Simon W. Poulton

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.

Ferruginous Conditions Dominated Later Neoproterozoic Deep-Water Chemistry

Earths surface chemical environment has evolved from an early anoxic condition to the oxic state we have today. Transitional between an earlier Proterozoic world with widespread deep-water anoxia and a Phanerozoic world with large oxygen-utilizing animals, the Neoproterozoic Era [1000 to 542 million years ago (Ma)] plays a key role in this history. The details of Neoproterozoic Earth surface oxygenation, however, remain unclear. We report that through much of the later Neoproterozoic (<742 ± 6 Ma), anoxia remained widespread beneath the mixed layer of the oceans; deeper water masses were sometimes sulfidic but were mainly Fe2+-enriched. These ferruginous conditions marked a return to ocean chemistry not seen for more than one billion years of Earth history.

Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes

Geochemical data suggest that oxygenation of the Earth’s atmosphere occurred in two broad steps. The first rise in atmospheric oxygen is thought to have occurred between ∼2.45 and 2.2 Gyr ago, leading to a significant increase in atmospheric oxygen concentrations and concomitant oxygenation of the shallow surface ocean. The second increase in atmospheric oxygen appears to have taken place in distinct stages during the late Neoproterozoic era (∼800–542 Myr ago), ultimately leading to oxygenation of the deep ocean ∼580 Myr ago, but details of the evolution of atmospheric oxygenation remain uncertain. Here we use chromium (Cr) stable isotopes from banded iron formations (BIFs) to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth’s atmosphere–hydrosphere system. The geochemical behaviour of Cr is highly sensitive to the redox state of the surface environment because oxidative weathering processes produce the oxidized hexavalent [Cr(VI)] form. Oxidation of reduced trivalent [Cr(III)] chromium on land is accompanied by an isotopic fractionation, leading to enrichment of the mobile hexavalent form in the heavier isotope. Our fractionated Cr isotope data indicate the accumulation of Cr(VI) in ocean surface waters ∼2.8 to 2.6 Gyr ago and a likely transient elevation in atmospheric and surface ocean oxygenation before the first great rise of oxygen 2.45–2.2 Gyr ago (the Great Oxidation Event). In ∼1.88-Gyr-old BIFs we find that Cr isotopes are not fractionated, indicating a decline in atmospheric oxygen. Our findings suggest that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. In the late Neoproterozoic, we observe strong positive fractionations in Cr isotopes (δ53Cr up to +4.9‰), providing independent support for increased surface oxygenation at that time, which may have stimulated rapid evolution of macroscopic multicellular life.

The transition to a sulphidic ocean ∼ 1.84 billion years ago

The Proterozoic aeon (2.5 to 0.54 billion years (Gyr) ago) marks the time between the largely anoxic world of the Archean (> 2.5 Gyr ago) and the dominantly oxic world of the Phanerozoic (< 0.54 Gyr ago). The course of ocean chemistry through the Proterozoic has traditionally been explained by progressive oxygenation of the deep ocean in response to an increase in atmospheric oxygen around 2.3 Gyr ago. This postulated rise in the oxygen content of the ocean is in turn thought to have led to the oxidation of dissolved iron, Fe(II), thus ending the deposition of banded iron formations (BIF) around 1.8 Gyr ago. An alternative interpretation suggests that the increasing atmospheric oxygen levels enhanced sulphide weathering on land and the flux of sulphate to the oceans. This increased rates of sulphate reduction, resulting in Fe(II) removal in the form of pyrite as the oceans became sulphidic. Here we investigate sediments from the ∼1.8-Gyr-old Animikie group, Canada, which were deposited during the final stages of the main global period of BIF deposition. This allows us to evaluate the two competing hypotheses for the termination of BIF deposition. We use iron–sulphur–carbon (Fe–S–C) systematics to demonstrate continued ocean anoxia after the final global deposition of BIF and show that a transition to sulphidic bottom waters was ultimately responsible for the termination of BIF deposition. Sulphidic conditions may have persisted until a second major rise in oxygen between 0.8 to 0.58 Gyr ago, possibly reducing global rates of primary production and arresting the pace of algal evolution.

Sulfide oxidation and iron dissolution kinetics during the reaction of dissolved sulfide with ferrihydrite

Abstract The reaction between synthetic ferrihydrite and dissolved sulfide was studied in artificial seawater and 0.42 M NaCl at 25 °C over the pH range 4.0–8.2. Electron transfer between solid phase Fe(III) and surface-complexed sulfide results in the reduction of Fe(III) and the formation of elemental sulfur. Subsequent formation of solid phase FeS occurs following dissolution of Fe(II) and reaction with dissolved sulfide. However, the majority of the Fe(II) produced at pH 7.5 remained associated with the oxide surface on the time-scale of these experiments. Rates of both sulfide oxidation and Fe(II) dissolution (in mol l−1 min−1) were expressed in terms of an empirical rate equation of the form: R=k i ( H 2 S ) t=0 0.5 A where ki represents the apparent rate constants for the oxidation of sulfide (kS) or the dissolution of Fe2+ (kFe), (H2S)t=0 is the initial sulfide concentration (in mol l−1) and A is the initial ferrihydrite surface area (in m2 l−1). The rate constant, kS, for the oxidation of sulfide in seawater at pH 7.5 is 8.4×10−6±0.9×10−6 mol0.5 l0.5 m−2 min−1, with the rate of sulfide oxidation being approximately 15 times faster than the rate of Fe(II) dissolution (given that the ratio of sulfide oxidized to Fe(II) produced is 2:1; kFe=1.1×10−6±0.2×10−6 mol0.5 l0.5 m−2 min−1). The determination of a fractional order with regard to the initial dissolved sulfide concentration occurs because reaction rates are dependent on the availability of reactive surface sites; the more reactive surface sites become saturated with sulfide at relatively low ferrihydrite to dissolved sulfide ratios. In many natural sulfidic environments, the iron oxide to dissolved sulfide ratio is expected to be lower than during this laboratory study. Thus, surface saturation will exert an important influence on reaction rates in nature.

Ocean acidification and the Permo-Triassic mass extinction

Ocean acidification and mass extinction The largest mass extinction in Earths history occurred at the Permian-Triassic boundary 252 million years ago. Several ideas have been proposed for what devastated marine life, but scant direct evidence exists. Clarkson et al. measured boron isotopes across this period as a highly sensitive proxy for seawater pH. It appears that, although the oceans buffered the acidifiying effects of carbon release from contemporary pulses of volcanism, buffering failed when volcanism increased during the formation of the Siberian Traps. The result was a widespread drop in ocean pH and the elimination of shell-forming organisms. Science, this issue p. 229 A rapid injection of massive amounts of carbon into the atmosphere acidified the oceans, causing mass extinction. Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.

Pathways for Neoarchean pyrite formation constrained by mass-independent sulfur isotopes

It is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the Neoarchean. It is puzzling, therefore, that the sulfur isotope record of the Neoarchean is characterized by a signal of atmospheric mass-independent chemistry rather than a strong overprint by sulfate reducers. Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.

Green rust formation controls nutrient availability in a ferruginous water column

Iron-rich (ferruginous) conditions were a prevalent feature of the ocean throughout much of Earth’s history. The nature of elemental cycling in such settings is poorly understood, however, thus hampering reconstruction of paleoenvironmental conditions during key periods in Earth evolution. This is particularly true regarding controls on nutrient bioavailability, which is intimately linked to Earth’s oxygenation history. Elemental scavenging during precipitation of iron minerals exerts a major control on nutrient cycling in ferruginous basins, and the predictable nature of removal processes provides a mechanism for reconstructing ancient ocean chemistry. Such reconstructions depend, however, on precise knowledge of the iron minerals formed in the water column. Here, we combine mineralogical and geochemical analyses to demonstrate formation of the mixed-valence iron mineral, green rust, in ferruginous Lake Matano, Indonesia. Carbonated green rust (GR1), along with signifi cant amounts of magnetite, forms below the chemocline via the reduction of ferrihydrite. Further, we show that uptake of dissolved nickel, a key micronutrient required for methanogenesis, is signifi cantly enhanced during green rust formation, suggesting a major control on methane production in ancient ferruginous settings.

Turbidite depositional influences on the diagenesis of Beecher's Trilobite Bed and the Hunsrück Slate; sites of soft tissue pyritization

Chemical signatures of enrichment of highly reactive iron, and framboid size distributions, are reported in turbidite sediments that host soft tissue pyritization (Beechers Trilobite Bed, Upper Ordovician, and the Hunsrück Slate, Lower Devonian). These signatures demonstrate that the sediment of Beechers Trilobite Bed was enriched in highly reactive iron prior to turbidite transport but that no enrichment was present in the Hunsrück Slate. Turbidite transport and re-sedimentation altered framboid size distributions. Small diameter framboids (< 5 μm) that formed in the sediment at the pre-transport site were lost during transport due to oxidation and/or size sorting. Larger diameter framboids (∼5 –15 μm) that formed at the pre-transport site were transported without alteration. The oxidation of the original small framboid population formed highly reactive iron (oxyhydr)oxides that were reduced during post-transport suboxic diagenesis to produce porewaters rich in dissolved iron. In some turbidites a bimodal framboid population resulted where a minor population of small diameter framboids, produced by limited sulfate reduction at the post-transport site, was added to the transported large diameter population. Soft tissue pyritization in this setting was facilitated by the presence of suboxic, iron rich porewaters where dissolved sulfide formed during soft tissue decay, confining iron sulfide precipitation to the decay site.

The use of hydrous iron(III) oxides for the removal of hydrogen sulphide in aqueous systems

The potential for iron (hydr)oxides to remove dissolved hydrogen sulphide from seawater has been examined under flow-through conditions. Ferrihydrite (a hydrous iron (III) oxide) was stabilised by precipitation onto zeolite pellets, and rates of sulphide removal were determined under laboratory conditions at pH 8.5. Sulphide removal kinetics were dependent on the initial sulphide concentration, substrate mass and flow rate. The experimental data suggest that these parameters can be optimised to result in the rapid and effective removal of hydrogen sulphide. The results from laboratory experiments compared favourably with sulphide removal kinetics determined in a series of experiments performed online in a recirculating mariculture production system. However, the presence in solution of ligands such as phosphate may also significantly affect reaction rates; a 50% reduction in sulphide removal rate for substrate removed from an online system was partly attributed to phosphate adsorption. The formation of a more crystalline, less reactive iron (hydr)oxide in recharged substrate was the likely result of FeS oxidation, which may also have contributed to the observed reduction in sulphide removal rates. Ferrihydrite-coated zeolite would appear to provide an efficient, low-cost method for sulphide removal, which is particularly suited to relatively small-scale aqueous flow-through systems. The reaction of iron (hydr)oxides with dissolved sulphide is also accompanied by a distinct colour change due to the formation of black FeS(s) which, under appropriate conditions, may be used as a rapid indicator of sulphidic conditions.