Multiple sulfur isotope constraints on the modern sulfur cycle

- High precision measurements of multiple sulfur isotopes from modern marine sulfate.
- Exploration of the riverine multiple sulfur isotope flux into the ocean.
- Model of the composition and flux of pyrite burial in modern marine sediments.
- Constrain the gross proportional pyrite burial flux to between 10% and 45%.

We present 28 multiple sulfur isotope measurements of seawater sulfate (δ34SSO4 and Δ33SSO4) from the modern ocean over a range of water depths and sites along the eastern margin of the Pacific Ocean. The average measured δ34SSO4 is 21.24‰ (±0.88‰,2σ) with a calculated Δ33SSO4 of +0.050‰ (±0.014‰,2σ). 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 δ34Spyrite, which is poorly constrained, but instead uses new information about the relationship between δ34S and δ33S 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%.