Sampling and Analysis of Dissolved Iron

Jinchun Yuan and Christopher I. Measures

Abstract

Sea water samples have been collected from Go-Flo bottles, Niskin bottles, and directly into plastic bottles using a bamboo pole sampler since HOT-37. The iron concentration of these samples has been measured using an on-board FLA analysis system. The surface samples taken with bamboo pole yield low iron concentrations (0.4-1.6 nM) consistent with values reported in the literature for this area. In contrast, the Go-Flo samples yield the highest iron concentrations which range from 1.4 nM to more than 37 nM. The profiles from these bottles are not oceanographically consistent and vary erratically between cruises. By comparison the samples taken from Niskin bottles mounted on the CTD rosette yield comparatively low iron concentrations and occanographically consistent. The surface Niskin sample agrees well with those collected by the pole method. Therefore, although we believe samples from Niskin bottles can be contamination-free, variation has been found between CTD casts on same cruise at same station indicating some level of random contamination. The first few casts at Station ALOHA have been found to have high iron concentrations and to be somewhat scattered. Later casts at the station show lower and more consistent Fe concentrations. It appears that the Niskin bottles benefit from the large amount of rinsing they receive during the multiple CTD casts; the Go-Flo bottles may also benefit from a similar rinsing process. Initial Fe data suggest that there is a temporal variation in the surface water Fe concentrations and that there may be a linkage to the phosphate concentration.