DISSOLVED ALUMINIUM AND MANGANESE IN THE POLAR OCEANS

R. Middag1, H.J.W. de Baar1,2, M. Klunder 1, P. Laan1
1 - Royal Netherlands Institute for Sea Research, Department of Biological Oceanography, Den Burg, The Netherlands
2 - Department of Marine Biology, University of Groningen, Groningen, Netherlands
rmiddag@nioz.nl


Samples were collected for dissolved Aluminium (Al) and for dissolved Manganese (Mn) with an ultraclean all-titanium CTD system (de Baar et al. 2008, Marine Chemistry, 111 (1-2), 4-21) during the IPY-GEOTRACES program of the ARK XXII/2 and ANT XXIV/3 expeditions aboard RV Polarstern. These are the first ever comprehensive datasets of dissolved Al and Mn in the polar oceans.
Surface concentrations of Al in the Arctic Ocean indicate that neither fluvial nor atmospheric inputs were significant. The deep Al concentrations provide evidence that water from the Barents Sea and Kara Sea strongly influence the deep waters of the Eurasian Basin by deep slope convection. Based on the Al and Silicate distributions it appeared that the deep Makarov Basin is influenced by both Canada Basin water flowing in over the Mendeleyev Ridge and Eurasian Basin water flowing in over the Lomonosov Ridge. Moreover, Al showed a nutrient type profile and a strong correlation was found between Al and the nutrient Silicate (Si). The Al/Si ratio derived from the slope of the Al-Si relation in the intermediate water of the Arctic Ocean can (at least partly) be explained by the dissolution of biogenic silica. There appears to be a strong biological influence on the cycling and distribution of Al in the Arctic Ocean through the formation and dissolution of biogenic silica. In the Southern Ocean and Weddell Sea the Al concentrations were much lower and no correlation with Silicate was found. However, like in the Arctic Ocean, surface concentrations indicate atmospheric input was insignificant in the Southern Ocean. The deep Al concentrations show that deep water formation also increases the Al concentration in the deep Southern Ocean and Weddell Sea.
Concentrations of Mn were elevated in the surface waters of the Arctic Ocean, especially in areas with apparent fluvial and/or shelf influence. Deep waters are extremely depleted in Mn, except for a distinct signal of hydrothermal input which was also visible in the dissolved Iron (Fe) concentrations, temperature and light transmission. Latter hydrothermal Mn input could be traced across a specific depth horizon in the entire Eurasian Basin. In the Southern Ocean also a site of suspected hydrothermal input with elevated Mn and Fe concentrations was observed. Surface concentrations of Mn in the Southern Ocean were extremely low and correlated well with low values of 234Th, nitrate and phosphate in the surface layer. This indicates the Mn concentrations in the surface layer are strongly influenced by primary production. Furthermore, based on the extremely depleted Mn values and previously published need of Mn of especially diatoms (Peers and Price 2004, Limnology and Oceanography 49 (5), 1774–1783), it appears Mn can not be ruled out as a factor if importance in explaining the High Nutrient Low Chlorophyll conditions in the Southern Ocean. Around 55° S the Mn surface concentrations were elevated and indeed this corresponded with less biological activity.
Willem Barentsz Poolinstituut

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