Placing an upper limit on cryptic marine sulphur cycling

Abstract

Aquantitative understanding of sources and sinks of fixed nitrogen in low-oxygenwaters is required to explain the role of oxygen-minimum zones (OMZs) incontrolling the fixed nitrogen inventory of the global ocean. Apparent imbalances in geochemical nitrogen budgets1 have spurred numerous studies to measure the contributions of heterotrophic and autotrophic N2-producing metabolisms(denitrification and anaerobicammonia oxidation, respectively)2,3.Recently, ‘cryptic’ sulphur cyclingwas proposed as a partial solution to the fundamental biogeochemical problem of closing marine fixed-nitrogen budgets in intensely oxygen-deficient regions4. The degree to which the cryptic sulphur cycle can fuel a loss of fixed nitrogen in themodern ocean requires the quantification of sulphur recycling inOMZsettings. Here we provide a new constraint for OMZ sulphate reduction based on isotopic profiles of oxygen (18O/16O) and sulphur (33S/32S, 34S/32S) in seawater sulphate through oxygenatedopen-oceanandOMZ-bearing water columns. When coupled with observations and models of sulphate isotope dynamics and data-constrainedmodel estimates ofOMZ water-mass residence time,wefindthat previous estimates for sulphurdriven remineralization and loss of fixed nitrogen from the oceans are near the upper limit for what is possible given in situ sulphate isotope data.