Dissolved Fe and Fe-binding organic ligands in the Mediterranean Sea – GEOTRACES G04
Gerringa, L.J.A.; Slagter, H.A.; Bown, J.; van Haren, H.; Laan, P. ; De Baar, H.J.W.; Rijkenberg, M.J.A. (2017). Dissolved Fe and Fe-binding organic ligands in the Mediterranean Sea – GEOTRACES G04. Mar. Chem. 194: 100-113. https://dx.doi.org/10.1016/j.marchem.2017.05.012
In: Marine Chemistry. Elsevier: Amsterdam. ISSN 0304-4203; e-ISSN 1872-7581, more
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| Author keywords |
GEOTRACES; Dissolved Fe; Organic ligands; Mediterranean Sea; Dust; Fe speciation |
| Abstract |
Dissolved Fe (DFe) and Fe-binding dissolved organic ligands were analysed during two GEOTRACES cruises inthe Mediterranean Sea in May and August 2013.DFe was relatively high near the surface probably due to atmospheric sources, whereas below 500–700 mdepth the concentrations were relatively low,< 0.4 nM, compared to typical concentrations of 0.6 nM at thesame depths in the Atlantic Ocean. These relatively low concentrations are probably due to scavenging andballasting by dust particles settling down through the water column. Especially in the Eastern Basin, and moreprominent in its northern part, distinct patches with high DFe, up to 8.40 nM, were found between 200 and3000 m depth. These patches were local, which indicates a point source and lateral transport from this source.Some of these patches coincided with sloping density lines indicating enforced along-frontal currents providinglateral transport of DFe. Sources are probably seamounts and mud volcanoes, which were found to exist at thesame depths as the elevated DFe. It is conceivable that a large eddy keeps infusions of DFe isolated from mixingwith other water masses. These infusions could originate from slopes or from downwards cascading materials outof canyons.Fe-binding dissolved organic ligands increase the solubility of Fe enabling high dissolved Fe concentrations,and hence longer residence time. These ligands had median total concentrations between [Lt] =0.77 and [Lt]= 1.74 nEq of M Fe and conditional stability constants between logK′=21.57 and logK′ =22.13 (N =156).Median values of [Lt] were higher in the upper 100 m and its median concentration increased from west to east.The [Lt] concentrations did not relate to water mass or DFe concentration. The ligands were nearly saturatedwith Fe where DFe was elevated near the surface and completely saturated, ratio [Lt]/DFe ≤1, in patches withhigh DFe at depth. The high DFe concentrations in these patches are extreme, if not even maximum, concentrationsas any surplus Fe with respect to the ligands will tend to precipitate. Calculated inorganic Fe concentrationsin the Mediterranean had minimum concentrations of 0.23 pM and below 100 m depth medianconcentrations that varied between 0.68 and 1.99 pM only. This suggests that the inorganic Fe concentration isthe result of a steady state between binding by organic ligands and scavenging processes. Thus scavenging willnot result in lower inorganic Fe concentrations and in this way the dissolved ligand concentration determines theconcentration of DFe in the Mediterranean Sea. |
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