ProjectB3
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Project B3: Archean life: Early life and ancient life-support systems on the Kaapvaal craton
French pi: P. Philippot
South African pi: M. de Wit (with H. Furnes in Norway)
Project Participants
- South Africa: Maarten de Wit, Eugene Grosch, AEON, Cape Town
- France: Pascal Philippot, Mark van Zuilen IPG-Paris
- Norway: Harald Furnes, Nicola MacLoughlin, Centre for Geobiology, Bergen
- Canada: Karlis Muelenbachs, Isotope centre, Edmonton.
Summary
Since the Archean and Palaeoproterozoic are dominated by volcanosedimentary successions that have experienced different degrees of metamorphism, many ongoing controversies exist regarding life in the Precambrian part of Earth history. It is now recognized that future studies of early life and the associated environmental conditions depends on better description of geological context, identification of hydrothermal and metamorphic processes, and detailed structural, isotopic and chemical description of mineral assemblages, stromatolitic microlaminae, and organic microstructures that are indigenous to and syngenetic with the stratigraphic rock record. The most serious problem for understanding the evolution of organisms and their biogeochemical environments from the ancient rock record has been the difficulty in obtaining important sequences of “fresh” rocks, i.e., rocks that have not been severely altered by post-depositional processes. In 2004, pristine drill core samples were recovered from key localities of the Archaen Pilbara Craton, Western Australia. Here we propose to take the advantage of this expertise to perform a new drilling project aimed at intersecting similar 3.3-3.5 billion year old rock succession in South Africa, from the Barberton greenstone belt. The important issue to be addressed concern: 1) the significance of traces (Ichnofossils) of microbial activity in the glassy (now recrystalised) margins of pillow lavas; 2) the significance of elemental sulfur disproportionation as one of the most primitive, rudimentary microbial metabolism on Early Earth and 3) the composition and temperature of hydrothermal fluids and Archaean seawater that have interacted with these rocks, and the origins of asociated carbonaceous material (biogenic vs inorganic). A wide variety of state-of-the-art bulk and in situ analytical techniques will be used to study biologic and geochemical processes in these pristine drill cores.
Scientific Project
Although several decades have passed since the first description of recognisable early Archaean microfossils (DUNLOP et al., 1978), morphology-focused imaging techniques of fossil-like objects and stable isotope (C, N, S) compositions of putative organisms have repeatedly failed to pose limits on the interpretation of the biogenic origin of the microstructures (BRASIER et al., 2002; MOJZSIS et al., 1996; SCHOPF et al., 2002; VAN ZUILEN et al., 2002). Additionally, several abiologic metamorphic and hydrothermal reactions have been identified that can produce kerogen and graphite (BRASIER et al., 2002; VAN ZUILEN et al., 2002), and specific abiologic processes have been described that can generate complex structures that resemble microfossils (GARCIA-RUIZ et al., 2003). In view of these uncertainties and controversies, it is clear that elucidating how and when life may have originated on Earth requires first to understand the conditions that prevailed early in Earth‟s history and the environments in which life may have appeared and later evolved. The recent findings of the Norwegian-led group of ichnofossils in the rims of the worlds oldest pillow lavas in South Africa and Australia (FURNES et al., 2004; 2007a,b ) has dramatically shown that rocks previously ignored in studies of early life (e.g. basaltic igneous rocks) now offer a new paleo-environment as habitats for early life. This holds great potential to track life back even further in time, and must be considered a profitable focus for such early life studies (FURNES et al., 2007a,b; MCLOUGHLIN ET AL., 2007).
Life on early Earth is likely to have gone through one or more hot ocean. It is possible that early life diversified near hydrothermal vents where only hyperthermophiles organisms would have survived. Archaean hydrothermal settings would have been varied, with abundant vent fields like black smokers and hydrothermal deposits rich in Fe, Cu, S (De Witt et al., 1982). Microscopic sulfides with low 34S/32S ratios in marine sulfate deposits from the 3,490 Myr-old Dresser Formation, Warrawoona Group, Australia, have been interpreted as evidence for the presence of early sulfate-reducing organisms on Earth (SHEN et al., 2001). This finding is at odd with the long standing consensual notion that the Archean atmosphere was essentially reduced and that oxygen reached appreciable levels around 2 to 2.4 billion years ago (HOLLAND, 1984). The great oxidation story was strengthened considerably in the recent years by the discovery that minerals in ancient rocks had unusual ratios of sulfur isotopes, a phenomenon known as mass-independent fractionation (MIF-S; FARQUHAR et al., 2000). The only known mechanism that can produce this effect is the breakup of sulfur dioxide by ultraviolet light in a low-oxygen atmosphere. The MIF-S isotopic signature is small or entirely absent in rocks younger than 2.4 billion years, suggesting that Earth‟s atmosphere has been oxygen-rich since that time. Recent results obtained from drill core samples from Australia show that the story may be much more complex then originally envisioned, however (Ohmoto et al., 2006; Ono et al., 2006; Anbar et al., 2007; Kaufman et al;, 2007). Using high resolution in situ isotopic techniques, Philippot et al. (2007) showed that the microscopic sulfides from the 3.5 Ga Dresser Foramtion have a mass independently-fractionated sulfur isotopic anomaly (∆33S) that differs from that of their host sulfate (barite). These microscopic sulfides cannot have been produced by sulfate reducing microbes, nor by abiologic processes that involve reduction of sulfate. Instead, they interpret the combined negative ∂34S and positive ∆33S signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur. This finding provides strong support in favor of a reduced environment at about 3.5 Ga.
Knowledge of the composition and temperature of hydrothermal fluids and seawater is central to elucidating the conditions pertaining to the development of life on early Earth. Cl/Br ratio in fluids is considered conservative in many geological settings and is therefore widely used to trace the origin of fluids (meteoric, oceanic, crustal, mantellic) in the rock record. Analysis of ancient fluid inclusions from the Kaapvaal Craton, South Africa, suggest that Cl/Br in Archaean 3.2 Gyr-old (DE RONDE et al., 1997) and paleoproterozoic 2.2 Gyr-old (GUTZMER et al., 2003) seawater was below present-day value and resulted from mantle buffering. These studies, however, were based on bulk fluid analyses (i.e., crush-leach) which can result in fluid mixing if several fluid generations occur in a single sample. Alternatively, detailed chemical analysis of individual fluid inclusions can be performed by Synchrotron Radiation X-ray micro-Fluorescence, thus allowing independent analysis of different fluids trapped in the same sample. Using this technique, (FORIEL et al., 2004) showed that fluid infiltrating pillow basalts at the base of the Dresser and Apex hydrothermal systems of the Pilbara Craton consisted of mixing trends of fluids with compositions ranging from typical modern seawater enriched in salt by 2 to 3 times (7 to 8 wt% salt) combined with hydrothermal fluid components enriched in Ba and transition metals (Fe, Cu, Zn). Furthermore, as introduced above, sulfur is a key element in Archaean geochemistry, as seawater sulfate concentration is used to constrain the oxidation state of the early ocean and atmosphere, and S may have been a key element for early metabolic processes and for the evolution of primitive life (SHEN et al., 2001, Philippot et al., 2007). Using in-situ SXRF technique, (FORIEL et al., 2004) estimated that the “North-Pole seawater component” contained low sulfate concentration (0–8 mM) compared to 28 mM in present-day ocean, which further supports the notion that early metabolisms used reduced (elemental S) rather than oxidized (sulfate) species as a source of energy.
Proposed Research Project
The aim of this project is to explore new frontiers of addressing the problem of primitive life by establishing links between hydrothermal fluids and seawater circulation, mineral crystallisation, sediment deposition and diagenesis/metamorphism, and the development of the earliest ecological niches. Specifically, we aim at constraining the composition, and the redox state of the primitive ocean and atmosphere, and to develop integrated geochemical studies involving structural, isotopic and chemical description of mineral assemblages, fluid inclusions and organic microstructures (microfossils?) that are indigenous to and syngenetic with the stratigraphic/hydrothermal rock record.
Subtopics:
- Microfossils in pillow margins - what was the optimum temperature-window for preservation?
- Elemental sulfur disproportionation; a widespread early Archean microbial metabolism?
- Interactions between fluids, rocks and microbes - Can we define chemical fingerprints?
- Archean ocean temperature and composition - hot or cold, strongly or middly saline, buffered by organic activity?
Three Proposed Drilling Targets
We take advantage of the expertise we gained while performing the Pilbara Drilling Project in 2004 (VAN KRANENDONK et al., 2006) to recover “fresh” continuous sections at two or three (depending of technical constraints and financial supports) key geologic formations of the Barberton, greenstone belt:
1- Upper Onverwacht Group
from the top of the Hooggenoeg Formation (>3,47Ga) into the base of the Kromberg Formation (>3.46 Ga).
This proposed section will penetrate the unconformity between a lower marine section of the pillow lavas and interbedded cherts, and the overlying subaerial conglomerates of the Kromberg Formation. The former is the type section of the biogenic (ichtofossils) structures from its pillow lava margins, and typifies Earth’s 3.4 Ga oceanic environments – including an early hydrothermal ocean-floor-type metamorphic overprint. This part of the section will allow the physio-chemical conditions of the biological communities in the pillow margins, and their preservation window to be better defined. The latter section suggests that, here, the top of the pillow lava pile was exposed to the Archean atmosphere at the unconformity. This will allow a variety of tests to be conducted concerning the composition of Archean atmosphere and biosphere conditions at that time.
2- Upper Kromberg Formation
This is a section of pillow lavas and sheet flow with interlayered cherts and chert xenoliths that overlies enigmatic early carbonated and silicified oceanic shear zones. Studies of fresh samples will allow better definition of the chemical alteration and microbial activities from an Archean marine bottom-surface into its underlying sub-surface fault zones. transition of sea water change. It will also likely solve the chemical composition and temperature of the associated seawater.
3- Lower Fig Tree Group
This section will penetrate basal shales interbedded with massive barite and jasper that ovelie altered pillow basalt. Although slightly younger (3.3 Ga), the Fig Tree Formation is lithologically similar to the 3.5 Gyr old chert-barite deposit of the Dresser Formation from which Philippot et al., (2007) described elemental sulfur disproportionation rather than sulfate reduction as a viable metabolism during the Early Archean and were Foriel et al (2004) found a “seawater” composition similar to modern seawater.
The Onverwacht and Warrawoona Groups in Pilbara represent the two unique remnants of early Archean seafloor settings that have experienced only low grade metamorphism, hence providing a unique opportunity to compare for the first time Archean ecosystems of similar ages from two different localities. The two first holes have obtained secured funding from Norwegian sources. The third hole will be supported by French sources.
Approach and Methodology
Technical aspects: drill holes of 200-300 meter depth are planned; technical details will be confirmed when the drilling specifics and a drilling contractor have been finalised.
Structural, chemical and isotopic analyses
- SEM, EPMA, TEM – AEON, Bergen, IPGP
- Fluid inclusion analysis - IPGP
- Raman spectroscopy, confocal microscopy – IPGP
- LA-ICP-MS – Bergen, AEON
- Synchrotron (SR-XRF, STXM), IPGP
- C, O isotopes analysis - Edmonton
- S isotopes analysis – CRPG, IPGP
- Sm-Nd isotopes – AEON
- Noble gases – CRPG
- Microbial diversity and sample contamination – IPGP, Orsay
Personnel
We have identified one black South African PhD student, Eugene Grosch, with suitable qualifications, and plan to advertise for at least one more student position for a three year period, to develop combined mineralogical and petrological approaches to perform an integrated microstructural and analytical study using various quantitative techniques including SEM, X-ray elemental mapping, cathodoluminescence imaging, EPM analysis and trace element analysis and in situ oxygen and S isotope analysis.
Eugene Grosch will be based at AEON, South Africa, IPGP, France, the new Centre of biogeological at Bergen University, Norway, and the stable isotope lab at the Univerisity of Edmonton, Canada.
In addition to principal participants from AEON, IPGP, Bergen University and University of Edmonton listed above, other participants involved in the Pilbara Drilling Project have notified their interests in participating to the project associated with drill hole 3. These include participants from IPGP (Magali Ader, Karim Benzerara, Jean Besse, Pierre Cartigny, Gaston Godard), CRPG (Bernard Marty, Marc Chaussidon, Béatrice Luais) and University Orsay (Purificacion Lopez-Garcia, David Moreira).
Norway, Canada and South Africa: Whole rock geochemistry and isotope analyses of basalts will be carried out in the new Geobiology Centre of Bergen (coordinators Profs H Furnes and R.D Pederson) and AEON in Cape Town (coordinator M de Wit). In Norway the petrography and electron microscopy and relevant geochemistry on the Ichnofossils will be undertaken by Dr Nicole McLouchlin. Stable isotope analyses (O, C) on minerals and whole rocks will be done in Edmonton, Canada, by Karlis Meulenbachs (no funds requested in this proposal).
Relevant References cited
- Anbar, A.D. et al., 2007. A Whiff of Oxygen Before the Great Oxidation Event? Science, 317: 1903-1907.
- Brasier M., Green O. R., Jephcoat A. P., Kleppe A., Van Kranendonk M. J., Lindsay J. F., Steele A., and Grassineau N. V. (2002) Questioning the evidence for Earth's oldest fossils. Nature 416, 76-81.
- De Ronde E. J., Channer D. M. D., Faure K., Bray C. J., and Spooner E. T. C. (1997) Fluid chemistry of Archean seafloor hydrothermal vents: Implications for the composition of circa 3.2 Ga seawater. Geochimica et Cosmochimica Acta 61, 4025-4042.
- de Wit M. J., Hart R., Martin A., and Abbott P. (1982) Archean abiogenic and probable biogenic structures associated with mineralized hydrothermal vent systems and regional metasomatism, with implications for greenstone belt studies. Economic Geology 77, 1783-1802.
- Dunlop J. S. R., Muir M. D., Milne V. A., and Groves D. I. (1978) A new microfossil assemblage from the Archaean of Western Australia. Nature 274, 676-678.
- Farquhar J., Bao H., and Thiemens M. H. (2000) Atmospheric influence of Earth's earliest sulfur cycle. Science 289, 756-758.
- Foriel J., Philippot P., Rey P., Somogyi A., Banks D., and Ménez B. (2004) Biological control of Cl/Br and low sulfate concentration in a 3.5-Gyr-old seawater from North Pole, Western Australia. Earth Planet.Sci. Lett. 228, 451-463.
- Furnes H, Banerjee NR, Muehlenbachs K, Staudigel H, de Wit MJ (2004) Early life recorded in Archean pillow lavas. Science 304:578-581
- Furnes H, Banerjee NR, Staudigel H, Muehlenbachs K, de Wit M, McLoughlin N, Van Kranendonk M (2007a) Bioalteration textures in recent to mesoarchean pillow lavas: A petrographic signature of subsurface life in oceanic igneous rocks. Precamb Research, 158, 156-176.
- Furnes, H., McLoughlin,N., Muehlenbachs,K., Banerjee, N., Staudigel, H., Dilek, H., de Wit, M., Van Kranendonk, M., Schiffman,P. (2007b). Oceanic pillow lavas and hyaloclastites as habitats for microbial life through time – A review. In: Dilek Y, Furnes H, Muehlenbachs K (eds) Links between geological processes, microbial activities and evolution of life, Springer Verlag. in press.
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