GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 97-12
Presentation Time: 9:00 AM-6:30 PM


XING, Chaochao1, LANG, Xianguo2, MA, Haoran1, PENG, Yang1, PENG, Yongbo3, NING, Meng1, CUI, Yixin1 and SHEN, Bing1, (1)School of Earth and Space Sciences, Peking University, Beijing, 100871, China, (2)CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing, 210008, China, (3)Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803

In the marine magnesium cycles, riverine input serves as the major source, while carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Thus, the Mg cycle would provide the key information about the interactions between continent system, ocean system, and Earth’s interior in Earth’s history. Reconstruction of marine Mg cycle can be approached with seawater Mg isotopic composition (d26Mgsw), because hydrothermal reactions involve with negligible or slightly negative fractionation (0.7‰) in d26Mg, while carbonate precipitation preferentially scavenge 24Mg (with isotopic fractionation up to 5‰). Seawater chemistry might be recorded in marine carbonate. However, it is unclear with carbonate component can be used to reconstruct d26Mgsw. As the most common component of carbonate, micrite has been widely used to reconstruct marine biogeochemical cycles (e.g. C and S cycles). However, micrite derives from multiple sources (micritization of carbonate, direct precipitation from seawater/porewater, and fragmentation of calcified algae), and carbonate precipitation (biogenic or abiotic) displays a wide range of fractionation in Mg isotopes, it is reasonable to speculate that marine micrite may not archive d26Mgsw. To test this hypothesis, we measured d26Mg of micrite (d26Mgmicrite) recognized in late Paleozoic limestone from South China. Based on the high resolution conodont biostratigraphic framework, d26Mgmicrite shows significant spatial (i.e. between different sections) and temporal (i.e. in stratigraphy) heterogeneities. The modeling results indicate that such d26Mgmicrite variations cannot be explained by the fluctuation of d26Mgsw values. Thus, our study indicates that micrite cannot be used to reconstruct d26Mgsw.


Keyword: micrite, seawater Mg isotopic composition