Persistence of full glacial conditions in the central Pacific until 15,000 years ago

The magnitude of atmospheric cooling during the Last Glacial Maximum and the timing of the transition into the current interglacial period remain poorly constrained in tropical regions, partly because of a lack of suitable climate records. Glacial moraines provide a method of reconstructing past temperatures, but they are relatively rare in the tropics. Here we present a reconstruction of atmospheric temperatures in the central Pacific during the last deglaciation on the basis of cosmogenic 3He ages of moraines and numerical modelling of the ice cap on Mauna Kea volcano, Hawaii--the only highland in the central Pacific on which moraines that formed during the last glacial period are preserved. Our reconstruction indicates that the Last Glacial Maximum occurred between 19,000 and 16,000 years ago in this region and that temperatures at high elevations were about 7 degrees C lower than today during this interval. Glacial retreat began about 16,000 years ago, but temperatures were still about 6.5 degrees C lower than today until 15,000 years ago. When combined with estimates of sea surface temperatures in the central Pacific Ocean, our reconstruction indicates that the lapse rate during the Last Glacial Maximum was higher than at present, which is consistent with the proposal that the atmosphere was drier at that time. Furthermore, the persistence of full glacial conditions until 15,000 years ago is consistent with the relatively late and abrupt transition to warmer temperatures in Greenland, indicating that there may have been an atmospheric teleconnection between the central Pacific and North Atlantic regions during the last deglaciation.     

Reversed flow of Atlantic deep water during the Last Glacial Maximum

The meridional overturning circulation (MOC) of the Atlantic Ocean is considered to be one of the most important components of the climate system. This is because its warm surface currents, such as the Gulf Stream, redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns, whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss, away from the atmosphere. Despite its significance for future climate, the operation of the MOC under contrasting climates of the past remains controversial. Nutrient-based proxies and recent model simulations indicate that during the Last Glacial Maximum the convective activity in the North Atlantic Ocean was much weaker than at present. In contrast, rate-sensitive radiogenic (231)Pa/(230)Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength. Here we show that the basin-scale abyssal circulation of the Atlantic Ocean was probably reversed during the Last Glacial Maximum and was dominated by northward water flow from the Southern Ocean. These conclusions are based on new high-resolution data from the South Atlantic Ocean that establish the basin-scale north to south gradient in (231)Pa/(230)Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of (231)Pa/(230)Th outside the North Atlantic basin will enhance our understanding of past ocean circulation, provided that spatial gradients are carefully considered. This broader perspective suggests that the modern pattern of the Atlantic MOC-with a prominent southerly flow of deep waters originating in the North Atlantic-arose only during the Holocene epoch.     

Past temperature and delta18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios

Determining the past record of temperature and salinity of ocean surface waters is essential for understanding past changes in climate, such as those which occur across glacial-interglacial transitions. As a useful proxy, the oxygen isotope composition (delta18O) of calcite from planktonic foraminifera has been shown to reflect both surface temperature and seawater delta18O, itself an indicator of global ice volume and salinity. In addition, magnesium/calcium (Mg/Ca) ratios in foraminiferal calcite show a temperature dependence due to the partitioning of Mg during calcification. Here we demonstrate, in a field-based calibration experiment, that the variation of Mg/Ca ratios with temperature is similar for eight species of planktonic foraminifera (when accounting for Mg dissolution effects). Using a multi-species record from the Last Glacial Maximum in the North Atlantic Ocean we found that past temperatures reconstructed from Mg/Ca ratios followed the two other palaeotemperature proxies: faunal abundance and alkenone saturation. Moreover, combining Mg/Ca and delta18O data from the same faunal assemblage, we show that reconstructed surface water delta18O from all foraminiferal species record the same glacial-interglacial change--representing changing hydrography and global ice volume. This reinforces the potential of this combined technique in probing past ocean-climate interactions.     

Oceanic Cd/P ratio and nutrient utilization in the glacial Southern Ocean

During glacial periods, low atmospheric carbon dioxide concentration has been associated with increased oceanic carbon uptake, particularly in the southern oceans. The mechanism involved remains unclear. Because ocean productivity is strongly influenced by nutrient levels, palaeo-oceanographic proxies have been applied to investigate nutrient utilization in surface water across glacial transitions. Here we show that present-day cadmium and phosphorus concentrations in the global oceans can be explained by a chemical fractionation during particle formation, whereby uptake of cadmium occurs in preference to uptake of phosphorus. This allows the reconstruction of past surface water phosphate concentrations from the cadmium/calcium ratio of planktonic foraminifera. Results from the Last Glacial Maximum show similar phosphate utilization in the subantarctic to that of today, but much smaller utilization in the polar Southern Ocean, in a model that is consistent with the expansion of glacial sea ice and which can reconcile all proxy records of polar nutrient utilization. By restricting communication between the ocean and atmosphere, sea ice expansion also provides a mechanism for reduced CO2 release by the Southern Ocean and lower glacial atmospheric CO2.     

Seawater subduction controls the heavy noble gas composition of the mantle

The relationship between solar volatiles and those now in the Earth's atmosphere and mantle reservoirs provides insight into the processes controlling the acquisition of volatiles during planetary accretion and their subsequent evolution. Whereas the light noble gases (helium and neon) in the Earth's mantle preserve a solar-like isotopic composition, heavy noble gases (argon, krypton and xenon) have an isotopic composition very similar to that of the modern atmosphere, with radiogenic and (in the case of xenon) solar contributions. Mantle noble gases in a magmatic CO2 natural gas field have been previously corrected for shallow atmosphere/groundwater and crustal additions. Here we analyse new data from this field and show that the elemental composition of non-radiogenic heavy noble gases in the mantle is remarkably similar to that of sea water. We challenge the popular concept of a noble gas 'subduction barrier'--the convecting mantle noble gas isotopic and elemental composition is explained by subduction of sediment and seawater-dominated pore fluids. This accounts for approximately 100% of the non-radiogenic argon and krypton and 80% of the xenon. Approximately 50% of the convecting mantle water concentration can then be explained by this mechanism. Enhanced recycling of subducted material to the mantle plume source region then accounts for the lower ratio of radiogenic to non-radiogenic heavy noble gas isotopes and higher water content of plume-derived basalts.     

Timing of the Last Glacial Maximum from observed sea-level minima

During the Last Glacial Maximum, ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today. But few direct estimates exist of the volume of the ice sheets, or the timing and rates of change during their advance and retreat. Here we analyse four distinct sediment facies in the shallow, tectonically stable Bonaparte Gulf, Australia--each of which is characteristic of a distinct range in sea level--to estimate the maximum volume of land-based ice during the last glaciation and the timing of the initial melting phase. We use faunal assemblages and preservation status of the sediments to distinguish open marine, shallow marine, marginal marine and brackish conditions, and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio-hydro-isostatic modelling. Our results indicate that from at least 22,000 to 19,000 (calendar) years before present, land-based ice volume was at its maximum, exceeding today's grounded ice sheets by 52.5 x 10(6) km. A rapid decrease in ice volume by about 10% within a few hundred years terminated the Last Glacial Maximum at 19,000 +/- 250 years.     

Links between climate and sea levels for the past three million years

The oscillations between glacial and interglacial climate conditions over the past three million years have been characterized by a transfer of immense amounts of water between two of its largest reservoirs on Earth -- the ice sheets and the oceans. Since the latest of these oscillations, the Last Glacial Maximum (between about 30,000 and 19,000 years ago), approximately 50 million cubic kilometres of ice has melted from the land-based ice sheets, raising global sea level by approximately 130 metres. Such rapid changes in sea level are part of a complex pattern of interactions between the atmosphere, oceans, ice sheets and solid earth, all of which have different response timescales. The trigger for the sea-level fluctuations most probably lies with changes in insolation, caused by astronomical forcing, but internal feedback cycles complicate the simple model of causes and effects.     

Forcing of wet phases in southeast Africa over the past 17,000 years

Intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast Africa since the Last Glacial Maximum, about 20,000?years ago. In particular, the relative importance of atmospheric and oceanic processes is not firmly established. Southward shifts of the intertropical convergence zone (ITCZ) driven by high-latitude climate changes have been suggested as a primary forcing, whereas other studies infer a predominant influence of Indian Ocean sea surface temperatures on regional rainfall changes. To address this question, a continuous record representing an integrated signal of regional climate variability is required, but has until now been missing. Here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro-climatology in southeast Africa during rapid climate changes over the past 17,000 years. Our results are based on a reconstruction of precipitation and river discharge changes, as recorded in a marine sediment core off the mouth of the Zambezi River, near the southern boundary of the modern seasonal ITCZ migration. Indian Ocean sea surface temperatures did not exert a primary control over southeast African hydrologic variability. Instead, phases of high precipitation and terrestrial discharge occurred when the ITCZ was forced southwards during Northern Hemisphere cold events, such as Heinrich stadial 1 (around 16,000?years ago) and the Younger Dryas (around 12,000?years ago), or when local summer insolation was high in the late Holocene, that is, during the past 4,000?years.     

Palaeovegetation. Diversity of temperate plants in east Asia

The exceptionally broad species diversity of vascular plant genera in east Asian temperate forests, compared with their sister taxa in North America, has been attributed to the greater climatic diversity of east Asia, combined with opportunities for allopatric speciation afforded by repeated fragmentation and coalescence of populations through Late Cenozoic ice-age cycles. According to Qian and Ricklefs, these opportunities occurred in east Asia because temperate forests extended across the continental shelf to link populations in China, Korea and Japan during glacial periods, whereas higher sea levels during interglacial periods isolated these regions and warmer temperatures restricted temperate taxa to disjunct refuges. However, palaeovegetation data from east Asia show that temperate forests were considerably less extensive than today during the Last Glacial Maximum, calling into question the coalescence of tree populations required by the hypothesis of Qian and Ricklefs.     

Dual modes of the carbon cycle since the Last Glacial Maximum

The most conspicuous feature of the record of past climate contained in polar ice is the rapid warming which occurs after long intervals of gradual cooling. During the last four transitions from glacial to interglacial conditions, over which such abrupt warmings occur, ice records indicate that the CO2 concentration of the atmosphere increased by roughly 80 to 100 parts per million by volume. But the causes of the atmospheric CO2 concentration increases are unclear. Here we present the stable-carbon-isotope composition (delta 13 CO2) of CO2 extracted from air trapped in ice at Taylor Dome, Antarctica, from the Last Glacial Maximum to the onset of Holocene times. The global carbon cycle is shown to have operated in two distinct primary modes on the timescale of thousands of years, one when climate was changing relatively slowly and another when warming was rapid, each with a characteristic average stable-carbon-isotope composition of the net CO2 exchanged by the atmosphere with the land and oceans. delta 13 CO2 increased between 16.5 and 9 thousand years ago by slightly more than would be estimated to be caused by the physical effects of a 5 degrees C rise in global average sea surface temperature driving a CO2 efflux from the ocean, but our data do not allow specific causes to be constrained.     

Low slip rates and long-term preservation of geomorphic features in Central Asia

In order to understand the dynamics of the India Asia collision zone, it is important to know the strain distribution in Central Asia, whose determination relies on the slip rates for active faults. Many previous slip-rate estimates of faults in Central Asia were based on the assumption that offset landforms are younger than the Last Glacial Maximum (approximately 20 kyr ago). In contrast, here we present surface exposure ages of 40 to 170 kyr, obtained using cosmogenic nuclide dating, for a series of terraces near a thrust at the northern margin of the Tibetan Plateau. Combined with the tectonic offset, the ages imply a long-term slip rate of only about 0.35 mm x yr(-1) for the active thrust, an order of magnitude lower than rates obtained from the assumption that the terraces formed after the Last Glacial Maximum. Our data demonstrate that the preservation potential of geomorphic features in Central Asia is higher than commonly assumed.     

末次冰盛期青藏高原冰川变化对亚洲气候的影响

以末次冰盛期(约2.6~1.9万年前)的气候为背景, 利用大气模式CAM4耦合陆面模式CLM4, 对青藏高原冰川规模扩大对气候产生的影响进行研究。结果表明, 末次冰盛期青藏高原冰川对北半球夏季的气候影响较显著, 除在冰川分布区引起显著的降温外, 通过遥相关作用, 还使得白令海峡附近显著升温。另外, 冰川产生的扰动会显著地增强南亚夏季风, 增加南亚地区降水。对比末次冰盛期与工业革命前时期不同气候态下青藏高原冰川规模扩大对气候的影响, 发现工业革命前时期的影响显著小于末次冰盛期, 说明青藏高原冰川对气候的影响与背景气候态有关。     

末次盛冰期南京降水记录中ENSO的演化

基于南京葫芦洞一支石笋TIMS-U系测年和纹层计数的交叉定年。获得丁末次盛冰期在24170～21222aBP时段石笋年纹层厚度的时间序列,主要反映了夏季风降水变率．小波分析揭示末次盛冰期连续3ka夏季风降水记录中厄尔尼诺一南方涛动（ENSO）周期和强度随时间的演化特征,该记录表明ENSO在末次盛冰期并非一直存在,有着高发期和平缓期。其爆发的频率和强度逐渐减弱。支持ENSO演化的“冰期阻尼效应”假说．     

Regional insolation forcing of late Quaternary climate change in the Southern Hemisphere

In agreement with the Milankovitch orbital forcing hypothesis it is often assumed that glacial-interglacial climate transitions occurred synchronously in the Northern and Southern hemispheres of the Earth. It is difficult to test this assumption, because of the paucity of long, continuous climate records from the Southern Hemisphere that have not been dated by tuning them to the presumed Northern Hemisphere signals. Here we present an independently dated terrestrial pollen record from a peat bog on South Island, New Zealand, to investigate global and local factors in Southern Hemisphere climate changes during the last two glacial-interglacial cycles. Our record largely corroborates the Milankovitch model of orbital forcing but also exhibits some differences: in particular, an earlier onset and longer duration of the Last Glacial Maximum. Our results suggest that Southern Hemisphere insolation may have been responsible for these differences in timing. Our findings question the validity of applying orbital tuning to Southern Hemisphere records and suggest an alternative mechanism to the bipolar seesaw for generating interhemispheric asynchrony in climate change.     

Spatial and temporal variations of C3/C4 relative abundance in global terrestrial ecosystem since the Last Glacial and its possible driving mechanisms

The primary factor controlling C 3 /C 4 relative abundance in terrestrial ecosystem since the Last Glacial has been widely debated. Now more and more researchers recognize that climate, rather than atmospheric CO 2 concentration, is the dominant factor. However, for a specific area, conflicting viewpoints regarding the more influential one between temperature and precipitation still exist. As temperature and precipitation in a specific area usually not only vary within limited ranges, but also covary with each other, it is difficult to get a clear understanding of the mechanism driving C 3 /C4 relative abundance. Therefore, systematic analysis on greater spatial scales may promote our understanding of the driving force. In this paper, records of C3/C4 relative abundance since the Last Glacial on a global scale have been reviewed, and we conclude that: except the Mediterranean climate zone, C3 plants predominated the high latitudes during both the Last Glacial and the Holocene; from the Last Glacial to the Holocene, C4 relative abundances increased in the middle latitudes, but decreased in the low latitudes. Combining with studies of modern process, we propose a simplified model to explain the variations of C3 /C4 relative abundance in global ecosystem since the Last Glacial. On the background of atmospheric CO2 concentration since the Last Glacial, temperature is the primary factor controlling C3/C4 relative abundance; when temperature is high enough, precipitation then exerts more influence. In detail, in low latitudes, temperature was high enough for the growth of C4 plants during both the Last Glacial and the Holocene; but increased precipitation in the Holocene inhibited the growth of C4 plants. In middle latitudes, rising temperature in the Holocene promoted the C4 expansion. In high latitudes, temperature was too low to favor the growth of C4 plants and the biomass was predominated by C3 plants since the Last Glacial. Our review would benefit interpretation of newly gained records of C3/C4 relative abundance from different areas and different periods, and has its significance in the understanding of the driving mechanisms of C3/C4 variations on longer timescales (e.g., since the late Miocene) with reliable records of temperature and atmospheric CO2 concentration.     

Noble gas isotopic compositions of deep carbonate rocks from the Tarim Basin

Abundances and isotopic compositions of noble gases (He, Ne, Ar, Kr) with various existence states in carbonate rocks from the Tacan1 Well have been investigated by means of the stepwise heating technique. The elemental abundance patterns of noble gases in the samples show the enrichment of heavy noble gases and depletion of ²⁰Ne relative to the atmosphere, which are designated as type-Ⅰand are similar to that observed in water, natural gases and sedimentary rocks. The ³He/⁴He ratios of deep carbonate samples at lower and medium temperature (300—700℃) and a majority of samples at higher temperature (1100—1500℃) steps are very similar to those of natural gases in the same strata in this area, this feature of radiogenic crustal helium shows that the Tazhong Uplift is relatively stable. However, significant helium and argon isotopic anomalies are found at the 1100℃ step in the Middle-Upper Ordovician carbonate rock, suggesting the incorporation of mantle-derived volatiles, this may be due to minor igneous minerals contained in sedimentary carbonate rocks. The ⁴⁰Ar/ ³⁶Ar ratios in the Cambrian carbonate rock are slightly higher than those in Ordovician carbonate rocks, which may reflect the influence of the chronologic accumulation effect of crust radiogenic ⁴⁰Ar. Argon isotopes of various existence states in source rocks are much more different, both ³⁸Ar/ ³⁶Ar and ⁴⁰Ar/ ³⁶Ar ratios at the higher temperature steps are higher than those at the lower temperature steps.     

H型钢控制冷却过程的应力场模拟

利用ANSYS软件建立H型钢模型并进行模拟,研究热轧H型钢轧后冷却过程中的热边界条件,分析H型钢在不同温度下应力应变场的分布及规律,为控制冷却系统的设计提供指导;分析不同冷却方式H型钢的力学性能,比较不同控冷方案,选择的控冷方案为:空冷5 s后,水冷5 s,再空冷5 s。根据应力产生机理和对比控制冷却不同水流密度的模拟结果,在相同冷却时间条件下,随着水流密度的增大,H型钢的温度逐渐降低,最大应力逐渐增大。     

The 'zero charge' partitioning behaviour of noble gases during mantle melting

Noble-gas geochemistry is an important tool for understanding planetary processes from accretion to mantle dynamics and atmospheric formation. Central to much of the modelling of such processes is the crystal-melt partitioning of noble gases during mantle melting, magma ascent and near-surface degassing. Geochemists have traditionally considered the 'inert' noble gases to be extremely incompatible elements, with almost 100 per cent extraction efficiency from the solid phase during melting processes. Previously published experimental data on partitioning between crystalline silicates and melts has, however, suggested that noble gases approach compatible behaviour, and a significant proportion should therefore remain in the mantle during melt extraction. Here we present experimental data to show that noble gases are more incompatible than previously demonstrated, but not necessarily to the extent assumed or required by geochemical models. Independent atomistic computer simulations indicate that noble gases can be considered as species of 'zero charge' incorporated at crystal lattice sites. Together with the lattice strain model, this provides a theoretical framework with which to model noble-gas geochemistry as a function of residual mantle mineralogy.     

Atlantic overturning responses to Late Pleistocene climate forcings

The factors driving glacial changes in ocean overturning circulation are not well understood. On the basis of a comparison of 20 climate variables over the past four glacial cycles, the SPECMAP project proposed that summer insolation at high northern latitudes (that is, Milankovitch forcing) drives the same sequence of ocean circulation and other climate responses over 100-kyr eccentricity cycles, 41-kyr obliquity cycles and 23-kyr precession cycles. SPECMAP analysed the circulation response at only a few sites in the Atlantic Ocean, however, and the phase of circulation response has been shown to vary by site and orbital band. Here we test the SPECMAP hypothesis by measuring the phase of orbital responses in benthic delta(13)C (a proxy indicator of ocean nutrient content) at 24 sites throughout the Atlantic over the past 425 kyr. On the basis of delta(13)C responses at 3,000-4,010 m water depth, we find that maxima in Milankovitch forcing are associated with greater mid-depth overturning in the obliquity band but less overturning in the precession band. This suggests that Atlantic overturning is strongly sensitive to factors beyond ice volume and summer insolation at high northern latitudes. A better understanding of these processes could lead to improvements in model estimates of overturning rates, which range from a 40 per cent increase to a 40 per cent decrease at the Last Glacial Maximum and a 10-50 per cent decrease over the next 140 yr in response to projected increases in atmospheric CO(2) (ref. 4).     

A stable argon compound

The noble gases have a particularly stable electronic configuration, comprising fully filled s and p valence orbitals. This makes these elements relatively non-reactive, and they exist at room temperature as monatomic gases. Pauling predicted in 1933 that the heavier noble gases, whose valence electrons are screened by core electrons and thus less strongly bound, could form stable molecules. This prediction was verified in 1962 by the preparation of xenon hexafluoroplatinate, XePtF6, the first compound to contain a noble-gas atom. Since then, a range of different compounds containing radon, xenon and krypton have been theoretically anticipated and prepared. Although the lighter noble gases neon, helium and argon are also expected to be reactive under suitable conditions, they remain the last three long-lived elements of the periodic table for which no stable compound is known. Here we report that the photolysis of hydrogen fluoride in a solid argon matrix leads to the formation of argon fluorohydride (HArF), which we have identified by probing the shift in the position of vibrational bands on isotopic substitution using infrared spectroscopy. Extensive ab initio calculations indicate that HArF is intrinsically stable, owing to significant ionic and covalent contributions to its bonding, thus confirming computational predictions that argon should form a stable hydride species with properties similar to those of the analogous xenon and krypton compounds reported before.