Photolysis and the dimethylsulfide (DMS) summer paradox in the Sargasso Sea

Abstract

Apparent quantum yields and rates of dimethylsulfide (DMS) photolysis were determined from Sargasso Sea seawater with the goal of assessing the extent to which photoreactions affect the unusually elevated upper ocean concentrations of DMS during the summer, the so-called DMS summer paradox. Apparent quantum yields determined with monochromatic radiation decrease exponentially with increasing wavelength and indicate that DMS photolysis is driven by ultraviolet (UV) radiation. The relative spectral partitioning differs between samples collected from the surface mixed layer (15 m) and from the chlorophyll a maximum (80 m), presumably because of differences in chromophoric dissolved organic matter (CDOM) quality (e.g., apparent degree of bleaching). Quantum yields are also temperature dependent, and an approximate doubling of photolysis rates occurs for a 20ЊC increase in temperature. The significance of DMS photolysis to upper ocean sulfur budgets is explored using a multiyear (1992-1994) DMS time series, concurrent irradiance determinations and temperature profiles, and estimates of CDOM absorption. Depth-integrated, mixed-layer DMS photolysis rates peak in the summer (15-25 mol m Ϫ2 d Ϫ1 ) and decline to Ͻ1 mol m Ϫ2 d Ϫ1 in the winter. These rates correspond to specific turnover rates of ϳ0.29 d Ϫ1 in the summer and Ͻ0.02 d Ϫ1 in the winter. Seasonal changes in solar radiation, temperature, and DMS concentrations drive the 30-fold differences in photolysis rates, overshadowing differences caused by photosensitizer (CDOM) quantity or quality (21-35%). These results demonstrate that although photolysis is not the primary driver of the summer paradox, it makes an important contribution to the time-depth pattern of DMS concentrations observed in the Sargasso Sea.The biogeochemical cycling of sulfur between the upper water column of the ocean and the atmospheric marine boundary layer has received a great deal of attention over the last several decades because of its implication in a cloud albedo feedback loop (e.g., Shaw 1983;Bates et al. 1987;Charlson et al. 1987). The biogenic production of dimethylsulfide (DMS) in the marine environment is an important source of atmospheric sulfur. Although DMS concentrations exhibit considerable spatial and temporal variability, DMS AcknowledgmentsThis work was supported by NASA under an Earth System Science Fellowship and the SIMBIOS Program. We are extremely grateful to John Dacey for the use of his DMS and DMSP time series datasets. The authors acknowledge George Westby and Todd Medovich (SUNY-ESF) for their assistance with laboratory photolysis studies; Rod Johnson, Paul Lethaby, Rachel Parsons, Chrissy Van Hilst, Karen Paterson, and the many BATS technicians for their assistance at sea; and Andrew Hall, Toby Westberry and two anonymous reviewers for their assistance with the development of this manuscript.

Revising upper-ocean sulfur dynamics near Bermuda: new lessons from 3 years of concentration and rate measurements

“…Downward irradiance was converted to scalar using a constant factor of 1.2 [30] and an average daily surface reflectance of 4 % was assumed. [31] When the integrated daily surface irradiance was not available for the sample day, the average daily surface irradiance for the cruise was used.…”

“…Although UV-A (320-400 nm) radiation is the primary contributor to DMS photolysis, UV-B (280-320 nm) contributes 20-30 % to total photolysis with peak photolysis rates occurring at 320 nm. [30] As the SPMR and SMSR sensors do not capture UV-B wavelengths, DMS photolysis rates with depth were determined using the integrated daily UV-A dose (324-412 nm) calculated from surface irradiance data and the 324 nm attenuation depth profile (K d (l 324 )). We acknowledge that not accounting for UV-B wavelengths will result in an underestimate of photolysis rates and a slightly deeper profile.…”

Photochemical oxidation of dimethylsulphide to dimethylsulphoxide in estuarine and coastal waters

“…Scaling to underwater irradiance appears justified given that water column DMS photo-oxidation was previously found to be dominated by UV-A wavelengths (Toole et al, 2003;Deal et al, 2005;Taalba et al, 2013). Estimating K d,350 from CDOM absorbance introduces some additional uncertainty, although previous work by Farmer et al (1993) in the Orinoco River plume found generally good agreement between K d,350 predicted from equation (6) and direct observations.…”

“…Previous studies reported a variable contribution from DMS photo-oxidation to overall DMS removal of around 6e70% (Kieber et al, 1996;Archer et al, 2002;Toole et al, 2003Toole et al, , 2004Bouillon et al, 2006;Simo, 2010, 2015), and indicated that photooxidation can typically dominate over losses through biological consumption and air-sea gas exchange during periods of strong stratification with shallow mixed layer depths (Toole et al, 2006;Galí and Simo, 2010). However, a recent meta-analysis of DMS cycling rates suggest that the combined contributions of photochemical and air-sea gas exchange losses generally fall below 20% (Galí and Simo, 2015).…”

“…The most rigorous calculations of photochemical DMS turnover in the water column are based on apparent quantum yields and wavelength resolved underwater irradiance (Toole et al, 2003;Bouillon et al, 2006;Taalba et al, 2013). Because we did not obtain wavelength resolved DMS photo-oxidation rates or underwater irradiance data, we constrained the effects of underwater light attenuation by scaling to mid-UV underwater irradiance at 350 nm.…”