δ34S and Δ33S records of Paleozoic seawater sulfate based on the analysis of carbonate associated sulfate

Abstract

This study updates the δ34S and Δ33S temporal records of Paleozoic and early Mesozoic seawater sulfate using analysis of carbonate associated sulfate of biogenic and whole rock carbonate. The time resolution of carbonate samples studied here is on the order of millions of years which is longer than the timescale for homogenization of seawater sulfate–sulfur (one tenth of a million years), but is comparable to the timescale for isotope evolution of sulfate in the oceans. This δ34S record confirms the long-term decreasing trend that is discernible over the 250 million year timescale of sampling, and the Δ33S record of seawater sulfate for the Paleozoic is consistent with an average value of −0.002±0.004‰ (2σ, 540–251 Ma) that is distinct from the positive Δ33S observed today and inferred for the rest of the Cenozoic. Both δ34S and Δ33S records of seawater sulfate suggest the presence of shorter-timescale variations that occur on timescales of tens of millions of years, arguably driven by changes in intensity of sulfide oxidation in cycling of sulfur and/or by rapid changes in sulfur influx to the oceans and its associated sulfur isotopes. The Permian–Triassic boundary marks a transition in co-trajectories of δ34S and Δ33S from in-phase to anti-phase. The biogeochemical forcing that causes this is unclear.This newly calibrated record remains consistent with an earlier assertion (Wu et al., 2010) that the sulfur isotope fractionation (Δ34SSW-PY) between oceanic sulfate (δ34SSW) and coeval sedimentary pyrite (δ34SPY) was smaller during the Paleozoic than in the Cenozoic, and reached lowest values during the Carboniferous. The δ34SIN and Δ33SIN estimates of influx sulfur to the oceans afforded by the isotope mass balance model for each geologic period throughout the Paleozoic reveal a change from higher positive values of δ34SIN (more negative values of Δ33SIN, approximately −0.030‰) to lower positive values of δ34SIN (slightly negative values of Δ33SIN, approximately −0.003‰). This change is interpreted to reflect the evolution of the Earth surface sulfur pool and the rapid recycling of newly formed evaporite sulfate minerals, as arguably supported by our estimates of integrated erosion rates over geologic time for pyrite and evaporite ((6.1–6.4)×10−9 a−1, and (7.7–7.8)×10−9 a−1, respectively).