Dissolvable Iron Research Articles

Iron Limitation and Biogeochemical Effects in Southern California Current Coastal Upwelling Filaments

AbstractIn the spring and summer, high rates of primary production occur in the California Current Ecosystem (CCE) when nutrients are supplied to the euphotic zone. During periods of intense coastal upwelling, a flux of the micronutrient iron comes from nearshore sedimentary sources. In this upwelling region, mesoscale filament features distribute iron laterally, leading to distinct iron‐influenced ecological zones. This study is the first to focus on the biogeochemical links between iron, the macronutrients, and particulates in coastal upwelling filaments. Broad spatial patterns of iron and biogenic silica concentrations, and proxies of iron‐stress of diatoms, support results from microcosm amendment studies conducted during CCE Long Term Ecological Research process cruises in the summers of 2017 and 2019. We found that the benthic boundary layer and shoreward filament endmember supply dissolved and total dissolvable iron to this feature, but rapid assimilation and sinking by biogenic particles (e.g., diatoms) depletes the surface concentrations. Subsequently, diatom blooms which form in recently upwelled water masses become iron limited over time, thereby affecting the ratios of surface macronutrient reservoirs and biogeochemical advective fluxes. The development of Fe‐limitation during lateral advection may lead to efficient carbon export downstream and offshore of the region with the highest phytoplankton growth rates and productivity.

Coastal N2 Fixation Rates Coincide Spatially With Nitrogen Loss in the Humboldt Upwelling System off Peru

AbstractMarine nitrogen (N2) fixation supports significant primary productivity in the global ocean. However, in one of the most productive regions of the world ocean, the northern Humboldt Upwelling System (HUS), the magnitude and spatial distribution of this process remain poorly characterized. This study presents a spatially resolved data set of N2 fixation rates across six coastal transects of the northern HUS off Peru (8°S–16°S) during austral summer. N2 fixation rates were detected throughout the waters column including within the Oxygen Minimum Zone (OMZ) between 12°S and 16°S. N2 fixation rates were highest where the subsurface OMZ (O2 < 20 μmol L−1) was most intense and estimated nitrogen (N) loss was highest. There, rates were measured throughout the water column. Hence, the vertical and spatial distribution of rates indicates a colocation of N2 fixation with N loss in the coastal productive waters of the northern HUS. Despite high phosphate and total dissolvable iron (TdFe) concentrations throughout the study area, N2 fixation was still generally low (1.19 ± 3.81 nmol L−1 d−1) and its distribution could not be directly explained by these two factors. Our results suggest that the distribution was likely influenced by a complex interplay of environmental factors including phytoplankton biomass and organic matter availability, and potentially iron, or other trace metal (co)‐limitation of both N2 fixation and primary production. In general, our results support previous conclusions that N2 fixation in the northern HUS plays a minor role as a source of new N and to replenish the regional N loss.

Enhanced Deposition of Atmospheric Soluble Iron by Intrusions of Marine Air Masses to East Antarctica

AbstractBio‐essential iron can relieve nutrient limitation and stimulate marine productivity in the Southern Ocean. The fractional iron solubility of aerosol iron is an important variable determining iron availability for biological uptake. However, estimates of dissolved iron (dFe; iron < 0.2 μm) and the factors driving the variability of fractional iron solubility in pristine air masses are largely unquantified. To constrain inputs of fractional iron solubility to remote East Antarctic waters, dFe, total dissolvable iron (TDFe), trace elements and refractory black carbon were analyzed in a 9‐year‐old snow pit (2005–2014) from a new ice core site at Aurora Basin North (ABN) in Wilkes Land, East Antarctica. Extremely low annual dFe deposition fluxes were estimated (0.2 × 10−6 g m−2 y−1), while annual TDFe deposition fluxes (70 × 10−6 g m−2 y−1) were comparable to other Antarctic sites. TDFe is dominantly sourced from mineral dust. Unlike coastal Antarctic sites where the variability of fractional iron solubility in modern snow is explained by a mixture of dust and biomass burning sources, dFe deposition and fractional iron solubility at ABN (ranging between 0.1% and 6%) is enhanced in episodic high precipitation events from synoptic warm air masses. Enhanced fractional iron solubility reaching the high elevation site at ABN is suggested through the mechanism of cloud processing of background mineral dust that modifies the dust chemistry and increases iron dissolution during long‐range transport. This study highlights a complex interplay of sources and processes that drive fractional iron solubility in pristine air masses.

Hydrothermal Exploration of the Southern Chile Rise: Sediment‐Hosted Venting at the Chile Triple Junction

AbstractWe report results from a hydrothermal plume survey along the southernmost Chile Rise from the Guamblin Fracture Zone to the Chile Triple Junction (CTJ) encompassing two segments (93 km cumulative length) of intermediate spreading‐rate mid‐ocean ridge axis. Our approach used in situ water column sensing (CTD, optical clarity, redox disequilibrium) coupled with sampling for shipboard and shore based geochemical analyses (δ3He, CH4, total dissolvable iron (TDFe) and manganese, (TDMn)) to explore for evidence of seafloor hydrothermal venting. Across the entire survey, the only location at which evidence for submarine venting was detected was at the southernmost limit to the survey. There, the source of a dispersing hydrothermal plume was located at 46°16.5’S, 75°47.9’W, coincident with the CTJ itself. The plume exhibits anomalies in both δ3He and dissolved CH4 but no enrichments in TDFe or TDMn beyond what can be attributed to resuspension of sediments covering the seafloor where the ridge intersects the Chile margin. These results are indicative of sediment‐hosted venting at the CTJ.

Major lithogenic contributions to the distribution and budget of iron in the North Pacific Ocean

Recent studies have elucidated that iron (Fe) is a critical trace metal that influences the productivity of marine ecosystems and the biogeochemical cycles of other elements in the modern ocean. However, our understanding of the biogeochemistry of Fe remains incomplete. Herein, we report basin-scale and full-depth sectional distributions of total dissolvable iron (tdFe), dissolved iron (dFe), and labile particulate iron (lpFe = tdFe – dFe) in the North Pacific Ocean, as observed during three cruises of the GEOTRACES Japan program. We found that lpFe dominates tdFe and is significantly correlated with labile particulate aluminum (lpAl): lpFe [nmol kg−1] = (0.544 ± 0.005) lpAl [nmol kg−1] + 0.11 ± 0.04, r2 = 0.968, n = 432. The results indicate a major lithogenic contribution to the distribution of particulate Fe. For dFe, the unique distribution is attributed to the combined effects of biogeochemical cycling, manganese reduction, and lithogenic contribution. Based on concurrent observations of Fe, Al, and manganese (Mn), we infer that the width of the boundary scavenging zone is approximately 500 km off the Aleutian shelf. We estimate the inventory of tdFe in the North Pacific as 1.1 × 1012 mol, which is approximately four times that of dFe. Our results emphasize the potential importance of lpFe in the ocean’s iron cycle.

Dissolved iron concentration in the recent snow of the Lambert Glacial Basin, Antarctica

Iron (Fe) is a limiting nutrient in the Southern Ocean (SO), and the input of atmospheric iron may fertilize the SO. Therefore, it is important to estimate the deposition and solubility of atmospheric iron in the Antarctica. For this purpose, we measured iron in a 3 m snow pit from the Lambert Glacial Basin (LGB), covering the period 2012–2017. We estimated an average annual dissolved iron (DFe) flux of 1.29 × 10−3 mg m−2 a−1 and an average total dissolvable iron (TDFe) flux of 0.14 mg m−2 a−1 for this site. The atmospheric fractional iron solubility (FFS) ranges from 0.01% to 21.47% (mean value 1.20%) and shows an inverse hyperbolic relationship with the TDFe concentration. This relationship suggests that atmospheric iron in the LGB comes from a mixture of mineral dust with low FFS and aerosol with high FFS (e.g. combustion aerosols and volcanic ash). Based on the mean value of the atmospheric iron deposit fluxes from both this and previous studies, we estimated that ∼0.013 Gg of dissolved atmospheric iron could be deposited in the seasonal sea ice zone (SSIZ) every year, potentially supporting an annual algal production of ∼2.52 × 1012 g C in the Antarctic coastal water. This is less than 1% of the annual primary production in the SSIZ of the SO estimated from satellite data. The result shows that, compared with other sources of iron such as iceberg and glacier meltwater, the dissolved atmospheric iron has very small effect on the annual primary production in the SSIZ.

Seasonal iron depletion in temperate shelf seas

AbstractOur study followed the seasonal cycling of soluble (SFe), colloidal (CFe), dissolved (DFe), total dissolvable (TDFe), labile particulate (LPFe), and total particulate (TPFe) iron in the Celtic Sea (NE Atlantic Ocean). Preferential uptake of SFe occurred during the spring bloom, preceding the removal of CFe. Uptake and export of Fe during the spring bloom, coupled with a reduction in vertical exchange, led to Fe deplete surface waters (<0.2 nM DFe; 0.11 nM LPFe, 0.45 nM TDFe, and 1.84 nM TPFe) during summer stratification. Below the seasonal thermocline, DFe concentrations increased from spring to autumn, mirroring NO3− and consistent with supply from remineralized sinking organic material, and cycled independently of particulate Fe over seasonal timescales. These results demonstrate that summer Fe availability is comparable to the seasonally Fe limited Ross Sea shelf and therefore is likely low enough to affect phytoplankton growth and species composition.

Regeneration dynamics of iron and nutrients from bay sediment into bottom water of Funka Bay, Japan

We studied iron remobilization and nutrient regeneration in bottom water of Funka Bay, Japan, bimonthly from August 2010 to December 2011. The bay basin (bottom depth, 92–96 m) is separated from the northwest Pacific Ocean at its mouth by a sill with a depth of 60 m. After a spring phytoplankton bloom during early March–early April, nutrients in bay bottom water tended to accumulate with time until August–September, and to increase gradually with depth during April–October, by the oxidative decomposition of settling particulate organic matter on the bay bottom. In contrast, the process of iron remobilization into bottom water of the bay is remarkably different from nutrient regeneration. The much higher concentrations of dissolved and total dissolvable iron near the bottom and the seasonally variable relationship between dissolved iron concentration and apparent oxygen utilization in bay bottom water likely reflect a balance between dissolved iron input and removal processes within the bay bottom water. The release of soluble Fe(II) from reducing bay sediments might induce the high concentrations of dissolved and total dissolvable iron in deep–bottom waters of Funka Bay and might be one of the most important sources of iron in Funka Bay. The upward transport of iron from the bay bottom to the surface water during the winter vertical mixing may play an important role in the supply of bioavailable iron for phytoplankton growth in the coastal waters.

Temporal variability of reactive iron over the Gulf of Alaska shelf

The Gulf of Alaska (GoA) shelf is a highly productive regime bordering the nitrate-rich, iron (Fe)-limited waters of the central GoA. The exchange between nitrate-limited, Fe-replete coastal waters and nitrate-rich, Fe-deplete offshore waters, amplified by mesoscale eddies, is key to the productivity of the region. Previous summer field studies have observed the partitioning of Fe in the coastal GoA as being heavily dominated by the particulate phase due to the high suspended particulate loads carried by glacial rivers into these coastal waters. Here we present new physico-chemical iron data and nutrient data from the continental shelf of the GoA during spring and late summer 2011. The late summer data along the Seward Line showed variable surface dissolved iron (DFe) concentrations (0.052nM offshore to 4.87nM inshore), within the range of previous observations. Relative to available surface nitrate, DFe was in excess (at Fe:C=50μmol:mol) inshore, and deficient (at Fe:C=20μmol:mol) offshore. Summer surface total dissolvable iron (TDFe, acidified unfiltered samples) was dominated by the acid-labile particulate fraction over the shelf (with a median contribution of only 3% by DFe), supporting previously observed Fe partitioning in the GoA. In contrast, our spring data from southeast GoA showed TDFe differently partitioned, with surface DFe (0.28–4.91nM) accounting on average for a much higher fraction (~25%) of the TDFe pool. Spring surface DFe was insufficient relative to available nitrate over much of the surveyed region (at Fe:C=50μmol:mol). Organic Fe-binding ligand data reveal excess concentrations of ligands in both spring and summer, indicating incomplete titration by Fe. Excess concentrations of an especially strong-binding ligand class in spring surface waters may reflect in-situ ligand production. Due to anomalous spring conditions in 2011, river flow and phytoplankton biomass during our spring sampling were lower than the expected May average. We argue our samples are likely more representative of early spring pre-bloom conditions, providing an idea of the possible physico-chemical partitioning of Fe in coastal GoA waters relevant to initial spring bloom dynamics.

Research on Production and Pyrophorisity of Iron Sulfur Compounds in Liquid Phase

The production and pyrophorisity of iron sulfur compounds in liquid phase were studied in order to get the spontaneous combustion rules of iron sulfur compounds producted by different ways. In the experiment, FeSO4·7H2O, FeCl2·4H2O, Fe (NO3) 3·9H2O react with (NH4)2Sx and Na2S·9H2O respectively,then the pyrophorisity were analyzed by the exothermic oxidation process of sulfuration productions. The results show that, the pyrophorisity of sulfuration productions formed from (NH4)2Sx and dissolvable iron salt is obviously higher than formed from Na2S and the same dissolvable iron salt. With the increase of dissolvable iron salt concentration, the oxidation pyrophorisity of sulfuration productions formed from (NH4)2Sx also increases. There is some pyrophorisity of sulfuration productions formed from Na2S and dissolvable iron salt. But the pyrophorisity is faint relatively.

Spatial and particle size distributions of atmospheric dissolvable iron in aerosols and its input to the Southern Ocean and coastal East Antarctica

AbstractTo characterize atmospheric dissolvable iron over the Southern Ocean (SO) and coastal East Antarctica (CEA), bulk and size‐segregated (0.056–18 µm in diameter) aerosols were collected from 34°S, 109°E to 69°S, 76°E and between 69°S, 76°E and 66°S, 110°E during a cruise during November 2010 to March 2011. Aerosols were analyzed for total dissolvable Fe and Fe(II) by UV/Visible spectroscopy and total Fe by inductively coupled plasma‐mass spectrometry. The average concentrations of total Fe were 19 ng m−3 (range: 10–38 ng m−3) over the SO and 26 ng m−3 (range: 14–56 ng m−3) in CEA. The average Fe(II) concentrations were 0.22 ng m−3 (range: 0.13–0.33 ng m−3) over the SO and 0.53 ng m−3 (range: 0.18–1.3 ng m−3) over CEA; the total dissolvable Fe followed the same trend. Over the SO, a single‐peak size distribution of Fe(II) existed. Over CEA, a bimodal size distribution of Fe(II) appeared, with the first peak at 0.32–0.56 µm and the second peak at 5.6–10 µm. Higher Fe concentrations over CEA than over the SO and the existence of coarse mode Fe(II) over CEA suggest potential dust sources in Antarctica. The fractional Fe(II) solubility ranged from 0.58% to 6.5% and decreased with total Fe concentration increase. The estimated atmospheric fluxes of Fe(II) were 0.007–0.092 mg m−2 yr−1 over the SO and 0.022–0.21 mg m−2 yr−1 in CEA. Total dissolvable Fe fluxes were 0.007–0.52 mg m−2 yr−1 over the SO. The atmospheric dissolvable Fe input contributes to the dissolved Fe pool in the SO surface waters.

Iron distribution and speciation in oxic and anoxic waters of the Baltic Sea

A survey to determine the distribution of iron in the Baltic Sea has been carried out to investigate the role of iron speciation at surface waters in the fertilization and initiation of summer diazotrophic cyanobacterial blooms. Levels of total dissolvable iron (II) (Fe(II)), total dissolved iron (Fediss) and iron associated with suspended particulate material (FeSPM) were measured in surface waters from all areas of the Baltic. Additionally, depth profiles were made at three characteristic stations for vertical process studies.The highest surface values of Fe(II) (~39nmolL−1), coinciding with maximum Fediss concentrations (~600nmolkg−1) and maximum FeSPM values (~250nmolL−1), were found in the Bothnian Bay, the region of the Baltic Sea with the lowest salinity. Fe(II) concentrations in the Baltic Proper ranged from between 0.6 and 4nmolL−1, with the minimum values in the transition area between the Baltic and the North Sea. In the depth profiles, we found Fe(II) maxima in anoxic layers (≈30nmolL−1 at 160m in the Gotland Basin), which coincided with maximum Fediss (~1200nmolkg−1 at 160m) and decreasing concentrations of FeSPM, revealing a pool of possibly bioavailable Fe(II) in deep waters. Net diffusion fluxes of 10.1μmolm−2day−1 for Fediss and 0.28μmolm−2day−1 for Fe(II) through the redoxcline were calculated. For Fe(II) a half-time of 162h was calculated for the oxygen minimum zone, while a half-time of 2min was calculated for the ~70m depth horizon, where oxygen is already present.This result indicated that the Fe(II) pool in the Gotland Basin deep waters can be neglected as a major Fe(II) source for the surface waters of the Baltic Proper.To our knowledge, this is the first comprehensive investigation of Fe speciation in oxic and anoxic Baltic Sea waters.

Reverse flow injection analysis method for catalytic spectrophotometric determination of iron in estuarine and coastal waters: A comparison with normal flow injection analysis

A method for determining iron in seawater had been developed by coupling reverse flow injection analysis (rFIA) and catalytic spectrophotometric detection with N,N-dimethyl-p-phenylenediamine dihydrochloride (DPD). With a seawater sample or a standard solution as the carrier, the mixture of DPD and buffer was injected into the carrier stream quantitatively and discretely. After mixing with H2O2, the DPD was oxidized to form two pink semiquinone derivatives that were monitored at 514nm wavelength with a reference at 700nm. The method detection limit was 0.40nmolL−1, lower than half of that of normal flow injection analysis (nFIA) method. The sample throughput was 10h−1 with triplicate determination, compared with 4h−1 for nFIA-DPD method. The analysis results of the certified seawaters CASS-4 (12.33±0.18nmolL−1) and NASS-5 (3.47±0.23nmolL−1) well agreed with the certified values (12.77±1.04 and 3.71±0.63nmolL−1, respectively). The typical precision of the method for a 2.97nmolL−1 iron sample was 4.49% (n=8). Interferences from copper and salinity were investigated. An instrument was assembled based on the proposed method and applied successfully to analyze total dissolvable iron (TDFe) in surface seawater samples collected from the Pearl River Estuary, the results of which revealed non-conservative behavior of TDFe during the estuarine mixing. Results for these samples with both rFIA-DPD and nFIA-DPD methods showed good agreement with each other. The proposed method was superior to the currently used nFIA-DPD method, particularly when it is adapted for field and in situ deployment, due to its lower reagent consumption, higher sample throughput and keeping the manifold tubing clean.

Estimating labile particulate iron concentrations in coastal waters from remote sensing data

Owing to the difficulties inherent in measuring trace metals and the importance of iron as a limiting nutrient for biological systems, the ability to monitor particulate iron concentration remotely is desirable. This study examines the relationship between labile particulate iron, described here as weak acid leachable particulate iron or total dissolvable iron, and easily obtained bio‐optical measurements. We develop a bio‐optical proxy that can be used to estimate large‐scale patterns of labile iron concentrations in surface waters, and we extend this by including other environmental variables in a multiple linear regression statistical model. By utilizing a ratio of optical backscatter and fluorescence obtained by satellite, we identify patterns in iron concentrations confirmed by traditional shipboard sampling. This basic relationship is improved with the addition of other environmental parameters in the statistical linear regression model. The optical proxy detects known temporal and spatial trends in average surface iron concentrations in Monterey Bay. The proxy is robust in that similar performance was obtained using two independent particulate iron data sets, but it exhibits weaker correlations than the full statistical model. This proxy will be a valuable tool for oceanographers seeking to monitor iron concentrations in coastal regions and allows for better understanding of the variability of labile particulate iron in surface waters to complement direct measurement of leachable particulate or total dissolvable iron.

Physical speciation of iron in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre

Distributions of total dissolvable iron (TDFe; unfiltered), dissolved iron (DFe; 0.2 μm filtered), and soluble iron (SFe; 0.02 μm filtered) were investigated during the BONUS‐GoodHope cruise in the Atlantic sector of the Southern Ocean (34°S/17°E–57°S/0°, February–March 2008). In the mixed layer, mean values of 0.43 ± 0.28 and 0.22 ± 0.18 nmol L−1 were measured for TDFe and DFe, respectively. In deeper waters, TDFe and DFe concentrations were 1.07 ± 0.68 and 0.52 ± 0.30 nmol L−1, respectively. DFe concentrations decreased from the north (subtropical waters) to the south (Weddell Sea Gyre). In the subtropical domain, dusts coming from Patagonia and southern Africa and inputs from the African continental margin may explain high DFe and TDFe concentrations in surface and intermediate waters. Results from numerical models gave support to these hypotheses. In the Antarctic Circumpolar Current domain, estimation of the median advective time of water masses suggests that sediment inputs from the Antarctic Peninsula, South America margin, and/or South Georgia Islands could be an important source of Fe. Except in the subtropical domain where 0.4–0.6 nmol L−1 of SFe were observed in the upper 1500 m, all stations exhibited values close to 0.1–0.2 nmol L−1 in surface and 0.3–0.5 nmol L−1 in deeper waters. For all stations, colloidal Fe (CFe) was a minor fraction of DFe in surface waters and increased with depth. Colloidal aggregation, sinking of CFe, and assimilation of SFe, followed by rapid exchange between the two fractions, are suspected to occur.

An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L−1 above the plateau and from 0.34 to 2.23 nmol L−1 offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329–770 nmol L−1). Apparent particulate iron (Feapp), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Feapp above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Feapp and DFe above the plateau, with a supply of 9.7±3.6×106 and 8.3±11.6×103 mol d−1, respectively. The residence times of 1.7 and 48 days estimated for Feapp and DFe respectively, indicate a rapid turnover in the surface water. A comparison between Feapp and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe)−1, which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.

Removal of iron from ilmenite by KOH leaching-oxalate leaching method

Oxalic acid was used for the removal of iron from the intermediates of ilmenite leached by KOH liquor. Various parameters, such as pH, temperature, initial oxalate concentration, and illumination were investigated. Meanwhile, it was found that orthorhombic crystal Ti2O2(OH)2(C2O4)·H2O formed as the leaching proceeded. Scanning electronic microscope (SEM) images implied that the formation of Ti2O2(OH)2(C2O4)·H2O with good crystallinity proceeded through three stages. Calcining Ti2O2(OH)2(C2O4)·H2O, anatase (350·C) or rutile (550·C) type TiO2 was obtained, respectively. Element analysis found that the calcined product contained 94.9% TiO2 and 2.5% iron oxide, but only about 1600 ppm dissolvable iron oxide was left, which indicates that oxalic acid was comparatively effective on iron oxide removal from the intermediates. Finally, an improved route was proposed for the upgrading of ilmenite into rutile.

オホーツク海中央海底の堆積物直上海水の全可溶マンガン・鉄濃度

オホーツク海中央部の堆積物直上海水中の全可溶マンガンや鉄濃度を調査する目的で，8本のマルチプルコア試料が採取された．堆積物直上海水のpH, 溶存酸素濃度，全可溶マンガン及び鉄量の測定を行なった．溶存酸素と全可溶鉄濃度には，強い負の相関が認められた．また，これまでの研究にて報告された結果と比べて，いくつかの地点では本研究で示された全可溶鉄量が比較的高いことが明らかとなった．以上から，これらの地点の海水－堆積物境界では比較的還元的な状態にあり，鉄が堆積物からが溶出していることが示唆される.

A long‐term, high‐resolution record of surface water iron concentrations in the upwelling‐driven central California region

Seven years of observations of surface water iron concentrations in the Monterey Bay region of central California reveal a consistent annual cycle dominated by the injection of high concentrations of particulate iron each spring. A companion study of the water column near the upwelling center at the north end of the bay clearly indicates a sedimentary source for the iron. Local river discharge, during winter storm events, results in the deposition of a fine‐grained sediment “fluff” layer along the shelf. The initial rapid shoaling of isotherms at the onset of upwelling in spring brings water from the fluff layer to the surface. Concentrations of iron in both surface and source waters are then quickly diminished although upwelling intensifies, bringing the highest concentration of nitrate to the surface ∼3 months later. The concentration of a number of constituents measured in bottom water samples collected during the companion study strongly suggests that the rapid decrease in surface water iron concentrations is due to the depletion of the fluff layer following initial isotherm shoaling. The decoupling of iron and nitrate supply sets up the potential for iron limitation. However, on the basis of average Fe/NO3 ratios, we conclude that the region is not often iron limited. Iron limitation was apparent only one summer at the offshore station. Summer chlorophyll concentrations are highly correlated to dissolvable (unfiltered) iron concentrations, evidence in support of the role of particulate iron in meeting the ecosystem's iron requirements.

Iron, manganese and aluminum in upper waters of the western South Pacific Ocean and its adjacent seas

Total dissolvable iron, manganese and aluminum distributions in upper waters were determined in the western South Pacific, Solomon Sea, Coral Sea, and Tasman Sea. In these oceanic regions, the surface aluminum distributions well reflect the atmospheric deposition pattern of mineral dust in the western South Pacific reported previously. Surface manganese distributions derive mainly from lateral transportation from the coastal sediments of western tropical islands. Compared to Mn and Al, the Fe distributions reflect the nutrient cycle in upper waters. Iron limitation over the vast South Pacific, as revealed by physiological features of phytoplankton, seems to be caused by low atmospheric dust deposition and low Fe:N ratios in deep waters. In the western South Pacific, with its unique geographic and oceanographic settings, the local sources of trace metals might considerably affect their biogeochemical cycles.