Triple-oxygen and sulfur isotopic evidence for diagenetic overprinting of carbonate-associated sulfate in Neoproterozoic samples from a drill core

Abstract:

Carbonate-associated sulfate (CAS) is used in many studies to reconstruct the isotopic composition of ancient seawater sulfate and to infer stages in the development of Earth’s oxygenated atmosphere. CAS is acid extractable and commonly is referred to as structurally substituted sulfate in carbonate minerals. Several recent studies, however, have raised concerns about sulfate overprinting during early or late diagenesis, including contamination by modern secondary atmospheric sulfate (SAS) and by sulfide oxidation during laboratory processing. To test for overprinting and contamination, we studied the isotopic composition of sulfate in a bedded carbonate succession of the Neoproterozoic Lantian Formation, South China. Materials were obtained from a drilling core (635Ma- 551Ma). Water-leachable sulfate (WLS), acid-leachable sulfate (ALS, i. e. extracted CAS), and chromium-reducible sulfur (CRS) were sequentially extracted out and triple oxygen isotopic compositions of WLS and ALS were analyzed as well as sulfur isotope of WLS, ALS, and CRS. We also analyzed the oxygen isotope of sulfate resulting from pyrite oxidation at a condition similar to the extraction of WLS and ALS in the laboratory and the δ¹⁸O value is at ~ –1.4‰ (VSMOW). The slightly negative ∆¹⁷O values of all WLS and ALS indicates that the ALS was not contaminated by sulfate of modern SAS. The WLS from the first 24 hours with consistently negative values of δ¹⁸O (about –11.0‰) and low δ³⁴S values (about +5‰) suggests that the WLS resulted from sulfide oxidation in water with very negative δ¹⁸O values, likely glacial melt-water in the distant past, which had likely soaked the whole stratigraphy of Lantian Formation for a long time. The WLS also comprised a significant fraction of ALS because both δ¹⁸O and δ³⁴S of ALS have wide ranges, from –6.9 to +15.8‰, and +12.7 to +31.7‰, respectively. More importantly, there is a strong positive correlation between δ¹⁸O and δ³⁴S of ALS. Our findings demonstrate that extracted CAS can be overprinted or contaminated by secondary sulfate during later diagenetic process. In addition, combining the triple oxygen isotope with sulfur isotope of WLS and ALS is a critical tool for unraveling the nature of CAS.