Recently, a team led by Academician Chen Jun and Professor Li Weiqiang, from the School of Earth Sciences and Engineering, Nanjing University, made substantial progress in tracing continental weathering process, with their study entitled "K isotopes as a tracer for continental weathering and geological K cycling," published on April 15, 2019 in Proceedings of the National Academy of Sciences of the United States of America, an prestigious international multidisciplinary science journal.
Continental silicate weathering removes CO2 from the atmosphere over multi-million-year timescales. Therefore, it is generally thought to be an important mechanism which regulates the Earth's climate. Whether this process was a driving force behind global climate change, or worked as a negative feedback which passively respond to climatic changes to maintain climatic stability in geological history has always been a big scientific question that remains unanswered. This is mainly due to the difficulty in quantifying the intensity of ancient continental silicate weathering.
Weathering proxies such as Chemical Index of Alteration (CIA) obtained through direct measurement of sediment only provide information on regional continental weathering process. Only the chemical indicators of ancient seawater solutes can be used to reconstruct the history of silicate weathering by tracing variations in chemical signals from continental rivers. However, existing solute weathering proxies eitherare prone to interference from carbonate or organic carbon weathering (e.g., Mg, Ca, Os, and Sr isotopes), or suggest a complicated relationship with weathering intensity (e.g., Li isotopes).
By measuring K isotopes in major rivers in mainland China, the team found that the K isotopic composition of the fluvial solute negatively correlated with the continental weathering intensity and could be used to reconstruct the continental weathering intensity. Based on this, the team further estimated that the average K isotopic composition of continental rivers was -0.22±0.04‰ using the K isotope - weathering intensity function. This provided important information for the estimation of modern K flux. With the Monte Carlo method, the team significantly reduced the uncertainty in the estimates of modern geological K flux (Figure 1).
Figure 1. A comparison between K flux and isotopic composition estimated in this study (words in white) and previously reported values (words in blue).
K, the only major element with multiple stable isotopes in the Group IA of the periodic table, participates in many natural processes. K isotope ratios are currently an important international research topic.
At present, many research teams including those from Harvard University, Princeton University and University of Washington are conducting research in this field.
Professor Li Weiqiang carried out an independent study of K isotope geochemistry with the support of the 1000 Talents Program. Since 2014 when he returned to China, he started to explore a method for high-precision K isotope measurement in geological samples. In 2016, He publish the method paperalmost at the same time as the Jacobsen's team of Harvard University did. He has had several papers published in many aspects such as the biological fractionation effect and the mineral fractionation effect on K isotope geochemistry. This time, Professor Li, together with the surficial geochemisry research team led by Academician Chen Jun, successfullyapply the potassium isotope approach on the research of continental weathering and global potassium cycling. This is a milestone, which indicate that Nanjing University has become an internationally recognized leader in the research of K isotopes.
Ph.D. student Li Shilei is the first author of the paper, Professor Li Weiqiang is the corresponding author, and the coauthors include Academician Chen Jun and Associate Professor Chen Yang, of MOE Key Laboratory of Surficial Geochemistry, Academician Maureen E. Raymo, of Columbia University, and Professor Brian L. Beard, of University of Wisconsin-Madison. The MOE Key Laboratory of Surficial Geochemistry, School of Earth Sciences and Engineering, Nanjing University, is the leading institution of this research, and the State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, is the corresponding institution. This work was advised by Academicians Chen Jun and Maureen E. Raymo and supported by the 1000 Talents Program, the Start-up Fund for Talent Introduction, Nanjing University, and the National Science Foundation of China Grants 41730101 and 41622301.