Primordial Degassing of Terrestrial Planets: the Case for Reduced Carbon Compounds on the Early Surfaces
A multiplicity of potential sources, degassing processes and events, and physical-chemical conditions leads to a bewildering array of scenarios and models for the outcome of degassing of the early atmospheres of terrestrial planets. For more than five decades geological, theoretical and experimental evidence seem to have converged on the conclusion that initially surviving terrestrial atmospheres were dominated by CO2- N2. Consideration of the chemical nature of the materials most likely to have been major contributors of planetary volatiles suggests not only the likelihood of substantial loss of the earliest atmosphere, but that most of the extant atmosphere probably resulted from the last 1-2% of planetary accretion. Detailed examination of the processes and physical-chemical conditions associated with late accretion suggest that Earth's earliest permanent atmosphere contained highly reduced carbon (and nitrogen?) species. The instability of methane and ammonia under ultraviolet radiation would have rapidly produced a large pool of condensed C-H-N-O compounds, rained out into a primordial ocean. Such a scenario inevitably leads to the production of pre-biotic compounds in concentrations especially favorable for the quick emergence of life. A large, early organic carbon reservoir at Earth's surface leads naturally to models of surface and near-surface chemistry more consistent with the geologic record than an atmosphere dominated by CO2-N2.
Role of Tectonomagmatic Processes in the Early Paleoproterozoic as a Trigger for Surface Environmental Changes and Evolution of Biosphere
It is known that ecological system in the Middle Paleoproterozoic experienced a fundamental change, which finally led to the appearance of multicellular organisms. Though organic life has been already existed in the Paleoarchean (Sergeev et al., 2007 and references herein), the multicellular organisms appeared only in the middle Paleoproterozoic about 1.6 Ga ago, subsequent to a fundamental change of tectononagmatic processes at 2.3-2.0 Ga ago, when early Precambrian high-Mg melts, derived from depleted mantle, gave way to the geochemically enriched Fe-Ti picrites and basalts, whose origin has been related to ascending of thermochemical mantle plumes. This new type of melts was characterized by elevated and high contents of Fe, Ti, Cu, P, Mn, alkalis (especially, Na), LREE, and other incompatible elements (Zr, Ba, Sr, U Th, F, and others). A large-scale influx of alkalies in the World Ocean presumably neutralized its water, making it more suitable for the life, while input of Fe-group metals, P, and other trace elements, which are required for metabolism and fermentation, rapidly expanded the possibility for the development of biosphere. This caused a rapid development of photosynthesizing cyanobacteria and, subsequently, the emergence of oxidizing atmosphere marked by formation of cupriferous red beds at all Precambrian shields, and generation of first hydrocarbon deposits (Melezhik et al., 2005). A drop in atmospheric CO2 presumably suppressed the greenhouse effect, while significant intensification of relief ruggedness caused by wide development of plate tectonics after 2 Ga resulted in the change of atmospheric circulation. All these processes finally led to the global glaciations. The latters commenced earlier, in the Paleproterozoic (Huronian glaciation), simultaneously with first manifestations of Fe--Ti basaltic magmatism, which came into force only in the middle Paleoproterozoic. Thus, a fundamental change in tectonomagmatic activity acted as the trigger for environmental changes and biospheric evolution, supplying a qualitatively new material on the Earth's surface.
The Delay in Oxidation of Earth's Atmosphere Following the Emergence of O2-producing Photosynthesis: an Explanation
The time of origin of O2-producing photosynthesis appears to be at least as early as 2.7 Ga, and possibly as early as 3.5 Ga, if stromatolitic fossils of those ages were produced by cyanobacteria. The record of mass- independent fractionation of sulphur isotopes indicates an abrupt rise in atmospheric oxygen at about 2.2-2.3 Ga, at least 400 Ma after the advent of oxygenic photosynthesis. This delay in rise of atmospheric oxygen in the face of the likely productivity of cyanobacteria has been a puzzle for several years, particularly if the main carbon species in the surface environment was CO2 as is widely assumed. Various possible nutrient limitations on cyanobacterial productivity have been suggested including Fe, Mo, fixed N and P. Examination of the geochemical availability of these limiting nutrients, as well as intrinsic biochemical inefficiency (i.e. primitive enzyme systems), suggests that none of these are capable of explaining the delay in surface oxidation. Consumption of photosynthetic oxygen by a surface reservoir of reducing substances appears to be insufficient as well, unless there was a large surface reservoir of reduced carbon compounds. However, with a large surface reservoir of reduced carbon compounds, the problem is directly resolved by the limited carbonate reservoir available for photosynthesis. The presence of such a reduced carbon reservoir appears to be not only possible, but likely, based on conditions attending late accretion and planetary degassing during the Hadean.
Fossil Biofilms, Microbial Communities and Phosphogenesis in the Paleoproterozoic Baraga Group, Michigan
Bacteria are important in the phosphorus cycle. They break down organic compounds releasing P, and they can create reducing microenvironments in which P is released from sedimentary oxides. Modern bacteria associated with phosphogenesis store polyphosphate intracellularly under aerobic conditions, which they release when conditions become anoxic. Although bacteria fossils are common in Phanerozoic phosphorite, the nature of their role in phosphogenesis in the Precambrian is unknown. Phosphatic sediments deposited in peritidal environments of the 1.85 Ga Baraga Group, Michigan contain an excellent record of such processes. This succession of marine clastic, iron formation, chert and phosphatic sedimentary rocks accumulated along the Nuna continental margin and ostensibly spans the sulfidic ocean transition. SEM examination of fossil biofilms revealed rod, spherical, and branching filamentous bacteria forms within phosphatic crusts lining fractures and peloids in supratidal to intertidal facies. Pyrite is rare but iron carbonate is abundant in these facies suggesting that these were not sulfate-reducing bacteria. Stromatolites are present in these environments, providing a source of oxygen, but conditions were likely reducing millimeters below the sediment surface. The bacterial communities documented here may have bridged the iron-redox interface. Phosphatized biofilms form meniscal bridges between sediment grains in supratidal facies. Inorganic francolite crystals sometimes engulf the fossils, demonstrating that the bacteria predate later cement crystals. Most prokaryotic life on Earth occurs in biofilms, however, there has been very little documentation of fossil biofilms, with the exception of cyanobacteria-formed stromatolites. Phosphogenesis began at this time because iron ferrihydroxide precursors of hematite and magnetite would have removed and sequestered phosphorus from the oceans, but as iron formation deposition ceased, dissolved P concentrations could have risen leading to phosphogenesis in shallow marine environments. These Proterozoic biofilms and associated fossil benthic communities are the oldest examples preserved within sedimentary phosphate minerals.
Redox-controlled U Cycle in Ancient Oceans Revealed by Black Shale Records
Redox-sensitive elements, such as U and Mo, are valuable proxies for oxygen availability in the ancient atmosphere and ocean. Scott et al. (2008) inferred three stages from the secular trend of Mo concentrations in organic matter-rich shales: 1) shales older than 2.2 Ga have low but above crustal average Mo concentrations; 2) shales ca. 2.2 Ga show a dramatic increase in Mo concentrations after the rise of atmospheric oxygen; 3) shales straddling the Precambrian-Cambrian boundary show a second rise in Mo concentrations. Both Mo and U are released during oxidative continental weathering but removed via different pathways from the ocean; Mo is predominantly enriched in shales deposited under euxinic conditions, whereas U only requires anoxic conditions to be scavenged from the water column. These elements therefore can provide complementary, but independent, information about the redox state of the ocean and atmosphere. Our compilation of U concentrations from >2.2 Ga organic matter-rich shales shows minor enrichments relative to average crust, consistent with the Mo trend. This observation is intriguing because neither U nor Mo would be carried to the ocean under fully anoxic conditions. However, under an Archean oxygen-free atmosphere, the fraction of oxygen produced by cyanobacteria would be consumed by oxidative crustal weathering. Thus, our data are relevant to the continuing debate over the timing of the first appearance of oxygenic photosynthesis and the transient presence of photosynthetically-produced oxygen in the Archean. Similar to trends in the Mo record, U concentrations increased in response to the rise of atmospheric oxygen, and once more close to the Precambrian-Cambrian boundary, reaching the highest enrichments seen in the geologic record. While the first increase in U concentrations likely reflects atmospheric oxygenation, the second rise possibly corresponds with deep-ocean ventilation and restriction of the areal extent of anoxic and euxinic settings, where Mo and U are scavenged. U and Mo seawater concentrations, and also their residence times, likely increased dramatically across these two oxygenation events.
Sedimentary Depositional Environment in the Nuvvuagittuq Greenstone Belt, Northeastern Superior Province, Canada
Our knowledge of Earth's primitive surface environment is limited by the very few outcrops of demonstrably sedimentary rocks we have available. The Nuvvuagittuq Greenstone Belt (NGB) is an Eoarchean/Hadean volcano-sedimentary assemblage emplaced prior to 3.75 Ga (Cates and Mojzsis 2007) and likely as early as 4.28 Ga (O'Neil et al. 2008). Its recent discovery extends the inventory of available outcrops and pushes back the limit of time that we previously had access to. The rocks included within the NGB may represent Earth's oldest sedimentary rocks in the world and, in this respect, may provide crucial information about Earth's primitive surface environment. Our recent sampling of the NGB reveals three distinct sedimentary assemblages comprised of a sulfide-rich quartzite and two different types of banded iron formations (BIF). Here our aim is to constrain protoliths of these sedimentary rocks, with the aim of deducing the original depositional environments. Accordingly, we have mapped the relationships between the different lithologies in detail. The two BIF horizons are a few meters thick and run sub parallel to each other, separated by a horizon of cummingtonite-bearing amphibolite (previously referred to as Faux-amphibolite). Both contain cm-scale quartz-, magnetite-, and amphibole-rich laminations, but differ distinctly from each other. BIF 1 is very magnetite rich with quartz and is relatively poor in sulfides, while BIF 2 has large amounts of sulfides and green amphibolite, but is very poor in magnetite. The quartzite horizon is considerably larger (reaching thicknesses of ∼ 50 m) and is mainly formed of coarse quartz grains with minor disseminated sulfide grains. In order to supplement this field observation, we started a detailed petrological description and performed chemical analyses on our sample set. At the conference we will propose possible environmental deposition conditions for these rocks and reconstruct possible sedimentary protoliths. In addition we will present preliminary sulfur and iron isotope analyses that will allow further comparisons between the three sedimentary units.
Origin of Peculiar Horizons from the Nuvvuagittuq Greenstone Belt: Testing the Conglomerate Hypothesis
Earth's earliest rock record is fragmentary, and often distorted by metamorphic changes suffered under great temperatures and pressures. Recent work may, however, have opened the door to more direct examination of the earliest Earth. Measurements of Nd-142 from rocks of the Nuvvuagittuq Greenstone Belt (NGB), N. Quebec yield an apparent 4.28 Ga isochron (O'Neil et al., 2008). These are potentially the oldest rocks on Earth and understanding the protoliths of the rocks of the NGB may help constrain Earth's earliest surface environment. Peculiar horizons of quartz-rich rocks within the NGB have been hypothesized to represent metamorphosed conglomerates. In order to evaluate this hypothesis, we apply a series of consistency tests aimed at observations ranging from the field to microscale. First, we made high-resolution (10m grid) maps of the purported conglomerate horizons, and interpreted the map pattern of these horizons for their conformity with sedimentary contacts. The horizons are continuous throughout the mapped area, and do not cross cut any other lithologies, which is necessary, but not sufficient, evidence for a sedimentary origin. Second, we examined the mineralogy and three-dimensional geometry of the potential clasts in large polished blocks. The potential clasts fall into just two different types: the dominant type is made up primarily of coarse grains of quartz; the second type is distinctly subordinate and is largely made up of fine-grained equigranular quartz and relict plagioclase with minor amounts of biotite. The dominant type can occur as lozenge-shaped pods up to 5 cm thick, while the subordinate type more commonly occurs as thin (<2 cm thick) undulose layers. Neither the mineralogies nor the geometries of the potential clasts offer certain indication of a sedimentary origin, though they are potentially consistent with one. Third, we are comparing the identity and chemistry of trace minerals within the potential clasts to that of the surrounding matrix. Preliminary results suggest that the matrix hosts many zircon, monazite and xenotime neoblasts as well as a variety of sulfide minerals, while most of the trace minerals in the dominant type of potential clasts are euhedral sulfides, with rare chromite-rich regions. Trace zircon and monazite are more common in the subordinate type of potential clasts, with zircon occasionally exhibiting that show a concentric but truncated zonation. And finally, we are also measuring sulfur isotope compositions of sulfides from the potential clasts and from the matrix. The key principle linking these two consistency tests is the presence or absence of mineralogical, mineral chemical, and isotopic heterogeneity.
Previously Unrecognized Paleo- to Mesoproterozoic Assemblage in the Northern Wernecke Mountains, Yukon
Despite numerous advances in the understanding of the Proterozoic evolution of Yukon over the past fifteen years, it appears that a mountainous area in the northern Wernecke Mountains is underlain by a previously "unrecognized" assemblage. The area lies within the Wernecke Proterozoic inlier near the confluence of Rapitan Creek and Bonnet Plume River, ∼180 km north-northeast of Mayo, Yukon. Previous regional mapping had identified the assemblage in question as the Katherine and Little Dal groups of the Mackenzie Mountains Supergroup (Neoproterozoic), overlain by the Slats Creek Formation (Cambrian). However, a preliminary U-Pb zircon date of ∼1.4 Ga on a diorite dyke in the lower part of the assemblage constrains the age of sedimentation of the lower part to pre-1.4 Ga, which is ∼0.4 Ga older than the Katherine and Little Dal groups. Potentially correlative units (those with ages ∼1.4 Ga) include the Wernecke Supergroup and possibly the Pinguicula Group; however, these units resemble neither the lower or upper parts of the "unrecognized assemblage." Consequently, the "unrecognized assemblage" does not correlate with any known successions of Proterozoic or Phanerozoic age in Yukon and represents, prior to ∼1.4 Ga, either (1) local sedimentation during a previously unidentified interval of deposition, or (2) emplacement of a Paleoproterozoic nappe or terrane. The "unrecognized assemblage" consists of a lower succession of white massive orthoquartzite below a massive carbonate interbedded with mudstone. The upper succession comprises a lower unit of dolostone, dolostone conglomerate and diamictite, and an upper unit of siliciclastic sandstone and granule conglomerate with abundant clasts of chert set in a carbonate matrix. Abundant north-trending, west-verging folds have deformed the entire "unrecognized assemblage." Preliminary mapping indicates that the assemblage is juxtaposed with the Mackenzie Mountains Supergroup along a north-northwest striking normal fault. We intend to characterize the assemblage through field work, petrology and detrital mineral geochronology, and determine if it has a Laurentian or exotic provenance.
Chemostratigraphy and geochronology of the Kaniapiskau Supergroup, Labrador Trough indicate a major tectonic reorganization event hidden in the first cycle
Deposition of the Paleoproterozoic Kaniapiskau Supergroup in the Labrador Trough started before 2.16 Ga and continued until shortly after ca. 1.88 Ga and is conventionally viewed as the result of three cycles of sedimentation in a single basin. The lower part of the first cycle is usually interpreted as rift to passive margin deposits, while the second and third cycles were likely deposited in the foreland basin. The age and depositional setting of the middle and upper parts of the lower cycle remains poorly constrained. We combine results of chemostratigraphic study of carbonates of the Kaniapiskau Supergroup with U-Pb geochronology of detrital zircons from the basal and middle parts of the first cycle to improve age constraints, understanding of tectonic setting, and correlation with other Paleoproterozoic basins along the eastern margin of the Superior Craton. The basal Chakonipau Formation has the youngest detrital zircon with an age of 2241 +/- 18 Ma, further constraining rifting between 2.22 and 2.16 Ga. The overlying lower part of the first cycle contains carbonates with highly positive carbon isotope values, typical for the 2.22-2.1 Ga Lomagundi Event. It is capped by black shales representing a flooding event and overlying turbiditic greywackes. Carbonates in the overlying upper part of the first cycle do not carry a 13C-enrichment and were therefore deposited after ca. 2.1 Ga. We infer a major break in sedimentation at the base of the Bacchus Formation consistent with its transgressive position over various units from the lower part of the first cycle. The flooding and deposition of immature sediments suggests a major reorganization in the basin likely related to the distal response to tectonic stresses. Similar events are also recognized in the Mistassini Basin, ~400 km SW along the margin of the Superior Craton and in other basins worldwide (e.g. on the Wyoming and Fennoscandian cratons), where black shales and turbidites straddle the end of the Lomagundi Event. If our interpretation is correct, a major stratigraphic break, likely accompanied by significant tectonic reorganization, separates the lower and upper parts of the first cycle. These parts of the first cycle, therefore, may need to be interpreted as the products of distinct sedimentary basins. An intriguing possibility is that the inferred tectonic reorganization may be responsible for the significant U mineralization that is restricted to the lower part of the first cycle. The restricted development of this U enrichment has long been puzzling in the light of the fact that it is conventionally assumed to be associated with the far younger Hudsonian Orogeny.