Investigation 3

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Investigation 3. Life Detection in the Ancient Terrestrial Rock Record

We propose to use the insights gained in the first two research components (Investigations 1 and 2) to deepen our understanding of the ancient rock record on Earth, with the goal of determining the co-evolution of the environment and a diverse range of microbial metabolisms. In addition to informing us about the evolution of life on Earth, such an approach provides an essential interpretive context for studies of ancient rocks on Mars, allowing significant progress to be made on developing the required interpretive context prior to Mars sample return. Our approach will blend direct studies of Precambrian biota (e.g., microfossils), with indirect indictors of microbial metabolisms and ambient environmental conditions, largely using isotopic tracers. A unique aspect of our proposed program is the breadth of isotopic tracers that will be used (C, O, Mg, S, Si, Fe, Mo, Sr, and U-Th-Pb), as well as application of state-of-the-art in situ methods (SIMS/ion microprobe, ultra-fast laser ablation) that will allow us to interrogate geologically complex samples in a manner not previously possible.


3.1 Indirect evidence for life: Evolution of the Earth’s surface environments

3.1.1 The Neoarchean: Environmental changes before the Great Oxidation Event (GOE)

Project 1: The iron redox cycle as recorded in BIFs and Ca-Mg carbonate platforms.

Lead: Clark Johnson, UW-Madison

Project 2: The C-S-Fe cycle in organic-rich shales.

Lead: John Valley, UW-Madison

Summary: Large excursions in C, S, Fe, and Mo isotopes during the ~300 my interval prior to the GOE point to fundamental shifts in biogeochemical cycles resulting from the expanding influence of oxygenic photosynthesis in Earth’s oceans. To date, geochemical studies of this important time interval have tended to focus on only a few isotopic proxies, and here we propose a comprehensive, coordinated approach on three lithologies from the Pilbara and Kaapvaal cratons: Ca-Mg platform carbonates, iron formations, and black shales, obtaining multiple isotopic data on the same samples, including coordinated in situ isotopic analysis. The goal is to produce a holistic understanding of the biological and paleoenvironmental changes that occurred in the time period leading up to the GOE.

Figure_3-2

Transition from Ca-Mg carbonates (Gamohaan Fm) to BIF deposition (Kuruman Fm), Kaapvaal craton (~2.5 Ga). Model of Czaja et al. (2012) interprets Fe and Mo isotope compositions of Ca-Mg carbonates to reflect the effects of extensive Fe oxide precipitation during BIF formation.

Figure_3-3

Left: Kerogen-associated pyrite from the 2.7 Ga Jeerinah Formation. Right: Single zoned pyrite from the 2.4 Ga Meteorite Bore Member, showing large ranges in δ34S values (Williford et al., 2011b). Scale bars are 10 microns.

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3.1.2 The Mesoarchean: Early continental margins and diversification of surface environments

Lead: Clark Johnson, UW-Madison

Summary: Relatively little studied by astrobiologists, the Mesoarchean era (3.2-2.8 Ga) seems likely to record the driving forces that established the great expansion of microbial diversity and climate that is famously recorded in younger rocks. A small, but important, database suggests that the atmosphere may have cycled between oxygen- and methane-bearing conditions, although such interpretations are controversial. In addition, the first occurrence of carbonates that have highly negative δ13C values, and iron formations and related rock that have negative δ56Fe values, suggest major changes in the C and Fe cycles. These changes may correlate with major climactic variations, including the oldest known glacial deposits, providing a view of the interplay between climate and the biosphere in the early Earth.

Figure_3-8

Mesoarchean marine sedimentary rocks, suburbs of Johannesburg. The Kapvaal Craton in South Africa preserves the best exposures of Mesoarchean rocks on Earth, and allows detailed studies of proximal to distal transitions.

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3.1.3 The Paleoarchean: Chert-Carbonate Associations – Formation conditions and constraints on surface environments

Project 1: Formation conditions of cherts from the Pilbara craton.

Lead: John Valley, UW-Madison

Project 2: Constraints on surface environments and metabolisms.

Lead: Clark Johnson, UW-Madison

Summary: The 3.2 to 3.5 Ga cherts and jaspers from the Pilbara and Kaapvaal cratons have been interpreted to contain the oldest evidence of life on Earth, as well as insights into surface environmental conditions. Understanding the environments of formation for these cherts is essential for understanding the evidence of early life and atmosphere that they may contain. We will determine the environment and conditions of formation for Paleoarchean cherts, jaspers, and associated carbonates. These results directly bear on the likelihood that these rocks could contain evidence for early life, and their applicability to constraining ancient atmospheric compositions and seawater chemistry and temperatures.

Figure_3-9

The 3.2 Ga Manzimnyama Jaspilite from the Fig Tree Group, Barberton Greenstone Belt, South Africa. Determining how the jasper (hematite+chert) formed through oxidation of reduced (ferrous) iron will provide constraints on ancient oxygen contents in shallow marine environments.

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3.1.4 Hadean and Paleoarchean impact events and implications for early life

Lead: Aaron Cavosie, Curtin University

Summary: Establishment of habitable environments on the early Earth was tempered by intermittent giant meteorite impacts that created surface conditions hostile to life. Understanding when surface environments were habitable requires better constraints on the timing and duration of early impact events. No intact Hadean or Archean impact structures have been identified. This project will search for evidence of impact events from the early Earth in the form of diagnostic impact shock microstructures in Hadean and Archean detrital zircons. Detrital shocked zircons that eroded from early giant impact structures are datable using U-Pb geochronology, and will allow constraints to be placed on the timing of early Earth impact events.

Figure_3-5

Detrital shocked zircons in modern fluvial sediments. (A) Vaal River in South Africa, eroded from the 2.02 Ga Vredefort Dome impact structure. (B) Onaping River in Ontario, eroded from 1.85 Ga Sudbury impact. Sources cited in text. BSE=back scattered electron, CL=cathodoluminescence. Arrows show orientation of planar fractures.

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3.2 Direct evidence for life: Microfossils and organic carbon

3.2.1 The Proterozoic, Part I: Microbial ecology

Lead: J. William Schopf, UCLA

Summary: Microfossils chert-permineralized are direct samples of past life, and a suite of 20 Precambrian assemblages will be studied using optical microscopy, confocal laser scanning microscopy, and Raman spectroscopy. The geologic history of these samples is diverse, ranging from essentially non-metamorphosed to middle greenschist facies, and H/C ratios vary from ~0.85 to <0.15. This suite therefore provides a reference frame for the effects of metamorphism on Raman spectroscopy and in situ C isotope analysis that is critical for work on Paleoarchean samples. In addition, we will explore shallow- and deep-water environments, which has not been done before, but is promising based on a newly discovered 2.3 Ga microbial assemblage from Western Australia: the first deep-water, non-photosynthetic, microbial sulfuretum discovered in the geological record.

3.2.2 The Proterozoic, Part II: Carbon isotope analysis of microfossils

Lead: John Valley, UW-Madison

Summary: As proof-of-concept, we propose in situ C isotope measurements of individual organic-walled chert-permineralized microfossils from 10 Proterozoic stromatolitic units that reflect a range of Raman Index of Preservation (RIP) based on the molecular structure of microfossil organic matter. We will thus assess the degree to which C isotope heterogeneity correlates with morphology based taxonomy and microfossil anatomy at varying states of organic matter preservation. We will also apply this approach to microfossils from the newly discovered Turee Creek sulfuretum (see above).

Figure_3-6

Backscattered electron (a,b) and transmitted light (c,d) images showing SIMS C isotope analyses of Leiosphaerida crassa (a,c) and Myxococcoides sp. microfossils located 8 mm apart in a single sample from the Chichkan Formation. δ13C values shown next to analytical pits; average and total range of δ13C is indicated at top right in (a) and (b). L. crassa has δ13C consistent with eukaryotic photosynthesis, whereas Myxococcoides sp. has δ13C consistent with cyanobacterial photosynthesis.

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3.2.3 The Neoarchean: Correlation of carbon and sulfur isotopes and molecular biomarkers as a tracer of microbial ecology

Lead: John Valley, UW-Madison

Summary: Molecular biomarker evidence for Archean aerobiosis is one of the most important constraints provided on the early biosphere, but has been questioned based on C isotope variability in different pools of sedimentary organic matter. Important advances in precision and accuracy of in situ C isotope measurements will address this controversy, and we propose a detailed in situ C isotope study of low maturity, drill core samples of Neoarchean age from Western Australia (RHDH2a, ABDP-9) that will be simultaneously analyzed for hydrocarbon biomarker distributions and ratios. Where pyrite is associated with organic matter, coordinated in situ analysis of δ34S and Δ33S by SIMS and δ56Fe by LA-ICP-MS will be done to place the biomarker results in the context of C-S-Fe redox cycling and element pathways.

Figure_3-7

Scanning electron micrographs showing in situ δ13C analysis by SIMS of two kerogen types in a single cm2 sample of the Tumbiana Formation (SV1 core, 55.3 m). Kerogen I (a) is associated with TiO2 (brightest areas in kerogenous domain) and has δ13C lower than the bulk value and consistent with biomass that incorporated methane. Kerogen II (b), associated with pyrite and clay minerals, has δ13C significantly higher than the bulk value and consistent with primary photosynthetic biomass. The δ13C value for bulk rock (open circle) and the range of δ13C for multiple in situ analyses of kerogen I (filled rectangle) and kerogen II (open rectangle) are shown below images.

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3.2.4 Archean microbial ecology, Part I: Microfossils

Lead: J. William Schopf, UCLA

Summary: The earliest fossil record of life on Earth is incompletely known: only 10 microfossil occurrences are known for sediments of 3.2 to 3.5 Ga in age, and only one of these has been studied in detail. We propose new studies of individual fossils of the 3.5 Ga Apex chert (Western Australia) and 3.4 Ga Kromberg Formation (South Africa), as well as several Neoarchean to Mesoarchean potentially fossiliferous black chert units that formed in demonstrably deep-water environments. The continued study of existing Archean microfossil localities, as well as the search for new localities that provide deep- and shallow-water perspectives, promises new advances in our understanding of microbial diversity on the early Earth.

3.2.5 Archean microbial ecology, Part II: Carbon isotope analysis of microfossils

Lead: John Valley, UW-Madison

Summary: Direct evidence for life in the Paleoarchean is rare and beset with controversy due to geological, geochemical, and morphological ambiguity. Biogenicity of putative microfossils and kerogen in the Apex Chert and Strelley Pool Formation has been assessed with a variety of techniques, but consensus remains elusive. Building on analytical developments enabling accurate in situ microanalysis of C isotopes, as well as the analysis of three dimensional and molecular structure of fossil organic matter, we propose a new look at the problem of Paleoarchean microfossils that will significantly advance our knowledge of the biogenicity and physiology of life inferred from Earth’s oldest cellular fossils.

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