Malcolm R. Walter

Stromatolite reef from the Early Archaean era of Australia

The 3,430-million-year-old Strelley Pool Chert (SPC) (Pilbara Craton, Australia) is a sedimentary rock formation containing laminated structures of probable biological origin (stromatolites). Determining the biogenicity of such ancient fossils is the subject of ongoing debate. However, many obstacles to interpretation of the fossils are overcome in the SPC because of the broad extent, excellent preservation and morphological variety of its stromatolitic outcrops—which provide comprehensive palaeontological information on a scale exceeding other rocks of such age. Here we present a multi-kilometre-scale palaeontological and palaeoenvironmental study of the SPC, in which we identify seven stromatolite morphotypes—many previously undiscovered—in different parts of a peritidal carbonate platform. We undertake the first morphotype-specific analysis of the structures within their palaeoenvironment and refute contemporary abiogenic hypotheses for their formation. Finally, we argue that the diversity, complexity and environmental associations of the stromatolites describe patterns that—in similar settings throughout Earths history—reflect the presence of organisms.

Morphological and ecological complexity in early eukaryotic ecosystems

Molecular phylogeny and biogeochemistry indicate that eukaryotes differentiated early in Earth history. Sequence comparisons of small-subunit ribosomal RNA genes suggest a deep evolutionary divergence of Eukarya and Archaea; C27–C29 steranes (derived from sterols synthesized by eukaryotes) and strong depletion of 13C (a biogeochemical signature of methanogenic Archaea) in 2,700 Myr old kerogens independently place a minimum age on this split. Steranes, large spheroidal microfossils, and rare macrofossils of possible eukaryotic origin occur in Palaeoproterozoic rocks. Until now, however, evidence for morphological and taxonomic diversification within the domain has generally been restricted to very late Mesoproterozoic and Neoproterozoic successions. Here we show that the cytoskeletal and ecological prerequisites for eukaryotic diversification were already established in eukaryotic microorganisms fossilized nearly 1,500 Myr ago in shales of the early Mesoproterozoic Roper Group in northern Australia.

Evidence for Low Sulphate and Anoxia in a Mid-Proterozoic Marine Basin

Many independent lines of evidence document a large increase in the Earths surface oxidation state 2,400 to 2,200 million years ago, and a second biospheric oxygenation 800 to 580 million years ago, just before large animals appear in the fossil record. Such a two-staged oxidation implies a unique ocean chemistry for much of the Proterozoic eon, which would have been neither completely anoxic and iron-rich as hypothesized for Archaean seas, nor fully oxic as supposed for most of the Phanerozoic eon. The redox chemistry of Proterozoic oceans has important implications for evolution, but empirical constraints on competing environmental models are scarce. Here we present an analysis of the iron chemistry of shales deposited in the marine Roper Basin, Australia, between about 1,500 and 1,400 million years ago, which record deep-water anoxia beneath oxidized surface water. The sulphur isotopic compositions of pyrites in the shales show strong variations along a palaeodepth gradient, indicating low sulphate concentrations in mid-Proterozoic oceans. Our data help to integrate a growing body of evidence favouring a long-lived intermediate state of the oceans, generated by the early Proterozoic oxygen revolution and terminated by the environmental transformation late in the Proterozoic eon.

Neoproterozoic stratigraphy of the Centralian Superbasin, Australia

Abstract The basement of central Australia includes the Arunta Block and the 1200-1100 Ma Musgrave Block succeeded by 1075-1000 Ma bimodal volcanic rifts and dolerite dykes. A regional hiatus at 1000-800 Ma, interpreted as reflecting a central post-volcanic (underplated) upland, was followed at 800 Ma by a second swarm of dykes in the Musgrave Block, Gawler Craton and Stuart Shelf, associated with the crustal sagging that initiated the Centralian Superbasin. The Centralian Superbasin is defined here to encompass the Neoproterozoic fill of the Amadeus, Georgina, Ngalia, Officer and Savory Basins. The intra-cratonic superbasin was disrupted internally 540–600 Ma ago (“Petermann Ranges Orogeny”) by a central uplift and associated thrusts, and by mid-Carboniferous (320 Ma) tectonism (“Alice Springs Orogeny”) to form the structural basins listed above. The stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions. New results from acritarch biostratigraphy and isotope chemostratigraphy, in conjunction with conventional lithostratigraphy and sequence analysis, allow better stratigraphic resolution than previously possible. Field observations in the recently discovered Savory Basin in Western Australia provide a basis for correlations with the adjacent basins.

The Ediacaran Period: A New Addition to the Geologic Time Scale

The International Union of Geological Sciences has approved a new addition to the geologic time scale: the Ediacaran Period. The Ediacaran is the first Proterozoic period to be recognized on the basis of chronostratigraphic criteria and the first internationally ratified, chronostratigraphically defined period of any age to be introduced in more than a century. In accordance with procedures established by the International Commission on Stratigraphy, the base of the Ediacaran Period is defined by a Global Stratotype Section and Point (GSSP) placed at the base of the Nuccaleena Formation cap carbonate directly above glacial diamictites and associated facies at Enorama Creek in the Flinders Ranges of South Australia. Its top is defined by the initial GSSP of the Cambrian Period. The new Ediacaran Period encompasses a distinctive interval of Earth history that is bounded both above and below by equally distinctive intervals. Both chemostratigraphic and biostratigraphic data indicate that the subdivision of the period into two or more series is feasible, and this should be a primary objective of continuing work by the Ediacaran Subcommission of the ICS.

Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact?

Biostratigraphic and chemostratigraphic studies of Australian late Neoproterozoic (Ediacarian) fossil plankton (acritarch) successions reveal a striking relationship between a radical palynofloral change, a short-lived negative excursion in the carbon isotope composition of kerogen, and a debris layer from the ca. 580 Ma Acraman bolide impact event. Palynomorphs changed from an assemblage dominated by long-ranging, simple spheroids to a much more diverse assemblage characterized by short-ranging, large, complex, process-bearing (acanthomorph) acritarchs, with the first appearance of 57 species. A marked negative carbon isotope excursion was followed by a steady rise coinciding with acanthomorph radiation. There are no apparent sedimentological controls on this radiation. Although the snowball Earth hypothesis predicts postglacial biotic change, radiation did not happen until long after the Marinoan glaciation and not until a second postglacial transgression. We propose that a global extinction and recovery event may have been associated with the Acraman bolide impact. Indications are that the Acraman event could rank with similar Phanerozoic major impact events.

Recognizing and Interpreting the Fossils of Early Eukaryotes

Using molecular sequence data, biologists cangenerate hypotheses of protistan phylogeny and divergence times. Fossils, however, provide our only direct constraints on the timingand environmental context of early eukaryotic diversification. Forthis reason, recognition of eukaryotic fossils in Proterozoic rocksis key to the integration of geological and comparative biologicalperspectives on protistan evolution. Microfossils preserved in shales of the ca. 1500 Ma Roper Group, northern Australia, display characters that ally them to the Eucarya, but, at present, attribution to any particular protistan clade is uncertain. Continuing research on wall ultrastructure and microchemistry promises new insights into the nature and systematic relationshipsof early eukaryotic fossils.

A possible chlorophycean affinity of some Neoproterozoic acritarchs

Abstract Two taxa of Neoproterozoic acritarchs of unknown affinity, Multifronsphaeridium pelorium and Species A, are analysed by electron microscopic (SEM, TEM) and chemical (micro-FTIR, pyrolysis GC–MS, thermal desorption–MS) methods. Both acritarch species are characterised by multi-branched processes and a remnant trilaminar sheath (TLS) structure. The TLS-bearing wall structures in these acritarchs suggest a possible biological affinity to chlorophyte algaenan. The molecular data obtained from the two acritarchs were generally similar and also consistent with a chlorophycean affinity. A significant aliphatic moiety is evident in these acritarchs as a short-chain series of n -alkene/alkane pyrolysates and prominent aliphatic IR bands. The restricted molecular-weight range ( 20 ) of the n -alkene/alkane doublets and the lack of isoprenoid and other branched alkanes in the pyrolysates suggest a low degree of branching in the aliphatic component of these acritarch macromolecules. The significant methyl (CH 3 ) IR signal was attributed to the terminal groups of short n -alkyl moieties. Alkylbenzenes, alkylphenols and alkylindoles were also significant pyrolysis products, indicating an aromatic component, although the latter two components may be attributed to artificially- and/or diagenetically-formed melanoidin moieties. The macromolecular structure of Multifronsphaeridium sp. and Species A consists of short n -alkylpolymethylenic chains, probably linked via ether/ester bonds, with possibly a small aromatic content. This study presents ultrastructural and molecular evidence of a genetic relationship between Neoproterozoic acritarchs and Chlorophyceae.

Lithofacies and biofacies of mid-Paleozoic thermal spring deposits in the Drummond Basin, Queensland, Australia

The Devonian to Carboniferous sinters of the Drummond Basin, Australia, are among the oldest well established examples of fossil subaerial hot springs. Numerous subaerial and subaqueous spring deposits are known from the geological record as a result of the occurrence of economic mineral deposits in many of them. Some are reported to contain fossils, but very few have been studied by paleobiologists; they represent an untapped source of paleobiological information on the history of hydrothermal ecosystems. Such systems are of special interest, given the molecular biological evidence that thermophilic bacteria lie near the root of the tree of extant life. The Drummond Basin sinters are very closely comparable with modern examples in Yellowstone National Park and elsewhere. Thirteen microfacies are recognisable in the field, ranging from high temperature apparently abiotic geyserite through various forms of stromatolitic sinter probably of cyanobacterial origin to ambient temperature marsh deposits. Microfossils in the stromatolites are interpreted as cyanobacterial sheaths. Herbaceous lycopsids occur in the lower temperature deposits.

Biogeochemistry of the 1640 Ma McArthur River (HYC) lead-zinc ore and host sediments, Northern Territory, Australia

Abstract The formation of the McArthur River lead-zinc deposit involves thermogenic or biologic oxidation of sedimentary organic matter, the products of which generated a massive stratiform sulfide ore body, and secondary carbonate and silica precipitates formed within the sediment pile down the flow pathway and above the reaction zone. The fine-grained texture of the mineralization indicates that primary ore texture is preserved, and coupled with the regional thermal maturity, indicate that this deposit is an ideal location to study organic matter signals related to ore formation and the sedimentary environment. Biomarker data point to a marine environment of deposition and are consistent with data previously collected from the host Barney Creek Formation in the adjacent Glyde Subbasin. An unusual biomarker distribution found in some samples from within two-orebody is considered to be related to the presence of sulfide-oxidizing bacteria. These organisms flourished after turbidite deposition, when oxygen in the upper part of the water column was mixed down to the sediment water interface. The biomarker data are supported by micropalaeontologic observations from the same samples and are consistent with intermittent oxygenation of the water column to the sediment water interface. This observation suggests an extension of the known occurrence of sulfide-oxidizing bacteria back in time by 800 million years, to 1640 Ma.