OS21G-01
High-Resolution Dissolved Fe and Al along 150 W in the North Pacific Ocean: Results from the CLIVAR Repeat Hydrography P16N Section
Dissolved Fe and Al in the upper 1000m are reported for samples collected from the North Pacific along 150° W, between 16° S and 57° N during the CLIVAR-CO2 Repeat Hydrography P16N section (Feb-Mar 2006). In the equatorial upwelling region between 4° N and 5° S dissolved Fe concentrations in the upper 100m ranged from 0.03 - 0.26 nmol L-1, with an average of 0.11 nmol L-1. Within this low Fe region, surface waters concentrations of nitrate and phosphate are ~5.7 μmol kg-1 and ~0.6 μmol kg-1, respectively. The Equatorial Undercurrent (EUC) which was sampled at 0.5 degree intervals showed a subsurface Al enrichment at ~120m. Dissolved Al levels range from ~2 to 8 nmol L-1, the maximum associated with the high velocity core of the EUC. In contrast, dissolved Fe concentrations show no corresponding enrichment in the EUC. Dissolved Fe concentrations in the surface waters range from 0.10 nmol L-1 in the North Equatorial Countercurrent at 5° N to ~0.35 nmol L-1 in the North Pacific subtropical gyre at 35° N. Elevated Al in the surface waters between ~25-33° N (~2 to 3 nmol Al L-1) confirms earlier reports of high values in this region including observations during the CLIVAR-CO2 Repeat Hydrography P2 section (2004). The extent and permanence of this feature suggests this is a region of frequent atmospheric deposition. Dissolved Fe concentrations in the surface water increase north of 35° N from 0.27-0.43 nmol L-1 approaching the continental shelf. In the surface waters near the Alaskan shelf region values of dissolved Fe rise to a maximum of ~0.7-1.1 nmol L-1 as a result of shelf input along the Aleutian Islands. These distributions show the importance of continental sources and atmospheric deposition in delivering Fe to regions of the North Pacific.
OS21G-02
Dissolved Iron and Aluminium Distributions in the Central and North Indian Ocean During CLIVAR-CO2 Repeat Hydrography I9N Transect
Dissolved Fe and Al were determined on seawater samples from the upper 1000 m during the CLIVAR I9N Repeat Hydrography expedition from 28S to 18N along 95E in the Indian Ocean (March-April 2007). Samples were collected at approximately one-degree intervals using a trace metal clean rosette based sampling system and dissolved Fe and Al were determined using shipboard Flow Injection Analysis. Between 28S and 5N, surface dissolved Fe was ~0.2 nM, increasing to 0.3-0.6 nM between ~5N and 18N. Al concentrations, a good tracer of atmospheric deposition, were 4.9-11.7 nM between 28S and ~5N, and then increased rapidly to 10.2-32.4 nM from ~5N to 18N. In the South Equatorial Counter Current, between 8S and 5N, mean dust deposition calculated from the surface Al imply a deposition of 0.66±0.07 g·m-2·yr-1, significantly less than reported predictions for this region. Between ~5N and 18N, the elevated Al values in surface waters were associated with decreasing salinity, implying that they were a result of a river input from the Ganges-Brahmaputra river system. Between 28S and ~20S at depths of 400-800 m, Fe concentrations were ~0.4 nM, and these increased significantly between ~20S and ~10S to ~0.75 nM. To the north of this region Fe values increased gradually reaching ~1.2 nM in the southern boundary of the Bay of Bengal, east of Sri Lanka. The dramatic increases seen in the deep Fe concentrations are coincident with the front associated with the low oxygen water emanating from the Bay of Bengal. An additional maximum between 8N and 12N appeared to be immediately downstream of Sri Lanka. Both the regional and general maxima appear to be the result of regeneration of vertically transported biological material produced in the relatively Fe-rich subsurface waters of this area. The Fe and Al distributions in the central and north Indian Ocean indicate that large areas are influenced by dust deposition and river input, and that there is a large subsurface region of enhanced Fe concentrations.
OS21G-03
Dynamics and Speciation of Dissolved Iron in the Sargasso Sea (BATS Region)
Data from 6 cruises in 2007-2008 and 3 cruises in 2003-2004 reveal a remarkable temporal and spatial variability in the distribution of dissolved iron (DFe, <0.4 μm) in the BATS region. In surface waters we observe a consistent seasonal cycle, with DFe increasing from ca. 0.1-0.3 nM in spring to ca. 0.4-2.0 nM in summer. The range of these seasonal extremes appears to reflect the extent of winter convective mixing and summer dust deposition, both of which are strongly linked to atmospheric circulation. However, surface DFe concentrations show significant (ca. two-fold) sub-mesoscale lateral variations that are not readily explained. Summer vertical profiles reveal pronounced DFe minima and sometimes maxima in the lower euphotic zone, which clearly reflect biological uptake and/or remineralization, and eddy-driven lateral gradients in these processes. We also see strong variability in the mesopelagic, with a DFe range of ca. 0.4- 1.0 nM at 1000 m depth, possibly reflecting variations in the composition and proportion of mode water, and mesoscale isopycnal displacements. Physicochemical speciation measurements indicate that the major fraction of DFe that accumulates in surface waters of the Sargasso Sea during summer is colloidal-sized Fe(III) complexed by strong, iron-binding organic ligands; dissolved Fe(II), on average, accounts for around 20% of DFe, with maximum concentrations of ca. 0.1 nM in the surface mixed layer and near 1000 m depth.
OS21G-04
Distributions of iron, aluminum and manganese in the westeern equatorial Pacific
Data for dissolved and total acid soluble iron, aluminum and manganese were collected in a zonal section along the equatorial Pacific from 140°W to 145°E from the surface to 1000 m with highest resolution through the depth range of the Equatorial Undercurrent (EUC). Samples were also collected in the New Guinea and New Ireland Coastal Undercurrents which are the southern source waters for the EUC. All three trace elements exhibited a subsurface maximum near the velocity core of the EUC, although the maximum for Fe was slightly deeper than those of Al and Mn. Al and Mn showed surface enrichment suggesting riverine or atmospheric inputs while Fe had no surface enrichment. All three metals exhibited strong zonal gradients. Total acid soluble Al in the EUC decreased from 10.7 nM at 145°E to 6.04 nM at 140°W. Total acid soluble Mn decreased from 0.92 nM to 0.45 nM from 145°E to 155°W, east of which no subsurface Mn maximum was observed. The Fe subsurface maxima appeared primarily in the particulate fraction, decreasing from 6.57 nM total acid soluble Fe at 145°E to 1.18 nM at 140°W, although the gradient was less smooth than that of Al and Mn. Metal concentrations were greatest offshore of New Guinea, with strongest enrichment downstream of the Sepik River. The zonal gradients and high shelf concentrations imply that continental sources of metals should be considered when modeling the oceanic Fe supply to the equatorial Pacific.
OS21G-05
Dissolved Iron Speciation in Oxic and Suboxic Water Column Profiles: Trends in the Distribution and Strength of Iron-Binding Ligands
The organic complexation of dissolved iron was examined in depth profiles to 250 m in the Northeast Pacific. Profiles for iron speciation were obtained from the southernmost line (line 93) of the CalCOFI station grid during summer 2007 and from stations along the west coast of Baja, California during summer 2008. These profiles follow the evolution of dissolved iron speciation from coastal to offshore waters in the Southern California Bight and in the suboxic water column of the Eastern Tropical North Pacific. Trends in stronger (L1) and weaker (L2) iron-binding organic ligand concentrations were examined with respect to depth, chlorophyll maxima, and water column oxygenation. In particular, iron speciation from a secondary deep chlorophyll maximum in the suboxic zone, dominated by Prochlorococcus species, was contrasted with the iron speciation measured in chlorophyll and biomass maxima observed in the oxygenated profiles of CalCOFI line 93.
OS21G-06
Fe(II) maxima are coincident with the secondary nitrite maxima in the three major oxygen minimum zones.
Fe(II) is thermodynamically unstable in the presence of oxygen at seawater pH. However, significant concentrations have been reported under various conditions in seawater, and attributed to a variety of non- equilibrium processes, including photochemical and biological reduction of Fe(III). Here, we report the widespread occurrence of Fe(II) coincident with the secondary nitrite maximum in the three main oxygen minimum zones in the world's oceans: the Arabian Sea in 2004 and 2007, and the Peru Upwelling and eastern tropical Pacific in 2005. While nitrite itself does not reduce Fe(III) to Fe(II), the nitrite maximum is associated with a dense microbial community that may reduce Fe(III). The presence of Fe(II) at the secondary nitrite maximum has important implications for Fe transport (total dissolved Fe accumulates there) and for the N cycle, since Fe is required for enzymes associated with denitrification and annamox. Fe(II) is generally absent above and below the secondary nitrite maxima, except when the water column is influenced by benthic sources.
OS21G-07
Sea ice-derived dissolved iron and its potential influence on the spring algal bloom in the Bering Sea
Observational and modeling studies in the Bering Sea indicate that changes in the seasonal ice cover and time of ice retreat influence open-water productivity. In particular, the timing of the spring bloom and phytoplankton community composition are affected. Dissolved iron (DFe) data in the water column and ice cores collected during the 2007- Bering Ecosystem Study (BEST) cruise indicate that the melting ice provided substantial DFe to the water column. Iron-binding organic ligand concentrations in surface waters were in excess of DFe concentrations. Additional Fe input from melting sea ice could be biologically important along the outer shelf and shelf break where in the absence of melting sea ice (or other external Fe inputs), insufficient DFe (< 1 nM) existed for the complete assimilation of available nitrate (> 20 uM) by large cells.
OS21G-08
Iron and Other Trace Metals in Southern Ocean Surface Waters - Indian Ocean Sector
It has now been shown that biological productivity in at least some regions of the Southern Ocean (SO) is limited by Fe supply rather than by the macronutrients N, P, or Si. Inferences of comparable status for the larger SO are limited by experimental evidence and the availability of dissolved metals data. We report here on distributions of bioactive and non-bioactive elements (<0.2µm Fe, Mn, Ti, Zn, Cu, Ni, V, Cr, Co, Cd and Pb) in surface waters of the Indian Ocean sector of the Southern Ocean (Polarstern ANTXXIII-9, Mar-Apr 2007). The sample transect extends from the region of Prydz Bay north to the Kerguelen Islands vicinity and then NW toward Cape Town to ~41S, a region encompassing a broad zonal maximum in SO Chlorophyll concentrations. This region has not been well studied for trace metals with the exception of the local vicinity of Kerguelen Islands, a natural Fe fertilization region. Parallel seawater samples were analyzed using solvent extraction GFAA, voltametry, and direct injection ICP-MS, allowing methodological comparison as well as oceanographic interpretation. We found that while NO3 and PO4 were high and near constant over much of the transect before decreasing at the northern end, some trace metals varied considerably. Dissolved Fe shows an overall S to N decrease, with values exceeding 1.0 nM near Prydz Bay and decreasing to ~ 0.1 nM in the north, but with substantial 2-3 fold variability over short spatial scales and a broad modest maximum just west of the Kerguelen Islands. We speculate that the relatively high Fe concentrations over much of this region reflect inputs of sedimentary Fe from the Kerguelen Plateau and the reduced biological uptake characterizing the seasonal timing, approximately 10 weeks after the Kerguelen bloom. Zinc and Mn show similar degrees of spatial variability and partially covary with Fe. Results for the full data set will be discussed in the context of SO hydrography, natural Fe fertilization, and controls on the distribution of the full suite of metals.