Variations of Younger Dryas atmospheric radiocarbon explicable without ocean circulation changes

The concentration of radiocarbon, 14C, in the atmosphere depends on its production rate by cosmic rays, and on the intensity of carbon exchange between the atmosphere and other reservoirs, for example the deep oceans. For the Holocene (the past approximately 11,500 years), it has been shown that fluctuations in atmospheric radiocarbon concentrations have been caused mostly by variations in the solar magnetic field. Recent progress in extending the radiocarbon record backwards in time has indicated especially high atmospheric radiocarbon concentrations in the Younger Dryas cold period, between 12,700 and 11,500 years before the present. These high concentrations have been interpreted as a result of a reduced exchange with the deep-ocean reservoir, caused by a drastic weakening of the deep-ocean ventilation. Here we present a high-resolution reconstruction of atmospheric radiocarbon concentrations, derived from annually laminated sediments of two Polish lakes, Lake Gosciaz and Lake Perespilno. These records indicate that the maximum in atmospheric radiocarbon concentrations in the early Younger Dryas was smaller than previously believed, and might have been caused by variations in solar activity. If so, there is no indication that the deep-ocean ventilation in the Younger Dryas was significantly different from today's.     

A signature of cosmic-ray increase in AD 774-775 from tree rings in Japan

Increases in (14)C concentrations in tree rings could be attributed to cosmic-ray events, as have increases in (10)Be and nitrate in ice cores. The record of the past 3,000 years in the IntCal09 data set, which is a time series at 5-year intervals describing the (14)C content of trees over a period of approximately 10,000 years, shows three periods during which (14)C increased at a rate greater than 3‰ over 10 years. Two of these periods have been measured at high time resolution, but neither showed increases on a timescale of about 1 year (refs 11 and 12). Here we report (14)C measurements in annual rings of Japanese cedar trees from ad 750 to ad 820 (the remaining period), with 1- and 2-year resolution. We find a rapid increase of about 12‰ in the (14)C content from ad 774 to 775, which is about 20 times larger than the change attributed to ordinary solar modulation. When averaged over 10 years, the data are consistent with the decadal IntCal (14)C data from North American and European trees. We argue that neither a solar flare nor a local supernova is likely to have been responsible.     

Changing boreal methane sources and constant biomass burning during the last termination

Past atmospheric methane concentrations show strong fluctuations in parallel to rapid glacial climate changes in the Northern Hemisphere superimposed on a glacial-interglacial doubling of methane concentrations. The processes driving the observed fluctuations remain uncertain but can be constrained using methane isotopic information from ice cores. Here we present an ice core record of carbon isotopic ratios in methane over the entire last glacial-interglacial transition. Our data show that the carbon in atmospheric methane was isotopically much heavier in cold climate periods. With the help of a box model constrained by the present data and previously published results, we are able to estimate the magnitude of past individual methane emission sources and the atmospheric lifetime of methane. We find that methane emissions due to biomass burning were about 45 Tg methane per year, and that these remained roughly constant throughout the glacial termination. The atmospheric lifetime of methane is reduced during cold climate periods. We also show that boreal wetlands are an important source of methane during warm events, but their methane emissions are essentially shut down during cold climate conditions.     

High-resolution carbon dioxide concentration record 650,000-800,000 years before present

Changes in past atmospheric carbon dioxide concentrations can be determined by measuring the composition of air trapped in ice cores from Antarctica. So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years. Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present. From previously published data and the present work, we find that atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout eight glacial cycles but with significantly lower concentrations between 650,000 and 750,000 yr before present. Carbon dioxide levels are below 180 parts per million by volume (p.p.m.v.) for a period of 3,000 yr during Marine Isotope Stage 16, possibly reflecting more pronounced oceanic carbon storage. We report the lowest carbon dioxide concentration measured in an ice core, which extends the pre-industrial range of carbon dioxide concentrations during the late Quaternary by about 10 p.p.m.v. to 172-300 p.p.m.v.     

Effect of trace metal availability on coccolithophorid calcification

The deposition of atmospheric dust into the ocean has varied considerably over geological time. Because some of the trace metals contained in dust are essential plant nutrients which can limit phytoplankton growth in parts of the ocean, it has been suggested that variations in dust supply to the surface ocean might influence primary production. Whereas the role of trace metal availability in photosynthetic carbon fixation has received considerable attention, its effect on biogenic calcification is virtually unknown. The production of both particulate organic carbon and calcium carbonate (CaCO3) drives the ocean's biological carbon pump. The ratio of particulate organic carbon to CaCO3 export, the so-called rain ratio, is one of the factors determining CO2 sequestration in the deep ocean. Here we investigate the influence of the essential trace metals iron and zinc on the prominent CaCO3-producing microalga Emiliania huxleyi. We show that whereas at low iron concentrations growth and calcification are equally reduced, low zinc concentrations result in a de-coupling of the two processes. Despite the reduced growth rate of zinc-limited cells, CaCO3 production rates per cell remain unaffected, thus leading to highly calcified cells. These results suggest that changes in dust deposition can affect biogenic calcification in oceanic regions characterized by trace metal limitation, with possible consequences for CO2 partitioning between the atmosphere and the ocean.     

Possible solar forcing of climate variability in the past 4000 years inferred from a proxy record at the southern margin of Tarim Basin

A ca. 4000a decadal-resolution climate proxy record of the mean grain-size from a lake (or mash) sediment at the southern margin of the Tarim Basin, correlates visually with the atmospheric 14C record from tree ring (residual △^14C, solar proxy) and the GISP2 ice core δ^18O record (temperature proxy). In general, △^14C maxima (solar minima) are coincident with cold (wet) periods in the study area and cold periods in the Greenland ice core. Power spectrum analysis revealed statistically significant periodicities of 196a, 121a, 97a, 62a, 45a and 33--30a, which are similar to those principal solar-oscillation periods as inferred from the atmospheric radiocarbon and other proxy records. Possible solar forcing is addressed to be the main driving forcing of climate change in the southern margin of Tarim Basin.     

Dust-climate couplings over the past 800,000 years from the EPICA Dome C ice core

Dust can affect the radiative balance of the atmosphere by absorbing or reflecting incoming solar radiation; it can also be a source of micronutrients, such as iron, to the ocean. It has been suggested that production, transport and deposition of dust is influenced by climatic changes on glacial-interglacial timescales. Here we present a high-resolution record of aeolian dust from the EPICA Dome C ice core in East Antarctica, which provides an undisturbed climate sequence over the past eight climatic cycles. We find that there is a significant correlation between dust flux and temperature records during glacial periods that is absent during interglacial periods. Our data suggest that dust flux is increasingly correlated with Antarctic temperature as the climate becomes colder. We interpret this as progressive coupling of the climates of Antarctic and lower latitudes. Limited changes in glacial-interglacial atmospheric transport time suggest that the sources and lifetime of dust are the main factors controlling the high glacial dust input. We propose that the observed approximately 25-fold increase in glacial dust flux over all eight glacial periods can be attributed to a strengthening of South American dust sources, together with a longer lifetime for atmospheric dust particles in the upper troposphere resulting from a reduced hydrological cycle during the ice ages.     

Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons

The organic matter found in sedimentary rocks must derive from many sources; not only from ancient primary producers but also from consumers and secondary producers. In all of these organisms, isotope effects can affect the abundance and distribution of 13C in metabolites. Here, by using an improved form of a previously described technique in which the effluent of a gas chromatograph is continuously analysed isotopically, we report evidence of the diverse origins of sedimentary organic matter. The record of 13C abundances in sedimentary carbonate and total organic carbon can be interpreted in terms of variations in the global carbon cycle. Our results demonstrate, however, that isotope variations within sedimentary organic mixtures substantially exceed those observed between samples of total organic carbon. Resolution of isotope variations at the molecular level offers a new and convenient means of refining views both of localized palaeoenvironments and of control mechanisms within the global carbon cycle.     

Short-term variations in the oxidizing power of the atmosphere

The hydroxyl radical is the predominant atmospheric oxidant, responsible for removing a wide range of trace gases, including greenhouse gases, from the atmosphere. Determination of trends and variability in hydroxyl radical concentrations is critical to understanding whether the 'cleansing' properties of the atmosphere are changing. The variability in hydroxyl radical concentrations on annual to monthly timescales, however, is difficult to quantify. Here we show records of carbon monoxide containing radiocarbon (14CO), which is oxidized by hydroxyl radicals, from clean-air sites at Baring Head, New Zealand, and Scott Base, Antarctica, spanning 13 years. Using a model study, we correct for known variations in production of 14CO (refs 6, 7), allowing us to exploit this species as a diagnostic for short term changes in hydroxyl radical concentrations. We find no significant long-term trend in hydroxyl radical concentrations but provide evidence for recurring short-term variations of around ten per cent persisting for a few months. We also find decreases in hydroxyl radical concentrations of up to 20 per cent, apparently triggered by the eruption of Mt Pinatubo in 1991 and by the occurrence of extensive fires in Indonesia in 1997.     

用树轮碳同位素年序列重建大气二氧化碳浓度

大气圈ＣＯ２浓度及其同位素组成是不断变化的。植物在同化大气ＣＯ２过程中产生碳同位素分馏,因此,树轮中稳定碳同位素比值（＾１３Ｃ／＾１２Ｃ）不仅是重建古气候的工具,也是过去大气ＣＯ２浓度变化的敏感指标器。利用采自浙江西天目山的两株柳杉树轮稳定碳同位素（δ＾１３Ｃ）组成年序列,对１９世纪中叶以来大气ＣＯ２浓度变化进行重建,其重建值与大气ＣＯ２浓度的实测值较吻合。     

Termination of global warmth at the Palaeocene/Eocene boundary through productivity feedback

The onset of the Palaeocene/Eocene thermal maximum (about 55 Myr ago) was marked by global surface temperatures warming by 5-7 degrees C over approximately 30,000 yr (ref. 1), probably because of enhanced mantle outgassing and the pulsed release of approximately 1,500 gigatonnes of methane carbon from decomposing gas-hydrate reservoirs. The aftermath of this rapid, intense and global warming event may be the best example in the geological record of the response of the Earth to high atmospheric carbon dioxide concentrations and high temperatures. This response has been suggested to include an intensified flux of organic carbon from the ocean surface to the deep ocean and its subsequent burial through biogeochemical feedback mechanisms. Here we present firm evidence for this view from two ocean drilling cores, which record the largest accumulation rates of biogenic barium--indicative of export palaeoproductivity--at times of maximum global temperatures and peak excursion values of delta13C. The unusually rapid return of delta13C to values similar to those before the methane release and the apparent coupling of the accumulation rates of biogenic barium to temperature, suggests that the enhanced deposition of organic matter to the deep sea may have efficiently cooled this greenhouse climate by the rapid removal of excess carbon dioxide from the atmosphere.     

大洋缺氧事件的碳稳定同位素响应

从碳稳定同位素组成及其分馏机理出发 ,系统探讨了大洋缺氧事件与海相碳酸盐和有机碳稳定同位素分馏之间的关系。缺氧事件期间 ,由于生物大批死亡和快速埋藏 ,其分解消耗海水中大量的溶解氧 ,引起大洋水体缺氧 ,富含 1 2 C的有机质从而得以大量保存 ;相应地大气和海水中富 1 3 C,同期海相碳酸盐岩碳同位素 δ值 (δ1 3C)正偏。在世界各地缺氧事件层内 ,无一例外地碳酸盐岩碳稳定同位素出现了不同程度的正偏 ,Cenomanian- Turonian 界线偏幅达～2‰。海相碳酸盐与有机质碳稳定同位素变化不仅可以提供地质历史中有机碳埋藏量的记录。研究全球碳循环变化 ,还可能追溯有机碳风化和埋藏速率的变化 ,定性地恢复大气 p CO2 变化。     

Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Niño

The stable isotope ratios of atmospheric CO(2) ((18)O/(16)O and (13)C/(12)C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere-atmosphere exchange fluxes affect the different atomic masses in a measurable way. Interpreting the (18)O/(16)O variability has proved difficult, however, because oxygen isotopes in CO(2) are influenced by both the carbon cycle and the water cycle. Previous attention focused on the decreasing (18)O/(16)O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration; a global increase in C(4) crops at the expense of C(3) forests; and environmental conditions, such as atmospheric turbulence and solar radiation, that affect CO(2) exchange between leaves and the atmosphere. Here we present 30 years' worth of data on (18)O/(16)O in CO(2) from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Ni?o/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Ni?o increases the (18)O/(16)O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO(2) by biosphere-atmosphere gas exchange. We show how the decay time of the El Ni?o anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Ni?o events, implying a shorter cycling time of CO(2) with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year, may be too low, and that a best guess of 150-175 petagrams of carbon per year better reflects the observed rapid cycling of CO(2). Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.     

Negligible glacial-interglacial variation in continental chemical weathering rates

Chemical weathering of the continents is central to the regulation of atmospheric carbon dioxide concentrations, and hence global climate. On million-year timescales silicate weathering leads to the draw-down of carbon dioxide, and on millennial timescales chemical weathering affects the calcium carbonate saturation state of the oceans and hence their uptake of carbon dioxide. However, variations in chemical weathering rates over glacial-interglacial cycles remain uncertain. During glacial periods, cold and dry conditions reduce the rate of chemical weathering, but intense physical weathering and the exposure of carbonates on continental shelves due to low sea levels may increase this rate. Here we present high-resolution records of the lead isotope composition of ferromanganese crusts from the North Atlantic Ocean that cover the past 550,000 years. Combining these records with a simple quantitative model of changes in the lead isotope composition of the deep North Atlantic Ocean in response to chemical weathering, we find that chemical weathering rates were two to three times lower in the glaciated interior of the North Atlantic Region during glacial periods than during the intervening interglacial periods. This decrease roughly balances the increase in chemical weathering caused by the exposure of continental shelves, indicating that chemical weathering rates remained relatively constant on glacial-interglacial timescales. On timescales of more than a million years, however, we suggest that enhanced weathering of silicate glacial sediments during interglacial periods results in a net draw-down of atmospheric carbon dioxide, creating a positive feedback on global climate that, once initiated, promotes cooling and further glaciation.     

Eocene bipolar glaciation associated with global carbon cycle changes

The transition from the extreme global warmth of the early Eocene 'greenhouse' climate approximately 55 million years ago to the present glaciated state is one of the most prominent changes in Earth's climatic evolution. It is widely accepted that large ice sheets first appeared on Antarctica approximately 34 million years ago, coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the world's oceans, and that glaciation in the Northern Hemisphere began much later, between 10 and 6 million years ago. Here we present records of sediment and foraminiferal geochemistry covering the greenhouse-icehouse climate transition. We report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from approximately 42 million years ago, with a permanent deepening 34 million years ago. The most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to 1.5 per mil, suggesting a lowering of global sea level through significant storage of ice in both hemispheres by at least 100 to 125 metres. Variations in benthic carbon isotope ratios of up to approximately 1.4 per mil occurred at the same time, indicating large changes in carbon cycling. We suggest that the greenhouse-icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago.     

Palaeoclimatology: the record for marine isotopic stage 11

The marine isotopic stage 11 (MIS 11) is an extraordinarily long interglacial period in the Earth's history that occurred some 400,000 years ago and lasted for about 30,000 years. During this period there were weak, astronomically induced changes in the distribution of solar energy reaching the Earth. The conditions of this orbital climate forcing are similar to those of today's interglacial period, and they rendered the climate susceptible to other forcing--for example, to changes in the level of atmospheric carbon dioxide. Here we use ice-core data from the Antarctic Vostok core to reconstruct a complete atmospheric carbon dioxide record for MIS 11. The record indicates that values for carbon dioxide throughout the interglacial period were close to the Earth's pre-industrial levels and that both solar energy and carbon dioxide may have helped to make MIS 11 exceptionally long. Anomalies in the oceanic carbonate system recorded in marine sediments at the time, for example while coral reefs were forming, apparently left no signature on atmospheric carbon dioxide concentrations.     

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.     

Net carbon dioxide losses of northern ecosystems in response to autumn warming

The carbon balance of terrestrial ecosystems is particularly sensitive to climatic changes in autumn and spring, with spring and autumn temperatures over northern latitudes having risen by about 1.1 degrees C and 0.8 degrees C, respectively, over the past two decades. A simultaneous greening trend has also been observed, characterized by a longer growing season and greater photosynthetic activity. These observations have led to speculation that spring and autumn warming could enhance carbon sequestration and extend the period of net carbon uptake in the future. Here we analyse interannual variations in atmospheric carbon dioxide concentration data and ecosystem carbon dioxide fluxes. We find that atmospheric records from the past 20 years show a trend towards an earlier autumn-to-winter carbon dioxide build-up, suggesting a shorter net carbon uptake period. This trend cannot be explained by changes in atmospheric transport alone and, together with the ecosystem flux data, suggest increasing carbon losses in autumn. We use a process-based terrestrial biosphere model and satellite vegetation greenness index observations to investigate further the observed seasonal response of northern ecosystems to autumnal warming. We find that both photosynthesis and respiration increase during autumn warming, but the increase in respiration is greater. In contrast, warming increases photosynthesis more than respiration in spring. Our simulations and observations indicate that northern terrestrial ecosystems may currently lose carbon dioxide in response to autumn warming, with a sensitivity of about 0.2 PgC degrees C(-1), offsetting 90% of the increased carbon dioxide uptake during spring. If future autumn warming occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon may be diminished earlier than previously suggested.     

Holocene Southern Ocean surface temperature variability west of the Antarctic Peninsula

The disintegration of ice shelves, reduced sea-ice and glacier extent, and shifting ecological zones observed around Antarctica highlight the impact of recent atmospheric and oceanic warming on the cryosphere. Observations and models suggest that oceanic and atmospheric temperature variations at Antarctica's margins affect global cryosphere stability, ocean circulation, sea levels and carbon cycling. In particular, recent climate changes on the Antarctic Peninsula have been dramatic, yet the Holocene climate variability of this region is largely unknown, limiting our ability to evaluate ongoing changes within the context of historical variability and underlying forcing mechanisms. Here we show that surface ocean temperatures at the continental margin of the western Antarctic Peninsula cooled by 3-4 °C over the past 12,000 years, tracking the Holocene decline of local (65° S) spring insolation. Our results, based on TEX(86) sea surface temperature (SST) proxy evidence from a marine sediment core, indicate the importance of regional summer duration as a driver of Antarctic seasonal sea-ice fluctuations. On millennial timescales, abrupt SST fluctuations of 2-4 °C coincide with globally recognized climate variability. Similarities between our SSTs, Southern Hemisphere westerly wind reconstructions and El Ni?o/Southern Oscillation variability indicate that present climate teleconnections between the tropical Pacific Ocean and the western Antarctic Peninsula strengthened late in the Holocene epoch. We conclude that during the Holocene, Southern Ocean temperatures at the western Antarctic Peninsula margin were tied to changes in the position of the westerlies, which have a critical role in global carbon cycling.     

基于小波变换的1～10 TeV宇宙线气象效应研究

利用小波变换,结合折叠周期分析方法,对西藏羊八井宇宙线观测站Tibet ASγ阵列1997年11月~1998年6月的实验记录数据进行周期分析,验证了气象参量,包括观测面处大气压和室外温度的日变化和半日变化.关联分析表明:宇宙线流强的周期变化与气压和温度的周期变化显著相关.