Carbon isotope ratios of Phanerozoic marine cements: Re-evaluating the global carbon and sulfur systems

Abstract

Original δ 13C values of abiotically precipitated marine cements from a variety of stratigraphic intervals have been used to document secular variations in the δ 13C values of Phanerozoic oceans. These, together with the ° 34S values of coeval marine sulfates, are used to examine the global cycling of carbon and sulfur. It is generally accepted that secular variation in δ 13C and δ 34S values of marine carbonates and sulfates is controlled by balanced oxidation-reduction reactions and that their long-term, steady-state variation can be predicted from the present-day isotopic fractionation ratio (Δ c/Δ s) the ratio of the riverine flux of sulfur and carbon ( F s/ F c). The predicted slope of the linear relation between δ 13C carb and δ 34S sulfate values is approximately −0.10 to −0.14. However, temporal variation observed in marine cement δ 13C values and the 6345 values of coeval marine sulfates produces a highly significant linear relation ( r 2 = 0.80; α > 95%) with a slope of −0.24; approximately twice the predicted value. This discordance suggests that either the Phanerozoic average riverine F s/ F c was 1.6–3.3 times greater than today's estimates or that an additional source of 34S-depleted sulfur or 13C-enriched carbon, other than continental reservoirs, was active during the Phanerozoic. This new relation between marine δ 13C and δ 34S values suggests that the flux of reduced sulfur, iron, and manganese from seafloor hydrothermal systems affects oceanic O 2 levels which, in turn, control the oxidation or burial of organic matter, and thus the δ 13C value of marine DIC. Therefore, the sulfur system (driven by seafloor hydrothermal systems) controls the carbon system rather than organic carbon burial controlling the response of δ 34S values (via formation of sedimentary pyrite). Secular variation of marine 87 Sr 86 Sr ratios and δ 13C values argues for a coupling of δ 34S and δ 34S values to variation in the relative contribution of seafloor hydrothermal and continental weathering fluxes. These trends indicate that the early Paleozoic was dominated by low temperature silicate weathering, whereas the Late Paleozoic to Modern was dominated by high temperature seawater-basalt interactions. Variation in Proterozoic δ 13C carb and δ 34S sulfate values produces a slope that is greater than that of the Phanerozoic ( −0.50 vs. −0.24). This steeper slope is consistent with other geochernical data that indicate relatively high seafloor hydrothermal fluxes during the late Precambrian. We speculate that the dramatic evolutionary changes of the Neoproterozoic-Paleozoic transition occur during a waning of seafloor hydrothermal fluxes and a concomitant decrease in O 2 consumption that permitted the oxygenation of seawater thought to be critical in metazoan evolution.