Revisiting Ediacaran Sulfur Isotope Chemostratigraphy With In Situ Nanosims Analysis Of Sedimentary Pyrite

Reconstructions of ancient sulfur cycling and redox conditions commonly rely on sulfur isotope measurements of sedimentary rocks and minerals. Ediacaran strata (635-541 Ma) record a large range of values in bulk sulfur isotope difference (Delta 34S) between carbonateassociated sulfate (delta 34SCAS) and sedimentary pyrite (delta 34Spy), which has been interpreted as evidence of marine sulfate reservoir size change in space and time. However, bulk delta 34Spy measurements could be misleading because pyrite forms under syngenetic, diagenetic, and metamorphic conditions, which differentially affect its isotope signature. Fortunately, these processes also impart recognizable changes in pyrite morphology. To tease apart the complexity of Ediacaran bulk delta 34Spy measurements, we used scanning electron microscopy and nanoscale secondary ion mass spectrometry to probe the morphology and geochemistry of sedimentary pyrite in an Ediacaran drill core of the South China block. Pyrite occurs as both framboidal and euhedral to subhedral crystals, which show largely distinct negative and positive delta 34Spy values, respectively. Bulk delta 34Spy measurements, therefore, reflect mixed signals derived from a combination of syndepositional and diagenetic processes. Whereas euhedral to subhedral crystals originated during diagenesis, the framboids likely formed in a euxinic seawater column or in shallow marine sediment. Although none of the forms of pyrite precisely record seawater chemistry, in situ framboid measurements may provide a more faithful record of the maximum isotope fractionation from seawater sulfate. Based on data from in situ measurements, the early Ediacaran ocean likely contained a larger seawater sulfate reservoir than suggested by bulk analyses.