Inputs of iron, manganese and aluminium to surface waters of the Northeast Atlantic Ocean and the European continental shelf

Abstract

International audienceDissolved Fe, Mn and Al concentrations (dFe, dMn and dAl hereafter) in surface waters and the water column of the Northeast Atlantic and the European continental shelf are reported. Following an episode of enhanced Saharan dust inputs over the Northeast Atlantic Ocean prior and during the cruise in March 1998, surface concentrations were enhanced up to 4 nmol L− 1 dFe, 3 nmol L− 1 dMn and 40 nmol L− 1 dAl and returned to 0.6 nmol L− 1 dFe, 0.5 nmol L− 1 dMn and 10 nmol L− 1 dAl towards the end of the cruise three weeks later. A simple steady state model (MADCOW, [Measures, C.I., Brown, E.T., 1996. Estimating dust input to the Atlantic Ocean using surface water aluminium concentrations. In: Guerzoni. S. and Chester. R. (Eds.), The impact of desert dust across the Mediterranean, Kluwer Academic Publishers, The Netherlands, pp. 301-311.]) was used which relies on surface ocean dAl as a proxy for atmospheric deposition of mineral dust. We estimated dust input at 1.8 g m− 2 yr− 1 (range 1.0-2.9 g m− 2 yr− 1) and fluxes of dFe, dMn and dAl were inferred. Mixed layer steady state residence times for dissolved metals were estimated at 1.3 yr for dFe (range 0.3-2.9 yr) and 1.9 yr for dMn (range 1.0-3.8 yr). The dFe residence time may have been overestimated and it is shown that 0.2-0.4 yr is probably more realistic. Using vertical dFe versus Apparent Oxygen Utilization (AOU) relationships as well as a biogeochemical two end member mixing model, regenerative Fe:C ratios were estimated respectively to be 20 ± 6 and 22 ± 5 μmol Fe:mol C. Combining the atmospheric flux of dFe to the upper water column with the latter Fe:C ratio, a 'new iron' supported primary productivity of only 15% (range 7%-56%) was deduced. This would imply that 85% (range 44-93%) of primary productivity could be supported by regenerated dFe. The open ocean surface data suggest that the continental shelf is probably not a major source of dissolved metals to the surface of the adjacent open ocean. Continental shelf concentrations of dMn, dFe, and to a lesser extent dAl, were well correlated with salinity and express mixing of a fresher continental end member with Atlantic Ocean water flowing onto the shelf. This means probably that diffusive benthic fluxes did not play a major role at the time of the cruise

Determination of Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in seawater using high resolution magnetic sector inductively coupled mass spectrometry (HR-ICP-MS)

“…The very low concentrations of Pb, particularly at the surface, are characteristic of this element and typical in areas of the open ocean which are distanced from anthropogenic influences [13,32]. Mn displays surface enrichment with depletion at depth, typical for this element [33,34]. Similar concentrations in surface waters and their distribution through the water column have been reported for these elements in the North Atlantic Ocean [11,26,32,33,[35][36][37][38].…”

Role of iron species in the photo-transformation of phenol in artificial and natural seawater

“…sediments) iron(II) could be released; when this is oxidized to Fe(III), the case is similar to addition of Fe(II) above. It precipitates in a form that differs from that formed when Fe(III) precipitates in estuarine waters or is supplied by atmospheric dust (de Jong, 2007). It is then expected that different haloderivatives are formed depending on the iron genesis.…”

“…The concentration of total dissolved iron in the oceans ranges from 0.01 to 10 nM, and includes both small soluble Fe species and colloidal forms (Nishioka, et al 2001;Sarthou, and Jeandel, 2001;de Jong et al, 2007); substantial portion of soluble Fe is present in the colloidal size range (80 to 90 % in near-surface waters) (Wu et al, 2001). Fe(II) in seawater is usually present as complexed species, such as FeCO 3 or FeCl + and, under oxic conditions at the pH of seawater, Fe(II) species are rapidly oxidized to Fe(III) by O 2 and H 2 O 2 , as Fe(III) is the thermodynamically stable form of iron in seawater, freshwater and most aqueous systems containing dissolved oxygen.…”

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

“…For shallow shelf and slope waters, rivers are anticipated to be only a small source because of extensive removal of Fe in low salinity waters during estuarine mixing (Sholkovitz et al 1978). Atmospheric inputs are most significant where dust inputs are large, and are directly delivered to the photic zone (de Jong et al 2007). However, sediments underlying shelf and slope waters are argued to be a major source of Fe that can ultimately be transferred to the ocean (Johnson et al 1999;Elrod et al 2004;Severmann et al 2010;Homoky et al 2012;Conway and John 2014;Dale et al 2015).…”