Reconstructing Sea Level Rise and Fall Since the NeoProterozoic

Reconstructing first-order changes in sea level during the Phanerozoic and Neoproterozoic using strontium isotopes

Authors:

van der Meer et al

Abstract:

The eustatic sea-level curves published in the seventies and eighties have supported scientific advances in the Earth Sciences and the emergence of sequence-stratigraphy as an important hydrocarbon exploration tool. However, validity of reconstructions of eustatic sea level based on sequence stratigraphic correlations has remained controversial. Proposed sea level curves differ because of site-to-site changes in local tectonics, depositional rates, and long-wavelength dynamic topography resulting from mantle convection. In particular, the overall amplitude of global Phanerozoic long-term sea level is poorly constrained and has been estimated to vary between ~ 400 m above present-day sea level to ~ 50 m below present-day sea level. To improve estimates of past sea level, we explore an alternative methodology to estimate global sea level change. We utilise the Phanerozoic-Neoproterozoic 87Sr/86Sr record, which at first order represents the mix of inputs from continental weathering and from mantle input by volcanism. By compensating for weathering with estimates of runoff from a 3D climate model (GEOCLIMtec), a corrected 87Sr/86Sr record can be obtained that solely reflects the contribution of strontium from mantle sources. At first order, the flux of strontium from the mantle through time is due to increases and decreases in the production of oceanic crust through time. Therefore, the changing levels of mantle-derived strontium can be used as a proxy for the production of oceanic lithosphere. By applying linear oceanic plate age distributions, we compute sea level and continental flooded area curves. We find that our curves are generally within the range of previous curves built on classical approaches. A Phanerozoic first order cyclicity of ~ 250 Myr is observed that may extend into the Neoproterozoic. The low frequency (i.e., on the order of 10 to 100 My) sea level curve that we propose, while open for improvement, may be used as baseline for refined sequence-stratigraphic studies at a global and basin scale.

Evidence Ediacaran Biota Members Survived Into the Cambrian

Transitional Ediacaran–Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution

Authors:

Yang et al

Abstract:

The Ediacaran–Cambrian transition records distinct evolutionary changes of metazoans. The Ediacaran fossils (i.e., Ediacara-type biota and tubular fossils) are contrasting with the diverse small skeletal fossils (SSFs) from the early Cambrian. The apparent dissimilarities hindered studies deciphering their evolutionary relationships. This also led to a popular assumption that there exists a mass extinction of the Ediacara biota and cloudinids at the Precambrian–Cambrian (Pc–C) boundary. Here we report for the first time a transitional fauna which consists of typical elements of Ediacaran, i.e. cloudinids and related tubicolous organisms, together with Cambrian SSFs including protoconodonts, anabaritids and siphogonuchitids from South China and Maly Karatau (Kazakhstan). The sediments yielding the transitional fauna are characterized by siliceous rocks in both regions and were previously considered to be unfossiliferous. Their chronostratigraphic assignment in South China has been debated for years. Based on the new fossil assemblage, the siliceous strata of the Daibu Member (Northeast Yunnan, South China) and the basal Kuanchuanpu Formation (South Shaanxi, South China) can be assigned to the earliest Cambrian SSF biozone (Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone) and thus can be considered of early Cambrian in age. A new subzone of the earliest SSF zone in eastern Yunnan (South China) is proposed herein defined as Ganloudina symmetrica–Rugatotheca typica Interval Subzone. The new fauna demonstrates that the cloudinids, originally confined to the late Ediacaran, persisted into the Cambrian Fortunian, and no major extinction event occurred at the Pc–C transition.

Earliest land plants created modern levels of atmospheric oxygen

Earliest land plants created modern levels of atmospheric oxygen

Authors:

Lenton et al

Abstract:

The progressive oxygenation of the Earth’s atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O2) first approached modern levels (∼21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435–392 Ma, and the appearance of fossil charcoal indicates O2 >15–17% by 420–400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burial—the net long-term source of O2. We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today’s global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2‰ increase in the carbonate carbon isotope (δ13C) record by ∼445 Ma, and predict a corresponding rise in O2 to present levels by 420–400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O2 levels.

pop sci write up.

Was Atmospheric Carbon dioxide Capped at 1000 ppm Since the Beginning of the Permian?

Was atmospheric CO2 capped at 1000 ppm over the past 300 million years?

Authors:

McElwain et al

Abstract:

Atmospheric carbon dioxide concentration has shifted dynamically over the Phanerozoic according to mass balance models and the majority of proxy estimates. A new paleo-CO2 proxy method underpinned by mechanistic understanding of plant stomatal, isotopic and photosynthetic responses to CO2 has provocatively claimed that maximum paleoatmospheric CO2 was capped at 1000 ppm for the majority of the past 300 million years. Here we evaluate the robustness of the new paleo-proxy CO2 model by testing its sensitivity to initial parameterization and to scaling factors employed to estimate paleophysiological function from anatomical and morphological traits. A series of sensitivity analyses find that the model is robust to modification in some of the constants employed, such as CO2 compensation point and mesophyll conductance, resulting in variability in paleo-CO2 estimates which are already accounted for in the error propagation of the model. We demonstrate high sensitivity in the model to key input parameters such as initial fossil plant assimilation rate, termed A0 and scaling factors used to estimate stomatal conductance from measurements of fossil stomata. Incorrect parameterization of A0 has resulted in under estimation of pCO2 by as much as 600 ppm. Despite these uncertainties, our analysis highlights that the new mechanistic paleo-CO2 proxy of Franks et al. (2014) has significant potential to derive robust and more accurate CO2 estimates from fossil plant stomata, as long as parameterization of A0 is strongly justified with species appropriate morphological and anatomical data. We highlight methods that can be used to improve current estimates of fossil plant assimilation rates, reduce uncertainty associated with implementation of the Franks et al. (2014) model and importantly add to understanding of patterns of plant productivity over the Phanerozoic, for which there currently is no consensus.

A More Precise Method for Determining the Speciation and Extinction Rates

A more precise speciation and extinction rate estimator

Author:

Alroy

Abstract:

A new turnover rate metric is introduced that combines simplicity and precision. Like the related three-timer and gap-filler equations, it involves first identifying a cohort of taxa sampled in the time interval preceding the one of interest (call the intervals i 0 and i 1 ). Taxa sampled in i 0 and i 1 are two-timers (t 2 ); those sampled in i 0 and i 2 but not i 1 are part-timers (p); and taxa sampled only in either i 1 , i 2 , or i 3 are newly notated here as either s 1 , s 2 , or s 3 . The gap-filler extinction proportion can be reformulated as (s 1 −s 3 )/(t 2 +p). The method proposed here is to substitute s 3 with the second-highest of the three counts when the expected ordering s 1 ≥s 2 ≥s 3 is violated. In simulation, this new estimator yields values that are highly correlated with those produced by the gap-filler equation but more precise. In particular, it rarely produces highly negative values even when sample sizes are quite small. It is mildly upwards biased when sampling is extremely poor and turnover rates are extremely low, but it is otherwise highly accurate. Examples of Phanerozoic extinction rates for four major marine invertebrate groups are given to illustrate the method’s improved precision. Based on the results, the procedure is recommended for general use.

Uncertainty in the Timing of Animal (Metazoan) Origins Strongly Suggest Molecular Clock Dating is Premature for use

Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales

Authors:

dos Reis et al

Abstract:

The timing of divergences among metazoan lineages is integral to understanding the processes of animal evolution, placing the biological events of species divergences into the correct geological timeframe. Recent fossil discoveries and molecular clock dating studies have suggested a divergence of bilaterian phyla greater than 100 million years before the Cambrian, when the first definite crown-bilaterian fossils occur. Most previous molecular clock dating studies, however, have suffered from limited data and biases in methodologies, and virtually all have failed to acknowledge the large uncertainties associated with the fossil record of early animals, leading to inconsistent estimates among studies. Here we use an unprecedented amount of molecular data, combined with four fossil calibration strategies (reflecting disparate and controversial interpretations of the metazoan fossil record) to obtain Bayesian estimates of metazoan divergence times. Our results indicate that the uncertain nature of ancient fossils and violations of the molecular clock impose a limit on the precision that can be achieved in estimates of ancient molecular timescales. For example, although we can assert that crown Metazoa originated during the Cryogenian (with most crown-bilaterian phyla diversifying during the Ediacaran), it is not possible with current data to pinpoint the divergence events with sufficient accuracy to test for correlations between geological and biological events in the history of animals. Although a Cryogenian origin of crown Metazoa agrees with current geological interpretations, the divergence dates of the bilaterians remain controversial. Thus, attempts to build evolutionary narratives of early animal evolution based on molecular clock timescales appear to be premature.

Primary Productivity Increases Have Driven the Increase in Ecological Diversity & Richness Through the Phanerozoic

Phanerozoic diversity and neutral theory

Authors:

Holland et al

Abstract:

Although Phanerozoic increases in the global richness, local richness, and evenness of marine invertebrates are well documented, a common explanation for these patterns has been difficult to identify. Evidence is presented here from marine invertebrate communities that there is a Phanerozoic increase in the fundamental biodiversity number (θ), which describes diversity and relative abundance distributions in neutral ecological theory. If marine ecosystems behave according to the rules of Hubbell's Neutral Theory of Biodiversity and Biogeography, the Phanerozoic increase in θ suggests three possible mechanisms for the parallel increases in global richness, local richness, and evenness: (1) an increase in the per-individual probability of speciation, (2) an increase in the area occupied by marine metacommunities, and (3) an increase in the density (per-area abundance) of marine organisms. Because speciation rates have declined over time and because there is no clear evidence for an increase in metacommunity area through the Phanerozoic, the most likely of these is an increase in the spatial density of marine invertebrates over the Phanerozoic, an interpretation supported by previous studies of fossil abundance. This, coupled with a Phanerozoic rise in body size, suggests that an increase in primary productivity through time is the primary cause of Phanerozoic increases in θ, global richness, local richness, local evenness, abundance, and body size.

No Real Increase in Oxygen Levels at the end of the NeoProterozoic

Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation

Authors:

Sperling et al

Abstract:

Sedimentary rocks deposited across the Proterozoic–Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities.

Better Fossil Makes Cambrian Ecdysozoan Hallucigenia’s Head Seem Even More Alien

Hallucigenia’s head and the pharyngeal armature of early ecdysozoans

Authors:

Smith et al

Abstract:

The molecularly defined clade Ecdysozoa comprises the panarthropods (Euarthropoda, Onychophora and Tardigrada) and the cycloneuralian worms (Nematoda, Nematomorpha, Priapulida, Loricifera and Kinorhyncha). These disparate phyla are united by their means of moulting, but otherwise share few morphological characters—none of which has a meaningful fossilization potential. As such, the early evolutionary history of the group as a whole is largely uncharted. Here we redescribe the 508-million-year-old stem-group onychophoran Hallucigenia sparsa from the mid-Cambrian Burgess Shale. We document an elongate head with a pair of simple eyes, a terminal buccal chamber containing a radial array of sclerotized elements, and a differentiated foregut that is lined with acicular teeth. The radial elements and pharyngeal teeth resemble the sclerotized circumoral elements and pharyngeal teeth expressed in tardigrades, stem-group euarthropods and cycloneuralian worms. Phylogenetic results indicate that equivalent structures characterized the ancestral panarthropod and, seemingly, the ancestral ecdysozoan, demonstrating the deep homology of panarthropod and cycloneuralian mouthparts, and providing an anatomical synapomorphy for the ecdysozoan supergroup.

If that doesn't look like an sfnal alien critter, IDK what would.

Mollusc Origins Traced to Ediacaran Fossil Kimberella

The origins of molluscs

Author:

Vinther

Abstract:

The interrelationships and evolutionary history of molluscs have seen great advances in the last decade. Recent phylogenetic studies have allowed alternative morphology-based evolutionary scenarios to be tested and, most significantly, shown that the aplacophorans are sister group to polyplacophorans (chitons), corroborating palaeontological and embryological evolutionary scenarios in which aplacophorans are secondarily simplified from a chiton-like ancestor. Aplacophoran morphology therefore does not represent the plesiomorphic condition for molluscs as a whole. The mollusc crown group radiated in the Early Cambrian, and rapidly thereafter, stem lineages to the major molluscan classes emerged: cephalopods, gastropods, bivalves (= pelecypods), monoplacophorans, rostroconchs (inferred stem scaphopods) and aculiferans. This attests to the fast, adaptive radiation of the crown group during the Cambrian explosion. Kimberella from the latest Ediacaran exhibits several molluscan traits, which justifies its position as a molluscan stem-group member, rather than as a more basal Lophotrochozoan. The interrelationships among the conchiferan molluscs are still a matter of contention and require further palaeontological and molecular phylogenetic scrutiny.

Ediacaran–Silurian Nanhua Foreland Basin From South China

Detrital provenance evolution of the Ediacaran–Silurian Nanhua foreland basin, South China

Authors:

Yao et al

Abstract:

We report here in-situ U–Pb and Hf isotopic results of detrital zircons from sixteen Cambrian–Silurian siliciclastic samples across the Nanhua foreland basin, South China. Together with published data from Ediacaran–Silurian sandstones in the region, we establish the temporal and spatial provenance evolution across the basin. Except for samples from northeast Yangtze, all other Ediacaran–Silurian samples exhibit a prominent population of 1100–900 Ma, moderate populations of 850–700 Ma and 650–490 Ma, and minor populations of 2500 Ma and 2000–1300 Ma, grossly matching that of crystalline and sedimentary rocks in northern India. Zircon Hf isotopes further reveal four episodes of juvenile crustal growth at 2.5 Ga, 1.8 Ga, 1.4 Ga and 1.0 Ga in the source regions. Utilizing the basin history and late Neoproterozoic to early Paleozoic paleogeography of South China, we conclude that the Ediacaran–Cambrian sediments in the Nanhua foreland basin were mainly sourced from northern India and adjacent orogens, and the Ordovician–Silurian sediments were derived from both locally recycled Ediacaran–Cambrian rocks and eroded Cathaysian basement. The Wuyi–Yunkai late-orogenic magmatic rocks also contributed to the Silurian sediments in the basin. The upper-Ordovician to Silurian samples in northeast Yangtze received higher proportions of local Cryogenian (850–700 Ma) magmatic rocks which were uplifted during late-Ordovician to Silurian time. We speculate that there was an Ediacaran–Cambrian collisional orogen between South China and northern India, shedding sediments to the early Nanhua foreland basin. Far-field stress during the late stage of this collisional orogeny triggered the Ordovician–Silurian intraplate Wuyi–Yunkai orogeny in South China, and erosion of the local Wuyi–Yunkai orogen further provided detritus to the late Nanhua foreland basin.

A Gondwanan Geological Suture in Ediacaran/Cambrian Madagascar Into India and Africa

The 580–520 Ma Gondwana suture of Madagascar and its continuation into Antarctica and Africa

Authors:

Boger et al

Abstract:

U–Pb age data from southwest Madagascar provides a compelling case that the pre-Gondwana Indian plate was stitched with the arc terranes of the Arabian Nubian Shield along a suture that closed between 580 Ma and 520 Ma. The key observations supportive of this interpretation are: (1) metamorphism dated to 630–600 Ma is manifested only on the west side of the suture in rocks that have affinities with the oceanic and island arc terranes of the Arabian Nubian Shield, or represent continental rocks welded to these terranes prior to the amalgamation of Gondwana, and (2) orogenesis at 580–520 Ma is manifest in rocks on both sides of the suture, an observation taken to mark the timing of collision and reflect spatial continuity across the suture. In southwest Madagascar the distribution of metamorphic ages places the suture along the Beraketa high-strain zone, the tectonic boundary between the Androyen and Anoysen domains. Similar age relationships allow for the extrapolation of this tectonic boundary into both East Antarctica and Africa.

Ca/SO4 Ratios From Seawater Evaporites Over the Course of the Phanerozoic

Calcium isotopes in evaporites record variations in Phanerozoic seawater SO4 and Ca

Authors:

Blättler et al

Abstract:

Reconstructing variations in the major element chemistry of seawater provides constraints on how the global carbon and sulfur cycles have changed over geological time, but archives for this information are rare. This work generates a new independent record of ancient seawater composition, in particular the relative abundance of Ca and SO4, through Ca isotope measurements in marine evaporites. Evaporite sequences that reach halite saturation record large Ca isotope variability if SO4 less than Ca, whereas a small range in Ca isotope ratios is observed if Ca less than SO4. Analyses of geological evaporites indicate SO4-rich seawater in the Neogene and Permian and Ca-rich seawater in the Cretaceous and Silurian. These results agree with previous reconstructions using fluid inclusions in halite, and demonstrate a new approach for extending our understanding of oceanic and geochemical evolution.

NeoProterozoic to Ordovician Strata in Bhutan

Documenting basin scale, geometry and provenance through detrital geochemical data: Lessons from the Neoproterozoic to Ordovician Lesser, Greater, and Tethyan Himalayan strata of Bhutan

Author:

McQuarrie et al

Abstract:

Detrital zircon (DZ) ages, augmented with εNd(0) and δ13C isotopic values from 18 new and 22 published samples collected from Lesser Himalayan (LH), Greater Himalayan (GH) and Tethyan Himalayan (TH) rocks in Bhutan, support deposition of > 7 km of sedimentary rock in late Cambrian–Ordovician time and provide a stratigraphic framework for the pre-collisional Indian margin. Youngest GH DZ grains become younger upsection from 900 Ma to 477 Ma. Youngest DZ grains in TH samples are ~ 490–460 Ma. Both the LH Jaishidanda Formation (Fm), and the LH Baxa Group overlie Paleoproterozoic LH rocks. The Jaishidanda Fm exhibits distinct populations of youngest DZ peaks, 475–550 Ma, and 800–1000 Ma. The Baxa Group (Manas, Pangsari, and Phuntsholing formations) contains youngest DZ peaks at both 500–525 Ma and 0.9–1.0 Ga. However, most samples from the Baxa Group in western Bhutan contain no grains younger than 1.8 Ga. Samples from the LH Paro Fm, which sits directly under the MCT in western Bhutan, have youngest DZ peaks at 0.5, 0.8, 1.0, 1.7, 1.8 Ga. εNd values generally match DZ spectra, with samples that contain old, youngest grain populations corresponding to more negative εNd signatures. The Paro Fm is an exception where εNd (0) values from quartzite samples are quite negative (− 19 to − 24) whereas the εNd (0) values from interbedded schist contain younger detritus (− 12 to − 17). δ13C values from the Jaishidanda, Paro and Manas formations have δ13C values (− 1.8 to + 6) suggestive of deposition over late Neoproterozoic to Ordovician time. δ13C values from the Pangsari Fm vary from − 2.8 to + 1.8, compatible with deposition in the early- to middle Neoproterozoic. The young, latest Cambrian–Ordovician grains preserved in TH, GH and LH rocks suggest that the late Cambrian–Ordovician orogeny, documented in GH rocks throughout the orogen, served as a significant sediment source in Bhutan.

Macroevolutionary Implications of Increased Marine Trophic Resources Through the Phanerozoic

Seafood through time revisited: the Phanerozoic increase in marine trophic resources and its macroevolutionary consequences

Authors:

Allmon et al

Abstract:

We review and synthesize multiple biotic and abiotic proxies for marine nutrient and food availability, primary productivity, and food quality (stoichiometry) and propose what their relationships may have been to macroevolutionary processes, especially speciation. This review confirms earlier suggestions that there has been an overall increase in marine primary productivity over the Phanerozoic, but indicates that the increase has been irregular and that present levels may not be the peak. We integrate these indicators into a new estimate of relative primary productivity in the global ocean through the Phanerozoic. We then combine multiple, frequently conflicting ecological-evolutionary hypotheses into a general model for how primary production may affect speciation over geological time scales. This model, an elaboration and extension of the “speciation cycle” previously proposed by Grant and Grant, attempts to explain why an increase in food supply sometimes is associated with decreased diversity, and at other times with increased diversification. We propose some simple tests for the application of this model to the fossil record.

Was There Increased Continental Weathering at the Ediacaran-Cambrian Boundary?

40Ar/39Ar dating of exceptional concentration of metals by weathering of Precambrian rocks at the Precambrian-Cambrian boundary

Authors:

Parnell et al

Abstract:

The sub-Cambrian surface, including diverse metalliferous deposits, shows evidence of intense weathering of Precambrian rocks to form supergene-enriched ores and metalliferous placers, followed by widespread peneplanation. Much of the metal would have been flushed to the Cambrian ocean during peneplanation. An 40Ar/39Ar age of 542.62 ± 0.38 Ma (1 sigma, full external precision, Renne et al., 2011) for metalliferous alteration clays in Scotland shows that this event occurred immediately prior to the Precambrian-Cambrian boundary. A negative δ53Cr isotopic signature for the clay is consistent with mobilization on land of redox sensitive metals by oxidative terrestrial weathering. This unprecedented flushing of metals from the weathered Precambrian surface would have contributed to the chemistry of the earliest Cambrian ocean at a time of marked faunal evolution.

Geochronological Constraints for Three Gorges, Weng'an and Chengjiang Areas, South China

New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges, Weng'an and Chengjiang areas, South China

Authors:

Okada et al

Abstract:

The Neoproterozoic and Cambrian were two of the most dramatic periods in the history of the Earth, because large multi-cellular animals first appeared then in the so-called “Cambrian Explosion”. To better understand this event, many paleontological and geochemical studies now focus on rocks in South China, because of the fossiliferous succession and good exposure. Since the recognition of the Yangtze Gorges and Chengjiang area as type localities of the Sinian (Ediacaran) and Meishucunian (Early Cambrian) Systems, both sections have been intensively investigated. In order to decipher the relationships between the evolution of life and surface environmental changes, it is necessary to understand their paleontological, geochemical and geo-chronological constraints.

This study presents new chronological constraints for the Cryogenian to Cambrian rocks in the Three Gorges, Weng'an and Chengjiang areas, South China. We discovered two tuff layers, one at the base of the Shuijingtuo Fm at Three Gorges and the other at the bottom of the Dahai Member in Chengjiang. In addition, we collected sandstones from Neoproterozoic strata in the Three Gorges, Chengjiang and Weng'an areas for provenance analysis. Zircons, separated from the tuff layers, provide new Nano-SIMS U–Pb ages of 526.4 ± 5.4 Ma at the base of the Shuijingtuo Fm, and 523.9 ± 6.7 Ma at the bottom of the Dahai Member. The tuffaceous beds occur at an unconformity, and nodules are common in the Three Gorges, Meishucun and Taoying sections, indicating that major and relatively wide-scale volcanic and sedimentological events occurred at ca. 525 Ma. Moreover, carbonate carbon isotope chemostratigraphies at Morocco, Siberia, Three Gorges and Meishucun display different characteristics during the Tommotian. One possibility is that the South China Ocean was separated from an outer ocean at that time. Detrital zircons in sandstones have age populations at ca. 2.7, 1.8, 1.6, 1.0 and from 0.9 to 0.75 Ga. indicating that the paleo-hinterland of the Nanhua and Kangdian rift basins was geologically complex. Despite the lack of ca. 1.6 Ga rocks in the Yangtze and Cathaysia Blocks, these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the Yangtze craton.

Understanding Oxygen's Rise in the PaleoAtmosphere

The rise of oxygen in Earth’s early ocean and atmosphere

Authors:

Lyons et al

Abstract:

The rapid increase of carbon dioxide concentration in Earth’s modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O2) spawned by early biological production. The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth’s history.

Once More With Feeling: Mass Extinction Periodicity Returns! Again! In Stereo!

Analysis of periodicity of extinction using the 2012 geological timescale

Authors:

Melott et al

Abstract:

Analysis of two independent data sets with increased taxonomic resolution (genera rather than families) using the revised 2012 timescale reveals that an extinction periodicity first detected by Raup and Sepkoski (1984) for only the post-Paleozoic actually runs through the entire Phanerozoic. Although there is not a local peak of extinction every 27 Myr, an excess of the fraction of genus extinction by interval follows a 27-Myr timing interval and differs from a random distribution at the p 0.02 level. A 27-Myr periodicity in the spectrum of interval lengths no longer appears, removing the question of a possible artifact arising from it. Using a method originally developed in Bambach (2006) we identify 19 intervals of marked extinction intensity, including mass extinctions, spanning the last 470 Myr (and with another six present in the Cambrian) and find that ten of the 19 lie within ±3 Myr of the maxima in the spacing of the 27-Myr periodicity, which differs from a random distribution at the p = 0.004 level. These 19 intervals of marked extinction intensity also preferentially occur during decreasing diversity phases of a well-known 62-Myr periodicity in diversity (16 of 19, p = 0.002). Both periodicities appear to enhance the likelihood of increased severity of extinction, but the cause of neither periodicity is known. Variation in the strength of the many suggested causes of extinction coupled to the variation in combined effect of the two different periodicities as they shift in and out of phase is surely one of the reasons that definitive comparative study of the causes of major extinction events is so elusive.

Plate Tectonic Influences on the Early History of Animal Evolution

Plate tectonic influences on Neoproterozoic-early Paleozoic climate and animal evolution

Authors:

Mckenzie et al

Abstract:

The initial diversification of animals paralleled some of the most dramatic episodes of climate and environmental change in Earth history. We compiled global Neoproterozoic-early Paleozoic detrital zircon age data to track spatiotemporal variations in continental arc systems to explore the influence of tectonic outgassing of CO2 on these climatic shifts. These data indicate that global continental arc systems were spatially reduced at the onset of the Cryogenian glacial interval, widespread during the Cambrian greenhouse, and reduced during Ordovician cooling. The Cambrian greenhouse was coincident with ecologically stressed conditions, whereas Ordovician global cooling was accompanied by a major biodiversification event. Thus, variation in the continental arc CO2 flux likely played a critical role in major climatic fluctuations, which profoundly influenced early animal evolution.