Laboratory data assessing the environmental factors which control photochemical alkyl nitrate production in seawater collected from the Southern Ocean, 2003-2006
Seawater samples were collected from 15m at the Rothera time series (RaTS) site from February 2005 until March 2007. During summer 2005-2006 samples were analysed using a GC-MS (gas chromatograph mass selective detector) set up in the Bonner laboratory. At all other times samples were stored on solid sorbents at -20C and returned to UEA for analysis. All trace gas seawater samples were extracted by purging and the resulting samples were analysed for selected alkyl nitrates and organohalogens using a GC-MSD. The instrument was set-up in the Bonner Laboratory at Rothera between November 2005 and April 2006. Sampling was not possible between mid-October and late December 2005 as the sea-ice remaining in the bay was not safe enough to traverse but was too abundant to launch a boat.
During passage from Stanley (Falklands Islands) to South Georgia, Stanley to Rothera and within the Western Core Box, 14 depth profiles, 70 surface seawater samples and 60 air samples were analysed for trace gas concentrations. In the Western Core Box, 6 CTD casts (3 off-shelf and 3 on-shelf) were carried out. Seawater and air samples were analysed for selected alkyl nitrates and organohalogens using a gas chromatograph - mass selective detector (GC-MSD). Data collected on biological and physical processes occurring within these regions was also used to compare alkyl nitrate and organo-halogen dynamics in these two geographically-close but hydrodynamically different ocean regions.
During 2005-2006 season, water samples were collected from Ryder Bay at the Rothera Time Series (RaTS) site. A biogeochemical profile of water samples was conducted at 5 depths: 0m (surface), 5m, 10m, 15m and 25m In addition to this, primary productivity measurements using 14C-sodium bicarbonate in conjunction with water-cooled light gradient (photosynthetron) and also nutrient uptake measurements using 15N labelled stable isotopes, were performed on the water samples.
Conductivity-Temperature-Depth casts were conducted at Ryder Bay and Marguerite Bay from the James Clark Ross (cruise numbers: JR112/113, JR136/137, JR155 and JR174). The CTD casts at each site were made at a range of depths from the bottom to the surface waters with samples collected for macronutrients, particulate biogenic silica,carbon and nitrogen for later analysis.
Two moorings were deployed and recovered using the James Clark Ross (cruise numbers: JR112/113, JR155). One was deployed in Ryder Bay at the Rothera Oceanographic and Biological Time Series (RaTS) oceanographic station midway between Rothera Point and Leonie Islands. The other was deployed in deep water in northern Marguerite Bay. The moorings were used to monitor particle flux, currents, salinity and temperature. Two sediment traps on each mooring were used to measure biogenic flux from surface waters to the seafloor and quantify changes down the water column.
Water samples were collected from Ryder Bay at the Rothera Time-Series (RaTS) site, using a small boat. A biogeochemical profile was performed at the following 5 depths: 0m (surface), 5m, 10m, 15m and 25m, for four RaTS events on: 7 January, 13 January, 19 January and 1 February 2005. Samples were taken for: phytoplankton nitrate, ammonium and urea uptake measurements, using stable isotope incubations and also primary production PvE curves were produced using 14C radioisotope incubations; Dissolved macronutrients (nitrate, silicate, phosphate, and urea); Surface dissolved iron. A scaled-down sampling RaTS event was performed by the Rothera Marine Assistants involving sampling for all above measurements once every 3 weeks at the 5m depth only at the RaTS site.
Box core samples were analysed for carbon and nitrogen isotopes, barium, aluminium and uranium and other lithogenic dust. Core dating was calculated and population analyses were used to determine species composition. The box cores were collected on the James Clark Ross (cruise numbers: JR112/113, JR136/137 and JR155) in January 2005 and December 2006. Swath bathymetry was also taken to identify optimum coring sites.
Conductivity-Temperature-Depth (CTD) casts were conducted to describe the physical structure of the water column and to provide water samples for nutrient and particulate matter analyses from the James Clark Ross. A CTD transect was also conducted on a cruise across the mouth of Marguerite Bay (it was the intention to cover the entire mouth, but sea ice prevented this from happening), and through the trough leading towards the mooring sites. These casts were used to sample nutrients (NO3-, NO2-, Si(OH)4); nutrient isotopes 13DIC (dissolved inorganic carbon) 15NO3; bulk organic POC/PON (particulate organic carbon/nitrogen); bulk organic delta 13POC and delta 15PON.
Sediment traps were deployed and recovered and CTD/rosette casts were taken at two sites, one in Ryder Bay and one in Marguerite Bay. Biogeochemical proxy signals of nutrients and organic matter were assessed in sea ice compared to the open water column. The flux of this material in time series sediment traps located to two depths below the sampling site was also followed. Past changes in these proxy signals in Ryder Bay were also investigated from box core material taken from the study site.