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The multi-disciplinary project Eddy Pump was designed to study processes which exert a control on the physical and biological pumps of carbon in the Southern Ocean. The data for Eddy Pump were collected 2012 during Polarstern Cruise ANT-XXVIII/3, which started January 7 in Cape Town and ended March 11 in Punta Arenas. The study sites visited were selected in order to represent different biogeographic regimes found along the Antarctic Circumpolar Current (ACC) in the Atlantic sector: 1. the central ACC with its regular separation in different frontal jets (investigated by a meridional transect along 10°E), 2. a transient months-long lasting large-scale bloom that occurred west of 10°W adjacent to chlorophyll-poor waters to the immediate east, and 3. the region in the wake of South Georgia, which regularly features a dense bloom presumably stimulated by iron released from that island and its shelf. After presenting an overview of the obtained first results we will put up for discussion possible explanations for the observed patterns.

Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin- and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux.

[1] The impact of iron fertilization on mesopelagic carbon (C) remineralization was assessed during the European Iron Fertilization Experiment (EIFEX) in the Southern Ocean by following the temporal change of excess particulate barium (also called biogenic or Ba(xs)) in the mesopelagic waters of a mesoscale eddy. Before the iron infusion the site was already sustaining a significant vertical flux of particles leading to organic remineralization in the mesopelagic. Approximately 35 d after the fertilization, mesopelagic Baxs contents provided evidence of changes in surface particulate export and of enhanced and particularly fast remineralization extending down to 1000 m. At the end of the experiment (day 36) both export and remineralization decreased. Organic carbon remineralization took place mainly in the lower part of the mesopelagic zone (500 - 1000 m) and increased more than 5-fold during EIFEX, reaching values up to 92 +/- 15 mg C m(-2) d(-1). However, such remineralization rates are not particularly high when compared to other values reported for the natural Southern Ocean during summer. Though export and remineralization reached peak values around day 34, remineralization integrated over the 150 to 1000 m layer accounted only for 13 +/- 1.4% of the export at 150 m, suggesting that a substantial amount of exported carbon reached deeper in the water column. Compared to natural blooms in high-nutrient low-chlorophyll (HNLC) waters of the Southern Ocean, the Fe-induced bloom during EIFEX resulted in a lower ratio of remineralized organic carbon over exported carbon, similar to what we recently observed for natural Fe-replete conditions prevailing above the Kerguelen Plateau. Unexpected similarities and phase lags of profiles in the mesopelagic between casts sampled inside and outside the fertilized patch during EIFEX indicated that eddy dynamics were a determinant in setting particle patterns on a broad spatial scale exceeding the extent of the fertilization, the

Surface active substances (SAS) in the water column were measured by voltammetry using the electrochemical probe o-nitrophenol (ONP) during EIFEX, a mesoscale open ocean iron enrichment experiment in the Southern Ocean. SAS levels were low throughout the experiment (<0.005 – 0.03 mg L−1 Triton X-100 equivalents). Initially SAS was extremely low in the photic zone, but as the phytoplankton bloom developed concentrations markedly increased throughout the upper 100 m (∼0.02 mg L−1 Triton X-100 equivalents). Highest concentrations of SAS (>0.02 mg L−1 Triton X-100 equivalents) were found at the end of the bloom particularly at density discontinuities where organic material may accumulate. Exudates from diatoms appeared to be the major source of SAS during EIFEX, either from direct extracellular release or in the action of being grazed upon by zooplankton.

Time series of sea ice draft in the Weddell Sea are evaluated together with hydrographic observations, satellite passive microwave data, and ice drift for estimation of the freshwater fluxes into and out of the Weddell Sea. Ice draft is measured with moored upward looking sonars since 1990 along two transects across the Weddell Gyre. One transect, extending from the tip of the Antarctic Peninsula to Kapp Norvegia, was sampled between 1990 and 1994 and covers the flow into and out of the southern Weddell Sea. The other transect, sampled since 1996 and extending from the Antarctic continent northward along the Greenwich meridian, covers the exchange of water masses between the eastern and the western Weddell Sea. In order to relate results obtained during the different time periods, empirical relationships are established between the length of the sea ice season, derived from the satellite passive microwave data and defined as the number of days per year with the sea ice concentration exceeding 15%, and (1) the annual mean ice draft and (2) the annual mean ice volume transport. By using these empirical relationships, estimates of annual mean ice drafts and ice volume transports are derived at all mooring sites for the period February 1979 through February 1999. Wind and current force a westward ice transport in the coastal areas of the eastern Weddell Sea and a northward ice transport in the west. During the 2-year period 1991/1992 the mean ice volume export from the Weddell Sea is (50 ± 19) × 103 m3 s−1. This freshwater export is representative for a longer-term (20-year) mean and exceeds the average amount of freshwater gained by precipitation and ice shelf melt by about 19×103 m3 s−1, yielding an upper bound for the formation rate of newly ventilated bottom water in the Weddell Sea of 2.6 Sv.