ALBERT

Description: In this study we present dissolved and particulate silicon isotope results in surface waters obtained as part of the GEOTRACES central Arctic Ocean section GN04 (2015) onboard POLARSTERN cruise PS94 (ARK-XXIX/3). The aim of the study was to investigate in more detail the influence of the Transpolar Drift on the Si cycle in the AO comparing areas with and without its influence.

Description: We report a comprehensive dataset of dissolved iron (Fe) comprising 482 values at 22 complete vertical profiles along a 1° latitudinal section at the Zero meridian. In addition a shorter high resolution (~00°09') surface section of the southernmost part of the transect (66°00' - 69°35' S) is presented. Within the upper surface mixed layer the concentrations of dissolved Fe vary between 0.1 and 0.3 nM. An inverse trend versus fluorescence suggests significant Fe removal by plankton blooms. Vertical mixing and upwelling are the most important supply mechanisms of iron from deep waters to the upper surface mixed layer. At lower latitude (42°S) there is a distinct maximum of 0.6-0.7 nM in the 2000-3000 m depth range due to inflow of North Atlantic Deep Water. In one region (55°S) elevated dissolved Fe found in the surface mixed layer is ascribed to the recent deposition of aeolian dust originating from South America. Close to the Antarctic continent there is an indication of Fe supply in surface waters from icebergs. In the deep waters there is a strong indication of a hydrothermal plume of dissolved Fe and Mn over the ridge in the Bouvet region (52-56°S). In the Weddell Gyre basin the dissolved Fe in the deep water is 0.47±0.16 nM in the eastward flow at ~56-62°S and is lower with a value of 0.34±0.14 nM in the westward flow at high ~62-69°S latitude. At the edge of the continental ice-sheet on the prime meridian, the continental margin of the Antarctic continent appears to be lesser source of dissolved Fe than in any other place in the world; this is likely because it is unique in being overlain by the extending continental ice-sheet that largely prevents biogeochemical cycling.

Description: Water samples were collected with 6.4 L Kemmerer bottles through a hole in the sea ice in several meters distance from the ship. Dimethylsulfide (DMS) was analyzed on a gas chromatograph (Varian 450C) which was coupled to a purge and trap system slightly modified from the method described in Kiene and Slezak (2006). Dimethylsulfoniopropionate (DMSP) was analyzed as DMS after alkali cleavage (Dacey and Blough, 1987) with the same analytical procedure as described for DMS. The nutrients phosphate (after Murphy & Riley, 1962), silicate (after Strickland & Parsons, 1968), nitrite and nitrate (after Grasshoff et al., 1983) were analyzed in an air-conditioned lab container with a continuous flow auto analyzer (Technicon TRAACS 800). In-between runs reproducibility better than 1.0 % , but typically 0.8% of its average value.

Description: The first comprehensive dataset (492 samples) of dissolved Mn in the Southern Ocean shows extremely low values of 0.04 up to 0.64 nM in the surface waters and a subsurface maximum with an average concentration of 0.31 nM (n=20; S.D.=0.08 nM). The low Mn in surface waters correlates well with the nutrients PO4 and NO3 and moderately well with Si(OH)4 and fluorescence. Furthermore, elevated concentrations of Mn in the surface layer coincide with elevated Fe and light transmission and decreased export (234Th/238U deficiency) and fluorescence. It appears that Mn is a factor of importance in partly explaining the HNLC conditions in the Southern Ocean, in conjunction with significant controls by the combination of Fe limitation and light limitation. No input of Mn from the continental margins was observed. This is ascribed to the protruding continental ice sheet that covers the shelf and shuts down the usual biological production, microbial breakdown and sedimentary geochemical cycling. The low concentrations of Mn in the deep ocean basins (0.07-0.23 nM) were quite uniform, but some elevations were observed. The highest deep concentrations of Mn were observed at the Bouvet Triple Junction region and coincided with high concentrations of Fe and are deemed to be from hydrothermal input. The deep basins on both sides of the ridge were affected by this input. In the deep Weddell Basin the input of Weddell Sea Bottom Water appears to be the source of the slightly elevated concentrations of Mn in this water layer.