Dominic Papineau

Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event

It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 2.4 Gyr ago. Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse, but this explanation is difficult to reconcile with the rock record. Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system. Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 2.7 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time. We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached ∼400 nM throughout much of the Archaean eon, but dropped below ∼200 nM by 2.5 Gyr ago and to modern day values (∼9 nM) by ∼550 Myr ago. Nickel is a key metal cofactor in several enzymes of methanogens and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane. A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it. The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere.

Composition and structure of microbial communities from stromatolites of Hamelin Pool in Shark Bay, Western Australia

ABSTRACT Stromatolites, organosedimentary structures formed by microbial activity, are found throughout the geological record and are important markers of biological history. More conspicuous in the past, stromatolites occur today in a few shallow marine environments, including Hamelin Pool in Shark Bay, Western Australia. Hamelin Pool stromatolites often have been considered contemporary analogs to ancient stromatolites, yet little is known about the microbial communities that build them. We used DNA-based molecular phylogenetic methods that do not require cultivation to study the microbial diversity of an irregular stromatolite and of the surface and interior of a domal stromatolite. To identify the constituents of the stromatolite communities, small subunit rRNA genes were amplified by PCR from community genomic DNA with universal primers, cloned, sequenced, and compared to known rRNA genes. The communities were highly diverse and novel. The average sequence identity of Hamelin Pool sequences compared to the >200,000 known rRNA sequences was only ∼92%. Clone libraries were ∼90% bacterial and ∼10% archaeal, and eucaryotic rRNA genes were not detected in the libraries. The most abundant sequences were representative of novel proteobacteria (∼28%), planctomycetes (∼17%), and actinobacteria (∼14%). Sequences representative of cyanobacteria, long considered to dominate these communities, comprised <5% of clones. Approximately 10% of the sequences were most closely related to those of α-proteobacterial anoxygenic phototrophs. These results provide a framework for understanding the kinds of organisms that build contemporary stromatolites, their ecology, and their relevance to stromatolites preserved in the geological record.

Global Biogeochemical Changes at Both Ends of the Proterozoic: Insights from Phosphorites

The distribution of major phosphate deposits in the Precambrian sedimentary rock record is restricted to periods that witnessed global biogeochemical changes, but the cause of this distribution is unclear. The oldest known phosphogenic event occurred around 2.0 Ga and was followed, after more than 1.3 billion years, by an even larger phosphogenic event in the Neoproterozoic. Phosphorites (phosphate-rich sedimentary rocks that contain more than 15% P(2)O(5)) preserve a unique record of seawater chemistry, biological activity, and oceanographic changes. In an attempt to emphasize the potentially crucial significance of phosphorites in the evolution of Proterozoic biogeochemical cycles, this contribution provides a review of some important Paleoproterozoic phosphate deposits and of models proposed for their origin. A new model is then presented for the spatial and temporal modes of occurrence of phosphorites along with possible connections to global changes at both ends of the Proterozoic. Central to the new model is that periods of atmospheric oxygenation may have been caused by globally elevated rates of primary productivity stimulated by high fluxes of phosphorus delivery to seawater as a result of increased chemical weathering of continental crust over geological timescales. The striking similarities in biogeochemical evolution between the Paleo- and Neoproterozoic are discussed in light of the two oldest major phosphogenic events and their possible relation to the stepwise rise of atmospheric oxygen that ultimately resulted in significant leaps in biological evolution.

Evidence for early life in Earth’s oldest hydrothermal vent precipitates

Although it is not known when or where life on Earth began, some of the earliest habitable environments may have been submarine-hydrothermal vents. Here we describe putative fossilized microorganisms that are at least 3,770 million and possibly 4,280 million years old in ferruginous sedimentary rocks, interpreted as seafloor-hydrothermal vent-related precipitates, from the Nuvvuagittuq belt in Quebec, Canada. These structures occur as micrometre-scale haematite tubes and filaments with morphologies and mineral assemblages similar to those of filamentous microorganisms from modern hydrothermal vent precipitates and analogous microfossils in younger rocks. The Nuvvuagittuq rocks contain isotopically light carbon in carbonate and carbonaceous material, which occurs as graphitic inclusions in diagenetic carbonate rosettes, apatite blades intergrown among carbonate rosettes and magnetite–haematite granules, and is associated with carbonate in direct contact with the putative microfossils. Collectively, these observations are consistent with an oxidized biomass and provide evidence for biological activity in submarine-hydrothermal environments more than 3,770 million years ago.

Biological carbon precursor to diagenetic siderite with spherical structures in iron formations

During deposition of Precambrian iron formation, the combined sedimentation of ferrihydrite and phytoplankton biomass should have facilitated Fe(III) reduction during diagenesis. However, the only evidence for this reaction in iron formations is the iron and carbon isotope values preserved in the authigenic ferrous iron-containing minerals. Here we show experimentally that spheroidal siderite, which is preserved in many iron formation and could have been precursor to rhombohedral or massive siderite, forms by reacting ferrihydrite with glucose (a proxy for microbial biomass) at pressure and temperature conditions typical of diagenesis (170 °C and 1.2 kbar). Depending on the abundance of siderite, we found that it is also possible to draw conclusions about the Fe(III):C ratio of the initial ferrihydrite-biomass sediment. Our results suggest that spherical to rhombohedral siderite structures in deep-water, Fe-oxide iron formation can be used as a biosignature for photoferrotrophy, whereas massive siderite reflects high cyanobacterial biomass loading in highly productive shallow-waters.

Needs and opportunities in mineral evolution research

Abstract Progress in understanding mineral evolution, Earth’s changing near-surface mineralogy through time, depends on the availability of detailed information on mineral localities of known ages and geologic settings. A comprehensive database including this information, employing the mindat.org web site as a platform, is now being implemented. This resource will incorporate software to correlate a range of mineral occurrences and properties vs. time, and it will thus facilitate studies of the changing diversity, distribution, associations, and characteristics of individual minerals as well as mineral groups. The Mineral Evolution Database thus holds the prospect of revealing mineralogical records of important geophysical, geochemical, and biological events in Earth history.

A Chronostratigraphic Division of the Precambrian: Possibilities and Challenges

Abstract: This chapter provides a review of events through Precambrian Earth history, with the aim of providing an up-to-date foundation on which to construct a chronostratigraphic revision of the Precambrian time scale. The guiding principles used to develop a revised Precambrian time scale follow Cloud’s vision to “…seek trend-related events that have affected the entire Earth over relatively short intervals of time and left recognizable signatures in the rock sequences of the globe…”, and apply Gould’s historical principles of directionality and contingency.

A Chronostratigraphic Division of the Precambrian

Abstract: This chapter provides a review of events through Precambrian Earth history, with the aim of providing an up-to-date foundation on which to construct a chronostratigraphic revision of the Precambrian time scale. The guiding principles used to develop a revised Precambrian time scale follow Cloud’s vision to “…seek trend-related events that have affected the entire Earth over relatively short intervals of time and left recognizable signatures in the rock sequences of the globe…”, and apply Gould’s historical principles of directionality and contingency.

Nanoscale petrographic and geochemical insights on the origin of the Palaeoproterozoic stromatolitic phosphorites from Aravalli Supergroup, India

Stromatolites composed of apatite occur in post-Lomagundi-Jatuli successions (late Palaeoproterozoic) and suggest the emergence of novel types of biomineralization at that time. The microscopic and nanoscopic petrology of organic matter in stromatolitic phosphorites might provide insights into the suite of diagenetic processes that formed these types of stromatolites. Correlated geochemical micro-analyses of the organic matter could also yield molecular, elemental and isotopic compositions and thus insights into the role of specific micro-organisms among these communities. Here, we report on the occurrence of nanoscopic disseminated organic matter in the Palaeoproterozoic stromatolitic phosphorite from the Aravalli Supergroup of north-west India. Organic petrography by micro-Raman and Transmission Electron Microscopy demonstrates syngeneity of the organic matter. Total organic carbon contents of these stromatolitic phosphorite columns are between 0.05 and 3.0 wt% and have a large range of δ(13) Corg values with an average of -18.5‰ (1σ = 4.5‰). δ(15) N values of decarbonated rock powders are between -1.2 and +2.7‰. These isotopic compositions point to the important role of biological N2 -fixation and CO2 -fixation by the pentose phosphate pathway consistent with a population of cyanobacteria. Microscopic spheroidal grains of apatite (MSGA) occur in association with calcite microspar in microbial mats from stromatolite columns and with chert in the core of diagenetic apatite rosettes. Organic matter extracted from the stromatolitic phosphorites contains a range of molecular functional group (e.g. carboxylic acid, alcohol, and aliphatic hydrocarbons) as well as nitrile and nitro groups as determined from C- and N-XANES spectra. The presence of organic nitrogen was independently confirmed by a CN(-) peak detected by ToF-SIMS. Nanoscale petrography and geochemistry allow for a refinement of the formation model for the accretion and phototrophic growth of stromatolites. The original microbial biomass is inferred to have been dominated by cyanobacteria, which might be an important contributor of organic matter in shallow-marine phosphorites.

7.7 The Earliest Phosphorites: Radical Change in the Phosphorus Cycle During the Palaeoproterozoic

Phosphate is an essential and growth-limiting nutrient required by all forms of life, as it is a key component of many important macro-molecules. These macro-molecules are involved in energy transport, information storage, and structural support functions include membrane lipids, proteins, and nucleic acids. The global phosphorus cycle, which includes only dissolved and solid phases without any gaseous components, is strongly influenced by biological processes (Gulbrandsen 1969; Jahnke 1992; Follmi 1996). Continental weathering and riverine discharges are the most important sources delivering both particulate and dissolved phosphate into the oceans (Froelich et al. 1982; Follmi 1995). Long-term changes in the phosphorus cycle, such as variations in sources, concentration of dissolved seawater phosphate, formation of phosphorite deposits, and sequestration in biomass, are linked with other biogeochemical cycles and track major changes in Earth’s environmental conditions (Sheldon 1980; Baturin 1982; Papineau 2010; Planavsky et al. 2010). Biologic influence upon the phosphorus cycle can be traced back to the early Archaean (Blake et al. 2010). Ancient biologic processing of phosphate is inferred from the oxygen isotope ratios of some phosphates in 3200–3500 Ma sediments that are similar to those of modern marine biogenic phosphates (Blake et al. 2010).