeking new ways of extracting information about past environments in terms of climate and ocean productivity changes constitutes another part of my research effort. On paleoclimate studies, our isotopic, chemical and mineralogical studies on lake sediments allow us to decipher the regional changes in climate and hydrology in great detail. From a mass balance of 18 O and water, and from an understanding of the factors controlling the d 13 C - d 18 O covariance in lake sediments, we attempt to deduce the volume change of a closed-basin lake, hence changes in regional precipitation versus evaporation. Such paleoclimatic reconstructions are being made in very high-resolution (subdecadal) for the Mono and Owens Lakes of the Great Basin in the western U.S. We further attempt to read high -resolution (interannual to subdecadal) paleoclimatic signals archived in the speleothem deposits of limestone caves. Results from our study stalagmites from a series of caves in in eastern China from 25 o to 40 o N enable us to tie the proxy (O, C and Sr isotopes) signals to past variations of the summer monsoon strength. We feel confident that this research will contribute to our understanding of not only the climate history of eastern Asia , but also the factors influencing the monsoon fluctuations that are rooted in the global climate variability. On paleoceanographic research, we have been critically evaluating the use of particle-reactive isotope pairs of 230 Th- 231 Pa and our newly proposed 26 Al- 10 Be as proxies for past oceanic productivity. These proxies have the advantage of being less affected by post-depositional dissolution/mobility, in addition to having known production ratio for each of the pairs. Recently, we used the global distribution of 10 Be to constrain the deep-water formation on centennial scales.
Also going on are two environmental research projects dealing with problems of societal concern of a more direct and immediate nature. Prior to the 1970s, coastal ocean off southern California received significant amounts of man-made organic contaminants such as chlorinated pesticides (DDTs), polychlorinated biphenyls (PCBs) and linear alkylbenzenes (LABs). Although the contaminants are largely buried by sedimentation, an investigation of their post-depositional movement and fate in the coastal marine environment is urgently needed. We have launched studies to address this issue in the Palos Verdes Shelf area and in the San Diego Bay . In collaboration with scientists from the Southern California Coastal Water Research Project (SCCWRP), we measured the organic contaminants and several naturally-occurring radioisotopes in seawater, sediment traps and sediment cores. As contaminants such as PCBs are largely bound to particles, we can use the particle-reactive radionuclides (e.g., 210 Po and 210 Pb) as tracers for pollutant movement and sediment dynamics in the coastal environment.
The second project related to environmental concerns is directed toward the characterization and quantitative assessment of subsurface transport of waste contaminants. The long-term migration behavior of pollutants, including radioactive waste, in underground aquifers can be understood through a study of the cumulative effects of transport over geological times, and these effects can be assessed from the naturally occurring U and Th decay-series disequilibria observed in the rock-water system. The decay series consist of elements with diverse chemical properties and with isotopes having a range of decay mean lives. Different geochemical behaviors of the nuclides lead to significant radioactive disequilibria between parents and daughters in the interstitial fluids and associated solids. Both thermodynamic and kinetic factors play a role in creating and maintaining the disequilibria. By modeling the local mass balance of the various radioisotopes, with constraints placed by their different decay rates and the parent-daughter relationships, one can derive information on kinetics of rock-water interaction under a given geochemical and hydrologic environment, on time scales of a few days to over a million years. Adopting this “natural analog”, multi-tracer approach, our current study sites include the Memphis Aquifer at the Shelby County area in western Tennessee and in and around the Nopal I uranium deposit at Peña Blanca in Chihuahua, Mexico. The studies provide information on (1) sorption-desorption rate constants and retardation factors of various radionuclides, (2) rates of precipitation and dissolution of rocks and their influence on radionuclide transport, and (3) transit time and flow path of groundwaters at the site. Such site-specific information is also of fundamental value for evaluating in-situ , long-term migration of radionuclides in the far field of a nuclear waste disposal site.
The above-outlined research programs are the collaborative undertakings of many researchers and institutions. The principal ones include:
University of Southern California : S. Luo, H. C. Li, L. Stott,
University of Tennessee : J. McCarthy, R. Gentry
Los Alamos National Laboratory: M. Murrell
Binghamton University : T. K. Lowenstein
University of Minnesota : R. L. Edwards
Southern California Coastal Water Research Project: E. Zeng, J, Peng,
Ohio State University : P. Berkman
Japan Marine Science and Technology Center : M. Kusakabe
Institute of Earth Sciences , Academia Sinica: T. Lee, C.A. Huh
National Cheng-Kung University : C. F. You
