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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2009.04.001 Byline: K.O. Buesseler (a), S. Pike (a), K. Maiti (a), C.H. Lamborg (a), D.A. Siegel (b), T.W. Trull (c) Keywords: Thorium-234; Particle flux; Sediment trap; Scavenging; North Pacific Abstract: An extensive.sup.234Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport In the Global Ocean) study. Total.sup.234Th:.sup.238U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while.sup.234Th:.sup.238U [approximately equal to]0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites -- even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of.sup.234Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in.sup.234Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of.sup.234Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30m) of excess.sup.234Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for.sup.234Th and/or with sediment traps. Also, the application of.sup.234Th as POC flux tracer relies on accurate sampling of particulate POC/.sup.234Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50[mu]m at ALOHA and >5-350[mu]m at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of.sup.234Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these.sup.234Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10's-100'skm at a resolution that is difficult to obtain with other methods. Author Affiliation: (a) Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1541, USA (b) Institute for Computational Earth System Science and Department of Geography, University of California, Santa Barbara, CA 93106, USA (c) Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania -- CSIRO Marine and Atmospheric Research, Hobart 7001, Australia Article History: Received 19 December 2008; Revised 27 March 2009; Accepted 6 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2005.09.003 Byline: D.A. Toole (a), D. Slezak (b)(c), R.P. Kiene (b)(c), D.J. Kieber (d), D.A. Siegel (a) Keywords: Sulfur compounds; Dimethylsulfide; Ultraviolet radiation; Photochemistry; Rate constants; Light attenuation; North Atlantic; Sargasso Sea; Gulf of Maine Abstract: The influence of solar radiation on springtime rates of photochemical and biological consumption of dimethylsulfide (DMS) in surface waters from the western Atlantic Ocean was examined by exposing 0.2[mu]m filtered and unfiltered surface seawater to natural sunlight at five depths in the upper 30m. Parallel deck incubations of 0.2[mu]m filtered seawater under various long-pass optical filters were also carried out to aid in assessing the wavelength dependence of DMS photolysis. DMS photolysis rate constants for mid-day exposure ([approximately equal to]10:30-17:30 local time) to surface irradiance ranged from 0.026 to 0.086h.sup.-1 and were highest in coastal and shelf waters. Photolysis rate constants decreased with increasing irradiation depth, in accordance with the attenuation of ultraviolet radiation (UVR, 280-400nm). Total DMS consumption rates (photochemical+biological) in unfiltered surface samples also decreased with increasing incubation depth and were larger than photolysis rates at nearly all depths and all stations. The decrease in photolysis rate constants with exposure depth was mirrored by biological DMS consumption rate constants that were severely inhibited at surface irradiances, and approached or exceeded dark rate constants at deeper exposure depths. Photolysis rates were 2-19 times greater than estimated biological consumption rates in the surface light exposed samples, while biological consumption rates were significantly larger than photolysis rates at incubation depths below the 1% light level for UV-B radiation (280-320nm). Total DMS loss rates increased up to nine-fold with UVR exposure, but changes in DMS concentrations were not strongly correlated to light dose, presumably due to parallel, light-mediated DMS production processes. The primary loss process for DMS depended mainly on the depth interval considered and the attenuation of UVR; in general, photochemical removal dominated shallow layers characterized by high UV-B intensities, whereas biological removal dominated in deeper layers where UV-B was absent, but UV-A (320-400nm) and visible (400-700nm) light fluxes were still relatively high. These results demonstrate that UVR exposure significantly influences the spatial and temporal pattern of DMS production and loss processes, and ultimately the DMS flux to the atmosphere. Author Affiliation: (a) Institute for Computational Earth System Science, University of California, Santa Barbara, CA 93106, USA (b) Department of Marine Sciences, University of South Alabama, Mobile, AL 36688, USA (c) Dauphin Island Sea Lab, Dauphin Island, AL 36528, USA (d) Chemistry Department, College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210, USA Article History: Received 19 November 2004; Revised 29 July 2005; Accepted 2 September 2005

Filtered and unfiltered surface seawater is exposed to natural sunlight at five depths in the upper 30m of the western Atlantic Ocean to examine the influence of solar radiation on springtime rates of photochemical and biological consumption of dimethylsulfide (DMS). Wavelength dependence of DMS photolysis are assessed and results demonstrate that ultraviolet radiation exposure significantly influences the spatial and temporal pattern of DMS production and loss processes.

The nature of colored dissolved material (CDM) light attenuation was characterized and its seasonal cycles in relation to hydrographic and biogeochemical processes in the Sargasso Sea were defined. Findings showed that CDM absorption made up an average of more than one-half of the total non-water absorption coefficient, while detrital particulate absorption was generally a minor constituent. The vertically integrated stock of CDM increased from the beginning of spring until the end of summer in 1994 and 1995. Near-surface values were depressed near the surface during summer.

The use of sediment traps over the past 20 years has provided oceanographers with information about the transport of metabolic energy and matter to the deep ocean. However, the interpretation of results is complicated by spatial variability in overlying waters. A new study uses Lagrangian analysis to investigate the characteristics of statistical funnels above a moored sediment trap array in the Sargasso Sea. A model of the trajectories that individual particles take before collection is also presented.

The article presents a recomputation of sinking particle trajectories in the Sargasso Sea. Sea surface particle origins were expressed as functions of sinking speed and trap depth. Adjustments made to original findings due to this recomputation are also discussed.