Tristan J. Horner

Nonspecific uptake and homeostasis drive the oceanic cadmium cycle

The global marine distributions of Cd and phosphate are closely correlated, which has led to Cd being considered as a marine micronutrient, despite its toxicity to life. The explanation for this nutrient-like behavior is unknown because there is only one identified biochemical function for Cd, an unusual Cd/Zn carbonic anhydrase. Recent developments in Cd isotope mass spectrometry have revealed that Cd uptake by phytoplankton causes isotopic fractionation in the open ocean and in culture. Here we investigate the physiochemical pathways that fractionate Cd isotopes by performing subcellular Cd isotope analysis on genetically modified microorganisms. We find that expression of the Cd/Zn carbonic anhydrase makes no difference to the Cd isotope composition of whole cells. Instead, a large proportion of the Cd is partitioned into cell membranes with a similar direction and magnitude of Cd isotopic fractionation to that seen in surface seawater. This observation is well explained if Cd is mistakenly imported with other divalent metals and subsequently managed by binding within the cell to avoid toxicity. This process may apply to other divalent metals, whereby nonspecific uptake and subsequent homeostasis may contribute to elemental and isotopic distributions in seawater, even for elements commonly considered as micronutrients.

Natural and Anthropogenic Cd Isotope Variations

Cadmium is a transition metal with eight naturally occurring isotopes that have atomic mass numbers of between 106 and 116. The large Cd isotope anomalies of meteorites have been subject to investigation since the 1970s, but improvements in instrumentation and techniques have more recently enabled routine studies of the smaller stable Cd isotope fractionations that characterize various natural and anthropogenic terrestrial materials. Whilst the current database is still comparatively small, pilot studies have identified two predominant mechanisms that routinely generate Cd isotope effects – partial evaporation/condensation and biological utilization. Processes that involve evaporation and condensation appear to be largely responsible for the Cd isotope fractionations of up to 1‰ (for 114Cd/110Cd) that have been determined for industrial Cd emissions, for example from ore refineries. Cadmium isotope measurements hence hold significant promise for tracing anthropogenic sources of this highly toxic metal in the environment. The even larger Cd isotope fractionations that have been identified in the oceans (up to 4‰ for 114Cd/110Cd) are due to biological uptake and utilization of dissolved seawater Cd. This finding confirms previous work, which identified Cd as an essential marine micronutrient that exhibits a phosphate-like distribution in the oceans. The marine Cd isotope fractionations are of particular interest, as they can be used to study micronutrient cycling and its impact on ocean productivity. In addition, they may also inform on past changes in marine nutrient utilization and how these are linked to global climate, if suitable archives of seawater Cd isotope compositions can be identified.

Persistence of deeply sourced iron in the Pacific Ocean

Significance The vertical supply of dissolved Fe (iron) is insufficient compared with the physiological needs of marine phytoplankton in vast swathes of the open ocean. However, the relative importance of the main sources of “new” Fe to the ocean—continental mineral dust, hydrothermal exhalations, and sediment dissolution—and their temporal evolution are poorly constrained. By analyzing the isotopic composition of Fe in marine sediments, we find that much of the dissolved Fe in the central Pacific Ocean originated from hydrothermal and sedimentary sources thousands of meters below the sea surface. As such, these data underscore the vital role of the oceans’ physical mixing in determining if any deeply sourced Fe ever reaches the Fe-starved surface-dwelling biota. Biological carbon fixation is limited by the supply of Fe in vast regions of the global ocean. Dissolved Fe in seawater is primarily sourced from continental mineral dust, submarine hydrothermalism, and sediment dissolution along continental margins. However, the relative contributions of these three sources to the Fe budget of the open ocean remains contentious. By exploiting the Fe stable isotopic fingerprints of these sources, it is possible to trace distinct Fe pools through marine environments, and through time using sedimentary records. We present a reconstruction of deep-sea Fe isotopic compositions from a Pacific Fe−Mn crust spanning the past 76 My. We find that there have been large and systematic changes in the Fe isotopic composition of seawater over the Cenozoic that reflect the influence of several, distinct Fe sources to the central Pacific Ocean. Given that deeply sourced Fe from hydrothermalism and marginal sediment dissolution exhibit the largest Fe isotopic variations in modern oceanic settings, the record requires that these deep Fe sources have exerted a major control over the Fe inventory of the Pacific for the past 76 My. The persistence of deeply sourced Fe in the Pacific Ocean illustrates that multiple sources contribute to the total Fe budget of the ocean and highlights the importance of oceanic circulation in determining if deeply sourced Fe is ever ventilated at the surface.

Analysis of high-precision vanadium isotope ratios by medium resolution MC-ICP-MS

We present and verify a new method to measure vanadium isotope ratios using a Thermo Scientific Neptune multi-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) operated in medium mass resolution mode. We collect masses 48 through 53 simultaneously using the L2, L1, Center, H1, H2 and H3 collectors. The Center cup is equipped with a 1012 Ω resistor, H1 is equipped with a 1010 Ω resistor, while the rest of the collectors have standard 1011 Ω resistors. Unlike previous low-resolution methods, the use of medium mass resolution (ΔM/M ∼ 4000) permits separation of V, Ti and Cr isotopes from all interfering molecular species representing combinations of C, N, O, S, Cl, and Ar. We show that the external reproducibility follows a power law function with respect to the number of V+ ions collected and achieve an external reproducibility of ±0.15‰ with total V+ ion beam intensities of ∼1 nA. The separation of interfering molecular species from the V mass spectrum reduces the V requirement for precise isotope data to as little as 200–300 ng V per analysis—a reduction of ∼90% compared with previous methods—making several low-V matrices amenable to V isotope analysis.

Pelagic barite precipitation at micromolar ambient sulfate

Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.The question of how significant barite deposits were able to form from early Earth’s low-sulfate seas remains controversial. Here, the authors show pelagic barite precipitation within a strongly barite-undersaturated ecosystem, highlighting the importance of particle-associated microenvironments.

Reply to Morel: Cadmium as a micronutrient and macrotoxin in the oceans

We thank Francois Morel for his interest in our study. Morel states that our conclusions are based on the approximate match between the Cd-isotope composition of cultured bacteria and the fractionation of Cd isotopes seen in seawater (1). This match is only a minor component of our argument, and we welcome the opportunity to reiterate our case.

Barium isotope evidence for pervasive sediment recycling in the upper mantle

Barium isotopes in mid-ocean ridge basalts reveal that the global upper mantle is contaminated with small amounts of sediment. The upper mantle, as sampled by mid-ocean ridge basalts (MORBs), exhibits significant chemical variability unrelated to mechanisms of melt extraction at ridges. We show that barium isotope variations in global MORBs vary systematically with radiogenic isotopes and trace element ratios, which reflects mixing between depleted and enriched MORB melts. In addition, modern sediments and enriched MORBs share similar Ba isotope signatures. Using modeling, we show that addition of ~0.1% by weight of sediment components into the depleted mantle in subduction zones must impart a sedimentary Ba signature to the overlying mantle and induce low-degree melting that produces the enriched MORB reservoir. Subsequently, these enriched domains convect toward mid-ocean ridges and produce radiogenic isotope variation typical of enriched MORBs. This mechanism can explain the chemical and isotopic features of enriched MORBs and provide strong evidence for pervasive sediment recycling in the upper mantle.

Publisher Correction : Pelagic barite precipitation at micromolar ambient sulfate

The original version of this Article contained an error in the barite saturation state equation in the fourth paragraph of the Introduction and incorrectly read ‘Ωbarite=({134Ba2+}⋅{SO42−})/Ksp)’. The correct version removes the superscript 134 next to ‘Ba2+’. This error has now been corrected in both the PDF and HTML versions of the Article.