Search

The role that dissolved iron (dFe) rich Circumpolar Deep Water (CDW) might play in sustaining the consistently observed discrete patches of high chlorophyll biomass over Pennell Bank (PB) and Mawson Bank (MB) in the Ross Sea, was investigated during January/February 2011. Over a 26-day period, hydrographic and trace metal clean water sampling was carried out adjacent to both of these banks, in some cases repeatedly. Particulate sampling was also accomplished at selected stations by in situ pumping. The results indicate that the dFe content of the CDW is in fact reduced by on-shelf mixing with Antarctic Surface Water as it transitions into modified CDW (MCDW). Our stations above PB, where the maximum bloom is encountered, show no evidence of MCDW presence. In contrast, above MB, where there is a smaller persistent bloom, MCDW was observed. Although both of these stations displayed the imprint of sedimentary Fe input connected to the strong tidal cycles above the banks, the stronger near-bottom density gradient that MCDW produces appears to contribute to reduced vertical mixing of the sedimentary source. Thus, ironically, the presence of MCDW may be hindering the Fe supply to the surface waters, rather than being the source, as originally hypothesized. [ABSTRACT FROM AUTHOR]

High-resolution dissolved Fe (dFe) and dissolved Mn (dMn) distributions were obtained using a trace metal clean rosette during the GEOTRACES GA03 zonal transect cruises (USGT10 and USGT11) across the North Atlantic Ocean. This manuscript provides a general overview of the dFe, as well as dMn and dissolved Al (dAl) distributions that reveal several Fe inputs at varying depths across the study region. Elevated dFe concentrations correlate with elevated dAl concentrations in the surface waters of the subtropical gyre, indicating a significant atmospheric source of Fe, in contrast there is no apparent significant dust source for Mn. In the subsurface waters, dFe maxima are a result of the remineralization process, as revealed by their correspondence with dissolved oxygen minima. Within the oxygen minimum, the ratio of dFe to apparent oxygen utilization (AOU) is lower than would be expected from the measured Fe content of surface water phytoplankton, suggesting that a significant amount of dFe that is remineralized at depth (~63–90%) is subsequently scavenged from the water column. The rate of remineralization, which is based on the slope of dFe:AOU plot, is similar across a wide area of the North Atlantic. In addition to the remineralization process, sedimentary inputs are apparent from elevated dMn signals in the eastern basin, particularly near the African coast. In the western basin, sedimentary input is also occurring along the advective flow path of the Upper Labrador Sea Water (ULSW), as ULSW transits along the North American continental shelf region. The largest dFe anomaly (~68 nM), which also corresponds to a dMn anomaly (up to ~33 nM) is seen in the neutrally buoyant hydrothermal plume sampled over the Mid-Atlantic Ridge, and that signal is visible for ~500 km to the west of the ridge. [ABSTRACT FROM AUTHOR]

We aimed to analyze the association between dietary iron intake and obesity assessed by BMI after adjustment for nutrient intake (macronutrients and fiber) and food groups. The study design was cross-sectional. Patients with type-2 diabetes (n = 1567; 63.1% males; mean age 62.3 ± 11.6 years) were included in the study. To assess diet, consumption of typical food groups was determined by a food frequency questionnaire. Obesity was defined as BMI ≥ 25 kg/m2. We performed a binary regression analysis between quartiles of iron intake and obesity by quartiles of age group. A direct linear association was found for the highest quartile of iron intake and obesity in the younger age group of 30 to 54 years (OR = 3.641, 95% CI = 1.020–12.990; p trend = 0.011). Multivariate analysis using food groups as opposed to nutrients revealed a positive trend for obesity in the younger age group after adjusting for lifestyle factors, energy intake and bean and vegetable intake (p trend = 0.023). In all participants, an inverse association was observed before adjustment by vegetable intake (OR = 0.453, 95% CI = 0.300–0.684; p trend = 0.001). Higher iron intake was associated with obesity independent of macronutrient and fiber intake but only in the youngest quartile of age group examined. [ABSTRACT FROM AUTHOR]

The carbon to iron (C:Fe) ratio of planktonic biomass constrains net production in iron-limited regions of the ocean and is an important parameter for predicting biomass production from iron inputs. On a cruise to the southern Drake Passage in July–August 2006, we used two approaches to determine the C:Fe ratio of planktonic material: dual-radiotracer labeling and net biomass production in iron-limited grow-out experiments. There was variability in C:Fe ratios among experiments, but values from the two methods overlapped with average values of 1.4×105 (mol:mol) for the radiotracer method and 1.7×105 for the net biomass production method. This is notable since the net biomass production method is a new approach to determine C:Fe ratios. Although it has potential issues related to bottle effects and sensitivity to trace contamination, the method avoids some of the questions associated with iron speciation and bioavailability since ambient iron supports production. Because light intensity is known to affect C:Fe ratios in phytoplankton through photosynthetic iron demands, we tested the effect of light level on C:Fe in Antarctic assemblages. In contrast to what is seen in many phytoplankton cultures, C:Fe ratios increased at low-light, but we suspect that this is due to initial photoinhibition of the low-light adapted winter assemblages at higher light levels. [ABSTRACT FROM AUTHOR]

Abstract: Horizontal advection, vertical mixing, and mixed-layer entrainment all affect iron concentrations and biological productivity in the Ona Basin, near the Shackleton Transverse Ridge (STR) in southern Drake Passage. Trace metal sampling in the region indicates that dissolved iron concentrations are significantly higher on the continental shelf near the Antarctic Peninsula and the South Shetland Islands than they are in the deep waters away from the shelf. Comparisons between satellite-derived sea surface height (SSH) and Chlorophyll-a (Chl-a) levels in the Ona Basin show correlation between Chl-a concentrations and horizontal advection of these iron-rich shelf waters during the months of November and December for the years 1997–2010. However, no significant correlations are found for January–April, while high Chl-a concentrations in the Ona Basin persist through March. Enhanced vertical (diapycnal) mixing and mixed-layer entrainment are considered as alternative mechanisms for delivering iron into the Ona Basin mixed layer and sustaining the high Chl-a concentrations. Estimates of iron flux based on in situ measurements of dissolved iron concentrations suggest that diapycnal mixing alone can supply iron to the base of the mixed layer at a rate of 64±2nmolm−2 day−1 during the summer. In addition, the summer mixed layer in the Ona Basin deepens from January to April, allowing for iron-rich water to be steadily entrained from below. Estimates based on monthly mixed-layer climatologies produce average daily entrainment rates ranging from 5 to 25nmolm−2 day−1. While neither diapycnal mixing nor entrainment alone is always sufficient to meet the estimated iron demand for the Ona Basin bloom, numerical simulation suggests that the combined effect of the two processes can consistently supply sufficient iron to sustain the bloom. [Copyright &y& Elsevier]

Dissolved iron (Fe) and manganese (Mn) share common sources and sinks in the global ocean. However, Fe and Mn also have different redox reactivity and speciation that can cause their distributions to become decoupled. The Arctic Ocean provides a unique opportunity to compare Fe and Mn distributions because the wide Arctic continental shelves provide significant margin fluxes of both elements, yet in situ vertical regeneration inputs that can complicate scavenging calculations are negligible under the ice of the Arctic Ocean, making it easier to interpret the fate of lateral gradients. We present here a large-scale case study demonstrating a three-step mechanism for Fe and Mn decoupling in the upper 400 m of the Western Arctic Ocean. Both Fe and Mn are released during diagenesis in porewaters of the Chukchi Shelf, but they become immediately decoupled when Fe is much more rapidly oxidized and re-precipitated than Mn in the oxic Chukchi Shelf water column, leading to Fe hosted primarily in the particulate phase and Mn in the dissolved phase. However, as these shelf fluxes are transported toward the shelf break and subducted into the subsurface halocline water mass, the loss rates of all species change significantly, causing further Fe and Mn decoupling. In the second decoupling step in the shelf break region, the dominant shelf species are removed rapidly via particle scavenging, with smallest soluble Fe (sFe < 0.02 µm) being least subject to loss, while colloidal Fe (0.02 µm < cFe < 0.2 µm), dissolved Mn (dMn), and non-lithogenic particulate Fe (pFe xs) are all lost at similarly rapid rates. In the third decoupling step, once these species are swept >1000 km offshore with the prevailing current into the low-particle waters of the open Arctic, cFe and dMn appear conserved, while pFe, dFe, and sFe are very slowly removed with variable log-scale distances of transport: pFe ≪ dFe < sFe. To assess the role of physicochemical speciation on these trends, we observed that Fe(II) was a small (∼7%) fraction of total dFe in the upper 400 m of the Arctic, even over the shelf (∼2%). Also, colloidal contribution to dFe was very low (∼20%) in the open Arctic, in contrast to dFe in the North Atlantic, which is composed much more by colloids (≥50%). Throughout the Western Arctic Ocean, Fe and Mn are thus decoupled as a result of distinct oxidation kinetics and different scavenging rates within high- and low-particle regimes. As the "scavengers of the sea", the relative distribution of particulate Fe and Mn phases across the Arctic Ocean shelf and slope, respectively, will play an important role in determining the distribution and ultimate sediment burial site for other scavenging-prone trace elements. Additionally, we suggest that the future effects of climate change, including loss of sea ice that could impact the formation of the halocline, might change distributions of Fe and Mn species in the future Western Arctic. [ABSTRACT FROM AUTHOR]

The size partitioning of dissolved Fe (dFe<0.2 µm) into soluble (sFe<0.02 µm) and colloidal (0.02 µm

Abstract: The Drake Passage region near Elephant Island in the Southern Ocean displays patchy phytoplankton blooms. To test the hypothesis that natural Fe addition from localized sources promoted phytoplankton growth here, a grid of stations (59°S to 62°S, 59°W to 53°W, as well as four stations in the eastern Bransfield Strait) were occupied from 12 February–24 March 2004. Phytoplankton abundance was measured using shipboard flow cytometry (70 stations), with abundances conservatively converted to biomass, and compared with measurements of dissolved iron (dFe) at a subset of stations (30 stations). Based on T–S property plots, stations were divided into Antarctic Circumpolar Current (ACC), Water On Shelf (WOS), Bransfield Strait (BS), and Mixed water stations, the latter representing locations with T–S properties intermediate between ACC and WOS stations. The highest integrated phytoplankton biomass was found at Mixed water stations, however, the highest integrated abundance was found at WOS stations, demonstrating that abundance and biomass do not necessarily show the same patterns. The distributions of nano- and micro-phytoplankton (<20 and >20μm diameter cells, respectively) were also examined, with nano- and micro-plankton contributing equally to the total biomass at WOS and BS stations, but micro-plankton representing ∼2/3 of the biomass at Mixed and ACC stations. Increased inventories of dFe did not always correspond to increases in phytoplankton biomass – rather stations with lower mean light levels in the mixed layer (<110μEinsteinsm−2 s−1) had lower biomass despite higher ambient dFe concentrations. However, where the mean light levels in the mixed layer were >110μEinsteinsm−2 s−1, total biomass shows a positive trend with dFe, as does micro-phytoplankton biomass, but neither regression is significant at the 95% level. In contrast, if just nano-phytoplankton biomass is considered as a function of dFe, there is a significant correlation (r 2=0.62). These data suggest a dual mechanism for the patterns observed in biomass: an increasing reservoir of dFe allows increased phytoplankton biomass, but biomass can only accumulate where the light levels are relatively high, such that light is not limiting to growth. [Copyright &y& Elsevier]