Sean A. Crowe

Photoferrotrophs thrive in an Archean Ocean analogue

Considerable discussion surrounds the potential role of anoxygenic phototrophic Fe(II)-oxidizing bacteria in both the genesis of Banded Iron Formations (BIFs) and early marine productivity. However, anoxygenic phototrophs have yet to be identified in modern environments with comparable chemistry and physical structure to the ancient Fe(II)-rich (ferruginous) oceans from which BIFs deposited. Lake Matano, Indonesia, the eighth deepest lake in the world, is such an environment. Here, sulfate is scarce (<20 μmol·liter−1), and it is completely removed by sulfate reduction within the deep, Fe(II)-rich chemocline. The sulfide produced is efficiently scavenged by the formation and precipitation of FeS, thereby maintaining very low sulfide concentrations within the chemocline and the deep ferruginous bottom waters. Low productivity in the surface water allows sunlight to penetrate to the >100-m-deep chemocline. Within this sulfide-poor, Fe(II)-rich, illuminated chemocline, we find a populous assemblage of anoxygenic phototrophic green sulfur bacteria (GSB). These GSB represent a large component of the Lake Matano phototrophic community, and bacteriochlorophyll e, a pigment produced by low-light-adapted GSB, is nearly as abundant as chlorophyll a in the lakes euphotic surface waters. The dearth of sulfide in the chemocline requires that the GSB are sustained by phototrophic oxidation of Fe(II), which is in abundant supply. By analogy, we propose that similar microbial communities, including populations of sulfate reducers and photoferrotrophic GSB, likely populated the chemoclines of ancient ferruginous oceans, driving the genesis of BIFs and fueling early marine productivity.

The methane cycle in ferruginous Lake Matano

In Lake Matano, Indonesia, the worlds largest known ferruginous basin, more than 50% of authigenic organic matter is degraded through methanogenesis, despite high abundances of Fe (hydr)oxides in the lake sediments. Biogenic CH₄ accumulates to high concentrations (up to 1.4 mmol L⁻¹) in the anoxic bottom waters, which contain a total of 7.4 × 10⁵ tons of CH₄. Profiles of dissolved inorganic carbon (ΣCO₂) and carbon isotopes (δ¹³C) show that CH₄ is oxidized in the vicinity of the persistent pycnocline and that some of this CH₄ is likely oxidized anaerobically. The dearth of NO₃⁻ and SO₄²⁻ in Lake Matano waters suggests that anaerobic methane oxidation may be coupled to the reduction of Fe (and/or Mn) (hydr)oxides. Thermodynamic considerations reveal that CH₄ oxidation coupled to Fe(III) or Mn(III/IV) reduction would yield sufficient free energy to support microbial growth at the substrate levels present in Lake Matano. Flux calculations imply that Fe and Mn must be recycled several times directly within the water column to balance the upward flux of CH₄. 16S gene cloning identified methanogens in the anoxic water column, and these methanogens belong to groups capable of both acetoclastic and hydrogenotrophic methanogenesis. We find that methane is important in C cycling, even in this very Fe-rich environment. Such Fe-rich environments are rare on Earth today, but they are analogous to conditions in the ferruginous oceans thought to prevail during much of the Archean Eon. By analogy, methanogens and methanotrophs could have formed an important part of the Archean Ocean ecosystem.

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.

Alteration of Iron-rich Lacustrine Sediments by Dissimilatory Iron-reducing Bacteria

The reactivity of trace elements in lake sediments towards microbial metal reduction was evaluated using spectroscopy, chemical extractions and incubations in a minimal media with the DIR bacterium Shewanella putrefaciens 200R. Micro-XRF measurements demonstrated the association of Cr, and Ni with Mn-rich phases. The onset of anaerobic conditions resulted in the rapid release of trace metals (Cr, Ni, Co) from the sediments with the progressive dissolution of a reactive Mn component. This fraction was approximately equivalent to that liberated by chemical extractions designed to operationally select for Mn phases. These results suggest that studies aiming to assess metal dissolution in anaerobic soils and sediments should attempt to discriminate between metals associated with Mn and Fe (hydr)oxides, the former being more reactive and likely dissolved to a greater extent.

Precise isotope ratio determination of common Pb using quadrupole LA-ICP-MS with optimized laser sampling conditions and a robust mixed-gas plasma

This work presents a method for the precise (0.2% RSE) determination of common (i.e. non-radiogenic) Pb isotope ratios using quadrupole Nd:YAG (266 nm) LA-ICP-MS at low (∼2 ppm) Pb concentrations. Laser sampling conditions significantly influence the precision of Pb isotope ratio measurements. This paper presents a set of optimum sampling conditions for the described system. Setting the laser focus above the sample surface significantly improves the precision of ratio measurements due to increased count rates and a reduction in the heterogeneity of particulate matter produced by fracturing at the site of ablation. In addition, using a more robust, mixed-gas (Ar–N2) plasma significantly increases sensitivity and reduces mass bias. With a mixed Ar–N2 plasma and optimized laser sampling conditions, single collector quadrupole LA-ICP-MS can be superior, for some applications (e.g., where micrometre-scale spatial resolution is important), to TIMS, and at low Pb concentrations is a cost-effective alternative to LA-MC-ICP-MS. The application of single detector, quadrupole LA-ICP-MS to the precise determination of common Pb isotope ratios in minerals has not been previously documented. This method is a powerful tool for use in isotope tracer studies of ore deposits and has potential applications to a range of environmental problems.

High-precision U–Pb age and geochemistry of the mineralized (Ni–Cu–Co) Suwar intrusion, YemenThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

High-precision U–Pb isotope dilution – thermal ionization mass spectrometry (ID–TIMS) geochronology on chemically abraded zircon grains from a noritic gabbro of the Ni-bearing Suwar mafic–ultramafic layered complex, northwestern Yemen, gives a mean 206Pb/238U age of 638.46xa0± 0.73 Ma (2σ; MSWDxa0= 1.4). At Wadi Qutabah, ∼30xa0km to the north, a similar mafic sample has an identical age of 638.58 ± 0.51xa0Ma (2σ; MSWDxa0= 0.32), which supports the possibility of there being a single, large intrusive complex with an estimated areal extent of ∼250xa0km2. This is supported by geochemical data of samples from each locality, which are postkinematic, gabbroic rocks that contain variable amounts of cumulus olivine, plagioclase, orthopyroxene, and ilmenite with intercumulus augite, hornblende, and Ni-sulphides. Straight rare Earth element (REE) patterns, Ba/La ∼30, Rb/Ba ∼0.04, and negative primitive-mantle-normalized P anomalies resemble EM1 (Enriched Mantle 1) of oceanic island basalts and Archean subcontinental mantle lit...

Preface to Bjørn Sundby’s Special Issue of Aquatic Geochemistry

This special issue of Aquatic Geochemistry is dedicated to the career of Bjorn Sundby (Fig. 1), Professor of Oceanography at the ‘‘Institut des Sciences de la Mer de Rimouski,’’ Canada. Bjorn earned his PhD in physical organic chemistry from the University of Bergen (Norway) in 1966. He started his professional career as an organic chemist for the ColgatePalmolive Company, where he worked to develop novel phosphate-free detergents in the face of the looming eutrophication crisis. Exploring his diverse interests while at Colgate, Bjorn enrolled in oceanography classes at Rutgers University and fell in love with the subject. In pursuit of a career in Oceanography, he then moved to Dalhousie University in Halifax to take on a postdoctoral position with Doug Loring. During this time, he met his wife Daniele Godbout. Ultimately, Bjorn took up a position as an oceanography research associate at the Universite du Quebec a Rimouski from 1974 to 1980, before becoming Professor of Oceanography from 1980 to 1984. He then returned to Europe to head the Department of Chemical Oceanography and Marine Pollution at the Netherlands Institute for Sea Research from 1984 to 1987, before coming back to Canada as Director of the Physical and Chemical Oceanography Branch at the Maurice Lamontagne Institute, Department of Fisheries and Oceans, Canada. With a mass of scientific achievements under his belt, Bjorn earned a prestigious Dr. Philos. Degree in aquatic geochemistry from the University of Bergen in 1987. From 1992 to his retirement in 2010, he was Professor of Oceanography at the Institut des Sciences de la Mer de Rimouski, QC. For nearly three decades, Bjorn has continued to challenge and change our view of sediment diagenesis, from the early models of a 1-dimensional steady-state system to our current picture of a 3D dynamic mosaic of biogeochemical reactions. One of his first major contributions was his work on manganese in the St. Lawrence Estuary. He was one of the