Deep Dissolved Iron Distributions in the Eastern North Atlantic in relation with Water Masses

Concentrations of dissolved iron (DFe) were measured in seawater in the eastern North Atlantic Ocean (Bay of Biscay). The cruise took place in March 2002 (R/V Pelagia) and followed a transect from the shelf break (47°01 80N, 5°36 50 W) in an offshore direction (46°00 00 N, 8°00 00 W, (4912 m)). This campaign was part of the European Union project IRONAGES. Deep water samples were collected using Go Flo bottles mounted on a kevlar line. Surface waters were sampled using a torpedo fish and a trace metal clean teflon bellows pump. Seawater was filtered on-line in a clean container through an 0.2 μm cartridge. The DFe concentrations were determined on board by FIA with on-line preconcentration (8-HQ resin) and chemiluminescence detection. The mean blank and detection limit during the cruise were 28 pM and 39 pM respectively. The DFe concentrations ranged from 0.23 to 1.5 nM throughout the entire water column. The lowest DFe concentrations (0.23-0.63 nM) were found in the surface waters (-3 m). The highest concentrations (1.5-1.3 nM) were observed close to the shelf break at depths between 500 and 1200 m. Important vertical mixing processes produced by internal waves and along slope currents have been reported in this area. These processes are thought to induce iron inputs from below in the water column leading to enhanced DFe concentrations. By contrast, an uniform DFe distribution (0.75 nM) was observed below 3000 m at the three offshore stations. There was no evidence of lateral advection of DFe from the continental margin at these depths. Interestingly, the deep iron profiles at the three offshore stations did not follow a nutrient type profiles. Between 1600 and 2500 m, a large plume of intermediate DFe concentrations (1.20 nM) was observed. This signature correlated well with special features in salinity (34.99) temperature (3.7°C) and dissolved oxygen ([O2] = 255μæM) distributions characteristic of a water mass identified as Labrador Sea Water (LSW). LSW takes its source in the surface of the Labrador Sea, before sinking, flowing out eastward and finally reaching the eastern Atlantic coast. The initial iron concentrations of the LSW, are expected to be high due to mixing with the coastal water of Canada. The transit time of the LSW through the Atlantic (5-10 years) is lower than iron residence time (50-100 years). This can explain the iron signature of the LSW along the eastern Atlantic coast. Mediterranean Outflow Sea water, MOW (9.7°C, S = 35.77), has been also identified in our studied area at depths between 600 m and 1400 m. MOW is expected to be iron enriched because of atmospheric Saharan inputs into the Mediterranean, however no clear evidence of this has been obtained from the DFe distributions. The iron signal of MOW may have been obscured by the influence of the adjacent margin coast, leading to no significant iron signature.