[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-P17] Stepwise dissolution of carbonate and refined analytical method for estimating phosphate concentration in Precambrian ocean

*Chiyori Nakajo¹, Kaiyu Wu¹, Yuichiro Ueno^1,2, Yusuke Sawaki³ (1.Department of Earth and Planetary Sciences, Institute of Science Tokyo, 2.Earth-Life Science Institute, Institute of Science Tokyo, 3.The University of Tokyo)

Keywords: marine phosphate, carbonate, Proterozoic, stepwise dissolution

Phosphorus is a primary limiting nutrient in the modern ocean and is essential for biological production. Therefore, marine phosphate concentration could be an important factor for understanding atmospheric oxygenation events at the early and the end of the Proterozoic eon. Previous studies have focused on P content in BIF to reconstruct marine phosphate concentrations (Canfield. 1998). However, phosphate partitioning into BIF is strongly influenced by cation concentrations and pH in the ocean at that time (Konhauser et al.,2007, jones et al.,2015). Alternatively, carbonate-associated phosphate (CAP) may provide independent estimates (Shimura et al.,2014, Ingalls et al., 2020), though the marine phosphate concentrations derived from BIF and CAP analyses show significant discrepancies, with variations sometimes reaching three order of magnitude. As a result, the estimation of marine phosphate concentrations in the Proterozoic is still a difficult problem. Here, we report a new stepwise dissolution method to extract CAP from carbonate minerals to obtain reliable and accurate CAP content in carbonate minerals. In this method, carbonate samples were partially dissolved using acetic acid at room temperature or 60°C (for samples containing dolomite), with the acid volume adjusted to dissolve 10% of the sample, and repeated the 10% dissolution 10 times to digest completely (Zhang et al., 2015).We tested the stepwise dissolution method to seven samples including (1) dolomite standard (JDo-1), (2) calcite standard (SRM-1d), (3) a 1:1 mixture of the (1) and (2). The elemental concentration was measured by ICP-MS for each step of the leachate. The results show that calcite was dissolved first and dolomite later after complete calcite dissolution. Based on the REE pattern, non-carbonate minerals dissolved up to the first 3 steps but not later. Often, extracted P content increases sharply at the last step, probably due to small apatite impurity dissolved out after complete digestion of carbonate. Therefore, analysis of all steps of dissolution obtained by stepwise dissolution rather than partial dissolution methods would allows us to evaluate the extracted phosphate phases in the carbonate sample, and thus is useful to determine reliable and accurate CAP concentration in carbonate, separated from P from non-carbonate phases.