Iron (Fe) limits or co-limits primary productivity and nitrogen fixation in large regions of the world's oceans, and the supply of Fe from hydrothermal vents to the deep ocean is now known to be ...extensive. However, the mechanisms that control the amount of hydrothermal Fe that is stabilized in the deep ocean, and thus dictate the impact of hydrothermal Fe sources on surface ocean biogeochemistry, are unclear. To learn more, we have examined the dispersion of total dissolvable Fe (TDFe), dissolved Fe (dFe) and soluble Fe (sFe) in the buoyant and non-buoyant hydrothermal plume above the Beebe vent field, Caribbean Sea. We have also characterized plume particles using electron microscopy and synchrotron based spectromicroscopy.
We show that the majority of dFe in the Beebe hydrothermal plume was present as colloidal Fe (cFe = dFe − sFe). During ascent of the buoyant plume, a significant fraction of particulate Fe (pFe = TDFe − dFe) was lost to settling and exchange with colloids. Conversely, the opposite was observed in the non-buoyant plume, where pFe concentrations increased during non-buoyant plume dilution, cFe concentrations decreased apparently due to colloid aggregation. Elemental mapping of carbon, oxygen and iron in plume particles reveals their close association and indicates that exchanges of Fe between colloids and particles must include transformations of organic carbon and Fe oxyhydroxide minerals. Notably, sFe is largely conserved during plume dilution, and this is likely to be due to stabilization by organic ligands, in contrast to the more dynamic exchanges between pFe and cFe.
This study highlights that the size of the sFe stabilizing ligand pool, and the rate of iron-rich colloid aggregation will control the amount and physico-chemical composition of dFe supplied to the ocean interior from hydrothermal systems. Both the ligand pool, and the rate of cFe aggregation in hydrothermal plumes remain uncertain and determining these are important intermediate goals to more accurately assess the impact of hydrothermalism on the ocean's carbon cycle.
This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
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•Soluble iron behaves quasi-conservatively in the hydrothermal plume as iron rich plume waters mix with low iron seawater.•Particulate iron is formed by the aggregation of colloidal iron as the hydrothermal plume disperses into the deep ocean.•Colloid aggregation is an important control on the amount of dissolved iron added to the ocean by hydrothermal vents.•Co-location of iron and carbon in particles implies that organic carbon mediates iron exchange between different species.
Although iron (Fe) is a key regulator of primary production over much of the ocean, many components of the marine iron cycle are poorly constrained, which undermines our understanding of climate ...change impacts. In recent years, a growing number of studies (often part of GEOTRACES) have used Fe isotopic signatures (δ56Fe) to disentangle different aspects of the marine Fe cycle. Characteristic δ56Fe endmembers of external sources and assumed isotopic fractionation during biological Fe uptake or recycling have been used to estimate relative source contributions and investigate internal transformations, respectively. However, different external sources and fractionation processes often overlap and act simultaneously, complicating the interpretation of oceanic Fe isotope observations. Here we investigate the driving forces behind the marine dissolved Fe isotopic signature (δ56Fediss) distribution by incorporating Fe isotopes into the global ocean biogeochemical model PISCES. We find that distinct external source endmembers acting alongside fractionation during organic complexation and phytoplankton uptake are required to reproduce δ56Fediss observations along GEOTRACES transects. δ56Fediss distributions through the water column result from regional imbalances of remineralization and abiotic removal processes. They modify δ56Fediss directly and transfer surface ocean signals to the interior with opposing effects. Although attributing crustal compositions to sedimentary Fe sources in regions with low organic carbon fluxes improves our isotope model, δ56Fediss signals from hydrothermal or sediment sources cannot be reproduced accurately by simply adjusting δ56Fe endmember values. This highlights that additional processes must govern the exchange and/or speciation of Fe supplied by these sources to the ocean.
Plain Language Summary
Iron is key in regulating biological activity over much of the ocean so that changes to iron availability affect the global climate. Despite this understanding, we have incomplete knowledge regarding the oceanic processes that cycle iron and the external supply mechanisms. In recent years, iron isotopes have been used to disentangle some of these aspects, thanks to characteristic isotopic endmembers of external sources and assumed isotopic fractionation during certain oceanic processes. However, the effects of external endmembers and oceanic fractionation often overlap, complicating the interpretation of iron isotope signatures in the water column. Here we investigate what drives the distribution of these signatures by incorporating iron isotopes into a global ocean biogeochemical model. We find to best reproduce observations, we needed to include distinct external source endmembers in our model as well as isotopic fractionation during two oceanic processes: iron uptake by phytoplankton and complexation by organic ligands. Other modeled processes affect the iron isotope distribution indirectly, namely remineralization of iron particles and abiotic removal of iron, which often have opposite effects. Poor model performance near some hydrothermal and sedimentary iron sources further indicates that our current understanding of these source processes, as reflected in our model representation, is too simple.
Key Points
External source endmembers and fractionation during phytoplankton uptake and organic complexation shape oceanic Fe isotope distribution
Imbalances in remineralization and abiotic removal rates produce regionally distinct dissolved Fe isotopic signatures
Hydrothermal and sedimentary Fe supply processes have additional controls that are not yet accounted for in global ocean models
Volcanogenic sediments are typically rich in Fe and Mn-bearing minerals that undergo substantial alteration during early marine diagenesis, however their impact on the global biogeochemical cycling ...of Fe and Mn has not been widely addressed. This study compares the near surface (0–20
cm below sea floor cmbsf) aqueous (<0.02
μm) and aqueous
+
colloidal here in after ‘dissolved’ (<0.2
μm) pore water Fe and Mn distributions, and ancillary O
2(aq),
NO
3
-
and solid-phase reactive Fe distributions, between two volcanogenic sediment settings: 1 a deep sea tephra-rich deposit neighbouring the volcanically active island of Montserrat and 2 mixed biosiliceous–volcanogenic sediments from abyssal depths near the volcanically inactive Crozet Islands archipelago. Shallow penetration of O
2(aq) into Montserrat sediments was observed (<1
cmbsf), and inferred to partially reflect oxidation of fine grained Fe(II) minerals, whereas penetration of O
2(aq) into abyssal Crozet sediments was >5
cmbsf and largely controlled by the oxidation of organic matter. Dissolved Fe and Mn distributions in Montserrat pore waters were lowest in the surface oxic-layer (0.3
μM Fe; 32
μM Mn), with maxima (20
μM Fe; 200
μM Mn) in the upper 1–15
cmbsf. Unlike Montserrat, Fe and Mn in Crozet pore waters were ubiquitously partitioned between 0.2
μm and 0.02
μm filtrations, indicating that the pore water distributions of Fe and Mn in the (traditionally termed) ‘dissolved’ size fraction are dominated by colloids, with respective mean abundances of 80% and 61%. Plausible mechanisms for the origin and composition of pore water colloids are discussed, and include prolonged exposure of Crozet surface sediments to early diagenesis compared to Montserrat, favouring nano-particulate goethite formation, and the elevated dissolved Si concentrations, which are shown to encourage fine-grained smectite formation. In addition, organic matter may stabilise authigenic Fe and Mn in the Crozet pore waters. We conclude that volcanogenic sediment diagenesis leads to a flux of colloidal material to the overlying bottom water, which may impact significantly on deep ocean biogeochemistry. Diffusive flux estimates from Montserrat suggest that diagenesis within tephra deposits of active island volcanism may also be an important source of dissolved Mn to the bottom waters, and therefore a source for the widespread hydrogenous MnO
x
deposits found in the Caribbean region.
Shelf sediments underlying temperate and oxic waters of the Celtic Sea (NW European Shelf) were found to have shallow oxygen penetrations depths from late spring to late summer (2.2–5.8 mm below ...seafloor) with the shallowest during/after the spring-bloom (mid-April to mid-May) when the organic carbon content was highest. Sediment porewater dissolved iron (dFe, <0.15 µm) mainly (>85%) consisted of Fe(II) and gradually increased from 0.4 to 15 µM at the sediment surface to ~ 100–170 µM at about 6 cm depth. During the late spring this Fe(II) was found to be mainly present as soluble Fe(II) (>85% sFe, <0.02 µm). Sub-surface dFe(II) maxima were enriched in light isotopes (δ⁵⁶Fe –2.0 to –1.5‰), which is attributed to dissimilatory iron reduction (DIR) during the bacterial decomposition of organic matter. As porewater Fe(II) was oxidised to insoluble Fe(III) in the surface sediment layer, residual Fe(II) was further enriched in light isotopes (down to –3.0‰). Ferrozine-reactive Fe(II) was found in surface porewaters and in overlying core top waters, and was highest in the late spring period. Shipboard experiments showed that depletion of bottom water oxygen in late spring can lead to a substantial release of Fe(II). Reoxygenation of bottom water caused this Fe(II) to be rapidly lost from solution, but residual dFe(II) and dFe(III) remained (12 and 33 nM) after >7 h. Iron(II) oxidation experiments in core top and bottom waters also showed removal from solution but at rates up to 5-times slower than predicted from theoretical reaction kinetics. These data imply the presence of ligands capable of complexing Fe(II) and supressing oxidation. The lower oxidation rate allows more time for the diffusion of Fe(II) from the sediments into the overlying water column. Modelling indicates significant diffusive fluxes of Fe(II) (on the order of 23–31 lmol m⁻² day⁻¹) are possible during late spring when oxygen penetration depths are shallow, and pore water Fe(II) concentrations are highest. In the water column this stabilised Fe(II) will gradually be oxidised and become part of the dFe(III) pool. Thus oxic continental shelves can supply dFe to the water column, which is enhanced during a small period of the year after phytoplankton bloom events when organic matter is transferred to the seafloor. This input is based on conservative assumptions for solute exchange (diffusion-reaction), whereas (bio)physical advection and resuspension events are likely to accelerate these solute exchanges in shelf-seas.
The island of South Georgia is situated in the iron (Fe)-depleted Antarctic
Circumpolar Current of the Southern Ocean. Iron emanating from its shelf
system fuels large phytoplankton blooms downstream ...of the island, but the
actual supply mechanisms are unclear. To address this, we present an
inventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore
waters, and the water column in the vicinity of South Georgia, alongside data
on zooplankton-mediated Fe cycling processes, and provide estimates of the
relative dissolved Fe (DFe) fluxes from these sources. Seafloor sediments,
modified by authigenic Fe precipitation, were the main particulate Fe source
to shelf bottom waters as indicated by the similar Fe ∕ Mn and Fe ∕ Al ratios for
shelf sediments and suspended particles in the water column. Less than 1 %
of the total particulate Fe pool was leachable surface-adsorbed (labile) Fe
and therefore potentially available to organisms. Pore waters formed the
primary DFe source to shelf bottom waters, supplying 0.1–44 µmol DFe m−2 d−1.
However, we estimate that only 0.41±0.26 µmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical
diffusive and advective mixing. Other trace metal sources to surface waters
included glacial flour released by melting glaciers and via zooplankton
egestion and excretion processes. On average 6.5±8.2 µmol m−2 d−1 of labile particulate Fe was supplied to the surface
mixed layer via faecal pellets formed by Antarctic krill (Euphausia superba), with a further 1.1±2.2 µmol DFe m−2 d−1
released directly by the krill. The faecal pellets released by krill included
seafloor-derived lithogenic and authigenic material and settled algal debris,
in addition to freshly ingested suspended phytoplankton cells. The Fe requirement of the phytoplankton blooms ∼ 1250 km
downstream of South Georgia was estimated as 0.33±0.11 µmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep
winter mixing, and aeolian dust estimated as ∼0.12 µmol m−2 d−1. We hypothesize that a substantial contribution of DFe was
provided through recycling of biogenically stored Fe following luxury Fe
uptake by phytoplankton on the Fe-rich shelf. This process would allow Fe to
be retained in the surface mixed layer of waters downstream of South Georgia
through continuous recycling and biological uptake, supplying the large
downstream phytoplankton blooms.
Iron is a scarce but essential micronutrient in the oceans that limits primary productivity in many regions of the surface ocean. The mechanisms and rates of Fe supply to the ocean interior are still ...poorly understood and quantified. Iron isotope ratios of different Fe pools can potentially be used to trace sources and sinks of the global Fe biogeochemical cycle if these boundary fluxes have distinct signatures. Seafloor hydrothermal vents emit metal rich fluids from mid-ocean ridges into the deep ocean. Iron isotope ratios have the potential to be used to trace the input of hydrothermal dissolved iron to the oceans if the local controls on the fractionation of Fe isotopes during plume dispersal in the deep ocean are understood. In this study we assess the behaviour of Fe isotopes in a Southern Ocean hydrothermal plume using a sampling program of Total Dissolvable Fe (TDFe), and dissolved Fe (dFe). We demonstrate that δ56Fe values of dFe (δ56dFe) within the hydrothermal plume change dramatically during early plume dispersal, ranging from −2.39±0.05‰ to −0.13±0.06‰ (2 SD). The isotopic composition of TDFe (δ56TDFe) was consistently heavier than dFe values, ranging from −0.31±0.03‰ to 0.78±0.05‰, consistent with Fe oxyhydroxide precipitation as the plume samples age. The dFe present in the hydrothermal plume includes stabilised dFe species with potential to be transported to the deep ocean. We estimate that stable dFe exported from the plume will have a δ56Fe of −0.28±0.17‰. Further, we show that the proportion of authigenic iron-sulfide and iron-oxyhydroxide minerals precipitating in the buoyant plume exert opposing controls on the resultant isotope composition of dissolved Fe passed into the neutrally buoyant plume. We show that such controls yield variable dissolved Fe isotope signatures under the authigenic conditions reported from modern vent sites elsewhere, and so ought to be considered during iron isotope reconstructions of past hydrothermalism from ocean sediment records.
Continental shelf sediments are globally important for biogeochemical activity. Quantification of shelf-scale stocks and fluxes of carbon and nutrients requires the extrapolation of observations made ...at limited points in space and time. The procedure for selecting exemplar sites to form the basis of this upscaling is discussed in relation to a UK-funded research programme investigating biogeochemistry in shelf seas. A three-step selection process is proposed in which (1) a target area representative of UK shelf sediment heterogeneity is selected, (2) the target area is assessed for spatial heterogeneity in sediment and habitat type, bed and water column structure and hydrodynamic forcing, and (3) study sites are selected within this target area encompassing the range of spatial heterogeneity required to address key scientific questions regarding shelf scale biogeochemistry, and minimise confounding variables. This led to the selection of four sites within the Celtic Sea that are significantly different in terms of their sediment, bed structure, and macrofaunal, meiofaunal and microbial community structures and diversity, but have minimal variations in water depth, tidal and wave magnitudes and directions, temperature and salinity. They form the basis of a research cruise programme of observation, sampling and experimentation encompassing the spring bloom cycle. Typical variation in key biogeochemical, sediment, biological and hydrodynamic parameters over a pre to post bloom period are presented, with a discussion of anthropogenic influences in the region. This methodology ensures the best likelihood of site-specific work being useful for up-scaling activities, increasing our understanding of benthic biogeochemistry at the UK-shelf scale.
Whilst Sediment Profile Imaging (SPI) is a very widely used technique in the regulatory assessment of seabed environmental health, and in the study of seafloor sediment-biology interactions, the ...potential for SPI images to be used in a geochemical context has not been rigorously assessed. Here we have examined relationships between colour and geochemistry in a sediment core collected from the Celtic Sea, North West European Shelf, that was digitally imaged and on which detailed geochemical analyses were also performed. Average oxygen penetration depth was 4.08 ± 0.72 mm, (n = 5), whilst the apparent redox potential discontinuity (aRPD) as determined by sediment colour change was at 78 mm. As iron (oxyhydr)oxides decreased with depth, black sulfide phases increased, and the aRPD most closely correlated with this geochemical change rather than the oxygen penetration depth. Colour analysis of the image showed a clear correlation of brightness with black FeS (acid volatile sulfide). There was a general correlation of iron oxide phases with orange colour in the upper part of the sediment profile, whilst.in the lower part of the core the orange oxide phases appeared to be obscured by the black FeS present. The sulfide-brightness relationship indicates colour analysis can provide an estimate of FeS, and potentially the carrying capacity for toxic metals such as cadmium, zinc and copper as sulfides in this type of sediment. Additionally, detailed geochemical analyses of SPI cores may provide new insights into the activity and impacts of infauna and the link with sediment biogeochemical cycles of carbon and nutrients.
•Sediment Profile Image directly linked to geochemical analyses.•Colour of image related to iron phases in this core.•Apparent Redox Potential Discontinuity (aRPD) not coincident with oxygen disappearance.•Caution needed in use of aRPD in benthic sediment quality assessment indices.
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