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Middag Et Al., 2011 Mn Antarctic.Pdf This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Deep-Sea Research II 58 (2011) 2661–2677 Contents lists available at ScienceDirect Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2 Dissolved manganese in the Atlantic sector of the Southern Ocean R. Middag a,n, H.J.W. de Baar a,b, P. Laan a, P.H. Cai c, J.C. van Ooijen a a Royal Netherlands Institute for Sea Research (Royal NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands b Department of Ocean Ecosystems, University of Groningen, Groningen, The Netherlands c State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China article info abstract Article history: The first comprehensive dataset (492 samples) of dissolved Mn in the Southern Ocean shows extremely low Received 21 October 2010 values of 0.04 up to 0.64 nM in the surface waters and a subsurface maximum with an average concentration Accepted 21 October 2010 of 0.31 nM (n¼20; S.D.¼0.08 nM). The low Mn in surface waters correlates well with the nutrients PO4 and Available online 19 November 2010 NO3 and moderately well with Si(OH)4 and fluorescence. Furthermore, elevated concentrations of Mn in the Keywords: surface layer coincide with elevated Fe and light transmission and decreased export (234Th/238U deficiency) Trace metals and fluorescence. It appears that Mn is a factor of importance in partly explaining the HNLC conditions in the GEOTRACES Southern Ocean, in conjunction with significant controls by the combination of Fe limitation and light Southern Ocean limitation. No input of Mn from the continental margins was observed. This is ascribed to the protruding Dissolved manganese continental ice sheet that covers the shelf and shuts down the usual biological production, microbial Biological uptake breakdown and sedimentary geochemical cycling. The low concentrations of Mn in the deep ocean basins Mn nutrient relation (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. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction contribute. Manganese (and other trace metals) can also reach the open ocean via other pathways; notably Mn can enter the water Manganese (Mn) is the 12th most abundant element in the column by reductive dissolution from (anoxic or suboxic) sediments earth’s crust (Wedepohl, 1995), yet in the open ocean it only exists in (e.g., Froelich et al., 1979; Johnson et al., 1992; Heggie et al., 1987; nanomolar concentrations. It occurs in natural waters as insoluble Pakhomova et al., 2007), lateral transportation from the marginal Mn oxides and as soluble Mn ions (Sunda and Huntsman, 1994). sediments (e.g., Landing and Bruland, 1980; Martin and Knauer, The latter are thermodynamically unstable in oxic seawater, with 1984; Johnson et al., 1992) or hydrothermal input (Klinkhammer relatively slow inorganic oxidation kinetics. However, this process is et al., 1977; Klinkhammer and Bender, 1980). accelerated by microbial mediation (Sunda and Huntsman, 1988 and For the remote Southern Ocean the fluvial input is not of references therein; Tebo et al., 2007 and references therein), and significance and atmospheric dust input is believed to be the main therefore Mn ions should oxidise to insoluble Mn oxides and be lost source of trace metals for the surface water of the open Southern from the water column via particulate scavenging and sinking Ocean (Sedwick et al., 1997). Atmospheric dust input can be direct (Sunda and Huntsman, 1994). This is observed below the photic (either dry (aeolian) or wet (precipitation)) dust deposition or indirect zone where the dissolved Mn concentrations decrease quickly with from the dissolution of accumulated dust particles in melting sea ice. increasing depth. However, in the surface layer a typical dissolved Hydrothermal Mn input has been shownintheBransfieldStraitnear Mn profile shows elevated concentrations (e.g., Landing and the Antarctic Peninsula (Klinkhammer et al., 2001). Close to the Bruland, 1980). It has been shown that the elevated dissolved Mn Antarctic continent and the Antarctic Peninsula, reductive dissolution surface concentrations are mainly the result of photo reduction of from (anoxic) sediments or land run off (i.e., ice melt) can be a source Mn oxides (Sunda et al., 1983; Sunda and Huntsman, 1988, 1994), for the dissolved Mn. Icebergs can transport entrained particles from but atmospheric input (Landing and Bruland, 1980; Baker et al., the Antarctic continent and from accumulated atmospheric input, 2006) or fluvial input (Aguilar-Islas and Bruland, 2006) may also derived of other continents, into the Southern Ocean. The surface concentrations of dissolved Mn are known to be very low in the Southern Ocean (Klinkhammer and Bender, 1980, Martin et al., 1990; n ¨ Corresponding author. Tel.: +31 222 369 464. Westerlund and Ohman, 1991), but no comprehensive distributions E-mail address: [email protected] (R. Middag). over the entire basin have thus far been studied. The reason for the low 0967-0645/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2010.10.043 Author's personal copy 2662 R. Middag et al. / Deep-Sea Research II 58 (2011) 2661–2677 concentrations of dissolved Mn in the Southern Ocean is believed to be 2004) and to some extent in the field (Buma et al., 1991; Coale, 1991a). its very limited input in combination with active removal by the However, the field experiments are ambiguous, as a response to Mn Antarctic algal community (Sedwick et al., 1997). addition is not always observed (Buma et al., 1991; Scharek et al., Mn is an essential trace nutrient, which is needed in enzymes for 1997; Sedwick et al., 2000), indicating that in some parts of the various biological processes in cells. Most notably Mn is needed in the Southern Ocean the ambient concentrations of dissolved Mn are in Photosystem II (PS II) for the splitting of water by photoautotrophs to adequate supply, i.e., not limited. To trace biological Mn uptake and supply electrons to the reaction centre of PS II (e.g., Sunda et al., 1983 subsequent export, a traditional tracer of particle export such as the and references therein; Raven, 1990). Recently it has been seen that disequilibrium between thorium-234 and uranium-238 (234Th/238U) Mn is also essential in the superoxide dismutase (SOD) enzymes of could be used. The 234Th/238U ratio is known equal to 1 at secular marine diatoms (Peers and Price, 2004; Wolfe-Simon et al., 2006). equilibrium while ratio values lower than 1 result from the removal of SOD destroys reactive oxygen species (ROS) such as superoxide and 234Th due to downward export of settling biogenic particles and hence hydroxyl radicals at the site of production, because these highly indicative of particle export. The 234Th is produced from radioac- reactive ROS cannot diffuse across cell membranes. Under iron (Fe) tive decay of the soluble 238U with a half-life of 4.47 Â 109 years, while deficient conditions the efficiency of PS II decreases (Greene et al., the half-life of the very particle reactive 234Th is only 24.1 days. The 1992; McKay et al., 1997), which presumably increases the formation resulting disequilibrium between the soluble parent 238Uandthe of ROS due to the less efficient flow of electrons through the measured daughter 234Th activity reflects the net rate of particle photosystem (Peers and Price, 2004). Thus the availability of Mn is export from the upper ocean on timescales of days to weeks (Cai et al., essential especially under Fe-deficient circumstances. 2008 and references therein). The Southern Ocean is considered to be a so-called High Nutrient Low Chlorophyll (HNLC) ocean (De Baar et al., 1995), where despite the abundance of the major nutrients for phytoplankton relatively 2. Methods little primary production takes place. It has been shown that Fe is one of the limiting factors in the Southern Ocean by several iron- 2.1. Sampling and processing fertilisation experiments in bottles (review by De Baar, 1994)and in situ (review by De Baar et al., 2005). Dissolved Mn has been Samples were collected in February–March 2008 aboard Polar- suggested to be potentially (co-) limiting (Martin et al., 1990; Brand, stern during expedition ANT XXIV/3at22tracemetal(TM)stations 1991), and this has been shown in lab experiments (Peers and Price, and 30 regular stations (see Middag et al., 2011 forthecoordinatesof Fig. 1. Trace metal sampling stations along the zero meridian and in the South East Atlantic Ocean during cruise ANT XXIV/3 from 10 February to 16 April 2008 from Cape Town (South Africa) to Punta Arenas (Chile) aboard R.V. Polarstern. Abbreviations in alphabetical order: AAZ: Antarctic Zone; ACC: Antarctic Circumpolar Current; PF: Polar Front; PFZ: Polar Frontal Zone; SAF: Sub Antarctic Front; SAZ: Sub Antarctic Zone; WF: Weddell Front; WG: Weddell Gyre.
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