Multiple sulfur isotope constraints on the modern sulfur cycle

We present 28 multiple sulfur isotope measurements of seawater sulfate ( δ 34 S SO 4 and Δ 33 S SO 4 ) from the modern ocean over a range of water depths and sites along the eastern margin of the Pacific Ocean. The average measured δ 34 S SO 4 is 21.24‰ ( ± 0.88 ‰ , 2 σ ) with a calculated Δ 33 S SO 4 of + 0.050 ‰ ( ± 0.014 ‰ , 2 σ ). With these values, we use a box-model to place constraints on the gross fraction of pyrite burial in modern sediments. This model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of δ 34 S pyrite , which is poorly constrained, but instead uses new information about the relationship between δ 34 S and δ 33 S in global marine sulfate. Our calculations indicate that the pyrite burial flux from the modern ocean is between 10% and 45% of the total sulfur lost from the oceans, with a more probable range between 20% and 35%.