Owen R. Green

Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event

In the Jurassic period, the Early Toarcian oceanic anoxic event (about 183 million years ago) is associated with exceptionally high rates of organic-carbon burial, high palaeotemperatures and significant mass extinction. Heavy carbon-isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon, which is isotopically light. In contrast, examples of light carbon-isotope values from marine organic matter of Early Toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12C versus 13 C (refs 1,2,5,6). Here, however, we report carbon-isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic, biospheric and atmospheric carbon reservoirs, and that this occurred despite the enhanced burial of organic carbon. We propose that—as has been suggested for the Late Palaeocene thermal maximum, some 55 million years ago—the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental-margin sediments.

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.

A fresh look at the fossil evidence for early Archaean cellular life

The rock record provides us with unique evidence for testing models as to when and where cellular life first appeared on Earth. Its study, however, requires caution. The biogenicity of stromatolites and ‘microfossils’ older than 3.0 Gyr should not be accepted without critical analysis of morphospace and context, using multiple modern techniques, plus rejection of alternative non-biological (null) hypotheses. The previous view that the co-occurrence of biology-like morphology and carbonaceous chemistry in ancient, microfossil-like objects is a presumptive indicator of biogenicity is not enough. As with the famous Martian microfossils, we need to ask not ‘what do these structures remind us of?’, but ‘what are these structures?’ Earths oldest putative ‘microfossil’ assemblages within 3.4–3.5 Gyr carbonaceous cherts, such as the Apex Chert, are likewise self-organizing structures that do not pass tests for biogenicity. There is a preservational paradox in the fossil record prior to ca 2.7 Gyr: suitable rocks (e.g. isotopically light carbonaceous cherts) are widely present, but signals of life are enigmatic and hard to decipher. One new approach includes detailed mapping of well-preserved sandstone grains in the ca 3.4 Gyr Strelley Pool Chert. These can contain endolithic microtubes showing syngenicity, grain selectivity and several levels of geochemical processing. Preliminary studies invite comparison with a class of ambient inclusion trails of putative microbial origin and with the activities of modern anaerobic proteobacteria and volcanic glass euendoliths.

Ediacarian sponge spicule clusters from southwestern Mongolia and the origins of the Cambrian fauna

Carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments (with Anabarites ) from southwestern Mongolia are of Ediacarian age. Spicule morphologies include bundles of oxeas arranged in three-dimensional quadrules, linked together at junctions by tetracts, pentacts, hexacts, or polyactines. All are referred to the Phylum Porifera, Class Hexactinellida. These sponge spicules provide the oldest remains that can be assigned without question to an extant phylum, and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record. It is suggested that siliceous and phosphatic members of the “Cambrian fauna” may have had their origins in eutrophic and outer shelf facies of the Late Proterozoic.

Cretaceous-Tertiary Carbonate Platform Evolution and the Age of the India-Asia Collision along the Ladakh Himalaya (Northwest India)

The India-Asia collision resulted in the formation and uplift of the Himalaya and the enhanced uplift of the Tibetan plateau. The transition from marine to continental facies within the Indus–Yarlung Tsangpo suture zone and along the northern margin of the Indian plate provides the most accurate method of dating the closure of the Tethys Ocean separating the Indian and Asian plates. Other indirect methods of dating the collision, such as paleomagnetism, dating the UHP metamorphism along the north margin of India, dating the youngest subduction-related granites along the southern margin of Asia, and dating the postorogenic Indus Molasse Group deposits within the suture zone, cannot provide such a precise or reliable age of collision. Ophiolite obduction onto the Indian passive margin occurred during the latest Cretaceous and predated initial collision of the two continental plates. Unconformities occur beneath the Late Maastrichtian Marpo Formation and beneath the Danian Stumpata Formation on the shelf and beneath the Upper Paleocene Sumda Formation in the suture zone. Stratigraphic and structural data from the Indian plate continental margin in the Ladakh and Zanskar Himalaya, northwest India, suggest that the final marine sediments were shallow marine limestones deposited during planktonic zone P8, corresponding to the Cusian stage of the late Lower Eocene (Ypresian) at 50.5 Ma. A regional unconformity across shelf and suture zones above these rocks marks the beginning of continental red bed deposition (Chulung-la and Nurla formations). The age of the final marine sediments is similar in Waziristan (northwest Pakistan) to the west and the South Tibet region to the east, suggesting that there was no significant diachroneity along the Indus–Yarlung Tsangpo suture zone. South of the Himalaya in the Hazara syntaxis, Pakistan, the youngest marine sediments correspond to nummulite-bearing limestones of the shallow benthic zone SBZ10 and planktonic foraminifera P7 zone (52–51 Ma). The timing of closure of Neo-Tethys between India and Asia corresponds closely to the ending of subduction-related granodiorite-granite magmatism along the Ladakh-Gangdese batholith (southern, Andean-type margin of the Asian plate) and precedes the drastic slowing of the northward drift of India. Continental fluvial-deltaic red beds unconformably overlie all marine sediments, both in the suture zone and along the north Indian plate margin.

Palaeobiology and evolution of the earliest agglutinated Foraminifera: Platysolenites, Spirosolenites and related forms

Tubular agglutinated fossils of Platysolenites antiquissimus Eichwald, 1860, P. cooperi n.sp. and Spirosolenites spiralis Glaessner, 1979 are examined from selected lower Cambrian successions in Avalonia (England; Wales; Newfoundland) and Baltica (Russia and Estonia). Platysolenites cooperi n.sp. is shown to extend below the first appearance of P. antiquissimus in the Neoproterozoic-Cambrian boundary stratotype region, SE Newfoundland, and to occur at higher stratigraphic levels in Wales and Finnmark. Taphonomic, teleological and ultrastructural studies on well-preserved material are consistent with a similar grade of organization and mode of life for P. antiquissimus and the living astrorhizacean foraminiferid Bathysiphon. However, agglutinated proloculi are here described from both rectilinear P. antiquissimus and coiled S. spiralis, which suggests that neither were typical astrorhizaceans.

Cortical growth marks reveal extended juvenile development in New Zealand moa

Cyclical growth marks in cortical bone, deposited before attainment of adult body size, reflect osteogenetic changes caused by annual rhythms and are a general phenomenon in non-avian ectothermic and endothermic tetrapods. However, the growth periods of ornithurines (the theropod group including all modern birds) are usually apomorphically shortened to less than a year, so annual growth marks are almost unknown in this group. Here we show that cortical growth marks are frequent in long bones of New Zealands moa (Aves: Dinornithiformes), a recently extinct ratite order. Moa showed the exaggerated K-selected life-history strategy formerly common in the New Zealand avifauna, and in some instances took almost a decade to attain skeletal maturity. This indicates that reproductive maturity in moa was extremely delayed relative to all extant birds. The two presently recognized moa families (Dinornithidae and Emeidae) also showed different postnatal growth rates, which were associated with their relative differences in body size. Both species of giant Dinornis moa attained their massive stature (up to 240 kg live mass) by accelerating their juvenile growth rate compared to the smaller emeid moa species, rather than by extending the skeletal growth period.

Photang thrust sheet: an accretionary complex structurally below the Spontang ophiolite constraining timing and tectonic environment of ophiolite obduction, Ladakh Himalaya, NW India

The pre-collisional tectonic evolution of the north Indian continental margin is best recorded in the few ophiolite complexes preserved, the largest of which occurs in the Spontang area of the Himalayas. Structural, sedimentological, palaeontological and geochemical work on the ophiolite and associated allochthonous thrust sheets has been carried out to constrain the timing and tectonic environment of ophiolite obduction. A distinct thrust sheet of accretionary complex rocks has been identified immediately underlying the ophiolite. Accreted units include thrust slices of tectonic melanges and alkaline basaltic lavas capped by limestones ranging from late Permian to late Cretaceous in age, interpreted as remnants of former seamounts. The accretionary complex formed above a north dipping intra-oceanic subduction zone during the Cretaceous, the Spontang ophiolite located in the hanging wall. Beneath the Photang thrust sheet, two further distinct, allochthonous thrust sheets of sedimentary melanges and continental slope deposits have been recognized. The structural relations of the allochthonous thrust sheets with the sediments of the north Indian margin have been mapped in detail and show clear evidence that obduction occurred in the late Cretaceous. At this time the Dras-Kohistan intra-oceanic arc had already collided with the southern Asian margin, over 1500 km to the north. Obduction of the Spontang ophiolite therefore records a separate tectonic episode in the Ladakh Himalaya.

The ∼3.4 billion-year-old strelley pool sandstone: A new window into early life on Earth

The recognition and understanding of the early fossil record on Earth is vital to the success of missions searching for life on other planets. Despite this, the evidence for life on Earth before ~3.0 Ga remains controversial. The discovery of new windows of preservation in the rock record more than 3.0 Ga would therefore be helpful to enhance our understanding of the context for the earliest life on Earth. Here we report one such discovery, a ~3.4 Ga sandstone at the base of the Strelley Pool Formation from the Pilbara of Western Australia, in which micrometre-sized tubular structures preserve putative evidence of biogenicity. Detailed geological mapping and petrography reveals the depositional and early diagenetic history of the host sandstone. We demonstrate that the depositional environment was conducive to life and that sandstone clasts containing putative biological structures can be protected from later metamorphic events, preserving earlier biological signals. We conclude from this that sandstones have an exciting taphonomic potential both on early Earth and beyond.

Making silica rock coatings in the lab: synthetic desert varnish

Desert varnish and silica rock coatings have perplexed investigators since Humboldt and Darwin. They are found in arid regions and deserts on Earth but the mechanism of their formation remains challenging (see Perry et al. this volume). One method of researching this is to investigate natural coatings, but another way is to attempt to produce coatings in vitro. Sugars, amino acids, and silicic acid, as well as other organic and (bio)organic compounds add to the complexity of naturally forming rock coatings. In the lab we reduced the complexity of the natural components and produced hard, silica coatings on basaltic chips obtained from the Mojave Desert. Sodium silicate solution was poured over the rocks and continuously exposed to heat and/or UV light. Upon evaporation the solutions were replenished. Experiments were performed at various pHs. The micro-deposits formed were analyzed using optical, SEM-EDAX, and electron microprobe. The coatings formed are similar in hardness and composition to silica glazes found on basalts in Hawaii as well as natural desert varnish found in US southwest deserts. Thermodynamic mechanisms are presented showing the theoretical mechanisms for overcoming energy barriers that allow amorphous silica to condense into hard coatings. This is the first time synthetic silica glazes that resemble natural coatings in hardness and chemical composition have been successfully reproduced in the laboratory, and helps to support an inorganic mechanism of formation of desert varnish as well as manganese-deficient silica glazes.