[B-CG07] Decoding the history of Earth: From Hadean to the present

[BCG07-03] In-situ analyses of Carbonate-associated phosphates: Implications for secular changes in phosphate contents of seawater through Earth history.

*Ryosuke Nagao¹, Yuki ISHIHARA¹, Satoshi Yoshida², Sota Niki³, Yuta Ijichi³, Yusuke Sawaki², NATSUKO ADACHI⁶, Takeshi Ohno⁵, Yuichiro Ueno⁴, Takafumi Hirata³, Tsuyoshi Komiya^1,2 (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo , 2.Graduate School of Arts and Sciences, The University of Tokyo, 3.Geochemical Research Center, The University of Tokyo, 4.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 5.Department of Chemistry, Faculty of Science, Gakushuin University, 6.Department of Geosciences, Osaka Metropolitan University)

Keywords:Precambrian, Stromatolite, Phosphorus, CAP (carbonate-associated phosphate), Seawater composition

Phosphorus is one of the most crucial nutrients to control primary productivity, and therefore its bioavailability significantly influences the redox state of the Earth’s biosphere through oxygenic photosynthesis. Although it is considered that oxygenic photosynthesis commenced at least before the “Great Oxygenation Event (GOE)”, it has also been widely accepted that the atmospheric oxygen level remained persistently low even after the GOE. This led to the idea that the primary productivity was severely limited by the low dissolved inorganic phosphate (DIP) levels through the Proterozoic (Derry 2015). Several attempts were conducted to estimate the DIP contents in the Precambrian ocean based on P contents in iron formations (IFs) and shales (Planavsky et al., 2010; Reinhard et al., 2017). However, the estimates are still under debate (e.g. Jones et al., 2015). Shimura et al. (2014) first proposed that P contents of carbonate minerals in ancient carbonate rocks can be used as a paleo-DIP proxy, and showed that the DIP level was quite high after the Marinoan glaciation. Recent studies also validated that the abundance of carbonate-associated phosphate (CAP), which is defined by phosphorus incorporated into the crystal lattice of carbonate minerals, is a useful proxy (Dodd et al., 2021). Ingalls et al. (2022) conducted whole-rock analyses by acid digestion for CAP in the Neoarchean carbonates and showed that the DIP level in the Neoarchean was higher than the present level. However, their data are highly scattered possibly due to contamination of either clastic materials or recrystallization, making the estimates still ambiguous. Therefore, we conducted in-situ analyses of phosphorus and other trace element contents of carbonate minerals using femtoseconds LA-ICP-MS equipped with a collision/reaction cell at the Geochemical Research Center, The University of Tokyo, Japan. Oxygen gas was used as a collision gas to reduce various polyatomic interferences. A sampling area during laser ablation was 70 µm × 70 µm for one data acquisition.

We analyzed carbonate rock samples with stromatolite or oolite textures from 2.7 Ga Tumbiana Fm., Fortescue Group, 2.4 Ga Kazput Fm., Turee Creek Group, 1.8 Ga Duck Creek Fm., Wyloo Group, 1.8 Ga Hearne Fm., Pethei Group, 1.4 Ga Tieling Fm., Jixian Group, and 1.0 Ga Khoraidi Fm., Kali Gandaki Group as well as some Phanerozoic carbonate rocks. We carefully selected analytical spots through microscopic and SEM-EDS observations to avoid tiny phosphate minerals and altered areas. The analytical spots range from 0.007 to 8 mmol/mol in P/(Ca+Mg) ratios. Several spikes were observed in signal intensity profiles for La and P. Their spikes could be attributed to the presence of phosphate minerals because they usually contain more La than carbonate minerals (Shimura et al., 2014).

Because their spikes can be interpreted as a sign of their contamination, data without their obvious spikes in the signal intensity profiles were used to estimate the CAP contents. The CAP/(Ca+Mg) ratios of the Precambrian carbonate minerals range from 0.02 to 0.15 mmol/mol, which is equivalent to or higher than those of modern shallow marine ooids of 0.01 to 0.08 mmol/mol (Dodd et al., 2021; Ingalls et al., 2022). The relatively high CAP contents in the Precambrian shallow marine carbonates suggest that the DIP in the Precambrian seawater was not severely limited, inconsistent with the “Precambrian phosphorus crisis” hypothesis.

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