Chasing iron bioavailability in the Southern Ocean: Insights from Phaeocystis antarctica and iron speciation.

Dissolved iron (dFe) availability limits the uptake of atmospheric CO2 by the Southern Ocean (SO) biological pump. Hence, any change in bioavailable dFe in this region can directly influence climate. On the basis of Fe uptake experiments with Phaeocystis antarctica, we show that the range of dFe bioavailability in natural samples is wider (<1 to ~200% compared to free inorganic Fe') than previously thought, with higher bioavailability found near glacial sources. The degree of bioavailability varied regardless of in situ dFe concentration and depth, challenging the consensus that sole dFe concentrations can be used to predict Fe uptake in modeling studies. Further, our data suggest a disproportionately major role of biologically mediated ligands and encourage revisiting the role of humic substances in influencing marine Fe biogeochemical cycling in the SO. Last, we describe a linkage between in situ dFe bioavailability and isotopic signatures that, we anticipate, will stimulate future research.

Chromium Cycling in Redox-Stratified Basins Challenges δ53Cr Paleoredox Proxy Applications.

Chromium stable isotope composition (δ53Cr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δ53Cr have not been assessed in modern redox-stratified basins. We present new chromium (Cr) concentration and δ53Cr data in dissolved, sinking particulate, and sediment samples from the redox-stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non-quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox-stratified basins show analogous patterns. This challenges assumptions from δ53Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non-quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ53Cr from oxidative continental weathering.

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean.

While chromium stable isotopes (δ53Cr) have received significant attention for their utility as a tracer of oxygen availability in the distant geological past, a mechanistic understanding of modern oceanic controls on Cr and δ53Cr is still lacking. Here we present total dissolved δ53Cr, concentrations of Cr (III) and total dissolved Cr, and net community productivity (NCP) from the North Pacific. Chromium concentrations show surface depletions in waters with elevated NCP, but not in lower productivity waters. Observed Cr deficits correspond well with calculated Cr export derived from NCP and Cr:C ratios of natural phytoplankton and marine particulates. Chromium (III) concentrations are stable over the diel cycle yet correlate with NCP, with maxima found in highly productive surface waters but not in lower productivity waters, indicating biological control on Cr (III). The relationship between Cr (III) and δ53Cr suggests that δ53Cr distributions may be controlled by the removal of isotopically light Cr (III) at an isotopic enrichment factor (∆53Cr) of -1.08‰ ± 0.25 relative to total dissolved δ53Cr, in agreement with the global δ53Cr-Cr fractionation factor (-0.82‰ ± 0.05). No perturbation to δ53Cr, Cr, or Cr (III) is observed in oxygen-depleted waters (~10 µmol/kg), suggesting no strong control by O2 availability, in agreement with other recent studies. Therefore, we propose that biological productivity is the primary control on Cr and δ53Cr in the modern ocean. Consequently, δ53Cr records in marine sediments may not faithfully record oxygen availability in the Late Quaternary. Instead, our data demonstrate that δ53Cr records may be a useful tracer for biological productivity.

A call for refining the role of humic-like substances in the oceanic iron cycle.

Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.

Undocumented water column sink for cadmium in open ocean oxygen-deficient zones.

Cadmium (Cd) is a micronutrient and a tracer of biological productivity and circulation in the ocean. The correlation between dissolved Cd and the major algal nutrients in seawater has led to the use of Cd preserved in microfossils to constrain past ocean nutrient distributions. However, linking Cd to marine biological processes requires constraints on marine sources and sinks of Cd. Here, we show a decoupling between Cd and major nutrients within oxygen-deficient zones (ODZs) in both the Northeast Pacific and North Atlantic Oceans, which we attribute to Cd sulfide (CdS) precipitation in euxinic microenvironments around sinking biological particles. We find that dissolved Cd correlates well with dissolved phosphate in oxygenated waters, but is depleted compared with phosphate in ODZs. Additionally, suspended particles from the North Atlantic show high Cd content and light Cd stable isotope ratios within the ODZ, indicative of CdS precipitation. Globally, we calculate that CdS precipitation in ODZs is an important, and to our knowledge a previously undocumented marine sink of Cd. Our results suggest that water column oxygen depletion has a substantial impact on Cd biogeochemical cycling, impacting the global relationship between Cd and major nutrients and suggesting that Cd may be a previously unidentified tracer for water column oxygen deficiency on geological timescales. Similar depletions of copper and zinc in the Northeast Pacific indicate that sulfide precipitation in ODZs may also have an influence on the global distribution of other trace metals.