Ariel D. Anbar

Tracing the stepwise oxygenation of the Proterozoic ocean

Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth’s atmosphere. Geochemical data suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.

A Whiff of Oxygen Before the Great Oxidation Event

High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 ± 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O2 in the environment more than 50 million years before the start of the Great Oxidation Event.

A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus

Evidence is offered for arsenate replacing phosphate as a molecular building block in a Mono Lake, California, bacterium. Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is theoretically possible that some other elements in the periodic table could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of the Halomonadaceae, isolated from Mono Lake, California, that is able to substitute arsenic for phosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins. Exchange of one of the major bio-elements may have profound evolutionary and geochemical importance.

Natural mass-dependent variations in the isotopic composition of molybdenum

We present the first observations of natural mass-dependent fractionation of the isotopic composition of molybdenum (Mo), using multi-collector inductively coupled plasma mass spectrometry. Variations in the isotopic composition of Mo are reported as δ97/95Mo (=((97Mo/95Mo)sample/(97Mo/95Mo)standard−1)×1000‰). External analytical precision of δ97/95Mo is 1‰ between sediments deposited under anoxic conditions (δ97/95Mo=+1.02 to +1.52‰ relative to our in-house standard) and ferromanganese nodules (δ97/95Mo=−0.63 to −0.42‰). δ97/95Mo of Pacific Ocean seawater (δ97/95Mo=+1.48‰) lies within the range of values for anoxic sediments, closest to modern Black Sea anoxic sediments. Molybdenites from continental ore deposits have intermediate δ97/95Mo ranging from −0.26 to +0.09‰. Variations in the abundances of 92Mo, 95Mo, 96Mo, 97Mo and 98Mo are consistent with mass-dependent fractionation. A sporadic unidentified interference occurs at mass 94 and 100Mo is not measured. We hypothesize that the δ97/95Mo offset between anoxic sediments and ferromanganese nodules results from Mo isotope fractionation during inefficient scavenging of Mo from seawater by Mn oxides under oxic conditions. The similarity in δ97/95Mo of anoxic sediments and seawater is consistent with the very efficient removal of Mo from seawater under anoxic conditions in the presence of H2S. The data can be interpreted in terms of a steady-state mass balance between the Mo flux into the oceans from the continents and the Mo flux out of the oceans into oxic and anoxic sediments. Such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of Mo to anoxic and oxic sediments. These findings suggest that δ97/95Mo in seawater may co-vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans, and that this variation may be recorded in δ97/95Mo of anoxic sediments. Hence, the Mo isotope system may be useful in paleoredox investigations.

Late archean biospheric oxygenation and atmospheric evolution

High-resolution geochemical analyses of organic-rich shale and carbonate through the 2500 million-year-old Mount McRae Shale in the Hamersley Basin of northwestern Australia record changes in both the oxidation state of the surface ocean and the atmospheric composition. The Mount McRae record of sulfur isotopes captures the widespread and possibly permanent activation of the oxidative sulfur cycle for perhaps the first time in Earths history. The correlation of the time-series sulfur isotope signals in northwestern Australia with equivalent strata from South Africa suggests that changes in the exogenic sulfur cycle recorded in marine sediments were global in scope and were linked to atmospheric evolution. The data suggest that oxygenation of the surface ocean preceded pervasive and persistent atmospheric oxygenation by 50 million years or more.

Molybdenum isotope fractionation during adsorption by manganese oxides

The isotopic composition of Mo (δ97/95Mo) in seawater is ∼2‰ heavier than Mo in marine ferromanganese crusts and nodules [Barling et al., Earth Planet. Sci. Lett. 193 (2001) 447–457; Siebert et al., Earth Planet. Sci. Lett. 211 (2003) 159–171]. To explore this phenomenon, we have conducted an experimental investigation into the mass-dependent fractionation of Mo isotopes during adsorption onto Mn oxyhydroxide. Two series of experiments were carried out: a ‘time series’, in which adsorption proceeded for 2–96 h; and a ‘pH series’ in which pH varied from 6.5 to 8.5. The extent of Mo adsorption by Mn oxyhydroxides decreases with increasing pH, a trend typical of anion adsorption, and takes 48 h to reach steady-state. Lighter Mo isotopes are preferentially adsorbed. Experimentally determined fractionation factors (αsoln–MnOx) exhibit no systematic variation with either time or experimental pH. The mean αsoln–MnOx for all experiments is 1.0018±0.0005 (2 S.D.). Comparison of the Mo isotopic data for experimental solutions and Mo adsorbed to Mn oxyhydroxide with predictions for ‘closed system’ equilibrium and Rayleigh fractionation models indicates that isotope fractionation occurs as a result of ‘closed system’ equilibrium exchange between dissolved and adsorbed Mo. The isotopic offset between dissolved and adsorbed Mo is comparable to that observed between Mo in seawater and Mo in ferromanganese nodules and crusts. It is therefore likely that adsorption of Mo to Mn oxyhydroxides is a significant factor in the fractionation of Mo isotopes in the oceans.

Ocean oxygenation in the wake of the Marinoan glaciation

Metazoans are likely to have their roots in the Cryogenian period, but there is a marked increase in the appearance of novel animal and algae fossils shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago. It has been suggested that an oxygenation event in the wake of the severe Marinoan glaciation was the driving factor behind this early diversification of metazoans and the shift in ecosystem complexity. But there is little evidence for an increase in oceanic or atmospheric oxygen following the Marinoan glaciation, or for a direct link between early animal evolution and redox conditions in general. Models linking trends in early biological evolution to shifts in Earth system processes thus remain controversial. Here we report geochemical data from early Ediacaran organic-rich black shales (∼635–630 million years old) of the basal Doushantuo Formation in South China. High enrichments of molybdenum and vanadium and low pyrite sulphur isotope values (Δ34S values ≥65 per mil) in these shales record expansion of the oceanic inventory of redox-sensitive metals and the growth of the marine sulphate reservoir in response to a widely oxygenated ocean. The data provide evidence for an early Ediacaran oxygenation event, which pre-dates the previous estimates for post-Marinoan oxygenation by more than 50 million years. Our findings seem to support a link between the most severe glaciations in Earth’s history, the oxygenation of the Earth’s surface environments, and the earliest diversification of animals.

Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish

The evolution of Earth’s biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550–560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.

Iron isotope fractionation during microbial reduction of iron: The importance of adsorption

In experiments investigating the causes of Fe isotope fractionation, the d 56/54 Fe value of Fe(II) remaining in solution (Fe(II)(aq)) after reduction of Fe(III) (goethite) by Shewanella putrefaciens is ;21.2‰ relative to the goethite, in agreement with previous research. The addition of an electron shuttle did not affect fractionation, suggesting that Fe isotope fractionation may not be related to the kinetics of the electron transfer. Furthermore, in abiotic, anaerobic FeCl2(aq) experiments in which approximately one-third of Fe(II)(aq) is lost from solution due to adsorption of Fe(II) onto goethite, the d 56/54 Fe value of Fe(II)(aq) remaining in solution is shifted by 20.8‰ relative to FeCl 2. This finding demonstrates that anaerobic nonbiological interaction between Fe(II) and goethite can generate signif- icant Fe isotope fractionation. Acid extraction of sorbed Fe(II) from goethite in experi- ments reveals that heavy Fe preferentially sorbs to goethite. Simple mass-balance modeling indicates that the isotopic composition of the sorbed Fe(II) pool is ;11.5‰ to 12.5‰ heavier than Fe in the goethite (;2.7‰-3.7‰ heavier than aqueous Fe(II)). Mass balance is also consistent with a pool of heavy Fe that is not released to solution during acid extraction.

A late archean sulfidic sea stimulated by early oxidative weathering of the continents

Of Ancient Iron and Oxygen Finding clues to understand the early evolution of ocean and atmospheric chemistry and its links to the evolution of life remains a daunting task. Often just a few rock samples provide our only evidence of what conditions on Earth were like long ago. Reinhard et al. (p. 713) combined iron speciation data from a 2.5-billion-year-old shale from Australia with sulfur isotope data from this and nearby formations to conclude that oxygen chemistry predominanty consisted of an anoxic sulfide–rich water column, instead of iron-rich oceans, as previously speculated. Thus, brief pulses of reduced iron from hydrothermal vents may have been responsible for the formation of nearby banded iron formations and may have provided enough buffering to prolong the appearance of atmospheric oxygen generated by the expansion of newly evolved cyanobacteria. Before Earth’s atmosphere became oxidizing, the oceans may have been sulfide-rich while receiving periodic pulses of iron. Iron speciation data for the late Archean Mount McRae Shale provide evidence for a euxinic (anoxic and sulfidic) water column 2.5 billion years ago. Sulfur isotope data compiled from the same stratigraphic section suggest that euxinic conditions were stimulated by an increase in oceanic sulfate concentrations resulting from weathering of continental sulfide minerals exposed to an atmosphere with trace amounts of photosynthetically produced oxygen. Variability in local organic matter flux likely confined euxinic conditions to midportions of the water column on the basin margin. These findings indicate that euxinic conditions may have been common on a variety of spatial and temporal scales both before and immediately after the Paleoproterozoic rise in atmospheric oxygen, hinting at previously unexplored texture and variability in deep ocean chemistry during Earth’s early history.