CARBONATE-ASSOCIATED SULFATE IN MESOPROTEROZOIC SUCCESSIONS AND MODERN CARBONATE SYSTEMS: EXAMINING RAPID SHIFTS IN PALEOENVIRONMENTAL CONDITIONS

Throughout the Phanerozoic, the balance between burial and oxidation of reduced forms of sulfur and, by inference, the corresponding effects on atmospheric oxygen are expressed as broad temporal shifts in the sulfur isotope composition of seawater sulfate. Gypsum provides a means of quantifying quantifying these relationships, but when gypsum is unavailable, trace amounts of carbonate-associated sulfate (CAS) can be used as a proxy for seawater sulfate in both modern and ancient carbonate systems.

Stratigraphically controlled CAS isotope measurements from carbonates of several Mesoproterozoic successions (Belt Supergroup, Montana; the Bylot Supergroup, arctic Canada; the Mescal Limestone, Arizona; and the Paradise Creek Formation, Australia) each vary by up to 10-12 permil across only a few hundred meters of section. These comparatively short-term shifts all show similar scales of isotopic variation, suggesting that sulfate concentrations in Mesoproterozoic ocean were dynamic and limited relative to Phanerozoic seawater and that these may be global signals.

An ongoing study of the isotope composition and concentration of sulfate-S in modern calcareous organisms from the Great Barrier Reef, Australia, and lime muds from Florida Bay validates the CAS method as a proxy for seawater sulfate. Calibration of this method in a wide range of modern carbonate environments is crucial in understanding the mechanisms and timing of sulfate incorporation and the potential for diagenetic reservoir effects. Despite evidence for dissolution and precipitation reactions within the first meter of sediment, CAS isotopic values in bulk lime mud may be dominated by an unevolved seawater signal. Consequently, downcore CAS isotope records from modern carbonate platforms can be used to refine bulk analytical approaches to ancient fine-grained carbonate sequences.