On the effects of circulation, sediment resuspension and biological incorporation by diatoms in an ocean model of aluminium, link to model data

The distribution of dissolved aluminium in the West Atlantic Ocean shows a mirror image with that of dissolved silicic acid, hinting at intricate interactions between the ocean cycling of Al and Si. The marine biogeochemistry of Al is of interest because of its potential impact on diatom opal remineralisation, hence Si availability. Furthermore, the dissolved Al concentration at the surface ocean has been used as a tracer for dust input, dust being the most important source of the bio-essential trace element iron to the ocean. Previously, the dissolved concentration of Al was simulated reasonably well with only a dust source, and scavenging by adsorption on settling biogenic debris as the only removal process. Here we explore the impacts of (i) a sediment source of Al in the Northern Hemisphere (especially north of ~ 40° N), (ii) the imposed velocity field, and (iii) biological incorporation of Al on the modelled Al distribution in the ocean. The sediment source clearly improves the model results, and using a different velocity field shows the importance of advection on the simulated Al distribution. Biological incorporation appears to be a potentially important removal process. However, conclusive independent data to constrain the Al / Si incorporation ratio by growing diatoms are missing. Therefore, this study does not provide a definitive answer to the question of the relative importance of Al removal by incorporation compared to removal by adsorptive scavenging.

Iron and aluminium and hydrogen peroxide measured on water bottle samples during Marion Dufresne cruise ANTARES-II

Total dissolvable iron (TDFe), particulate iron (PFe) and hydrogen peroxide (H2O2 measurements were performed along a N-S transect in the upper 250 m in the Southern Ocean (62°00E/66°42S - 49°00S, ANTARES II cruise, February 1994). TDFe was organically extracted (APDC/DDDC-chloroform) and analysed by Graphite Furnace Atomic Absorption Spectrometry (GFAAS), PFe was analysed by GFAAS following a strong mixed-acid leach, and H2O2 was analysed on board by fluorometry. The respective detection limits are equal to 0.13 nmol/kg, 0.02 nmol/kg, and 3.0 nmol/kg. TDFe concentrations vary from 0.4 to 6.2 nmol/kg and profiles are not completely depleted in the surface. PFe concentrations vary from 0.02 to 0.2 nmol/kg. Iron/carbon (Fe/C) uptake ratios for phytoplankton were calculated either from seawater or particle measurements. They are variable along the transect but are consistent when they could be compared. All the observed ratios are within the range of values proposed for the Fe/C uptake ratios by phytoplankton. Using our uptake ratio calculated in the Permanent Open Ocean Zone (4 x 10**?6 mol/mol), we estimate that the primary production which can be supported by the iron input flux into the surface waters is two times higher than the measured primary production in the same area. In the surface waters, H2O2 concentrations vary from 5.0 to 19.7 nmol/kg. Such low concentrations are due to strong vertical mixing, low dissolved organic matter concentrations and the latitude of the site.

(Table 2) Concentrations of dissolved aluminum in interstitial water from sediments of the Indian Ocean

Results are presented of application of laser stepwise photoionization of atoms in combination with thermal atomization of matter in vacuum for direct determination of aluminum dissolved in sea and interstitial waters. Dry residue from evaporation of 40 ?l sea water was atomized in a crucible at 1800°C, and aluminum atoms in the beam thus formed were energized into Rydberg state in two steps by two tunable dye laser beams; the atoms were then ionized by an electric pulse and resulting ions were recorded by secondary emission electron multiplier (ion detector). Ionic signal dependence on sample vaporization time was studied. The procedure is suggested for separating out a selective signal in a single measurement. Dissolved aluminum concentrations in interstitial waters of the Indian Ocean and in waters of the river-sea zone were determined using preliminarily plotted calibration characteristics for aluminum solutions in deionized and sea waters. The minimum detectable Al concentration in seawater was 1 ?g/l that corresponds to 40 pg of Al in a sample.

(Figure 2) Dissolved manganese concentrations during POLARSTERN Cruise ANT-XXIV/3

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.