Phosphate-associated sulfate in Lingula shells represents a potential archive for seawater sulfate composition

The marine sulfur cycle is highly sensitive to redox environments at the Earth's surface. Fluctuations in atmospheric pO2 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 (PO43−/SO42−) of ~76, and S and O isotopes of PAS (δ34SPAS and δ18OPAS) in each group show limited variations. For living Lingula shells, the mean values of δ34SPAS and δ18OPAS are +15.0 ± 1.5‰ and + 9.8 ± 0.4‰, respectively, which are ~6‰ lower and comparable to those of modern seawater sulfate compositions (δ34SSW and δ18OSW-SO4), probably due to vital effects. For fossil Lingula shells, δ18OPAS (14.1 ± 0.9‰) is identical to coeval evaporite. The range of δ34SPAS is encompassed by the values of both coeval evaporite and CAS, but the mean value of δ34SPAS (29.5 ± 0.9‰) is higher. Thus, fossil Lingula shells show a good potential for reconstructing δ34SSW and δ18OSW-SO4 values.