Average fluorescence and dissolved iron and Fe-binding ligand characteristics during POLARSTERN expedition ANT-XXIV/3 in 2008

Organic complexation of dissolved iron (dFe) was investigated in the Atlantic sector of the Southern Ocean in order to understand the distribution of Fe over the whole water column. The total concentration of dissolved organic ligands ([Lt]) measured by voltammetry ranged between 0.54 and 1.84 nEq of M Fe whereas the conditional binding strength (K') ranged between 10**21.4 and 10**22.8. For the first time, trends in Fe-organic complexation were observed in an ocean basin by examining the ratio ([Lt]/[dFe]), defined as the organic ligand concentration divided by the dissolved Fe concentration. The [Lt]/[dFe] ratio indicates the saturation state of the natural ligands with Fe; a ratio near 1 means saturation of the ligands leading to precipitation of Fe. Reversely, high ratios mean Fe depletion and show a high potential for Fe solubilisation. In surface waters where phytoplankton is present low dissolved Fe and high variable ligand concentrations were found. Here the [Lt]/[dFe] ratio was on average 4.4. It was especially high (5.6-26.7) in the HNLC (High Nutrient, Low Chlorophyll) regions, where Fe was depleted. The [Lt]/[dFe] ratio decreased with depth due to increasing dissolved Fe concentrations and became constant below 450 m, indicating a steady state between ligand and Fe. Relatively low [Lt]/[dFe] ratios (between 1.1 and 2.7) existed in deep water north of the Southern Boundary, facilitating Fe precipitation. The [Lt]/[dFe] ratio increased southwards from the Southern Boundary on the Zero Meridian and from east to west in the Weddell Gyre due to changes both in ligand characteristics and in dissolved iron concentration. High [Lt]/[dFe] ratio expresses Fe depletion versus ligand production in the surface. The decrease with depth reflects the increase of [dFe] which favours scavenging and (co-) precipitation, whereas a horizontal increase in the deep waters results from an increasing distance from Fe sources. This increase in the [Lt]/[dFe] ratio at depth shows the very resistant nature of the dissolved organic ligands.

Iron analysis in bottle profiles off Mauritania

Iron solubility measurements in the Mauritanian upwelling and the adjacent Open Ocean of the Tropical Atlantic show for all stations lower values in the surface mixed layer than at depth below the pycnocline. We attribute this distribution to a combination of loss terms, chiefly photo-oxidation of organic ligands in the surface, and supply terms, predominantly from the release of ligands from the decomposition of organic matter. Significant correlations with pH, oxygen and phosphate for all samples below the surface mixed layer indicate that biogenic remineralisation of organic matter results in the release of iron binding ligands into the dissolved phase. The comparison of the cFeS/PO4**3- ratio with other published data from intermediate and deep waters in the Pacific suggests an enhanced release of iron chelators in the more productive Mauritanian upwelling zone.

Stable isotope composition of Holocene benthic foraminifers from the Eastern Mediterranean Sea

Temporal and regional changes in paleoproductivity and paleoceanography in the eastern Mediterranean Sea during the past 12 kyr were reconstructed on the basis of the stable oxygen and carbon isotope composition of the epibenthic Planulina ariminensis and the shallow endobenthic Uvigerina mediterranea from three sediment cores of the Aegean Sea and Levantine Basin. The Younger Dryas is characterized by high d18O values, indicating enhanced salinities and low temperatures of deep water masses at all investigated sites. With the onset of the Holocene, d18O records show a continuous decrease towards the onset of sapropel S1 formation, mainly caused by a freshening and warming of surface waters at deep water formation sites. In the middle and late Holocene, the similarity of d18O values from the southern Aegean Sea and Levantine Basin suggests the influence of isotopically identical deep water masses. By contrast, slightly higher d18O values are observed the northern Aegean Sea, which probably point to lower temperatures of North Aegean deep waters. The epifaunal d13C records reveal clear changes in sources and residence times of eastern Mediterranean deep waters associated with period of S1 formation. Available data for the early and late phase of sapropel S1 formation and for the interruption around 8.2 kyr display drops by 0.5 and 1.5 per mil, indicating the slow-down of deep water circulation and enhanced riverine input of isotopically light dissolved inorganic carbon from terrestrial sources into the eastern Mediterranean Sea. The decrease in epifaunal d13C signals is particularly expressed in the southern Aegean Sea and Levantine Basin, while it is less pronounced in the northern Aegean Sea. This points to a strong reduction in deep water exchange rates in the southern areas, but the persistence of local deep water formation in the northern Aegean Sea. The d13C values of U. mediterranea records reveal temporal and regional differences in paleoproductivity during the past 12 kyr, with rather eutrophic and mesotrophic conditions in the North Aegean Sea and southeast Levantine Basin, respectively, while the South Aegean Sea is characterized by rather oligotrophic conditions. After S1 formation, increasing d13C values reflect a progressive decrease in surface water productivity in the eastern Mediterranean Sea during the middle and late Holocene. In the northern Aegean Sea, this time interval is marked by repetitive changes in organic matter fluxes documented by significant fluctuations in the d13C signal of U. mediterranea on millennial- to multi-centennial time scales. These fluctuations can be linked to short-term changes in river runoff driven by northern hemisphere climatic variability.