Gwyneth W. Gordon

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.

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.

Modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans

The increase in atmospheric oxygen ca. 2.4 Ga had a significant impact on the geochemical cycling of Fe. The history of environmental oxygenation may be recorded in the Fe isotope composition of Archean and Proterozoic sediments, but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations. Here we present Fe isotope data and iron/aluminum ratios from the Black Sea oxic shelf and euxinic basin. The isotope data demonstrate that shelf Fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin. We propose that there is net transport of isotopically light Fe from sediments of the shelf to those of the distal, anoxic basin, consistent with enrichments in reactive Fe seen in the deep basin. The low δ 56 Fe benthic Fe flux is generated during the coupling of microbial Fe(III) reduction or sulfidization with Fe 2+ aq oxidation. Low δ 56 Fe values reported previously from Late Archean sedimentary pyrites may be an isotopic fingerprint of analogous Fe redox cycling in the Late Archean oceans. This interpretation implies shallow-water Fe redox recycling in the Late Archean. We predict that the light isotopic compositions of the Late Archean will prove to be distinct from those of the Early Archean, before Fe redox cycling became an important process, and we infer that this difference may be related to the presence of oxygen in the surface ocean.

When do black shales tell molybdenum isotope tales

Molybdenum (Mo) isotopes in ancient sediments are promising recorders of global ocean paleoredox conditions. Organic-rich black shales can be used to reconstruct ancient ocean Mo isotope compositions if these sediments record the isotopic composition of contemporaneous seawater. Comparison of δ 98/95 Mo in two Devonian shale sequences of similar age, the New York Oatka Creek and Geneseo Formations, reveals that this assumption cannot be applied to all organic-rich shales. Although both sequences contain laminated intervals, elevated organic carbon, and enrichments of redox-sensitive metals, the mean δ 98/95 Mo differs systematically between the formations by ~0.59‰. Independent paleoredox indicators reveal that portions of the Oatka Creek Formation were deposited under pervasively euxinic (anoxic and sulfidic) conditions, whereas conditions during deposition of the Geneseo Formation were intermittently euxinic to suboxic (oxygen deficient but not sulfidic in the water column). We infer that reconstruction of ancient ocean δ 98/95 Mo from organic-rich shales requires independent verification of persistent local euxinia. With these considerations in mind, our data point to δ 98/95 Mo in the Devonian oceans ~0.6‰ lighter than in today9s oceans, consistent with expanded anoxia.

Resolution of inter-laboratory discrepancies in Mo isotope data: an intercalibration

The molybdenum (Mo) stable isotope system has been applied to a variety of geochemical and environmental problems. In the absence of a universally accepted zero-delta reference material, different groups report their data relative to their adopted in-house standards. Rigorous comparison of results generated in different laboratories using different analytical approaches is only possible if the in-house standards are of identical Mo isotope composition. To determine potential isotopic differences among various standards, the δ98Mo (98Mo/95Mo) values of ten Mo standard solutions were measured as part of this study. For six of these solutions, four laboratories carried out an intercalibration. In contrast to previous results, δ98Mo of various in-house standards were found to differ by up to 0.37‰. Renormalisation of our new and published Mo-isotope data available for seawater taken from various sites and the USGS rock reference material SDO-1 relative to NIST-SRM-3134, provides a much better agreement among reported δ98Mo values for these samples. Relative to NIST-SRM-3134, the δ98Mo of SDO-1 is 0.80 ± 0.14‰ (2s), while oxic, open-ocean seawater is characterised by an average δ98Mo of 2.09 ± 0.10‰ (2s). This intercalibration provides a solid platform for comparing and amending existing δ98Mo values. In addition, we recommend that future Mo isotope studies adopt NIST-SRM-3134 as a universal zero-delta reference material.

Characterization, Recovery Opportunities, and Valuation of Metals in Municipal Sludges from U.S. Wastewater Treatment Plants Nationwide

U.S. sewage sludges were analyzed for 58 regulated and nonregulated elements by ICP-MS and electron microscopy to explore opportunities for removal and recovery. Sludge/water distribution coefficients (KD, L/kg dry weight) spanned 5 orders of magnitude, indicating significant metal accumulation in biosolids. Rare-earth elements and minor metals (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) detected in sludges showed enrichment factors (EFs) near unity, suggesting dust or soils as likely dominant sources. In contrast, most platinum group elements (i.e., Ru, Rh, Pd, Pt) showed high EF and KD values, indicating anthropogenic sources. Numerous metallic and metal oxide colloids (<100-500 nm diameter) were detected; the morphology of abundant aggregates of primary particles measuring <100 nm provided clues to their origin. For a community of 1 million people, metals in biosolids were valued at up to US

Rapidly assessing changes in bone mineral balance using natural stable calcium isotopes

13 million annually. A model incorporating a parameter (KD × EF ×

Effect of biological soil crusts on soil elemental concentrations: implications for biogeochemistry and as traceable biosignatures of ancient life on land

Value) to capture the relative potential for economic value from biosolids revealed the identity of the 13 most lucrative elements (Ag, Cu, Au, P, Fe, Pd, Mn, Zn, Ir, Al, Cd, Ti, Ga, and Cr) with a combined value of US

Isotope fractionation during microbial metal uptake measured by MC-ICP-MS

280/ton of sludge.