Analytical Sciences

Leaching Methods for Ba Isotope Studies of Carbonates

Lili ZHANG,*1 Ruoyu SUN,*2 Yingzeng GONG,*3 Huimin YU,*1,*4 and Yi LIU*2

*1 CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
*2 Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai Rim, Institute of Surface-Earth System Science, Tianjin University, Tianjin 300350, China
*3 School of Earth Sciences, Yunnan University, Kunming, Yunnan 650500, China
*4 CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China, Hefei 230026, China

Barium (Ba) stable isotopes in carbonate rock have great potential to provide valuable information on environmental change and the biogeochemical cycles of oceans in the past. Ba in carbonate rock can exist in various phases, such as adsorbable and silicate-bound Ba. However, only the carbonate-bound phase is considered to record the Ba isotopic compositions of ambient seaweater. Here, we designed a two-step leaching experiment to obtain the carbonate-bound Ba in two typical carbonate rocks: limestone and cap dolostone. The results showed that after leaching by 1 mol L⁻¹ ammonium acetate, the carbonate-bound Ba extracted by mixed solution of 1.5 mol L⁻¹ acetic acid and 1 mol L⁻¹ ammonium acetate in each studied sample have indistinguishable isotope ratios in leaching time conditions between 12 and 72 h. More importantly, the carbonate-bound Ba isotope ratios were quite different from those of the residue (up to 10 times of analytic uncertainty, 2SD ≤ ±0.04‰) after leaching in three out of four leaching experiments, indicating that noncarbonated fraction could overprint a primary seawater signal. Our sequential leaching techniques could improve targeting of carbonate-bound Ba isotope signatures in various carbonate rocks to trace the Ba cycling in the oceans.

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