Dr. Xiangli Wang

Bio

Assistant Professor, University of South Alabama
Senior Marine Scientist, Dauphin Island Sea Lab
Ph.D. 2013, Department of Geology, University of Illinois at Urbana-Champaign
 

Emphasis: Marine Geochemistry, Redox Reactions, Metal Isotopes


Research Interests

I am interested in understanding how biosphere and geosphere interact with each other in the modern time, and how they have co-evolved over geological time. My research examines these questions primarily through the lens of isotope geochemistry. Specifically, I study how redox-sensitive metal isotope systems (e.g. chromium, uranium, molybdenum, iron etc.) in seawater are recorded in marine sediments, and how they are modified during early and later diagenesis. I then apply this knowledge to interpreting redox-sensitive metal isotope data in ancient sediments in order to reconstruct paleoceanographic redox conditions, and tie it to broader questions such as climate and tectonic perturbations.

Why reduction-oxidation (redox for short) reaction is important:

Redox reactions are the main means for life to extract energy from the environment. A central player in redox reactions is molecular oxygen. The ambient level of oxygen determines the types of ecosystems: anaerobic vs. aerobic. For instance, low oxygen conditions in seawater can lead to “dead zones” we see today, or mass extinctions that occurred in the geological past. Therefore, knowledge about the evolution of oxygen level in the ocean is critical for understanding the coevolution of life and environment.

Why redox-sensitive metal isotope systems are important?

Measuring the oxygen content in the modern ocean is simply a matter of deploying a set of sensors. Measuring oxygen level in the paleocean, however, is tricky. We have to turn to marine sediments as a historical book to estimate what oxygen level was like in the overlying water column. Fortunately, some redox-sensitive metals are well suited for this application. For instance, chromium (Cr) is isotopically fractionated (i.e. 53Cr/52Cr ratio is shifted) during reduction/oxidation reactions, and the isotopic ratio is then preserved in sediments.

Redox-sensitive metal isotope systems as redox proxies have been introduced into our geochemical toolbox only since ten years ago, largely due to analytical constraint. However, the advent of modern mass spectrometers allows measurement of isotopic ratios at sub-tenth of a permil. This precise measurement allows us to detect small isotopic shifts induced by redox reactions. However, such small isotopic shifts could easily be overprinted by non-redox reactions during deposition, or by later diagenetic alterations. Therefore, there is great need for work designed to understand the basic fractionation mechanisms during uptake from seawater and during early diagenesis, since this knowledge is essential for correctly interpreting metal isotope signals recorded in sedimentary rocks. And this is an important part of my research.

Publications

Saad E.M., Wang X.L., Planavsky N.J., Reinhard C.T., Tang Y. Accepted. Chromium isotope fractionation induced by ligand-promoted mobilization of Cr(III)-containing mineral.

Tarhan L.G., Planavsky N.J., Wang X.L., Bellefroid E., Droser M.L. and Gehling J.G. Accepted. The Late-Stage ‘Ferruginization’ of the Ediacara Member (Rawnsley Quartzite, South Australia): Insights from Uranium Isotopes. Geobiology.

Cole D.B., Wang X.L., Qin L., Planavsky N.J. 2017. Chromium isotopes geochemistry. In Earth Science Series – Encyclopedia of Geochemistry.

Wu W.H., Wang X.L., Reinhard C.T., Planavsky N.J. 2017. Chromium isotope systematics in the Connecticut River. Chemical Geology. Vol. 456, p. 98-111.

Qin L.P., Wang X.L. 2017. Chromium isotope geochemistry. In: Teng F.Z., Watkins J.M., and Dauphas N. (Eds.), Measurements, Theories and Application of Non Traditional Stable Isotopes. Reviews in Mineralogy and Geochemistry. Vol. 82, p. 379-408.

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