Placing an upper limit on cryptic marine sulphur cycling

Area of Science:

• Marine Biogeochemistry
• Oceanography
• Geochemistry

Background:

• Fixed nitrogen is crucial for marine ecosystems, but its global ocean inventory is not fully understood.
• Oxygen-minimum zones (OMZs) significantly impact oceanic fixed nitrogen levels.
• Geochemical nitrogen budgets in OMZs show apparent imbalances, prompting research into nitrogen loss mechanisms.

Purpose of the Study:

• To quantify the role of sulfur cycling in fixed nitrogen loss within OMZs.
• To provide new constraints on sulfate reduction in OMZs using isotopic analysis.
• To assess the potential contribution of cryptic sulfur cycling to oceanic nitrogen budgets.

Main Methods:

• Analysis of oxygen and sulfur isotope ratios ((18)O/(16)O, (33)S/(32)S, (34)S/(32)S) in seawater sulfate.
• Investigating isotopic profiles across oxygenated and OMZ-bearing water columns.
• Integrating isotopic data with sulfate isotope dynamics models and OMZ water-mass residence times.

Main Results:

• New constraints on sulfate reduction in OMZs were established using multi-isotope seawater sulfate analysis.
• Previous estimates for sulfur-driven remineralization and nitrogen loss in OMZs were found to be at the upper limit of possibility.
• In situ sulfate isotope data suggest that cryptic sulfur cycling's contribution to fixed nitrogen loss is constrained.

Conclusions:

• The study provides critical data for understanding nitrogen cycling in OMZs.
• Sulfur-driven remineralization and fixed nitrogen loss in OMZs may be less significant than previously estimated.
• Accurate quantification of sulfur recycling is essential for closing marine fixed-nitrogen budgets.