The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area,South China

The Ediacaran period was one of the most important times for the evolution of life. However, the scarcity of well-preserved outcrops of Ediacaran rocks still leaves ambiguity in decoding ambient surface environmental changes and biological evolution.The Ediacaran strata in South China are almost continuously exposed, comprise mainly carbonate rocks with subordinate black shales and sandstones, and they contain many fossils, suitable for study of environmental and biological changes in the Ediacaran. We conducted drilling through the Doushantuo Fm at four sites in the Three Gorges area to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed ⁸⁷Sr/⁸⁶Sr and ⁸⁸Sr/⁸⁶Sr ratios of the fresh carbonate rocks, selected on the basis of microscopic observations and the geochemical signatures of Sr contents, Mn/Sr and Rb/Sr ratios, and δ¹⁸O values, with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS).The chemostratigraphy of the ⁸⁷Sr/⁸⁶Sr ratios of the drilled samples displays a smooth curve and two large positive shifts during Ediacaran time. The combination of the detailed chemostratigraphies of δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values and Mn and Fe contents enables us to decode the surface environmental changes and their causes in the Ediacaran. The first large positive excursion of ⁸⁷Sr/⁸⁶Sr occurred together with negative δ¹³C and positive δ¹⁸O excursions. The higher ⁸⁷Sr/⁸⁶Sr values indicate an enhancement of continental weathering, whereas the positive δ¹⁸O excursion suggests global cooling. Global regression due to global cooling enhanced the oxidative decay of exposed marine organic sediments and continental weathering. Accelerated influx of nutrients promoted primary productivity, resulting in oxidation of dissolved organic carbon (DOC), whereas active sulfate reduction due to a higher sulfate influx from the continents caused remineralization of the large DOC, both of which caused a negative δ¹³C anomaly. The 580 Ma Gaskiers glaciation accounts for the close correlation among the positive ⁸⁷Sr/⁸⁶Sr, negative δ¹³C and positive δ¹⁸O excursions.The second large positive shift of ⁸⁷Sr/⁸⁶Sr firstly accompanied a positive δ¹³C excursion, and continued through the Shuram δ¹³C negative excursion. The positive correlation of δ¹³C and ⁸⁷Sr/⁸⁶Sr values is consistent with an enhanced continental weathering rate due to continental collisions that built Trans-Gondwana mountain chains, and with a higher primary activity due to the enhancement of continental weathering and consequent higher nutrient contents in seawater. The accompanied increase in Mn and Fe contents implies a gradual decline of the seawater oxygen content due to more active aerobic respiration and oxidation of reductive materials flowing in the oceans. In the Shuram excursion, higher ⁸⁷Sr/⁸⁶Sr values and a transition from increase to decrease in Mn and Fe contents were accompanied by the large negative δ¹³C excursion. The higher ⁸⁷Sr/⁸⁶Sr values are the first compelling evidence for enhanced continental weathering, which was responsible for the large δ¹³C anomaly through the remineralization of the DOC by more active sulfate reduction due to a higher sulfate influx. Higher Mn and Fe contents in the early and middle stages of the excursion suggest a decline in the oxygen content of seawater due to oxidative decay of the DOC, whereas in the late stages the decrease in Mn and Fe contents is consistent with oceanic oxygenation.The emergence of Ediacara biota after the Gaskiers glaciation and the prosperity of the latest Ediacaran is concomitant with the formation of more radiogenic seawater with high ⁸⁷Sr/⁸⁶Sr values, suggesting that enhanced continental weathering, and the consequent higher influx of nutrients, played an important role in biological evolution.     

Age constraints on the Palaeoproterozoic Lomagundi–Jatuli Event in Zimbabwe: Zircon geochronology of the Magondi Supergroup

The ca. 2.2–2.1 Ga Magondi Supergroup on the Zimbabwe Craton in Southern Africa is mainly composed of sedimentary rocks deposited in a rift basin/passive continental margin, which record a unique episode in carbon isotope perturbation called the Lomagundi–Jatuli Event (LJE). This study reports new U–Pb ages of detrital zircons from the Deweras and Lomagundi groups of the Magondi Supergroup, and of igneous zircons from underlying granitoids, to constrain the timing of the LJE and to identify the provenance of the Magondi Supergroup. Most analysed detrital zircon grains range in ages between ca. 2.9 and 2.6 Ga. Three ca. 2.3–2.2 Ga detrital zircons from sandstone of the Deweras Group, with the youngest ²⁰⁷Pb‐²⁰⁶Pb age of 2,216 ± 22 Ma, indicate the onset of LJE in the Zimbabwe Craton was almost simultaneous to that in Fennoscandia and the Superior Craton, supporting the global synchronicity of the LJE.     

http://www.sciencedirect.com/science/article/pii/S1674987114000681

We present the disaster-forced biological evolution model as a general framework that includes Darwinian "phylogenic gradualism",Eldredge-Gould's "punctuated equilibrium",mass extinctions,and allopatric.parapatric,and sympatric speciation.It describes how reproductive isolation of organisms is established through global disasters due to supernova encounters and local disasters due to radioactive volcanic ash fall-outs by continental alkaline volcanism.Our new evolution model uniquely highlights three major factors of disaster-forced speciation:enhanced mutation rate by higher natural radiation level,smaller population size,and shrunken habitat size(i.e.,isolation among the individual populations).We developed a mathematical model describing speciation of a half-isolated group from a parental group,taking into account the population size(N_e),immigration rate(m),and mutation rate(μ).The model gives a quantitative estimate of the speciation,which is consistent with the observations of speciation speed.For example,the speciation takes at least 10~5 generations,if mutation rate is less than10~(-3) per generation per individual.This result is consistent with the previous studies,in which μ is assumed to be 10~(-3)-10~(-5).On the other hand,the speciation is much faster(less than 10~5 generations)for the case that μ is as large as 0.1 in parapatric conditions(m μ).Even a sympatric(m ~ 1) speciation can occur within 10~3 generations,if mutation rate is very high(μ~1 mutation per individual per generation),and if N_e 20-30.Such a high mutation rate is possible during global disasters due to supernova encounters and local disasters due to radioactive ash fall-outs.They raise natural radiation level by a factor of 100-1000.Such rapid speciation events can also contribute to macro-evolution during mass extinction events,such as observed during the Cambrian explosion of biodiversity.A similar rapid speciation(though in a much smaller scale) also has been undergoing in cichlid fishes and great African apes in the last several tens of thousand years in the current African rift valley,including the origin of humankind due to the radioactive ash fall-outs by continental alkaline volcanism.     

http://www.sciencedirect.com/science/article/pii/S1674987114000267

Habitable Trinity is a newly proposed concept of a habitable environment.This concept indicates that the coexistence of an atmosphere(consisting largely of C and N),an ocean(H and O).and a landmass(supplier of nutrients) accompanying continuous material circulation between these three components driven by the Sun is one of the minimum requirements for life to emerge and evolve.The life body consists of C,0,H,N and other various nutrients,and therefore,the presence of water,only,is not a sufficient condition.Habitable Trinity environment must be maintained to supply necessary components for life body.Our Habitable Trinity concept can also be applied to other planets and moons such as Mars,Europa,Titan,and even exoplanets as a useful index in the quest for life-containing planetary bodies.     

Evidences of increasing primary production in the ocean by Stommel's perpetual salt fountain

The American physical oceanographer Henry Stommel and co-workers proposed “the perpetual salt fountain” and suggested the possibility of upwelling deep seawater without an energy source. In the open ocean, deep seawater containing rich nutrients becomes a source of primary production. Previously, we have tested Stommel's hypothesis by numerical simulations and in ocean experiments, and confirmed the upwelling of a perpetual salt fountain. In the present study, we conducted an open-ocean experiment in the Philippines Sea, and succeeded to demonstrate an increase in chlorophyll concentration. The chlorophyll concentration at the pipe outlet was much greater than that in the surrounding seawater. Satellite ocean-color image around the pipe was analyzed, and the signal of artificial upwelling is investigated. Composite analysis of satellite chlorophyll image indicates an increased surface chlorophyll distribution in the vicinity of pipe position, in which the increasing signal is much larger than the expected production based on nutrient supply. Although the problem must be further discussed, this increased signal is shown to be statistically significant. This mechanism may contribute to effective utilization of fishery resources in subtropical oligotrophic region.     

Frontiers in early Earth history and primordial life——Part Ⅱ

正Following the publication of Part Ⅰ of the series on"Frontiers in early Earth history and primordial life"(Maruyama and Santosh,2017),we present the second part in this volume compiling state-of-the-art research focusing on issues related to the early evolutionary history of the Earth and life.The first paper by Imaeda and Ebisuzaki(2018)on"Single planet formation regime in the high-ionization environment:     

Secular change in lifetime of granitic crust and the continental growth: A new view from detrital zircon ages of sandstones

U-Pb ages of detrital zircons were newly dated for 4 Archean sandstones from the Pilbara craton in Australia, Wyoming craton in North America, and Kaapvaal craton in Africa. By using the present results with previously published data, we compiled the age spectra of detrital zircons for 2.9, 2.6, 2.3,1.0, and0.6 Ga sandstones and modern river sands in order to document the secular change in age structure of continental crusts through time. The results demonstrated the following episodes in the history of continental crust:(1) low growth rate of the continents due to the short cycle in production/destruction of granitic crust during the Neoarchean to Paleoproterozoic(2.9-23 Ga),(2) net increase in volume of the continents during Paleo-to Mesoproterozoic(2.3-1.0 Ga), and(3) net decrease in volume of the continents during the Neoproterozoic and Phanerozoic(after 1.0 Ga). In the Archean and Paleoproterozoic, the embryonic continents were smaller than the modern continents, probably owing to the relatively rapid production and destruction of continental crust. This is indeed reflected in the heterogeneous crustal age structure of modern continents that usually have relatively small amount of Archean crusts with respect to the post-Archean ones. During the Mesoproterozoic, plural continents amalgamated into larger ones comparable to modern continental blocks in size. Relatively older crusts were preserved in continental interiors, whereas younger crusts were accreted along continental peripheries.In addition to continental arc magmatism, the direct accretion of intra-oceanic island arc around continental peripheries also became important for net continental growth. Since 1.0 Ga, total volume of continents has decreased, and this appears consistent with on-going phenomena along modern active arc-trench system with dominant tectonic erosion and/or arc subduction. Subduction of a huge amount of granitic crusts into the mantle through time is suggested, and this requires re-consideration of the mantle composition and heterogeneity.     

http://www.sciencedirect.com/science/article/pii/S167498711200148X

The Earth was born from a giant impact at 4.56 Ga. It is generally thought that the Earth subsequently cooled, and hence shrunk, over geologic time. However, if the Earth's convection was double-layered, there must have been a peak of expansion during uni-directional cooling. We computed the expansion-contraction effect using first principles mineral physics data. The result shows a radius about 120 km larger than that of the present Earth immediately after the consolidation of the magma-ocean on the surface, and subsequent shrinkage of about 110 km in radius within about 10 m.y., followed by gradual expansion of 11 km in radius due to radiogenic heating in the lower mantle in spite of cooling in the upper mantle in the Archean. This was due to double-layered convection in the Archean with final collapse of overturn with contraction of about 8 km in radius, presumably by the end of the Archean. Since then, the Earth has gradually cooled down to reduce its radius by around 12 km. Geologic evidence supports the late Archean mantle overturn ca. 2.6 Ga, such as the global distribution of super-liquidus flood basalts on nearly all cratonic fragments (>35 examples). If our inference is correct, the surface environment of the Earth must have undergone extensive volcanism and emergence of local landmasses, because of the thin ocean cover (3–5 km thickness). Global unconformity appeared in cratonic fragments with stromatolite back to 2.9 Ga with a peak at 2.6 Ga. The global magmatism brought extensive crustal melting to yield explosive felsic volcanism to transport volcanic ash into the stratosphere during the catastrophic mantle overturn. This event seems to be recorded by sulfur mass-independent fractionation (SMIF) at 2.6 Ga. During the mantle overturn, a number of mantle plumes penetrated into the upper mantle and caused local upward doming of by ca. 2–3 km which raised local landmasses above sea-level. The consequent increase of atmospheric oxygen enabled life evolution from prokaryotes to eukaryotes by 2.1 Ga, or even earlier in the Earth history.     

Structural controls on the 1998 volcanic unrest at Iwate volcano: Relationship between a shallow, electrically resistive body and the possible ascent route of magmatic fluid

Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18 km in length and oriented E–W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4 km. The surface resistive layer (~ several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200 °C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5–3 km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200 °C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes.     

An examination of the presence and topography of the D″ discontinuity under the Russia–Kazakhstan border region using seismic waveform data from a deep earthquake in Spain

The D″ layer, which is located atop the core–mantle boundary, has long been an area of focus for global seismology studies. A widely used approach to study the discontinuities in the D″ layer involves the use of the SdS phases between the S and ScS phases, which requires that certain stringent conditions be satisfied with respect to an epicentral distance and earthquake depth. Therefore, this approach is only practical for investigating the presence and topography of velocity interfaces in certain local regions around the world. The Russia–Kazakhstan border region has been a “blind spot” with respect to this detection method. The seismic network deployed in the northeastern margin of the Tibetan Plateau has recorded relatively clear SdS phases for the M _S 6.3 earthquake that occurred in Spain on April 11, 2010, allowing this blind spot to be studied. This paper compares the observed waveforms and synthetics and uses the travel times of the relevant phases to obtain a D″ discontinuity depth between 2,610 and 2,740 km in the examined area. This study provides the first results regarding the depth of the D″ layer discontinuity for this region and represents an important addition to the global studies of the D″ layer.