A carbon isotope challenge to the snowball Earth

The snowball Earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the Neoproterozoic era, in a self-enhanced glaciation caused by the high albedo of the ice-covered planet. In a hard-snowball picture, the subsequent rapid unfreezing resulted from an ultra-greenhouse event attributed to the buildup of volcanic carbon dioxide (CO(2)) during glaciation. High partial pressures of atmospheric CO(2) (pCO2; from 20,000 to 90,000?p.p.m.v.) in the aftermath of the Marinoan glaciation (～635?Myr ago) have been inferred from both boron and triple oxygen isotopes. These pCO2 values are 50 to 225 times higher than present-day levels. Here, we re-evaluate these estimates using paired carbon isotopic data for carbonate layers that cap Neoproterozoic glacial deposits and are considered to record post-glacial sea level rise. The new data reported here for Brazilian cap carbonates, together with previous ones for time-equivalent units, provide estimates lower than 3,200?p.p.m.v.--and possibly as low as the current value of ～400?p.p.m.v. Our new constraint, and our re-interpretation of the boron and triple oxygen isotope data, provide a completely different picture of the late Neoproterozoic environment, with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard-snowball Earth.     

CO2 and the end-Triassic mass extinction.

The end of the Triassic period was marked by one of the largest and most enigmatic mass-extinction events in Earth's history and, with few reliable marine geochemical records, terrestrial sediments offer an important means of deciphering environmental changes at this time. Tanner et al. describe an isotopic study of Mesozoic fossil soils which suggests that the atmospheric concentration of carbon dioxide (pCO2) across the Triassic-Jurassic boundary was relatively constant (within 250 p.p.m.v.), but this is inconsistent with high-resolution evidence from the stomatal characters of fossil leaves. Here I show that the temporal resolution of the fossil-soil samples may have been inadequate for detecting a transient rise in pCO2. I also show that the fossil-soil data are consistent with a large increase in pCO2 across the Triassic-Jurassic boundary when variations in the stable carbon isotope (denoted as delta13C) in terrestrial plant leaves are taken into account. These factors suggest that the linkage between pCO2, global warming and the end-Triassic mass extinction remains intact.     

Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary

The Eocene/Oligocene boundary, at about 33.7 Myr ago, marks one of the largest extinctions of marine invertebrates in the Cenozoic period. For example, turnover of mollusc species in the US Gulf coastal plain was over 90% at this time. A temperature change across this boundary--from warm Eocene climates to cooler conditions in the Oligocene--has been suggested as a cause of this extinction event, but climate reconstructions have not provided support for this hypothesis. Here we report stable oxygen isotope measurements of aragonite in fish otoliths--ear stones--collected across the Eocene/Oligocene boundary. Palaeo-temperatures reconstructed from mean otolith oxygen isotope values show little change through this interval, in agreement with previous studies. From incremental microsampling of otoliths, however, we can resolve the seasonal variation in temperature, recorded as the otoliths continue to accrete new material over the life of the fish. These seasonal data suggest that winters became about 4 degrees C colder across the Eocene/Oligocene boundary. We suggest that temperature variability, rather than change in mean annual temperature, helped to cause faunal turnover during this transition.     

Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2

Photosynthesis by marine phytoplankton in the Southern Ocean, and the associated uptake of carbon, is thought to be currently limited by the availability of iron. One implication of this limitation is that a larger iron supply to the region in glacial times could have stimulated algal photosynthesis, leading to lower concentrations of atmospheric CO2. Similarly, it has been proposed that artificial iron fertilization of the oceans might increase future carbon sequestration. Here we report data from a whole-ecosystem test of the iron-limitation hypothesis in the Southern Ocean, which show that surface uptake of atmospheric CO2 and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbon and ocean nutrients, forced with atmospheric iron fluxes to the region derived from the Vostok ice-core dust record. During glacial periods, predicted magnitudes and timings of atmospheric CO2 changes match ice-core records well. At glacial terminations, the model suggests that forcing of Southern Ocean biota by iron caused the initial approximately 40 p.p.m. of glacial-interglacial CO2 change, but other mechanisms must have accounted for the remaining 40 p.p.m. increase. The experiment also confirms that modest sequestration of atmospheric CO2 by artificial additions of iron to the Southern Ocean is in principle possible, although the period and geographical extent over which sequestration would be effective remain poorly known.     

Extraterrestrial amino acids in Cretaceous/Tertiary boundary sediments at Stevns Klint, Denmark

Since the discovery nearly a decade ago that Cretaceous/Tertiary (K/T) boundary layers are greatly enriched in iridium, a rare element in the Earth's crust, there has been intense controversy on the relationship between this Ir anomaly and the massive extinction of organisms ranging from dinosaurs to marine plankton that characterizes the K/T boundary. Convincing evidence suggests that both the Ir spike and the extinction event were caused by the collision of a large bolide (greater than 10 km in diameter) with the Earth. Alternative explanations claim that extensive, violent volcanism can account for the Ir, and that other independent causes were responsible for the mass extinctions. We surmise that the collision of a massive extraterrestrial object with the Earth may have produced a unique organic chemical signature because certain meteorites, and probably comets, contain organic compounds which are either rare or non-existent on the Earth. In contrast, no organic compounds would be expected to be associated with volcanic processes. Here we find that K/T boundary sediments at Stevns Klint, Denmark, contain both alpha-amino-isobutyric acid [AIB,(CH3)2CNH2COOH] and racemic isovaline [ISOVAL, CH3CH2(CH3)CNH2COOH], two amino acids that are exceedingly rare on the Earth but which are major amino acids in carbonaceous chondrites. An extraterrestrial source is the most reasonable explanation for the presence of these amino acids.     

Two episodes of microbial change coupled with Permo/Triassic faunal mass extinction

Microbial expansion following faunal mass extinctions in Earth history can be studied by petrographic examination of microbialites (microbial crusts) or well-preserved organic-walled microbes. However, where preservation is poor, quantification of microbial communities can be problematic. We have circumvented this problem by adopting a lipid biomarker-based approach to evaluate microbial community changes across the Permo/Triassic (P/Tr) boundary at Meishan in South China. We present here a biomarker stratigraphic record showing episodic microbial changes coupled with a high-resolution record of invertebrate mass extinction. Variation in the microbial community structure is characterized by the 2-methylhopane (2-MHP) index (a ratio of the abundance of cyanobacterial biomarkers to more general bacterial biomarkers). Two episodes of faunal mass extinction were each preceded by minima in the 2-MHP index, followed by strong maxima, likely reflecting microbial responses to the catastrophic events that caused the extinction and initiated ecosystem changes. Hence, both cyanobacterial biomarker and invertebrate fossil records provide evidence for two episodes of biotic crisis across the P/Tr boundary.     

Atmospheric carbon dioxide concentrations over the past 60 million years

Knowledge of the evolution of atmospheric carbon dioxide concentrations throughout the Earth's history is important for a reconstruction of the links between climate and radiative forcing of the Earth's surface temperatures. Although atmospheric carbon dioxide concentrations in the early Cenozoic era (about 60 Myr ago) are widely believed to have been higher than at present, there is disagreement regarding the exact carbon dioxide levels, the timing of the decline and the mechanisms that are most important for the control of CO2 concentrations over geological timescales. Here we use the boron-isotope ratios of ancient planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used to reconstruct atmospheric CO2 concentrations. We estimate CO2 concentrations of more than 2,000 p.p.m. for the late Palaeocene and earliest Eocene periods (from about 60 to 52 Myr ago), and find an erratic decline between 55 and 40 Myr ago that may have been caused by reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial. Since the early Miocene (about 24 Myr ago), atmospheric CO2 concentrations appear to have remained below 500 p.p.m. and were more stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately 15 and 3 Myr ago.     

A carbon isotope record of CO2 levels during the late Quaternary

Analyses of gases trapped in continental ice sheets have shown that the concentration of CO2 in the Earth's early atmosphere increased from 180 to 280 p.p.m. during the most recent glacial-interglacial transition. This change must have been driven by an increase in the concentration of CO2 dissolved in the mixed layer of the ocean. Biochemical and physiological factors associated with photosynthetic carbon fixation in this layer should lead to a relationship between concentrations of dissolved CO2 and the carbon isotopic composition of phytoplanktonic organic material, such that increased atmospheric CO2 should enhance the difference in 13C content between dissolved inorganic carbon and organic products of photosynthesis. Here we show that a signal related to atmospheric CO2 levels can be seen in the isotope record of a hemipelagic sediment core, which we can correlate with the CO2 record of the Vostok ice core. Calibration of the relationship between isotope fractionation and CO2 levels should permit the extrapolation of CO2 records to times earlier than those for which ice-core records are available.     

Thresholds for Cenozoic bipolar glaciation

The long-standing view of Earth's Cenozoic glacial history calls for the first continental-scale glaciation of Antarctica in the earliest Oligocene epoch ( approximately 33.6 million years ago), followed by the onset of northern-hemispheric glacial cycles in the late Pliocene epoch, about 31 million years later. The pivotal early Oligocene event is characterized by a rapid shift of 1.5 parts per thousand in deep-sea benthic oxygen-isotope values (Oi-1) within a few hundred thousand years, reflecting a combination of terrestrial ice growth and deep-sea cooling. The apparent absence of contemporaneous cooling in deep-sea Mg/Ca records, however, has been argued to reflect the growth of more ice than can be accommodated on Antarctica; this, combined with new evidence of continental cooling and ice-rafted debris in the Northern Hemisphere during this period, raises the possibility that Oi-1 represents a precursory bipolar glaciation. Here we test this hypothesis using an isotope-capable global climate/ice-sheet model that accommodates both the long-term decline of Cenozoic atmospheric CO(2) levels and the effects of orbital forcing. We show that the CO(2) threshold below which glaciation occurs in the Northern Hemisphere ( approximately 280 p.p.m.v.) is much lower than that for Antarctica ( approximately 750 p.p.m.v.). Therefore, the growth of ice sheets in the Northern Hemisphere immediately following Antarctic glaciation would have required rapid CO(2) drawdown within the Oi-1 timeframe, to levels lower than those estimated by geochemical proxies and carbon-cycle models. Instead of bipolar glaciation, we find that Oi-1 is best explained by Antarctic glaciation alone, combined with deep-sea cooling of up to 4 degrees C and Antarctic ice that is less isotopically depleted (-30 to -35 per thousand) than previously suggested. Proxy CO(2) estimates remain above our model's northern-hemispheric glaciation threshold of approximately 280 p.p.m.v. until approximately 25 Myr ago, but have been near or below that level ever since. This implies that episodic northern-hemispheric ice sheets have been possible some 20 million years earlier than currently assumed (although still much later than Oi-1) and could explain some of the variability in Miocene sea-level records.     

The life span of the biosphere revisited

A decade ago, Lovelock and Whitfield raised the question of how much longer the biosphere can survive on Earth. They pointed out that, despite the current fossil-fuel induced increase in the atmospheric CO2 concentration, the long-term trend should be in the opposite direction: as increased solar luminosity warms the Earth, silicate rocks should weather more readily, causing atmospheric CO2 to decrease. In their model, atmospheric CO2 falls below the critical level for C3 photosynthesis, 150 parts per million (p.p.m.), in only 100 Myr, and this is assumed to mark the demise of the biosphere as a whole. Here, we re-examine this problem using a more elaborate model that includes a more accurate treatment of the greenhouse effect of CO2, a biologically mediated weathering parameterization, and the realization that C4 photosynthesis can persist to much lower concentrations of atmospheric CO2(<10 p.p.m.). We find that a C4-plant-based biosphere could survive for at least another 0.9 Gyr to 1.5 Gyr after the present time, depending respectively on whether CO2 or temperature is the limiting factor. Within an additional 1 Gyr, Earth may lose its water to space, thereby following the path of its sister planet, Venus.     

Volatile content of lunar volcanic glasses and the presence of water in the Moon's interior

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO(2), H(2)O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.     

Atmospheric carbon dioxide concentrations before 2.2 billion years ago

The composition of the Earth's early atmosphere is a subject of continuing debate. In particular, it has been suggested that elevated concentrations of atmospheric carbon dioxide would have been necessary to maintain normal surface temperatures in the face of lower solar luminosity in early Earth history. Fossil weathering profiles, known as palaeosols, have provided semi-quantitative constraints on atmospheric oxygen partial pressure (pO2) before 2.2 Gyr ago. Here we use the same well studied palaeosols to constrain atmospheric pCO2 between 2.75 and 2.2 Gyr ago. The observation that iron lost from the tops of these profiles was reprecipitated lower down as iron silicate minerals, rather than as iron carbonate, indicates that atmospheric pCO2 must have been less than 10(-1.4) atm--about 100 times today's level of 360 p.p.m., and at least five times lower than that required in one-dimensional climate models to compensate for lower solar luminosity at 2.75 Gyr. Our results suggest that either the Earth's early climate was much more sensitive to increases in pCO2 than has been thought, or that one or more greenhouse gases other than CO2 contributed significantly to the atmosphere's radiative balance during the late Archaean and early Proterozoic eons.     

Carbon isotope negative excursions across the Triassic-Jurassic boundary interval in the Tibetan Himalayas

Fossils from sections at Germig in the Tibetan Himalayas allow the establishment of ammonoid assemblages ranging from uppermost Triassic to basal Jurassic. The carbon isotope profile from these sections shows a majority of positive values in the Rhaetian, and negative values within the Hettangian intervals. Pronounced negative excursions between the Triassic and Jurassic boundary corresponds to the end-Triassic extinction. A stepwise pattern of the end-Triassic extinction is demonstrated by the bivalves and ammonoids. However, the warm episode persisting throughout from late Triassic to basal Hettangian in the Tibetan Himalayas, indicated by subtropic and tropic forms remains to be interpreted.     

Carbon isotope negative excursions across the Triassic-Jurassic boundary interval in the Tibetan Himalayas

Fossils from sections at Germig in the Tibetan Himalayas allow the establishment of ammonoid assemblages ranging from uppermost Triassic to basal Jurassic. The carbon isotope profile from these sections shows a majority of positive values in the Rhaetian, and negative values within the Hettangian intervals. Pronounced negative excursions between the Triassic and Jurassic boundary corresponds to the end-Triassic extinction. A stepwise pattern of the end-Triassic extinction is demonstrated by the bivalves and ammonoids. However, the warm episode persisting throughout from late Triassic to basal Hettangian in the Tibetan Himalayas, indicated by subtropic and tropic forms remains to be interpreted.     

Volcanic carbon dioxide vents show ecosystem effects of ocean acidification

The atmospheric partial pressure of carbon dioxide (p(CO(2))) will almost certainly be double that of pre-industrial levels by 2100 and will be considerably higher than at any time during the past few million years. The oceans are a principal sink for anthropogenic CO(2) where it is estimated to have caused a 30% increase in the concentration of H(+) in ocean surface waters since the early 1900s and may lead to a drop in seawater pH of up to 0.5 units by 2100 (refs 2, 3). Our understanding of how increased ocean acidity may affect marine ecosystems is at present very limited as almost all studies have been in vitro, short-term, rapid perturbation experiments on isolated elements of the ecosystem. Here we show the effects of acidification on benthic ecosystems at shallow coastal sites where volcanic CO(2) vents lower the pH of the water column. Along gradients of normal pH (8.1-8.2) to lowered pH (mean 7.8-7.9, minimum 7.4-7.5), typical rocky shore communities with abundant calcareous organisms shifted to communities lacking scleractinian corals with significant reductions in sea urchin and coralline algal abundance. To our knowledge, this is the first ecosystem-scale validation of predictions that these important groups of organisms are susceptible to elevated amounts of p(CO(2)). Sea-grass production was highest in an area at mean pH 7.6 (1,827 (mu)atm p(CO(2))) where coralline algal biomass was significantly reduced and gastropod shells were dissolving due to periods of carbonate sub-saturation. The species populating the vent sites comprise a suite of organisms that are resilient to naturally high concentrations of p(CO(2)) and indicate that ocean acidification may benefit highly invasive non-native algal species. Our results provide the first in situ insights into how shallow water marine communities might change when susceptible organisms are removed owing to ocean acidification.     

鄂尔多斯盆地西缘差异抬升的裂变径迹证据

目的探讨鄂尔多斯盆地西缘从南至北不同地区的差异抬升时期及抬升速率。方法利用磷灰石和锆石裂变径迹年龄的综合分析,研究盆地西缘的构造差异抬升。结果西缘北部汝箕沟地区中生代以来有两次较大的抬升时期,分别为晚白垩世和始新世,抬升速率分别为29.5 m/Ma和46.5 m/Ma。中部石沟驿地区抬升时期较早,为晚侏罗世和晚白垩世,抬升速率分别为40.0m/Ma和21.9 m/Ma。南部的差异抬升最为强烈,最早的抬升时期为晚侏罗世,在炭山地区表现明显;早白垩世末—晚白垩世南部地区发生整体抬升;中新世末期六盘山地区发生快速抬升。罗山、炭山地区相对抬升速率和后期抬升速率为46.3 m/Ma和25 m/Ma,六盘山地区则分别为22.5m/Ma和283.3 m/Ma。结论在鄂尔多斯盆地西部,最早的抬升时期为晚侏罗世,晚三叠世并没有抬升事件,故西部前陆盆地的形态始显现于晚侏罗世。西部的差异抬升导致了不同地区前陆盆地构造发展的不平衡。     

A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles

To understand better the link between atmospheric CO2 concentrations and climate over geological time, records of past CO2 are reconstructed from geochemical proxies. Although these records have provided us with a broad picture of CO2 variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO2 concentrations, I reconstruct past atmospheric CO2 concentrations. For the past 300 Myr, only two intervals of low CO2 (<1,000 p.p.m.v.) are inferred, both of which coincide with known ice ages in Neogene (1-8 Myr) and early Permian (275-290 Myr) times. But for most of the Mesozoic era (65-250 Myr), CO2 levels were high (1,000-2,000 p.p.m.v.), with transient excursions to even higher CO2 (>2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO2 (refs 1, 2) and palaeotemperature records, but suggest that CO2 reconstructions based on carbon isotope proxies may be compromised by episodic outbursts of isotopically light methane. These results support the role of water vapour, methane and CO2 in greenhouse climate warming over the past 300 Myr.     

下扬子地区盆地的"四层楼"结构及其动力学机制

下扬子地区震旦纪以来经历了不同的大地构造与地球动力学背景 ,盆地演化相应地经历了震旦纪—中三叠世海盆→晚三叠世—中侏罗世沿江前陆盆地→晚侏罗世—早白垩世火山岩盆地→晚白垩世—早第三纪陆相伸展盆地。下扬子地区震旦纪—中三叠世海盆发育于伸展性被动大陆边缘。沿江前陆盆地形成于扬子与华北板块碰撞活动中的前陆变形带上。火山岩盆地出现于区域性走滑剪切和环太平洋岩浆弧背景下。库拉—太平洋板块高角度、高速正面向东亚大陆下的俯冲 ,造成了岩石圈上拱拉张 ,从而产生了晚白垩世—早第三纪的陆相伸展盆地。随着西太平洋边缘弧后盆地的出现及印度与欧亚板块的碰撞 ,中国东部遭受近东西向挤压 ,从而结束了下扬子地区的盆地演化历史。     

Triassic-Jurassic atmospheric CO2 spike.

I question the claim by Tanner et al. that atmospheric CO2 levels remained constant across the Triassic-Jurassic boundary on the grounds of problems with stratigraphic completeness and contamination with atmospheric methane. Because methanogenic CH4 has a light isotope composition and oxidizes readily to CO2, methane-clathrate dissociation and oxidation events cannot be detected by palaeobarometers that use the carbon-isotope composition of palaeosol carbonate.     

Bird-like fossil footprints from the Late Triassic

The study of fossilized footprints and tracks of dinosaurs and other vertebrates has provided insight into the origin, evolution and extinction of several major groups and their behaviour; it has also been an important complement to their body fossil record. The known history of birds starts in the Late Jurassic epoch (around 150 Myr ago) with the record of Archaeopteryx, whereas the coelurosaurian ancestors of the birds date back to the Early Jurassic. The hind limbs of Late Triassic epoch theropods lack osteological evidence for an avian reversed hallux and also display other functional differences from birds. Previous references to suggested Late Triassic to Early Jurassic bird-like footprints have been reinterpreted as produced by non-avian dinosaurs having a high angle between digits II and IV and in all cases their avian affinities have been challenged. Here we describe well-preserved and abundant footprints with clearly avian characters from a Late Triassic redbed sequence of Argentina, at least 55 Myr before the first known skeletal record of birds. These footprints document the activities, in an environment interpreted as small ponds associated with ephemeral rivers, of an unknown group of Late Triassic theropods having some avian characters.