Parallel, high-resolution carbon and sulfur isotope records of the evolving Paleozoic marine sulfur reservoir

Abstract

Links between the biogeochemical cycles of carbon and sulfur are expressed in the evolving stable isotope composition of the ocean. Carbonate rocks record the inorganic carbon isotope composition of the oceanic reservoir through geological time, along with the sulfate sulfur isotope composition preserved as carbonate-associated sulfate (CAS). An inverse relationship exists between the first-order carbon and sulfur records of the Paleozoic; however, the isotope curves also reveal higher-frequency variations superimposed on the first-order (10 7–10 8 yr) trend. Because of links between carbon and sulfur cycling, high-resolution, CAS-based S isotope data have the potential to shed essential light on the mechanisms behind carbon isotope excursions observed in the geologic record. Results from the Late Cambrian SPICE event at Shingle Pass, Nevada, show parallel positive C (5‰) and S (25‰) isotope excursions, likely recording a large-scale marine organic carbon burial event with sympathetic pyrite burial. Carbon and sulfur records from other events throughout the Paleozoic reveal different and evolving relationships. A Silurian–Devonian boundary section at Strait Creek, West Virginia, reveals a positive C excursion of 6‰ but with an enigmatic, apparently negative S excursion (15‰) antithetic to the C shift. By contrast, a Kinderhookian–Osagean (Early Mississippian) section in the Confusion Range, Utah, shows invariant S isotope ratios across a C isotope excursion of 6‰. The coupling of the high-resolution C–S isotope records may have weakened in the mid-to-late Paleozoic with decreasing sensitivity of the seawater S isotope reservoir to flux changes as the marine sulfate concentrations increased through the Paleozoic. The progressively decreasing isotopic variability of CAS across three positive carbon excursions suggests that the Paleozoic was transitional between a Proterozoic ocean with rapid isotopic variability and thus low sulfate concentration and the comparatively more stable isotopic properties of a relatively more sulfate-rich Mesozoic and Cenozoic ocean. Furthermore, the emergence of the terrestrial ecosystems during the late middle and late Paleozoic yielded new loci of organic burial that also likely played a central role in the decoupling carbon and sulfur isotope records. Organic carbon burial in these sulfate-limited environments occurred in the absence of significant pyrite burial.