•Ni and δ60Ni data depth profiles from South Atlantic Ocean transect.•Isotopically homogeneous deep ocean; fractionation to heavy values in the upper water column.•Phytoplankton mediated ...fractionation in the photic zone, but with small fractionation factor.•Ecologically controlled changes in Ni-nutrient systematics across Antarctic Polar Front.•Ni systematics provide another example of the biogeochemical divide in the Southern Ocean.
Nickel (Ni) is important for a number of enzymes in oceanic phytoplankton. It has received less attention than some other bioactive metals because it is not reduced to extremely low dissolved concentrations in the photic zone. However, there are strong indications in previous studies that this residual pool is not bio-available. Oceanic Ni isotope data are still scarce, but have great potential for understanding this issue, as well as for understanding the Ni mass balance of the oceans now and in the past. Here, we present new concentration and isotope data for the UK GEOTRACES section at 40°S in the Atlantic (GA10). Nickel concentration data show typical nutrient-like profiles, slightly modified by variable pre-formed concentrations in sub-surface water masses, e.g. North Atlantic Deep Water. Nickel isotopes, in common with findings in previous studies, are homogeneous beneath 500 m, at about +1.3‰ in δ60Ni, in samples with Ni concentrations above 3-3.5 nM. The surface South Atlantic, however, has concentrations below 3 nM, and shows significantly higher δ60Ni, up to +1.74‰, that are closely anti-correlated with Ni concentrations.
The data for the deep South Atlantic dissolved pool, with a δ60Ni = 1.31 ± 0.12‰ (average and 2SD) confirm the homogeneity of the global deep ocean, which previous data demonstrate extends all the way to the surface in the upwelling zone of the Southern Ocean south of the Polar Front. This Ni isotope composition is significantly heavier than known inputs to the oceanic dissolved pool. This mass balance requires an isotopically light sink that may be represented by sedimentary Mn-oxide associated Ni. The magnitude of the isotope fractionation implied by the upper ocean data is not consistent with plausible potential abiotic removal processes. Rather, these data are best explained by biological uptake. However, consideration of the detailed relationships between Ni concentrations and isotope compositions requires that a substantial portion of the oceanic dissolved Ni pool is not bio-available. The data are consistent either with a small preference for the light isotope during uptake (about 0.1‰) or two distinct pools of dissolved Ni, one bio-available and one strongly bound in organic complexes, with limited isotopic exchange between them. Patterns of co-variation in Ni concentrations and isotopes with the major nutrients point to strong contrasts across the Polar Front of the Southern Ocean, contrasts that exhibit both similarities and differences with those for the major nutrients and other trace metal micronutrients. South of the Polar Front, Ni is taken up in modest amounts by diatoms, without isotope fractionation. North of the Polar Front the data are most consistent with cyanobacteria as the dominant control on Ni uptake, leading to significant coupled abundance and isotope variation.
A novel method, combining isotope dilution with standard additions, was developed for the analysis of eight elements (Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb) in seawater. The method requires just 12
mL of ...sample and employs an off-line pre-concentration step using the commercially available chelating resin Toyopearl AF-Chelate-650M prior to determination by high resolution inductively coupled plasma magnetic sector mass spectrometry (ICP-MS). Acidified samples were spiked with a multi-element standard of six isotopes (
57Fe,
62Ni,
65Cu,
68Zn,
111Cd and
207Pb) enriched over natural abundance. In addition, standard additions of a mixed Co and Mn standard were performed on sub-sets of the same sample. All samples were irradiated using a low power (119
mW
cm
−2; 254
nm) UV system, to destroy organic ligands, before pre-concentration and extraction from the seawater matrix. Ammonium acetate was used to raise the pH of the 12
mL sub-samples (off-line) to pH 6.4
±
0.2 prior to loading onto the chelating resin. The extracted metals were eluted using 1.0
M Q-HNO
3 and determined using ICP-MS. The method was verified through the analysis of certified reference material (NASS-5) and the SAFe inter-comparison samples (S1 and D2), the results of which are in good agreement with the certified and reported consensus values. We also present vertical profiles of the eight metals taken from the Bermuda Atlantic Time Series (BATS) station collected during the GEOTRACES inter-comparison cruise in June 2008.
Copper (Cu) is both an essential micronutrient and toxic to photosynthesizing microorganisms at low concentrations. Its dissolved vertical distribution in the oceans is unusual, being neither a ...nutrient-type nor scavenged-type element. This distribution is attributed to biological uptake in the surface ocean with remineralisation at depth, combined with strong organic complexation by dissolved ligands, scavenging onto particles, and benthic sedimentary input. We present coupled dissolved and particulate phase Cu isotope data along the UK-GEOTRACES South Atlantic section, alongside higher resolution dissolved and particulate phase Cu concentration measurements. Our dissolved phase isotope data contribute to an emerging picture of homogeneous deep ocean δ65Cu, at about +0.65‰ (relative to NIST SRM 976). We identify two pools of Cu in the particulate phase: a refractory, lithogenic pool, at about 0‰, and a labile pool accessed via a weak acidic leach, at about +0.4‰. These two pools are comparable to those previously observed in sediments. We observe deviations towards lighter δ65Cu values in the dissolved phase associated with local enrichments in particulate Cu concentrations along the continental slopes, and in the surface ocean. Copper isotopes are thus a sensitive indicator of localised particle-associated benthic or estuarine Cu inputs. The measurement of Cu isotopes in seawater is analytically challenging, and we call for an intercalibration exercise to better evaluate the potential impacts of UV-irradiation, storage time, and different analytical procedures.
Despite recent advances in observational data coverage, quantitative constraints on how different physical and biogeochemical processes shape dissolved iron distributions remain elusive, lowering ...confidence in future projections for iron-limited regions. Here we show that dissolved iron is cycled rapidly in Pacific mode and intermediate water and accumulates at a rate controlled by the strongly opposing fluxes of regeneration and scavenging. Combining new data sets within a watermass framework shows that the multidecadal dissolved iron accumulation is much lower than expected from a meta-analysis of iron regeneration fluxes. This mismatch can only be reconciled by invoking significant rates of iron removal to balance iron regeneration, which imply generation of authigenic particulate iron pools. Consequently, rapid internal cycling of iron, rather than its physical transport, is the main control on observed iron stocks within intermediate waters globally and upper ocean iron limitation will be strongly sensitive to subtle changes to the internal cycling balance.
•Pb concentrations and isotope ratios presented for GEOTRACES section GA06.•Northern and southern hemisphere water masses have distinct Pb isotope ratios.•Pb isotope ratios consistent with ...ventilation timescales of northern water masses.•Mixing complicates interpretation of Pb distributions in southern water masses.
Anthropogenic emissions have dominated marine Pb sources during the past century. Here we present Pb concentrations and isotope compositions for ocean depth profiles collected in the eastern Tropical Atlantic Ocean (GEOTRACES section GA06), to trace the transfer of anthropogenic Pb into the ocean interior. Variations in Pb concentration and isotope composition were associated with changes in hydrography. Water masses ventilated in the southern hemisphere generally featured lower 206Pb/207Pb and 208Pb/207Pb ratios than those ventilated in the northern hemisphere, in accordance with Pb isotope data of historic anthropogenic Pb emissions. The distributions of Pb concentrations and isotope compositions in northern sourced waters were consistent with differences in their ventilation timescales. For example, a Pb concentration maximum at intermediate depth (600–900 m, 35 pmol kg−1) in waters sourced from the Irminger/Labrador Seas, is associated with Pb isotope compositions (206Pb/207Pb = 1.1818–1.1824, 208Pb/207Pb = 2.4472–2.4483) indicative of northern hemispheric emissions during the 1950s and 1960s close to peak leaded petrol usage, and a transit time of ∼50–60 years. In contrast, North Atlantic Deep Water (2000–4000 m water depth) featured lower Pb concentrations and isotope compositions (206Pb/207Pb = 1.1762–1.184, 208Pb/207Pb = 2.4482–2.4545) indicative of northern hemispheric emissions during the 1910s and 1930s and a transit time of ∼80–100 years. This supports the notion that transient anthropogenic Pb inputs are predominantly transferred into the ocean interior by water mass transport. However, the interpretation of Pb concentration and isotope composition distributions in terms of ventilation timescales and pathways is complicated by (1) the chemical reactivity of Pb in the ocean, and (2) mixing of waters ventilated during different time periods. The complex effects of water mass mixing on Pb distributions is particularly apparent in seawater in the Tropical Atlantic Ocean which is ventilated from the southern hemisphere. In particular, South Atlantic Central Water and Antarctic Intermediate Water were dominated by anthropogenic Pb emitted during the last 50–100 years, despite estimates of much older average ventilation ages in this region.
Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well ...understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3-23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean.
This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus ...particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.
Significance The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for approximately 40% of the global ocean; however, our knowledge of how it is transferred between chemical states and pools is poorly constrained. Here we utilize the isotopic composition of dissolved and particulate iron to fingerprint its transformation in the surface ocean by abiotic and biotic processes. Photochemical and biological reduction and dissolution of particulate iron in the surface ocean appear to be key processes in regulating its supply and bioavailability to marine biota. Iron isotopes offer a new window into our understanding of the internal cycling of Fe, thereby allowing us to follow its biogeochemical transformations in the surface ocean.
The supply and bioavailability of iron (Fe) controls primary productivity and N2 fixation in large parts of the global ocean. An important, yet poorly quantified, source to the ocean is particulate ...Fe (pFe). Here we present the first combined dataset of particulate, labile‐particulate (L‐pFe), and dissolved Fe (dFe) from the (sub)tropical North Atlantic. We show a strong relationship between L‐pFe and dFe, indicating a dynamic equilibrium between these two phases whereby particles “buffer” dFe and maintain the elevated concentrations observed. Moreover, L‐pFe can increase the overall “available” (L‐pFe + dFe) Fe pool by up to 55%. The lateral shelf flux of this available Fe was similar in magnitude to observed soluble aerosol‐Fe deposition, a comparison that has not been previously considered. These findings demonstrate that L‐pFe is integral to Fe cycling and hence plays a role in regulating carbon cycling, warranting its inclusion in Fe budgets and biogeochemical models.
Key Points
Labile‐particulate iron has a key role in iron cycling and can increase the overall “available” iron pool by up to 55% in the OMZ
A strong relationship between particles and dissolved iron indicates that L‐pFe “buffers” the elevated dFe concentrations observed
Lateral shelf transport of available Fe (L‐pFe + dFe) supplied a similar magnitude of iron as atmospheric sources
Anthropogenic emissions completely overwhelmed natural marine lead (Pb) sources during the past century, predominantly due to leaded petrol usage. Here, based on Pb isotope measurements, we reassess ...the importance of natural and anthropogenic Pb sources to the tropical North Atlantic following the nearly complete global cessation of leaded petrol use. Significant proportions of up to 30-50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters, reflecting the success of the global effort to reduce anthropogenic Pb emissions. The observation of mineral dust derived Pb in surface waters is governed by the elevated atmospheric mineral dust concentration of the North African dust plume and the dominance of dry deposition for the atmospheric aerosol flux to surface waters. Given these specific regional conditions, emissions from anthropogenic activities will remain the dominant global marine Pb source, even in the absence of leaded petrol combustion.
The South Indian Subtropical Gyre (SISG) is one of the least studied gyre systems of the world ocean with respect to trace elements. Here we report dissolved (<0.45μm) Fe and Al measurements ...collected during two high-resolution US-CLIVAR CO2 Repeat Hydrography sections, which transected the upper 1000m of the gyre zonally along ~32°S (I05) and meridionally along ~30°E (I06S). Particulate Fe and Al concentrations in waters influenced by the Agulhas Current are also presented. The distributions of dissolved Fe and Al in the gyre are primarily impacted by mineral dust deposition at the surface and the large-scale circulation patterns of the gyre at depth. Using mean mixed layer dissolved Al concentrations, we estimate that the deposition and partial dissolution of mineral dust emanating from South Africa and Australia vary from 60 to 685mg (dust) m−2yr−1 across the 32°S transect. This translates into a dust source of dissolved Fe ranging from 1.7 to 20μmolFem−2y−1. The zonal patterns of aeolian deposition and those of N*, an indirect geochemical tracer for nitrogen fixation, show remarkable similarities along the I05 transect, suggesting that aeolian delivery of Fe may regulate nitrogen fixation rates in the SISG. In the western SISG (west of 60°E), which receives some of the highest aeolian Fe fluxes of the 32°S section, the Fe:AOU ratio in Indian Central Water was elevated relative to that observed in the Indian Central Water occupying the eastern Indian Ocean. These elevated Fe:AOU ratios may reflect the remineralization of Fe-rich organic material from nitrogen fixing organisms at the western end of the basin. Below the mixed layer, the distribution of dissolved Al appears to trace the principal features of the large-scale circulation of the SISG. Elevated subsurface concentrations of dissolved Al (>4nM) in the southwest Indian Ocean west of 45–50°E are most likely sustained by leakage of Al-rich waters from the Agulhas Return Current. Along the southeast African margin, the elevated particulate Fe (up to 230nM) and Al (up to 690nM) concentrations reflect the resuspension and transport of shelf sediments by the highly energetic Agulhas Current. However, while the particulate inputs at the margin are massive and appear to supply modest amounts of dissolved Fe, the distribution of dissolved Al is decoupled from the particulate phase. This observation suggests that the elevated subsurface dissolved Al concentrations observed near the African shelf are not the result of sediment resuspension processes occurring in situ along I05 but are more likely an advected signal originating from the upper reaches of the Agulhas Current.
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•Dissolved Fe and Al are impacted by dust deposition and water circulation.•Total dust deposition is estimated using mean mixed layer dissolved Al.•Aeolian Fe deposition may regulate N-fixation along 32°S in the south Indian gyre.•Elevated subsurface Al in the western gyre is sourced from Agulhas Return Current.•Particulate and dissolved Al distributions are decoupled in the Agulhas Current.
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