Resolving sources of marine dissolved organic matter (DOM) via novel sulfur isotope analyses

Abstract:

Dissolved organic matter (DOM) is a critical reservoir of carbon in the ocean, impacting marine food webs, nutrient cycling, and global climate on annual to geological time scales. Sulfur was recently discovered to be a significant component of this pool, elevating DOM to the largest reservoir of organic sulfur in the oceans by several orders of magnitude. This pool of dissolved organic sulfur (DOS) is at least as dynamic as DOM, and increasingly recognized as central to the ocean’s sulfur cycle. The biogeochemical significance of DOS is also substantial, through climate feedbacks, trace metal speciation, and microbial ecology. Tracing DOS may provide insights to these important processes as well as new information on the sources and sinks of DOM. A first order question regarding DOS is the source of organic sulfur, with competing theories of biotic and abiotic reactions. If DOS has a biological origin, through labile metabolites like peptides, its sulfur isotopic composition should resemble biomass and by extension, marine sulfate, with enriched (δ34S ~21‰) values. Instead, if DOS forms from abiotic sulfurization reactions, either in anoxic basins or sediments, bulk isotope values should reflect depleted pore-water sulfide (δ34S <-10‰). Through isotope mass balance, we can use measured δ34S of DOS to calculate the relative contribution from each source. A necessary first step towards realizing these measurements is method development: adapting existing DOM and δ34S procedures for DOS. We will present progress using a standard developed in collaboration with Scripps Institution of Oceanography, with precise δ34S (0.3‰) measurements on samples as low as 1 µgS (~200 µg DOM). Using a novel EA-IRMS with a ramped GC column and programmable flow rates, we can simultaneously determine δ34S, δ13C, µg C, µg S, and C/S ratios of a given sample within a 20 minute acquisition. Analytical and procedural blanks will also be discussed, as well as preliminary results applying the method to wide-ranging samples across ocean basins.