Sulfur isotope homogeneity of oceanic DMSP and DMS Alon Amrani a,1 , Ward Said-Ahmad a , Yeala Shaked a,b , and Ronald P. Kiene c,d a Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel; b Interuniversity Institute for Marine Sciences, Eilat 88103, Israel; c Department of Marine Sciences, University of South Alabama, Mobile, AL 36688; and d Dauphin Island Sea Lab, Dauphin Island, AL 36528 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved October 8, 2013 (received for review July 9, 2013) Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics. To constrain the contribution of oceanic DMS to aerosols we established the sulfur isotope ratios ( 34 S/ 32 S ratio, δ 34 S) of DMS and its precursor, dimethylsulfoniopro- pionate (DMSP), in a range of marine environments. In view of the low oceanic concentrations of DMS/P, we applied a unique method for the analysis of δ 34 S at the picomole level in individual com- pounds. Surface water DMSP collected from six different ocean provinces revealed a remarkable consistency in δ 34 S values ranging between +18.9 and +20.3‰. Sulfur isotope composition of DMS analyzed in freshly collected seawater was similar to δ 34 S of DMSP, showing that the in situ fractionation between these species is small (<+1‰). Based on volatilization experiments, emission of DMS to the atmosphere results in a relatively small fractionation (-0.5 ± 0.2‰) compared with the seawater DMS pool. Because δ 34 S values of oceanic DMS closely reflect that of DMSP, we con- clude that the homogenous δ 34 S of DMSP at the ocean surface re- presents the δ 34 S of DMS emitted to the atmosphere, within +1‰. The δ 34 S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmospheric sulfate, thereby enabling estimation of the DMS contribution to aerosols. oceanic sulfur cycle | phytoplankton | sulfate aerosols | compound specific | MC-ICPMS D imethylsulfoniopropionate (DMSP), a metabolite of marine phytoplankton, is one of the major organosulfur compounds produced in the oceans (1). One of its degradation products, dimethylsulfide (DMS), is volatile and supersaturated in all marine surface waters. Large amounts of DMS (0.55–1.1 Tmol S·y -1 ) are released from the ocean to the atmosphere (2), where it contributes to the formation and growth of aerosol particles (3). Over remote oceans, distant from anthropogenic sulfur inputs, DMS is the major source of nonsea-salt sulfate aerosol (4, 5). The oxidation of DMS to submicron sulfate aerosols was suggested to increase cloud-condensation nuclei and the albedo of clouds, leading to a potential climate feedback loop operating through the DMS-producing biota in the surface ocean (6). Although evidence for the direct, local climate feedback via DMS is limited (7), DMS likely contributes, together with organic and sea-spray aerosol sources, to the complex dynamics of climate-active aerosols in the lower atmosphere (3, 7). Sulfur isotope measurements ( 34 S/ 32 S ratio, δ 34 S) may offer a way to constrain the contribution of ocean-derived DMS to global sulfur cycling and aerosol budgets. Recently, Oduro et al. (8) used a multistep extraction of large seawater volumes (e.g., 50 L) coupled with Raney nickel dehydrosulfurization and subsequent fluorination for the S isotope analysis of DMSP in intertidal macroalgae (+17.3 to +19.3‰) and estuarine phyto- plankton blooms (+19 to +20‰). However, there are still no direct DMS and DMSP δ 34 S data in oligotrophic oceans owing to their typically low concentrations of 0.5–5 and 5–80 nM, re- spectively (9). We recently developed a sensitive method for δ 34 S analysis in individual compounds at the picomole level, which couples gas chromatography (GC) to multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) (10). Based on this method we further developed a simple and robust technique requiring only a few milliliters of seawater for DMS and DMSP isotopic analysis, using purge-and-trap of seawater and subsequent injection of DMS into a GC coupled to MC-ICPMS (11). Here we describe the δ 34 S values of DMSP obtained in five oligotrophic regions and one mesotrophic ocean region, encom- passing a wide range of hydrological, meteorological, and bi- ological conditions. Whereas DMSP can be reliably preserved for months (12, 13), DMS had to be analyzed within 24 h of sample collection to minimize storage artifacts. Taking advantage of our geographical proximity to the Red Sea and the Mediterranean, we analyzed the δ 34 S of freshly collected DMS together with DMSP to evaluate the in situ isotopic fractionation resulting from their multiple transformations. We then examined the S isotopic frac- tionation during DMS volatilization in the laboratory. Taken to- gether, our data reveal rather uniform δ 34 S values of surface ocean DMSP and small S isotopic fractionation during DMSP degra- dation and DMS volatilization. These data provide a constraint on the marine contribution of the δ 34 S value of DMS emitted to the atmosphere. Results and Discussion Homogeneity of DMSP Sulfur Isotopes in Seawater from Different Ocean Basins. To evaluate the range of DMSP δ 34 S values in seawater, we sampled six open ocean sites around the globe in- cluding Hawaii [St. HOT (Hawaii Ocean Time-series)], Bermuda (hydrostation S), East Mediterranean (Haifa), Eilat (northern Gulf of Aqaba, Red Sea), Gulf of Mexico (Mobile Bay–Alabama shelf), and the north Atlantic between Greenland and Iceland, hereafter called Greenland (Fig. 1A; see also SI Appendix, S1–S10 for site description and raw data). These sites were chosen to encompass variations in ocean basin (Atlantic, Pacific, and Indian), latitude (tropical, subtropical, and temperate), biogeochemical provinces (oligotrophic and mesotrophic), season, and phytoplankton Significance Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global at- mosphere, where it mediates aerosol dynamics and may affect climate. Sulfur isotope ratios ( 34 S/ 32 S) offer a way to estimate oceanic DMS contribution to aerosols. We used a unique method for the analysis of 34 S/ 32 S of DMS and its precursor, dimethylsulfoniopropionate (DMSP), in a range of marine envi- ronments. Surface water collected from six different ocean provinces revealed a remarkable consistency in 34 S/ 32 S ratios of DMS and DMSP ranging between +18.9 and +20.3‰. The 34 S/ 32 S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmo- spheric sulfate, thereby enabling estimation of the DMS contri- bution to aerosols. Author contributions: A.A., Y.S., and R.P.K. designed research; A.A., W.S.-A., Y.S., and R.P.K. performed research; A.A. contributed new reagents/analytic tools; A.A., W.S.-A., Y.S., and R.P.K. analyzed data; and A.A., W.S.-A., Y.S., and R.P.K. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. 1 To whom correspondence should be addressed. E-mail: alon.amrani@mail.huji.ac.il. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1312956110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1312956110 PNAS Early Edition | 1 of 6 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES