Astronomical pacing of late Palaeocene to early Eocene global warming events

At the boundary between the Palaeocene and Eocene epochs, about 55 million years ago, the Earth experienced a strong global warming event, the Palaeocene-Eocene thermal maximum. The leading hypothesis to explain the extreme greenhouse conditions prevalent during this period is the dissociation of 1,400 to 2,800 gigatonnes of methane from ocean clathrates, resulting in a large negative carbon isotope excursion and severe carbonate dissolution in marine sediments. Possible triggering mechanisms for this event include crossing a threshold temperature as the Earth warmed gradually, comet impact, explosive volcanism or ocean current reorganization and erosion at continental slopes, whereas orbital forcing has been excluded. Here we report a distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge, which we term the Elmo horizon. Using orbital tuning, we estimate deposition of the Elmo horizon at about 2 million years after the Palaeocene-Eocene thermal maximum. The Elmo horizon has similar geochemical and biotic characteristics as the Palaeocene-Eocene thermal maximum, but of smaller magnitude. It is coincident with carbon isotope depletion events in other ocean basins, suggesting that it represents a second global thermal maximum. We show that both events correspond to maxima in the approximately 405-kyr and approximately 100-kyr eccentricity cycles that post-date prolonged minima in the 2.25-Myr eccentricity cycle, implying that they are indeed astronomically paced.     

Eocene global warming events driven by ventilation of oceanic dissolved organic carbon

'Hyperthermals' are intervals of rapid, pronounced global warming known from six episodes within the Palaeocene and Eocene epochs (～65-34 million years (Myr) ago). The most extreme hyperthermal was the ～170 thousand year (kyr) interval of 5-7 °C global warming during the Palaeocene-Eocene Thermal Maximum (PETM, 56?Myr ago). The PETM is widely attributed to massive release of greenhouse gases from buried sedimentary carbon reservoirs, and other, comparatively modest, hyperthermals have also been linked to the release of sedimentary carbon. Here we show, using new 2.4-Myr-long Eocene deep ocean records, that the comparatively modest hyperthermals are much more numerous than previously documented, paced by the eccentricity of Earth's orbit and have shorter durations (～40?kyr) and more rapid recovery phases than the PETM. These findings point to the operation of fundamentally different forcing and feedback mechanisms than for the PETM, involving redistribution of carbon among Earth's readily exchangeable surface reservoirs rather than carbon exhumation from, and subsequent burial back into, the sedimentary reservoir. Specifically, we interpret our records to indicate repeated, large-scale releases of dissolved organic carbon (at least 1,600 gigatonnes) from the ocean by ventilation (strengthened oxidation) of the ocean interior. The rapid recovery of the carbon cycle following each Eocene hyperthermal strongly suggests that carbon was re-sequestered by the ocean, rather than the much slower process of silicate rock weathering proposed for the PETM. Our findings suggest that these pronounced climate warming events were driven not by repeated releases of carbon from buried sedimentary sources, but, rather, by patterns of surficial carbon redistribution familiar from younger intervals of Earth history.     

Long-term decline of global atmospheric ethane concentrations and implications for methane

After methane, ethane is the most abundant hydrocarbon in the remote atmosphere. It is a precursor to tropospheric ozone and it influences the atmosphere's oxidative capacity through its reaction with the hydroxyl radical, ethane's primary atmospheric sink. Here we present the longest continuous record of global atmospheric ethane levels. We show that global ethane emission rates decreased from 14.3 to 11.3 teragrams per year, or by 21 per cent, from 1984 to 2010. We attribute this to decreasing fugitive emissions from ethane's fossil fuel source--most probably decreased venting and flaring of natural gas in oil fields--rather than a decline in its other major sources, biofuel use and biomass burning. Ethane's major emission sources are shared with methane, and recent studies have disagreed on whether reduced fossil fuel or microbial emissions have caused methane's atmospheric growth rate to slow. Our findings suggest that reduced fugitive fossil fuel emissions account for at least 10-21 teragrams per year (30-70 per cent) of the decrease in methane's global emissions, significantly contributing to methane's slowing atmospheric growth rate since the mid-1980s.     

Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model

The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. About half of the current emissions are being absorbed by the ocean and by land ecosystems, but this absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change. Here we present results from a fully coupled, three-dimensional carbon-climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr(-1) is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.     

Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming

Large uncertainties in the budget of atmospheric methane, an important greenhouse gas, limit the accuracy of climate change projections. Thaw lakes in North Siberia are known to emit methane, but the magnitude of these emissions remains uncertain because most methane is released through ebullition (bubbling), which is spatially and temporally variable. Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia. We show that ebullition accounts for 95 per cent of methane emissions from these lakes, and that methane flux from thaw lakes in our study region may be five times higher than previously estimated. Extrapolation of these fluxes indicates that thaw lakes in North Siberia emit 3.8 teragrams of methane per year, which increases present estimates of methane emissions from northern wetlands (< 6-40 teragrams per year; refs 1, 2, 4-6) by between 10 and 63 per cent. We find that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and estimate that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in our study region by 58 per cent. Furthermore, the Pleistocene age (35,260-42,900 years) of methane emitted from hotspots along thawing lake margins indicates that this positive feedback to climate warming has led to the release of old carbon stocks previously stored in permafrost.     

Banach空间中一阶初值问题整体解的存在性

在紧型条件下研究了一般Banach空间中一阶微分方程初值问题整体解的存在性。利用非紧性测度的性质与凝聚映射的Sadovskii不动点定理， 获得了2个整体解的存在性结果。     

Dendritic spikes as a mechanism for cooperative long-term potentiation

Strengthening of synaptic connections following coincident pre- and postsynaptic activity was proposed by Hebb as a cellular mechanism for learning. Contemporary models assume that multiple synapses must act cooperatively to induce the postsynaptic activity required for hebbian synaptic plasticity. One mechanism for the implementation of this cooperation is action potential firing, which begins in the axon, but which can influence synaptic potentiation following active backpropagation into dendrites. Backpropagation is limited, however, and action potentials often fail to invade the most distal dendrites. Here we show that long-term potentiation of synapses on the distal dendrites of hippocampal CA1 pyramidal neurons does require cooperative synaptic inputs, but does not require axonal action potential firing and backpropagation. Rather, locally generated and spatially restricted regenerative potentials (dendritic spikes) contribute to the postsynaptic depolarization and calcium entry necessary to trigger potentiation of distal synapses. We find that this mechanism can also function at proximal synapses, suggesting that dendritic spikes participate generally in a form of synaptic potentiation that does not require postsynaptic action potential firing in the axon.     

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.     

Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan

Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar (ref. 1) and, similar to the Earth, N2 is the main component of the atmosphere. Methane is the second most important component, but it is photodissociated on a timescale of 10(7) years (ref. 3). This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for a more solid (perhaps icy) surface. We also find a circular structure approximately 30 km in diameter that does not resemble any features seen on other icy satellites. We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titan's atmosphere.     

物理扰动等环境因素对底泥磷释放的影响

以总磷、溶解性总磷、正磷酸盐含量等3个指标研究了物理扰动、微生物活性、温度和培养时间等4种环境因素对富营养化河道底泥磷释放的影响.结果表明,环境因素对水体总磷变化的影响效果明显,而对另外两种形态的磷影响较小.对各种处理及其交互作用的差异显著性分析表明,4种环境因素对底泥磷释放影响作用的大小依次为:物理扰动、培养时间、微生物活性、温度,温度主要通过影响微生物活性间接作用于底泥磷释放过程.     

CDK-dependent phosphorylation of BRCA2 as a regulatory mechanism for recombinational repair

Inherited mutations in BRCA2 are associated with a predisposition to early-onset breast cancers. The underlying basis of tumorigenesis is thought to be linked to defects in DNA double-strand break repair by homologous recombination. Here we show that the carboxy-terminal region of BRCA2, which interacts directly with the essential recombination protein RAD51, contains a site (serine 3291; S3291) that is phosphorylated by cyclin-dependent kinases. Phosphorylation of S3291 is low in S phase when recombination is active, but increases as cells progress towards mitosis. This modification blocks C-terminal interactions between BRCA2 and RAD51. However, DNA damage overcomes cell cycle regulation by decreasing S3291 phosphorylation and stimulating interactions with RAD51. These results indicate that S3291 phosphorylation might provide a molecular switch to regulate RAD51 recombination activity, providing new insight into why BRCA2 C-terminal deletions lead to radiation sensitivity and cancer predisposition.     

Glycosyl-phosphatidylinositol linkage as a mechanism for cell-surface expression of immunoglobulin D.

The B-cell antigen receptor of the IgM and IgD class is a multimeric complex consisting of the membrane-bound form of the immunoglobulin molecule and two other proteins, Ig-alpha and Ig-beta. The Ig-alpha and Ig-beta proteins form a disulphide-linked alpha/beta heterodimer and are encoded by the mb-1 (ref 9, 10) and B29 genes, respectively. Surface expression of the membrane-bound IgM molecule requires assembly with the alpha/beta heterodimer. The IgD molecule, however, can be expressed on the cell surface in an alpha/beta-dependent and -independent form. We show here that in the alpha/beta-independent form the IgD molecule is anchored in the plasma membrane through a glycosyl-phosphatidylinositol linker. In the presence of the alpha/beta heterodimer, most of the otherwise glycosyl-phosphatidylinositol-linked IgD molecule is expressed on the cell surface as transmembrane proteins.     

Direct observation of a submarine volcanic eruption from a sea-floor instrument caught in a lava flow

Our understanding of submarine volcanic eruptions has improved substantially in the past decade owing to the recent ability to remotely detect such events and to then respond rapidly with synoptic surveys and sampling at the eruption site. But these data are necessarily limited to observations after the event. In contrast, the 1998 eruption of Axial volcano on the Juan de Fuca ridge was monitored by in situ sea-floor instruments. One of these instruments, which measured bottom pressure as a proxy for vertical deformation of the sea floor, was overrun and entrapped by the 1998 lava flow. The instrument survived-being insulated from the molten lava by the solidified crust-and was later recovered. The data serendipitously recorded by this instrument reveal the duration, character and effusion rate of a sheet flow eruption on a mid-ocean ridge, and document over three metres of lava-flow inflation and subsequent drain-back. After the brief two-hour eruption, the instrument also measured gradual subsidence of 1.4 metres over the next several days, reflecting deflation of the entire volcano summit as magma moved into the adjacent rift zone. These findings are consistent with our understanding of submarine lava effusion, as previously inferred from seafloor observations, terrestrial analogues, and laboratory simulations.     

Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs

Natural hydrocarbons are largely formed by the thermal decomposition of organic matter (thermogenesis) or by microbial processes (bacteriogenesis). But the discovery of methane at an East Pacific Rise hydrothermal vent and in other crustal fluids supports the occurrence of an abiogenic source of hydrocarbons. These abiogenic hydrocarbons are generally formed by the reduction of carbon dioxide, a process which is thought to occur during magma cooling and-more commonly-in hydrothermal systems during water-rock interactions, for example involving Fischer-Tropsch reactions and the serpentinization of ultramafic rocks. Suggestions that abiogenic hydrocarbons make a significant contribution to economic hydrocarbon reservoirs have been difficult to resolve, in part owing to uncertainty in the carbon isotopic signatures for abiogenic versus thermogenic hydrocarbons. Here, using carbon and hydrogen isotope analyses of abiogenic methane and higher hydrocarbons in crystalline rocks of the Canadian shield, we show a clear distinction between abiogenic and thermogenic hydrocarbons. The progressive isotopic trends for the series of C1-C4 alkanes indicate that hydrocarbon formation occurs by way of polymerization of methane precursors. Given that these trends are not observed in the isotopic signatures of economic gas reservoirs, we can now rule out the presence of a globally significant abiogenic source of hydrocarbons.     

合肥盆地南缘油气来源的地球化学示踪

合肥盆地已经发现了多套烃源岩,但有效烃源岩的发育层位仍是合肥盆地未解决的重要问题。文章根据所发现沥青的常规地球化学分析,结合地质研究,推断出可能的烃源岩发育层位是下侏罗统防虎山组或下白垩统黑石渡组;利用沥青和可能源岩抽提物饱和烃及芳烃的分子地球化学对比,确定了下白垩统黑石渡组泥岩是霍山沥青的母源,解决了长期以来合肥盆地是否存在有效烃源岩的问题,证实了下白垩统黑石渡组是合肥盆地的烃源岩,为下一步油气勘探提供了依据。     

A global synthesis reveals biodiversity loss as a major driver of ecosystem change

Evidence is mounting that extinctions are altering key processes important to the productivity and sustainability of Earth's ecosystems. Further species loss will accelerate change in ecosystem processes, but it is unclear how these effects compare to the direct effects of other forms of environmental change that are both driving diversity loss and altering ecosystem function. Here we use a suite of meta-analyses of published data to show that the effects of species loss on productivity and decomposition--two processes important in all ecosystems--are of comparable magnitude to the effects of many other global environmental changes. In experiments, intermediate levels of species loss (21-40%) reduced plant production by 5-10%, comparable to previously documented effects of ultraviolet radiation and climate warming. Higher levels of extinction (41-60%) had effects rivalling those of ozone, acidification, elevated CO(2) and nutrient pollution. At intermediate levels, species loss generally had equal or greater effects on decomposition than did elevated CO(2) and nitrogen addition. The identity of species lost also had a large effect on changes in productivity and decomposition, generating a wide range of plausible outcomes for extinction. Despite the need for more studies on interactive effects of diversity loss and environmental changes, our analyses clearly show that the ecosystem consequences of local species loss are as quantitatively significant as the direct effects of several global change stressors that have mobilized major international concern and remediation efforts.     

Structure of a methyl-coenzyme M reductase from Black Sea mats that oxidize methane anaerobically

The anaerobic oxidation of methane (AOM) with sulphate, an area currently generating great interest in microbiology, is accomplished by consortia of methanotrophic archaea (ANME) and sulphate-reducing bacteria. The enzyme activating methane in methanotrophic archaea has tentatively been identified as a homologue of methyl-coenzyme M reductase (MCR) that catalyses the methane-forming step in methanogenic archaea. Here we report an X-ray structure of the 280?kDa heterohexameric ANME-1 MCR complex. It was crystallized uniquely from a protein ensemble purified from consortia of microorganisms collected with a submersible from a Black Sea mat catalysing AOM with sulphate. Crystals grown from the heterogeneous sample diffract to 2.1?? resolution and consist of a single ANME-1 MCR population, demonstrating the strong selective power of crystallization. The structure revealed ANME-1 MCR in complex with coenzyme M and coenzyme B, indicating the same substrates for MCR from methanotrophic and methanogenic archaea. Differences between the highly similar structures of ANME-1 MCR and methanogenic MCR include a F(430) modification, a cysteine-rich patch and an altered post-translational amino acid modification pattern, which may tune the enzymes for their functions in different biological contexts.