1. Phosphate-associated sulfate in Lingula shells represents a potential archive for seawater sulfate composition.
- Author
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Xiang, Kai, Ma, Haoran, Sun, Yuanlin, Yuan, Zhiwei, and Shen, Bing
- Subjects
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SULFUR cycle , *SEAWATER composition , *SURFACE of the earth , *SULFATES - Abstract
The marine sulfur cycle is highly sensitive to redox environments at the Earth's surface. Fluctuations in atmospheric p O 2 level or marine redox environments are commonly associated with changes in seawater sulfate concentration, sulfate sulfur (S) and oxygen (O) isotopes. Thus, the marine S cycle through the Earth's ancient past provides a first-order constraint on the redox evolution of the Earth's surface. Reconstructions of the marine S cycle have been approached through analyses of various sulfate-bearing geological materials, such as sedimentary barite, evaporitic sulfate (e.g., gypsum), carbonate associated sulfate (CAS), and phosphate-associated sulfate (PAS) in phosphatic ores. These geological materials have their own pros and cons. In this study, we focus on PAS in Lingula brachiopods, which possess phosphatic shells and have a long evolutionary history since Cambrian times. Biogenic phosphate typically has a higher sulfate concentration than carbonate and phosphate ores, and is less vulnerable to diagenetic alteration. Here, we analyzed modern Lingula shells from the South China Sea and fossil Lingula shells collected from the Late Devonian Xiejingsi Formation of South China. Living and fossil Lingula shells have similar P/S ratios (PO 4 3−/SO 4 2−) of ∼76, and S and O isotopes of PAS (δ34S PAS and δ18O PAS) in each group show limited variations. For living Lingula shells, the mean values of δ34S PAS and δ18O PAS are +15.0 ± 1.5‰ and + 9.8 ± 0.4‰, respectively, which are ∼6‰ lower and comparable to those of modern seawater sulfate compositions (δ34S SW and δ18O SW-SO4), probably due to vital effects. For fossil Lingula shells, δ18O PAS (14.1 ± 0.9‰) is identical to coeval evaporite. The range of δ34S PAS is encompassed by the values of both coeval evaporite and CAS, but the mean value of δ34S PAS (29.5 ± 0.9‰) is higher. Thus, fossil Lingula shells show a good potential for reconstructing δ34S SW and δ18O SW-SO4 values. • Sulfate from modern Lingula shells, with stable P/S ratio, δ34S, and δ18O, can reconstruct δ34S and δ18O of seawater sulfate. • Seawater sulfate δ34S reconstructed by fossil Lingula shells are higher than the δ34S of coeval evaporite and CAS. • Phosphate associated sulfate from Lingula shells might be a more reliable achieve of seawater sulfate than evaporite and CAS. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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