The North Atlantic receives the highest aerosol (dust) input of all the oceanic basins. Dust deposition provides essential bioactive elements, as well as pollution-derived elements, to the surface ...ocean. The arid regions of North Africa are the predominant source of dust to the North Atlantic Ocean. In this study, we describe the elemental composition (Li, Na, Mg, Al, P, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Cd, Sn, Sb, Cs, Ba, La, Ce, Nd, Pb, Th, U) of the bulk aerosol from samples collected during the US-GEOTRACES North Atlantic Zonal Transect (2010/11) in order to highlight the differences between a Saharan dust end-member and the reported elemental composition of the upper continental crust (UCC), and the implications this has for identifying trace element enrichment in aerosols across the North Atlantic basin. As aerosol titanium (Ti) is less soluble than aerosol aluminum (Al), it is a more conservative tracer for lithogenic aerosols and trace element-to-Ti ratios. However, the presence of Ti-rich fine aerosols can confound the interpretation of elemental enrichments, making Al a more robust tracer of aerosol lithogenic material in this region.
•First basin-wide measurements of plankton metal quotas in the N. Atlantic Ocean.•Fe and Mn quotas significantly higher on western side of section.•Cu and Ni quotas significantly elevated on eastern ...side of section.•Evidence for Al scavenging by biogenic silica.•Dissolved ratios not an accurate measure of cellular Fe quotas.
Phytoplankton contribute significantly to global C cycling and serve as the base of ocean food webs. Phytoplankton require trace metals for growth and also mediate the vertical distributions of many metals in the ocean. We collected bulk particulate material and individual phytoplankton cells from the upper water column (<150m) of the North Atlantic Ocean as part of the US GEOTRACES North Atlantic Zonal Transect cruise (GEOTRACES GA03). Particulate material was first leached to extract biogenic and potentially-bioavailable elements, and the remaining refractory material was digested in strong acids. The cruise track spanned several ocean biomes and geochemical regions. Particulate concentrations of metals associated primarily with lithogenic phases (Fe, Al, Ti) were elevated in surface waters nearest North America, Africa and Europe, and elements associated primarily with biogenic material (P, Cd, Zn, Ni) were also found at higher concentrations near the coasts. However metal/P ratios of labile particulate material were also elevated in the middle of the transect for Fe, Ni, Co, Cu, and V. P-normalized cellular metal quotas measured with synchrotron X-ray fluorescence (SXRF) were generally comparable to ratios in bulk labile particles but did not show mid-basin increases. Manganese and Fe ratios and cell quotas were higher in the western part of the section, nearest North America, and both elements were more enriched in bulk particles, relative to P, than in cells, suggesting the presence of labile oxyhydroxide particulate phases. Cellular Fe quotas thus did not increase in step with aeolian dust inputs, which are highest near Africa; these data suggest that the dust inputs have low bioavailability. Copper and Ni cell quotas were notably higher nearest the continental margins. Overall mean cellular metal quotas were similar to those measured in the Pacific and Southern Oceans except for Fe, which was approximately 3-fold higher in North Atlantic cells. Cellular Fe quotas are in-line with those measured in laboratory cultures at comparable Fe concentrations. Particulate Zn, Cu, Ni, and Co are primarily associated with cellular material, but less than 30% of labile particulate Fe and Mn are biogenic. Particulate Al was primarily associated with lithogenic material, but the labile fraction was highly correlated with P, as well as with biogenic silica, suggesting that some particulate Al (perhaps around 20%) may occur adsorbed to biogenic material. Cellular element maps indicate that externally scavenged Fe was not a significant fraction of the metal associated with live phytoplankton, but adsorbed or precipitated phases are likely to be important in particulate detrital material. Such abiotic scavenging, along with differential remineralization of cellular nutrients in the water column, results in estimates of cellular metal/nutrient ratios from dissolved concentrations that significantly underestimate the ratios in phytoplankton. These data demonstrate the response of phytoplankton to the unique metal inputs to the North Atlantic Ocean.
Trace elements sustain biological productivity, yet the significance of trace element mobilization and export in subglacial runoff from ice sheets is poorly constrained at present. Here, we present ...size-fractionated (0.02, 0.22, and 0.45 μm) concentrations of trace elements in subglacial waters from the Greenland Ice Sheet (GrIS) and the Antarctic Ice Sheet (AIS). Concentrations of immobile trace elements (e.g., Al, Fe, Ti) far exceed global riverine and open ocean mean values and highlight the importance of subglacial aluminosilicate mineral weathering and lack of retention of these species in sediments. Concentrations are higher from the AIS than the GrIS, highlighting the geochemical consequences of prolonged water residence times and hydrological isolation that characterize the former. The enrichment of trace elements (e.g., Co, Fe, Mn, and Zn) in subglacial meltwaters compared with seawater and typical riverine systems, together with the likely sensitivity to future ice sheet melting, suggests that their export in glacial runoff is likely to be important for biological productivity. For example, our dissolved Fe concentration (20,900 nM) and associated flux values (1.4 Gmol y−1) from AIS to the Fe-deplete Southern Ocean exceed most previous estimates by an order of magnitude. The ultimate fate of these micronutrients will depend on the reactivity of the dominant colloidal size fraction (likely controlled by nanoparticulate Al and Fe oxyhydroxide minerals) and estuarine processing. We contend that ice sheets create highly geochemically reactive particulates in subglacial environments, which play a key role in trace elemental cycles, with potentially important consequences for global carbon cycling.
Patients with chronic coronary disease were randomly assigned to receive 0.5 mg of colchicine once daily or matching placebo. The incidence of the composite end point of cardiovascular death, ...spontaneous myocardial infarction, ischemic stroke, or ischemia-driven coronary revascularization was significantly lower with colchicine than with placebo.
The mineralogy and oxidation state of aerosol iron (Fe) play important roles in controlling aerosol Fe solubility and consequent bioavailability in seawater. In this study, the spatial variability of ...Fe mineralogy and oxidation states in aerosols collected during the US GEOTRACES Western Arctic cruise (GN01) were determined using synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy. Both Fe(II) minerals (biotite, ilmenite) and Fe(III) minerals (ferrihydrite, hematite, Fe(III) phosphate) were found in these samples. However, aerosol Fe mineralogy and solubility observed during this cruise varied spatially and can be grouped into three clusters based on the air masses that affected aerosols collected in different regions: (1) biotite-enriched particles (87 % biotite, 13 % hematite) with the air masses passing over Alaska, showing relatively low Fe solubility (4.0 ± 1.7 %); (2) ferrihydrite-enriched particles (82 % ferrihydrite, 18 % ilmenite) collected in the remote Arctic air, showing relatively high Fe solubility (9.6 ± 3.3 %); (3) the fresh dust derived from North America and Siberia, primarily dominated by hematite (41 % hematite, 25 % Fe(III) phosphate, 20 % biotite, 13 % ferrihydrite), showing relatively low Fe solubility (5.1 ± 3.5). A significant positive correlation was found between Fe oxidation state and Fe fractional solubility, suggesting that long-range transport could modify iron (hydr) oxide such as ferrihydrite through atmospheric processing, influencing aerosol Fe solubility and consequently Fe bioavailability in the remote Arctic Ocean.
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•Western Arctic Ocean mineral aerosols are rich in hematite, ferrihydrite, while Alaska's aerosols predominantly contain biotite.•Aerosol Fe oxidation state is more oxidized with increasing latitude.•A significant correlation was found between Fe oxidation state and Fe fractional solubility.
Microbial N
fixation (diazotrophy) represents an important nitrogen source to oligotrophic peatland ecosystems, which are important sinks for atmospheric CO
and are susceptible to the changing ...climate. The objectives of this study were (i) to determine the active microbial group and type of nitrogenase mediating diazotrophy in an ombrotrophic
-dominated peat bog (the S1 peat bog, Marcell Experimental Forest, Minnesota, USA); and (ii) to determine the effect of environmental parameters (light, O
, CO
, and CH
) on potential rates of diazotrophy measured by acetylene (C
H
) reduction and
N
incorporation. A molecular analysis of metabolically active microbial communities suggested that diazotrophy in surface peat was primarily mediated by
(
and
). Despite higher concentrations of dissolved vanadium (V 11 nM) than molybdenum (Mo 3 nM) in surface peat, a combination of metagenomic, amplicon sequencing, and activity measurements indicated that Mo-containing nitrogenases dominate over the V-containing form. Acetylene reduction was only detected in surface peat exposed to light, with the highest rates observed in peat collected from hollows with the highest water contents. Incorporation of
N
was suppressed 90% by O
and 55% by C
H
and was unaffected by CH
and CO
amendments. These results suggest that peatland diazotrophy is mediated by a combination of C
H
-sensitive and C
H
-insensitive microbes that are more active at low concentrations of O
and show similar activity at high and low concentrations of CH
Previous studies indicate that diazotrophy provides an important nitrogen source and is linked to methanotrophy in
-dominated peatlands. However, the environmental controls and enzymatic pathways of peatland diazotrophy, as well as the metabolically active microbial populations that catalyze this process, remain in question. Our findings indicate that oxygen levels and photosynthetic activity override low nutrient availability in limiting diazotrophy and that members of the
(
) catalyze this process at the bog surface using the molybdenum-based form of the nitrogenase enzyme.
Iron (Fe) and copper (Cu) are essential cofactors for microbial metalloenzymes, but little is known about the metalloenyzme inventory of anaerobic marine microbial communities despite their ...importance to the nitrogen cycle. We compared dissolved O2, NOFormula: see text, NOFormula: see text, Fe and Cu concentrations with nucleic acid sequences encoding Fe and Cu-binding proteins in 21 metagenomes and 9 metatranscriptomes from Eastern Tropical North and South Pacific oxygen minimum zones and 7 metagenomes from the Bermuda Atlantic Time-series Station. Dissolved Fe concentrations increased sharply at upper oxic-anoxic transition zones, with the highest Fe:Cu molar ratio (1.8) occurring at the anoxic core of the Eastern Tropical North Pacific oxygen minimum zone and matching the predicted maximum ratio based on data from diverse ocean sites. The relative abundance of genes encoding Fe-binding proteins was negatively correlated with O2, driven by significant increases in genes encoding Fe-proteins involved in dissimilatory nitrogen metabolisms under anoxia. Transcripts encoding cytochrome c oxidase, the Fe- and Cu-containing terminal reductase in aerobic respiration, were positively correlated with O2 content. A comparison of the taxonomy of genes encoding Fe- and Cu-binding vs. bulk proteins in OMZs revealed that Planctomycetes represented a higher percentage of Fe genes while Thaumarchaeota represented a higher percentage of Cu genes, particularly at oxyclines. These results are broadly consistent with higher relative abundance of genes encoding Fe-proteins in the genome of a marine planctomycete vs. higher relative abundance of genes encoding Cu-proteins in the genome of a marine thaumarchaeote. These findings highlight the importance of metalloenzymes for microbial processes in oxygen minimum zones and suggest preferential Cu use in oxic habitats with Cu > Fe vs. preferential Fe use in anoxic niches with Fe > Cu.
Cobalt (Co) is an important bioactive trace metal that is the metal
cofactor in cobalamin (vitamin B12) which can limit or co-limit
phytoplankton growth in many regions of the ocean. Total dissolved ...and
labile Co measurements in the Canadian sector of the Arctic Ocean during the
U.S. GEOTRACES Arctic expedition (GN01) and the Canadian International Polar
Year GEOTRACES expedition (GIPY14) revealed a dynamic biogeochemical cycle
for Co in this basin. The major sources of Co in the Arctic were from shelf
regions and rivers, with only minimal contributions from other freshwater
sources (sea ice, snow) and eolian deposition. The most striking feature
was the extremely high concentrations of dissolved Co in the upper 100 m,
with concentrations routinely exceeding 800 pmol L−1 over the shelf
regions. This plume of high Co persisted throughout the Arctic basin and
extended to the North Pole, where sources of Co shifted from primarily
shelf-derived to riverine, as freshwater from Arctic rivers was entrained in
the Transpolar Drift. Dissolved Co was also strongly organically complexed
in the Arctic, ranging from 70 % to 100 % complexed in the surface and deep
ocean, respectively. Deep-water concentrations of dissolved Co were
remarkably consistent throughout the basin (∼55 pmol L−1), with concentrations reflecting those of deep Atlantic water and
deep-ocean scavenging of dissolved Co. A biogeochemical model of Co cycling
was used to support the hypothesis that the majority of the high surface Co
in the Arctic was emanating from the shelf. The model showed that the high
concentrations of Co observed were due to the large shelf area of the
Arctic, as well as to dampened scavenging of Co by manganese-oxidizing (Mn-oxidizing)
bacteria due to the lower temperatures. The majority of this scavenging
appears to have occurred in the upper 200 m, with minimal additional
scavenging below this depth. Evidence suggests that both dissolved Co (dCo) and labile Co (LCo) are increasing over time on the Arctic shelf, and these limited temporal results are consistent
with other tracers in the Arctic. These
elevated surface concentrations of Co likely lead to a net flux of Co out of
the Arctic, with implications for downstream biological uptake of Co in the
North Atlantic and elevated Co in North Atlantic Deep Water. Understanding
the current distributions of Co in the Arctic will be important for
constraining changes to Co inputs resulting from regional intensification of
freshwater fluxes from ice and permafrost melt in response to ongoing
climate change.
The Arctic region is undergoing significant changes in climate, with a notable decrease in summertime sea ice coverage over the past three decades. This trend means an increasing proportion of Arctic ...Ocean surface waters can receive direct deposition of material from the atmosphere, potentially influencing marine biogeochemical cycles and delivery of pollutants to the Arctic ecosystem. Here, we present aerosol concentrations of selected trace elements (Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) measured during the US GEOTRACES Western Arctic cruise (GN01, also known as HLY1502) in August–October 2015. Concentrations of “lithogenic” elements (Al, Ti, V, Mn, Fe, and Co) were similar to those measured in remote and predominantly marine-influenced air masses in previous studies, reflecting the remoteness of the Arctic Ocean from major dust sources. Concentrations of Ni, Cu, Zn, Pb, and Cd showed significant enrichments over crustal values, and were often of similar magnitude to concentrations measured over the North Atlantic in air masses of North American or European provenance. We use 7Be inventory and flux data from GN01 to estimate a bulk atmospheric deposition velocity during the study period, and combine it with our aerosol concentrations to calculate atmospheric deposition fluxes of the trace elements in the Arctic region during late summer. The resulting estimates for mineral dust and Fe deposition fall at the low end of global estimates and confirm the Arctic Ocean as a low-dust environment during the summer months.
This article is part of a special issue entitled: Conway GEOTRACES - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
•We report bulk aerosol concentrations of multiple trace elements from the western Arctic Ocean during summer 2015.•Deposition fluxes are calculated using a bulk deposition velocity calculated from aerosol and snow 7Be data.•Summertime atmospheric deposition fluxes of mineral dust and Fe to the Arctic are low relative to other oceanic regions.
The measurable supply of 232Th to the ocean can be used to derive the supply of other elements, which is more difficult to quantify directly. The measured inventory of an element divided by the ...derived supply yields a replacement time estimate, which in special circumstances is related to a residence time. As a proof of concept, Th‐based supply rates imply a range in the replacement times of the rare earth elements in the North Atlantic that is consistent with the chemical reactivity of rare earth elements related to their ionic charge density. Similar estimates of replacement times for the bioactive trace elements (Fe, Mn, Zn, Cd, Cu, and Co), ranging from <5 years to >50,000 years, demonstrate the broad range of elemental reactivity in the ocean. Here we discuss how variations in source composition, fractional solubility ratios, or noncontinental sources, such as hydrothermal vents, lead to uncertainties in Th‐based replacement time estimates. We show that the constraints on oceanic replacement time provided by the Th‐based calculations are broadly applicable in predicting how elements are distributed in the ocean and for some elements, such as Fe, may inform us on how the carbon cycle may be impacted by trace element supply and removal.
Key Points
Thorium‐232 supply rates produce reasonable ocean replacement times for the rare earth elements
Iron residence time in the North Atlantic may be less than 6 years
Thorium‐232 supply rates may provide an accurate basis for the supply of many other elements for constraining biogeochemical models
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