Netherlands Polar Data Center

Dissolved iron (DFe) concentrations of 40 profile stations and 7 surface stations were measured directly on board by an automated Flow Injection Analysis (FIA) after a modified method of De Jong et al. 1998. In addition, unfiltered samples from 17 profiles and 7 surface stations were acidified and stored to determine the total Fe concentrations in the NIOZ laboratory after 6-12 months of dissolution (see table 1). Filtered (0.2µm) and acidified (pH 1.8) seawater was concentrated on a column containing aminodiacetid acid (IDA). This material binds only transition metals and not the interfering salts. After washing the column with ultra pure water, the column is eluted with diluted acid. After mixing with luminol, peroxide and ammonium, the oxidation of luminol with peroxide is catalyzed by iron and a blue light is produced and detected with a photon counter. The amount of iron is calculated using a standard calibration line, where a known amount of iron is added to low iron containing seawater. Using this calibration line a number of counts per nM iron is obtained. Samples were analyzed in duplicate sample bottles, each of which was measured in triplicate, and average DFe concentrations and standard deviation are given. Concentrations of DFe measured on the NBP0901 cruise ranged from 30 pM up to 0.7 nM. The standard deviation varied between 0% and 10% (the latter being exceptional), but was generally < 5% in samples with DFe concentrations higher than 0.1nM. Since samples containing less than 0.1nM DFe are near the detection limit of the system; the standard deviation of these measurements was sometimes high (<30%). The average blank was determined at 0.024nM±0.010nM and was defined as a sample loaded for 10 seconds and measured daily. The average limit of detection, 0.009±0.008 was defined as 3*standard deviation of the mean blank and measured daily. To better understand the day to day variation duplicate sample bottles were measured at least 24h later. The differences between these measurements were rather large, in the order of 5-20%, while the largest differences were measured in samples with low DFe concentrations. To correct for this day to day variation a so-called lab standard sample was measured daily. All data will be corrected for the mean average of this value after the cruise and all data presented so far are uncorrected for this day to day variation. The consistency of the FIA system over the course of the day was verified using a drift standard. The drift was observed to be less than 7% and no corrections have been made for this drift. A certified SAFe standard (Johnson et al. 2007) for the long term consistency and absolute accuracy was measured at a regular basis. Preliminary results The profile from station 16 clearly shows high iron input near the PIG (Fig. 5). DFe concentrations are high throughout the entire water column and indicate a constant input of DFe into the polynya. Profile 107, which is in the central Pine Island Polynya, shows much lower values throughout the first 300 meter which can be explained by the high phytoplankton abundance in the surface water. DFe concentrations during a transect from Pine Island Glacier (PIG) (101 S,30W) through the central Pine Island Polynya in north westwards direction clearly show the high DFe concentrations near the PIG and the rapid decrease of the DFe in the polynya, especially in waters shallower than 50m (Fig. 6). Extreme low DFe values in the central polynya correlate with the biological productive area as observed from the fluorometer data and Chl a measurements.