Ussher
Simon J.
Ussher
Simon J.
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PreprintCommunity-wide intercomparison exercise for the determination of dissolved iron in seawater( 2005-07-29) Bowie, Andrew R. ; Achterberg, Eric P. ; Croot, Peter L. ; Baar, Hein J. W. de ; Laan, Patrick ; Moffett, James W. ; Ussher, Simon J. ; Worsfold, Paul J.The first large-scale international intercomparison of analytical methods for the determination of dissolved iron in seawater was carried out between October 2000 and December 2002. The exercise was conducted as a rigorously “blind” comparison of 7 analytical techniques by 24 international laboratories. The comparison was based on a large volume (700 L), filtered surface seawater sample collected from the South Atlantic Ocean (the “IRONAGES” sample), which was acidified, mixed and bottled at sea. Two 1 L sample bottles were sent to each participant. Integrity and blindness were achieved by having the experiment designed and carried out by a small team, and overseen by an independent data manager. Storage, homogeneity and time-series stability experiments conducted over 2.5 years showed that interbottle variability of the IRONAGES sample was good (<7%), although there was a decrease in iron concentration in the bottles over time (from 0.8-0.5 nM) before a stable value was observed. This raises questions over the suitability of sample acidification and storage. For the complete dataset of 45 results (after excluding 3 outliers not passing the screening criteria), the mean concentration of dissolved iron in the IRONAGES sample was 0.59±0.21 nM, representing a coefficient of variation (%CV) for analytical comparability (“community precision”) of 36% (1s), a significant improvement over earlier exercises. Within-run precision (5-10%), inter-run precision (15%) and inter-bottle homogeneity (<7%) were much better than overall analytical comparability, implying the presence of: (1) random variability (inherent to all intercomparison exercises); (2) errors in quantification of the analytical blank; and (3) systematic inter-method variability, perhaps related to secondary sample treatment (e.g. measurement of different physicochemical fractions of iron present in seawater) in the community dataset. By grouping all results for the same method, analyses performed using flow injection – luminol chemiluminescence (with FeII detection after sample reduction) [Bowie et al., 1998. Anal. Chim. Acta 361, 189] and flow injection – catalytic 3 spectrophotometry (using the reagent DPD) [Measures et al., 1995. Mar. Chem. 50, 3] gave significantly (P=0.05) higher dissolved iron concentrations than analyses performed using isotope dilution ICPMS [Wu and Boyle, 1998. Anal. Chim. Acta 367, 183]. There was, however, evidence of scatter within each method group (CV up to 59%), implying that better uniformity in procedures may be required. This paper does not identify individual data and should not be viewed as an evaluation of single laboratories. Rather it summarises the status of dissolved iron analysis in seawater by the international community at the start of the 21st century, and can be used to inform future exercises including the SAFE iron intercomparison study in the North Pacific in October 2004.
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ArticleIron in the Sargasso Sea (Bermuda Atlantic Time-series Study region) during summer : eolian imprint, spatiotemporal variability, and ecological implications(American Geophysical Union, 2005-10-13) Sedwick, Peter N. ; Church, Thomas M. ; Bowie, Andrew R. ; Marsay, Christopher M. ; Ussher, Simon J. ; Achilles, K. M. ; Lethaby, Paul ; Johnson, Rodney J. ; Sarin, M. M. ; McGillicuddy, Dennis J.We report iron measurements for water column and aerosol samples collected in the Sargasso Sea during July-August 2003 (summer 2003) and April-May 2004 (spring 2004). Our data reveal a large seasonal change in the dissolved iron (dFe) concentration of surface waters in the Bermuda Atlantic Time-series Study region, from ∼1–2 nM in summer 2003, when aerosol iron concentrations were high (mean 10 nmol m−3), to ∼0.1–0.2 nM in spring 2004, when aerosol iron concentrations were low (mean 0.64 nmol m−3). During summer 2003, we observed an increase of ∼0.6 nM in surface water dFe concentrations over 13 days, presumably due to eolian iron input; an estimate of total iron deposition over this same period suggests an effective solubility of 3–30% for aerosol iron. Our summer 2003 water column profiles show potentially growth-limiting dFe concentrations (0.02–0.19 nM) coinciding with a deep chlorophyll maximum at 100–150 m depth, where phytoplankton biomass is typically dominated by Prochlorococcus during late summer.
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ArticleControls on dissolved cobalt in surface waters of the Sargasso Sea : comparisons with iron and aluminum(American Geophysical Union, 2012-05-19) Shelley, Rachel U. ; Sedwick, Peter N. ; Bibby, Thomas S. ; Cabedo-Sanz, P. ; Church, Thomas M. ; Johnson, Rodney J. ; Macey, A. I. ; Marsay, Christopher M. ; Sholkovitz, Edward R. ; Ussher, Simon J. ; Worsfold, Paul J. ; Lohan, Maeve C.Dissolved cobalt (dCo), iron (dFe) and aluminum (dAl) were determined in water column samples along a meridional transect (~31°N to 24°N) south of Bermuda in June 2008. A general north-to-south increase in surface concentrations of dFe (0.3–1.6 nM) and dAl (14–42 nM) was observed, suggesting that aerosol deposition is a significant source of dFe and dAl, whereas no clear trend was observed for near-surface dCo concentrations. Shipboard aerosol samples indicate fractional solubility values of 8–100% for aerosol Co, which are significantly higher than corresponding estimates of the solubility of aerosol Fe (0.44–45%). Hydrographic observations and analysis of time series rain samples from Bermuda indicate that wet deposition accounts for most (>80%) of the total aeolian flux of Co, and hence a significant proportion of the atmospheric input of dCo to our study region. Our aerosol data imply that the atmospheric input of dCo to the Sargasso Sea is modest, although this flux may be more significant in late summer. The water column dCo profiles reveal a vertical distribution that predominantly reflects ‘nutrient-type’ behavior, versus scavenged-type behavior for dAl, and a hybrid of nutrient- and scavenged-type behavior for dFe. Mesoscale eddies also appear to impact on the vertical distribution of dCo. The effects of biological removal of dCo from the upper water column were apparent as pronounced sub-surface minima (21 ± 4 pM dCo), coincident with maxima in Prochlorococcus abundance. These observations imply that Prochlorococcus plays a major role in removing dCo from the euphotic zone, and that the availability of dCo may regulate Prochlorococcus growth in the Sargasso Sea.