Precambrian Asteroid Impacts: Changing Earth History or Just Marking Time?
Some see impacts by large extraterrestrial bodies as major catalysts for change in the biosphere, atmosphere, hydrosphere, and even lithosphere throughout Earth history. If so, evidence of large impacts should correlate with major stratigraphic changes. We assess this proposition mainly using distal ejecta layers in the Hamersley and Griqualand West Basins of Western Australia and South Africa, respectively. At least 7 different formations contain distal ejecta layers in these two well-preserved Neoarchean-Paleoproterozoic successions. Each layer contains many sand-size spherules of former silicate melt and (where analyzed) up to a few percent ET material. Although replaced, many spherules show relict textures typical of basaltic melts. Each layer probably represents an impactor roughly the size of the end-Cretaceous object hitting oceanic instead of continental crust. Distal ejecta layers are well suited for assessing the environmental effects of impacts as they are always found in stratigraphic context. In the Hamersley Basin, 4 formations contain spherule layers that range in age from 2.63-2.49 Ga. The 2.63 layers (Jeerinah, Carawine) appear to have been formed by the same impact. Neither of the layers coincides exactly with a dramatic stratigraphic change, but both are close. The Jeerinah layer is near, but below, the base of the oldest large BIF, whereas the Carawine layer is near, but above, the base of one of Earth's first large carbonate platforms. The 2 younger layers (Wittenoom, Dales Gorge) are both in mid-formation and unrelated to significant stratigraphic shifts. The story is similar in the Griqualand West Basin. The oldest spherule layer (Monteville - possibly from the same impact as the Carawine and Jeerinah layers) is near, but above, the base of a large carbonate platform. The 2 younger layers (Reivilo, Kuruman) are in mid-formation and unrelated to major stratigraphic shifts. At least 5 spherule layers formed from 3.47-3.24 Ga are preserved in the Pilbara and Barberton successions. A majority of these layers probably do not coincide with major stratigraphic shifts. One layer is close to a fundamental shift from volcanic to sedimentary deposition in the Barberton, but it appears to be above the actual transition rather than at or below it. In summary, the settings of Precambrian spherule layers appear to lend little support to the theory that large impacts typically cause major changes in Earth surface environments. The true worth of spherule layers may be as high-precision time planes across environmental gradients in individual basins and to link Precambrian successions globally. One problem using spherule layers for widespread correlation is their scarcity. At Phanerozoic impact rates, Precambrian strata ought to contain about 40 layers. The fact that almost all layers have been found in strata deposited during 2 narrow time windows raises the possibility of major changes in the flux of impactors since the end of the Late Heavy Bombardment. The idea of secular changes in the source of impactors is supported by geochemical evidence that the Paleoarchean layers were produced by carbonaceous chondritic objects whereas the Neoarchean to Paleoproterozoic layers were produced by ordinary to enstatite chondritic objects. The discovery of the Sudbury layer around Lake Superior proves that more layers may be found even in intensively studied areas. To reliably determine whether or not large impacts are routinely changing Earth history, more geologists need to add distal ejecta layers to their check list.
Coupled Iron and Sulfur Isotope Constraints on the Archean and Paleoproterozoic Ocean Redox State
The rise of atmospheric oxygen level by ca. 2.3 Ga have led to dramatic shifts in the iron and sulfur oceanic cycles. Past studies of non-mass dependent and mass dependent sulfur isotope record in sedimentary sulfides over geological time have placed important constraints on biogeochemical cycle of sulfur and evolution of Precambrian ocean chemistry. Recently, we applied a similar time-record approach to explore potential changes in Fe isotope composition of pyrite in black shales. Although the underlying mechanisms for Fe isotope fractionation in organic-rich sediments are debated, we identified direct link between the rise of atmospheric oxygen and changes in the Fe ocean cycle suggesting that Fe isotopes are useful proxies to the past ocean redox state. Since biogeochemical cycles of Fe and S are closely coupled in marine systems, Fe-limitation and S-limitation for pyrite formation in black shales should leave imprint on the isotopic record of both elements. Coupled Fe and S isotope systematics of Devonian pyrite display a range of 50‰ in δ34S values whereas δ56Fe values vary between - 1.0 and +0.1‰ consistent with Fe isotope variations in modern marine sediments. Similarly, pyrite in the 1.88 Ga Gunflint Formation has δ34S values ranging from - 32‰ to +10‰ and displays a range of δ56Fe values between 0 to - 0.4‰. In contrast, Archean black shales (e.g. Manjeri Fm., Belingwe Belt and Jeerinah Fm., Hamersley Basin) display a smaller range of δ34S values between together with ubiquitous non-mass dependent S-isotope fractionation but a larger range of δ56Fe values from - 3.5 to +0.2‰. A transitional period between ca. 2.3 and ca. 1.8 Ga is marked by a larger spread of δ34S values from - 34 to +28‰, disappearance of MIF and a larger range of δ56Fe values from - 1.7 to +1.1‰. These results confirm that after the rise of atmospheric oxygen by ca. 2.3 Ga, Paleoproterozoic ocean became stratified and gradually affected by an increase of seawater sulfate concentration in an Fe- limited system whereas pre-2.3 Ga ocean was S-limited and characterized by extensive Fe-oxide precipitation and biogeochemical redox cycling.
Paleoproterozoic Carbon Isotope Excursion: Updating the Evidence
The isotopic composition of Paleoproterozoic sedimentary carbonates provide evidence for a major perturbation in the carbon cycle, that affected the surface environments between 2.2 and 2.1 Ga. During that interval, the δ13C values of dissolved inorganic carbon in the oceans increased to 10 ‰, and even higher. Carbon isotope records from sedimentary successions in different shield areas indicate that the excursion was global in character. The positive excursion implies a major increase in the fractional burial rate of organic carbon, and it appears to have been one step in the irreversible oxidation of the atmosphere, oceans and the surface environments between 2.4 and 2.0 Ga. The most complete stratigraphic sections representing the time of the carbon isotope excursion come from the Fennoscandian Shield. The beginning of the excursion is still poorly constrained, from 2.3 to 2.2 Ga. However, the termination of the excursion is well defined in several sedimentary successions to have occurred between 2115±6 and 2062±2 Ma. A new U-Pb date of 2106±8 Ma from the pyroclastic Hirsimaa Formation confirms the older data and, at the same time, provides a more accurate estimate for the time, when the oceans were returning back to the normal δ13C values of about 0 ‰. The general form of the Paleoproterozoic carbon isotope record appears to be different from that of the Neoproterozoic record. The Neoproterozoic δ13C curve is presented by long-lasting periods with positive δ13C values punctuated by sharp negative minima, often associated with glacial intervals. In contrast, the Paleoproterozoic δ13C record is characterized by a single positive major excursion, lasting for 100 Ma or more and clearly postdating the Paleoproterozoic glacial periods.
The Lomagundi Event Marks Post-Pasteur Point Evolution of Aerobic Respiration: A Hypothesis
All published early Earth carbon cycle models assume that aerobic respiration is as ancient as oxygenic photosynthesis. However, aerobic respiration shuts down at oxygen concentrations below the Pasteur Point, (.01 of the present atmospheric level, PAL). As geochemical processes are unable to produce even local oxygen concentrations above .001 PAL, it follows that aerobic respiration could only have evolved after oxygenic photosynthesis, implying a time gap. The evolution of oxygen reductase-utilizing metabolisms presumably would have occupied this interval. During this time the PS-II-generated free oxygen would have been largely unavailable for remineralization of dissolved organic carbon and so would have profoundly shifted the burial ratio of organic/inorganic carbon. We argue that the sequential geological record of the Makganyene (Snowball?) glaciation (2.3-2.22), the exessively aerobic Hekpoort and coeval paleosols, the Lomagundi-Jatuli carbon isotopic excursion (ending 2.056 Ga), and the deposition of concentrated, sedimentary organic carbon (shungite) mark this period of a profoundly unbalanced global carbon cycle. The Kopp et al. (2005) model for oxyatmoversion agrees with phylogenetic evidence for the radiation of cyanobacteria followed closely by the radiation of gram-negative lineages containing magnetotactic bacteria, which depend upon vertical oxygen gradients. These organisms include delta-Proteobacteria from which the mitochondrial ancestor originated. The Precambrian carbon cycle was rebalanced after a series of biological innovations allowed utilization of the high redox potential of free oxygen. Aerobic respiration in mitochondria required the evolution of a unique family of Fe-Cu oxidases, one of many factors contributing to the >210 Myr delay between the Makganyene deglaciation and the end of the Lomagundi-Jatuli event. We speculate that metalliferious fluids associated with the eruption of the Bushveld complex facilitated evolution of these proteins, allowing mitochondrial endosymbiosis and ending the Lomagundi-Jatuli event at 2.056 Ga.
The 1850 Ma Sudbury Impact Event in Northern Michigan and Changing Paleoproterozoic Oceanography
An ejecta layer produced by the Sudbury impact event at ca. 1.85 Ga occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, 2 to 4-m-thick horizon accumulated during the onset of sulfidic ocean conditions, an event thought to coincide with the demise of continental margin-type iron formation and the end of the Earth's first phosphogenic episode. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Lithofacies associations in the Baraga Group indicate that ejecta was deposited in a peritidal setting along this portion of the Nuna margin. Deeper water hemipelagic paleoenvironments contain interbedded iron formation and variably pyritiferous prodelta siltstone and sandstone. A sharp increase in pyrite abundance and a pronounced 6 ‰ positive shift in pyrite δ34S immediately following ejecta deposition suggest that the transition to sulfidic ocean conditions occurred just after the impact. Thus, this transition appears to have been rapid, likely occurring at ca. 1.85 Ga. Although the change to a sulfidic ocean is generally regarded as marking the beginning of a long lull in eukaryote evolution, the effects of a bolide impact during this critical stage in Earth history have not been addressed. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan has yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life.
A Coupled Carbon Cycle - Climate Model of Neoproterozoic Glaciation: the Influence of Precipitation, Continental Configuration and Stochastic Perturbations to the Carbon Cycle or Solar Insolation
In Peltier et al. (Nature 450, 813-818, 2007), we described a negative feedback mechanism that could prevent the occurrence of complete planetary glaciation. The idea was corroborated by numerical simulation using a simple ice-sheet coupled climate model that was in turn coupled to a model of the carbon cycle. However, many conditions in the model were simple and ideal. Therefore, here we investigate more generally the influence of precipitation rate, continental configuration and shorter timescale perturbations on the nature of solutions to this dynamical system. It is found that (1) when precipitation rate is reduced, the period of the cycles of the coupled system becomes longer and the maximum land ice volume is reduced. (2) The hysteresis loop in the solution space that is required for the existence of oscillatory solutions exists for a series of different continental configurations. (3) When large amplitude stochastic perturbations are applied to the concentration of carbon dioxide, to mimic the variability of the carbon cycle that was not considered in our two- box carbon cycle model, and to incorporate the fact that the influence of orbital perturbations may be significantly amplified by the internal variability of the complex Earth system as has been the case in the Late Quaternary, cyclic behavior of the coupled model is recovered in circumstances in which it would otherwise not occur. So long as the amplitude of the stochastic perturbations is not too large, a ˇ°hard snowballˇ± state never forms.