Nathalie V. Grassineau

Questioning the evidence for Earth's oldest fossils.

Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from ∼3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer–Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.

Implications of a 3.472–3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt, South Africa for the UV environmental conditions on the early Earth

Modelling suggests that the UV radiation environment of the early Earth, with DNA weighted irradiances of about three orders of magnitude greater than those at present, was hostile to life forms at the surface, unless they lived in specific protected habitats. However, we present empirical evidence that challenges this commonly held view. We describe a well-developed microbial mat that formed on the surface of volcanic littoral sediments in an evaporitic environment in a 3.5–3.3 Ga-old formation from the Barberton greenstone belt. Using a multiscale, multidisciplinary approach designed to strongly test the biogenicity of potential microbial structures, we show that the mat was constructed under flowing water by 0.25 μm filaments that produced copious quantities of extracellular polymeric substances, representing probably anoxygenic photosynthesizers. Associated with the mat is a small colony of rods–vibroids that probably represent sulphur-reducing bacteria. An embedded suite of evaporite minerals and desiccation cracks in the surface of the mat demonstrates that it was periodically exposed to the air in an evaporitic environment. We conclude that DNA-damaging UV radiation fluxes at the surface of the Earth at this period must either have been low (absorbed by CO2, H2O, a thin organic haze from photo-dissociated CH4, or SO2 from volcanic outgassing; scattered by volcanic, and periodically, meteorite dust, as well as by the upper layers of the microbial mat) and/or that the micro-organisms exhibited efficient gene repair/survival strategies.

Antiquity of the biological sulphur cycle: evidence from sulphur and carbon isotopes in 2700 million-year-old rocks of the Belingwe Belt, Zimbabwe.

Sulphur and carbon isotopic analyses on small samples of kerogens and sulphide minerals from biogenic and non-biogenic sediments of the 2.7 × 109 years(Ga)-old Belingwe Greenstone Belt (Zimbabwe) imply that a complex biological sulphur cycle was in operation. Sulphur isotopic compositions display a wider range of biological fractionation than hitherto reported from the Archaean. Carbon isotopic values in kerogen record fractionations characteristic of rubisco activity, methanogenesis and methylotrophy, and possibly anoxygenic photosynthesis. Carbon and sulphur isotopic fractionations have been interpreted in terms of metabolic processes in 2.7Ga prokaryote mat communities, and indicate the operation of a diverse array of metabolic processes. The results are consistent with models of early molecular evolution derived from ribosomal RNA.

Palaeosol control on groundwater flow and pollutant distribution: the example of arsenic.

The consumption of groundwater polluted by arsenic (As) has a severe and adverse effect on human health, particularly where, as happens in parts of SE Asia, groundwater is supplied largely from fluvial/deltaic aquifers. The lateral distribution of the As-pollution in such aquifers is heterogeneous. The cause of the heterogeneity is obscure. The location and severity of the As-pollution is therefore difficult to predict, despite the importance of such predictions to the protection of consumer health, aquifer remediation, and aquifer development. To explain the heterogeneity, we mapped As-pollution in groundwater using 659 wells across 102 km(2) of West Bengal, and logged 43 boreholes, to reveal that the distribution of As-pollution is governed by subsurface sedimentology. Across 47 km(2) of contiguous palaeo-interfluve, we found that the shallow aquifer (<70 mbgl) is unpolluted by As (<10 μg/L) because it is capped by an impermeable palaeosol of red clay (the last glacial maximum palaeosol, or LGMP, of ref 1 ) at depths between 16 and 24 mbgl. The LGMP protects the aquifer from vertical recharge that might carry As-rich water or dissolved organic matter that might drive reduction of sedimentary iron oxides and so release As to groundwater. In 55 km(2) of flanking palaeo-channels, the palaeosol is absent, so invasion of the aquifer by As and dissolved organic matter can occur, so palaeo-channel groundwater is mostly polluted by As (>50 μg/L). The role of palaeosols and, in particular, the LGMP, has been overlooked as a control on groundwater flow and pollutant movement in deltaic and coastal aquifers worldwide. Models of pollutant infiltration in such environments must include the appreciation that, where the LGMP (or other palaeosols) are present, recharge moves downward in palaeo-channel regions that are separated by palaeo-interfluvial regions where vertical recharge to underlying aquifers cannot occur and where horizontal flow occurs above the LGMP and any aquifer it caps.

FERNS AND FIRES: EXPERIMENTAL CHARRING OF FERNS COMPARED TO WOOD AND IMPLICATIONS FOR PALEOBIOLOGY, PALEOECOLOGY, COAL PETROLOGY, AND ISOTOPE GEOCHEMISTRY

Abstract We report the effects of charring on the ferns Osmunda, Pteridium, and Matteucia with coniferous wood (Sequoia) for comparison. Like charred wood, charred ferns shrink, become black and brittle with a silky sheen, and retain three-dimensional cellular structure. Ferns yield recognizable charcoal (up to 800°C) that could potentially survive in the fossil record enabling reconstruction of ancient fire-prone vegetation containing ferns. Charred fossils of herbaceous ferns would indicate surface fires. Like charred wood, cell-wall layers of charred ferns homogenize, and their reflectance values increase with rising temperature. Charcoalified fragments of thick-walled cells from conifer wood or fern tissues are indistinguishable and so cannot be used to infer the nature of source vegetation. Charred conifer wood and charred fern tissues show a relationship between mean random reflectance and temperature of formation and can be used to determine minimum ancient fire temperatures. Both charred conifer wood and charred fern tissues show some tendency toward increasingly lighter δ13C values up to charring temperatures of 600°C, which should be taken into account in analyses of δ13C in charcoals. Charred fern tissues consistently have significantly more depleted δ13C values (≤4‰) than charred wood. Therefore, if an analysis of δ13C through time included fern charcoal among a succession of wood charcoals, any related shifts in δ13C could be misinterpreted as atmospheric changes or misused as isotope stratigraphic markers. Thus, charcoals of comparable botanical origin and temperatures of formation should be used in order to avoid misinterpretations of shifts in δ13C values.

Recent fluid processes in the Kaapvaal Craton, South Africa: coupled oxygen isotope and trace element disequilibrium in polymict peridotites

Abstract Oxygen-isotope mapping of thin sections of polymict peridotite xenoliths shows that significant oxygen isotope disequilibrium is preserved on a sub-millimetre scale in primary and secondary minerals. Primary porphyroblastic phases (e.g., olivine, orthopyroxene, garnet, diopside) tend to have higher δ 18 O ratios than secondary minerals (e.g., mica, ilmenite, neoblastic olivine, orthopyroxene rims). Polymict minerals have a lower oxygen isotope composition than ‘average mantle’ (δ 18 O=5.2±0.3‰) and show clear evidence of inter- and intra-mineral oxygen isotope disequilibrium. Disequilibrium is also evident in the elemental geochemistry of the mantle minerals and a general correlation exists between oxygen isotopes and major (Si, Mg, Ca, Fe) and trace elements (Ce, Cr, Zr, Nb, REE). The interpretation that isotopic heterogeneity may relate to melt processes is supported by δ 18 O zonation in garnets, significant isotopic variation close to secondary veins, δ 18 O (primary phases)>δ 18 O (secondary phases) and oxygen isotope disequilibria in many minerals. In addition, a positive correlation between δ 18 O and grain size indicates a role for deformation processes as a result of diffusion reactions perhaps inextricably linked to melt processes. We suggest that polymict peridotites formed as a result of movement along mantle shear zones which led to the juxtaposition of minerals of varied provenance. Contemporaneous melt transfer reacted with these mantle breccias and rapid entrainment by ‘kimberlite’ meant that any associated mineral disequilibrium was very effectively preserved.

Geochemical evidence for combustion of hydrocarbons during the K-T impact event.

It has been proposed that extensive wildfires occurred after the Cretaceous–Tertiary (K-T) impact event. An abundance of soot and pyrosynthetic polycyclic aromatic hydrocarbons (pPAHs) in marine K-T boundary impact rocks (BIRs) have been considered support for this hypothesis. However, nonmarine K-T BIRs, from across North America, contain only rare occurrences of charcoal yet abundant noncharred plant remains. pPAHs and soot can be formed from a variety of sources, including partial combustion of vegetation and hydrocarbons whereby modern pPAH signatures are traceable to their source. We present results from multiple nonmarine K-T boundary sites from North America and reveal that the K-T BIRs have a pPAH signature consistent with the combustion of hydrocarbons and not living plant biomass, providing further evidence against K-T wildfires and compelling evidence that a significant volume of hydrocarbons was combusted during the K-T impact event.

Sedimentological control on Mn, and other trace elements, in groundwater of the bengal delta

To reveal what controls the concentration and distribution of possibly hazardous (Mn, U, Se, Cd, Bi, Pb) and nonhazardous (Fe, V, Mo, PO(4)) trace elements in groundwater of the Bengal delta, we mapped their concentrations in shallow groundwater (<60 mbgl) across 102 km(2) of West Bengal. Only Mn is a potential threat to health, with 55% of well water exceeding 0.3 mg/L, the current Indian limit for drinking water in the absence of an alternate source, and 75% exceeding the desirable limit of 0.1 mg/L. Concentrations of V are <3 μg/L. Concentrations of U, Se, Pb, Ni, Bi, and Cd, are below WHO guideline values. The distributions of Fe, Mn, As, V, Mo, U, PO(4), and δ(18)O in groundwater reflect subsurface sedimentology and sources of water. Areas of less negative δ(18)O reveal recharge by sources of evaporated water. Concentrations of Fe, As, Mo, and PO(4) are high in palaeo-channel groundwaters and low in palaeo-interfluvial groundwaters. Concentrations of U, V, and Mn, are low in palaeo-channel groundwaters and high in palaeo-interfluvial groundwaters. Concentrations of Fe and Mn are highest (18 and 6 mg/L respectively) at dual reduction-fronts that form strip interfaces at depth around the edges of palaeo-interfluvial aquifers. The fronts form as focused recharge carries dissolved organic carbon into the aquifer margins, which comprise brown, iron-oxide bearing, sand. At the Mn-reduction front, concentrations of V and Mo reach peak concentrations of 3 μg/L. At the Fe-reduction front, concentrations of PO(4) and As reach concentrations 3 mg/L and 150 μg/L respectively. Many groundwaters contain >10 mg/L of Cl, showing that they are contaminated by Cl of anthropogenic origin and that organic matter from in situ sanitation may contribute to driving reduction.

Stable isotopes in the Archaean Belingwe belt, Zimbabwe: evidence for a diverse microbial mat ecology

Abstract Sulphide-rich sediments, stromatolitic limestones and tidal-flat deposits in the late Archaean (2.7 Ga) Manjeri and Cheshire Formations, Belingwe greenstone belt, Zimbabwe show evidence for complex and extensive prokaryotic mat communities, including (1) shallow-water coastal sulphur mats; (2) mats, probably in somewhat deeper water; (3) nearby stromatolites that lived by oxygenic photosynthesis in shallow coastal settings. Petrological and geochemical (rare earth element; REE) evidence, coupled with high-resolution stable isotope results, identifies several complex interdependent metabolic consortia of bacteria and archaea. These microbial consortia would have exchanged nutrients and products both locally within prokaryotic mats and more widely via the waters of the Belingwe basin. This isotopic, sedimentological and REE evidence for a complex ecology of bacteria and archaea is consistent with metabolic inferences from rRNA phylogeny and is direct evidence that a diverse prokaryotic community, managing carbon on a global scale, had evolved by the late Archaean.

Archean (3.33 Ga) microbe-sediment systems were diverse and flourished in a hydrothermal context

Interacting, diverse microbe-sediment systems exist in natural environments today but have not yet been recognized in the oldest records of life on Earth (older than 3.3 Ga) because of lack of distinctive biomarker molecules and patchy preservation of microbial paleocommunities. In an in-situ outcrop- to microbial-scale study, we have differentiated probable phototrophic, chemolithotrophic, and chemo-organotrophic fossil microbial signatures in a nearshore volcanogenic sedimentary setting in 3.33 Ga rocks of the Josefsdal Chert, Barberton greenstone belt, South Africa, while demonstrating the importance of contemporaneous hydrothermal activity. Hydrothermal fluids, as a nutrient source, strongly controlled the development and distribution of the microbial communities and, as a silicifying agent, contributed to their rapid fossilization. We thus show that intricate microbe-sediment systems are deep-rooted in time and that at least some early life may indeed have been thermophilic.