Decrease In Global Temperature Research Articles

The Middle-Upper Eocene in the Northern Plateau of the Bahariya Depression, Western Desert (Egypt) and correlation with other Tethyan sectors

This study describes the paleoenvironmental evolution of the Middle-Upper Eocene succession exposed along the northern plateau of the Bahariya Depression (Western Desert, Egypt). Based on detailed field observations and facies analysis, twenty-one facies were recognized and grouped into five facies associations: tidal flat to restricted marine (FA1), lagoonal (FA2), ramp crest (FA3), middle ramp (FA4), and siliciclastic-dominated foreshore–shoreface (FA5). These facies associations were developed in a gently dipping mixed siliciclastic-carbonate ramp which was connected with siliciclastic coastal-shelf depositional system. The biogenic assemblages described in the Middle-Upper Eocene deposits show a mixing of photozoan and heterozoan associations, indicating euphotic to mesophotic inner to mid-ramp settings under mesotrophic-oligotrophic conditions. The succession shows a transition from photozoan to heterozoan carbonates and a shift towards inner ramp settings. The formation and the evolution of the northern Bahariya platform reflect the complex interplay of synsedimentary tilting due to post-Middle Eocene inversion phase, high-frequency sea-level changes, transition from warm to humid climate and paleoecological parameters. The sedimentation pattern including distinct lateral changes in facies and thicknesses are largely controlled by tectonics. Long-term sea level changes were responsible for the unconformities associated with major regressions. The Middle-Upper Eocene strata display a characteristic up-section increase in clastic input from the hinterlands through tectonic uplift and/or a change to a cooling climate. The shift to a colder climate, which was a result of a global temperature decrease, led to the maximum northward retreat of the Neo-Tethys Ocean. During the Eocene epoch, carbonates dominated by Larger Benthic Foraminifera (LBF) were prevalent along the entire Tethyan margins. This was linked to a decline in coral reef formations. Comparisons with other southern and western-central Tethyan sectors showed that small coral reef build-ups in widely scattered localities could exist in broad LBF-rich ramps. Consequently, zooxanthellate corals were only able to flourish locally in marginal environments under specific ecological conditions.

Late Cretaceous evolution of chemical weathering at the northeastern South American margin inferred from mineralogy and Hf-Nd isotopes

The origin of the global temperature decrease that initiated the last greenhouse-icehouse transition about 90 million years ago still remains debated. Among the potential processes driving long-term climate evolution over million-year timescales, this study investigates the possible impact of the uplift-weathering connection on the late Cretaceous atmospheric CO2 drawdown and associated cooling.We analysed a marine sediment record from the Demerara margin, using Nd and Hf isotopes in clay-size detrital fractions (ΔɛHf(t)clay) together with bulk and clay mineralogy and major element abundances to reconstruct the evolution of tectonic uplift and chemical weathering intensity in northeastern South America during the late Cretaceous. Our data indicate that silicate weathering intensified on the northeastern South American margin during the middle Campanian and the Maastrichtian, concomitant with an uplift phase of the Guiana craton. We propose that the tectonic pulse highlighted by apatite fission track data, argon/argon dating and primary silicate mineral evolution, was accompanied by an accelerated chemical weathering, which presumably acted as a sink for atmospheric CO2. By contrast, during the earlier late Turonian to early Campanian, period of relative tectonic quiescence, climate most likely acted as the main driver controlling the evolution of chemical weathering intensity. The main phase of enhanced weathering induced by the uplift of the northeastern South American margin occurred during the Campanian, thereby postdating the onset of global seawater temperature decrease (late Turonian). By inference, this regional event is unlikely to have triggered late Cretaceous cooling but it may have acted as an additional factor stabilizing colder climate conditions at that time.

Middle Miocene Climate Transition as reflected by changes in ichnofacies and palaeosols from Patagonia, Argentina

The ichnofacies and palaeosols from the La Pava and Collón Curá formations were studied to shed light on the evolution of terrestrial ecosystems from Patagonia (Argentina) during the Middle Miocene Climate Transition. At Paso del Sapo, Cerrito Terciario and El Petiso localities (Chubut province), the La Pava Formation included Coprinisphaera ispp., Rosellichnus patagonicus, Rebuffoichnus casamiquelai, Feoichnus challa and burrows of spiders and earthworms, representing a case of the Rosellichnus ichnoassemblage of the Coprinisphaera Ichnofacies. Some of the components of this ichnoassemblage, scarce or absent in Cenozoic formations older than La Pava, are present in extant Patagonian habitats, pointing in the direction of extant invertebrate fauna. At El Petiso locality, the Collón Curá Formation included Celliforma ispp., Rosellichnus patagonicus, Fictovichnus ispp., Teisseirei barattinia and Teisseirei paladinco isp. nov., representing a case of the Celliforma Ichnofacies. The La Pava Formation comprises brownish, argillic, granular and strongly-developed palaeosols (Mollisols and Alfisols) formed in mildly stable to long-lasting landscapes, whereas the Collón Curá Formation comprises calcic, tuffaceous, paler and weakly-developed palaeosols (Andisols, Aridisols) formed in unsteady and ashy environments. The ichnofacies and palaeosols from the La Pava Formation indicate wooded grasslands developed in seasonal, subhumid and temperate-warm conditions, whereas those from the Collón Curá Formation indicate a habitat with reduced plant cover similar to semiarid and temperate-warm dry woodlands or bushlands. This shift towards more arid conditions in Patagonian landscapes would have been coeval with climatic changes corresponding to the Middle Miocene Climate Transition, a lapse of global temperature decrease associated with extensive drying.

Spatial-temporal evolution and diversification inSisyrinchium(Iridaceae) with emphasis on abiotic drivers

AbstractEvolutionary and ecological processes that influenced the assembly of the New World flora are best understood through investigation of spatio-temporal processes of specific lineages, but some groups still lack a historical overview. Here, we produced a well-sampled dated tree, reconstructed ancestral ranges and performed diversification analyses for Sisyrinchium (Iridaceae) to elucidate its evolution in the Americas. Eight molecular markers and samples representing its full geographical range and morphological diversity were used to estimate divergence times with a Bayesian relaxed clock with secondary calibrations. Ancestral range reconstruction under likelihood methods and diversification analyses were performed. Sisyrinchium originated in a broad range including the Andes and Mesoamerica in the Mid-Miocene. Diversification at high elevations occurred in the early diverging lineages, which feature the highest extinction rates. Increase in diversification rate was detected during the Pliocene/Pleistocene after the colonization of lower elevations. Later sympatric speciation in south-eastern Brazil was followed by movements to other regions, including a long-dispersal event to North America. Higher extinction rates were followed by movements to lower elevations, with periods of accelerated Andean orogeny and global temperature decrease. Our results indicate that palaeoclimate and changes in elevational range influenced diversification in Sisyrinchium.

Formation of clays, ferrihydrite, and possible salts in Hydrae Chasma, Mars

We used multiple orbital data sets to analyze the morphology, composition, and stratigraphy of clays and other hydrated deposits within Hydrae Chasma. Fe/Mg-smectites are concentrated along the lower floor of the chasma, adjacent to pit craters. There are at least two distinct smectites that vary in their morphology and spectral properties, with a more Mg-rich smectite associated with a thin brighter upper layered deposit and a more Fe-rich smectite in a lower darker massive deposit. We also identified a younger hydrated unit with spectral features similar to ferrihydrite. The spectra of this unit lack an absorption between 2.28 and 2.31 µm, but display a drop in reflectance near 2.3 µm. The ferrihydrite-type detections correspond to cleaner surfaces of light-toned deposits further north and higher in elevation than the smectites. A few small outcrops in the southern chasma floor appear brighter than the clays and display an additional narrow absorption at 2.4 µm in combination with the smectite features, which may indicate a smectite plus perchlorate mixture. There are no valleys along the plateau that intersect Hydrae Chasma and, consequently, any water that created the Fe/Mg-smectites and ferrihydrite must have been from groundwater in the subsurface or surface water sourced from within the chasma. A terraced fan and associated valley to the north of the clays provide support for melting snow/ice within the chasma and this same water may have also altered materials lower in elevation where the ferrihydrite-bearing unit formed under colder conditions than the smectites. The change in stratigraphy from smectites to ferrihydrite is consistent with a decrease in global temperature during the Hesperian.

Cretaceous calcareous dinoflagellate cysts as recorder of δ44/40Caseawater and paleo-temperature using Sr/Ca thermometry

We evaluate the potential of calcareous dinoflagellates as archives for Sr/Ca-based paleo-temperature reconstructions and δ44/40Caseawater fluctuations on sediments from Ocean Drilling Program Expedition 113 (Hole 690C, Weddell Sea, Southern Ocean). Between 73 and 68 Ma, Sr/Ca ratios of two Cretaceous dinoflagellate species, Pirumella krasheninnikovii and Orthopithonella globosa show a pronounced decrease, consistent with a significant drop in sea-surface temperature as reflected by the δ18O of planktic foraminifers. The apparent temperature sensitivity of the dinoflagellate cysts' Sr/Ca is ~0.06 and ~0.08 mmol/mol °C−1, using δ18O-derived paleo sea-surface temperatures, which is significant and large enough to resolve paleoenvironmental temperature changes at current analytical precision. As the chemical composition of the cyst calcite appears to have a good preservation, the Sr/Ca of calcareous dinoflagellates has a potential to serve as paleo-temperature proxy, although the chemical composition of the Cretaceous seawater and potential impacts on the cyst geochemistry is pending further inspection.The Ca isotope composition of the two dinoflagellate species shows identical trends of increasing δ44/40Ca between 73 and 67 Ma. The planktic foraminifer Archaeoglobigerina australis and the benthic foraminifer Nuttallides truempyi reveal the same increase of about 0.4‰ but are offset relative to the dinoflagellates by about +0.5‰, presumably due to species-specific Ca isotope fractionation. Bulk carbonate sediment shows significant scatter, likely caused by changes in faunal composition and does not reproduce the trend revealed by the dinoflagellate and foraminifer records. These observations demonstrate the importance of taxon-specific records and careful determination of fractionation factors of selected archives and highlight complications arising from utilizing less suitable archives, such as bulk sediments, for δ44/40Caseawater reconstructions. Our records indicate strong changes in the marine Ca cycle associated with the global temperature decrease towards the end of the Cretaceous.

Calcareous nannofossil fluxes and size fluctuations in the middle Eocene (48–39 Ma) from Ocean Drilling Program (ODP) Site 1209 in the tropical Pacific Ocean

Pelagic carbonates constitute the majority of the ocean sediments, particularly since the Jurassic, and record climatic and oceanic changes. Cenozoic sediments record glacial and interglacial periods, as well as thermal maxima, such as the PETM (Paleocene-Eocene Thermal Maximum) at ca. 56Ma or the MECO (Middle Eocene Climatic Optimum) that dates back to ca. 40Ma. The Paleogene period shows variations in seawater temperature and atmospheric CO2, as well as fluctuations of the carbonate compensation depth (CCD). In order to understand the actions and feedbacks between climate (regulated amongst others by temperature, partial pressure of CO2, and atmospheric belts), CCD fluctuations and pelagic carbonate producers, the fluxes, assemblages and sizes of calcareous nannofossils were studied from sediments dated between 48 and 39Ma, including the MECO interval. Calcareous nannofossils were analysed in from the ODP (Ocean Drilling Project) Leg 198 Site 1209A, Shatsky Rise located in the tropical Pacific. The previously proposed age model for Site 1209 has been slightly modified based on new calcareous nannofossil biostratigraphy recalibrated according to the GTS 2012. In a long-term perspective, calcareous nannofossil assemblages and taxa size show significant changes during the middle Eocene. Our data suggest that relatively oligotrophic conditions occurred between 48 and 45Ma, interrupted by higher trophic conditions between 45 and 39Ma, although oligotrophy occurred again during the MECO. Discoaster spp. show smaller sizes from 48 to 39Ma, mirroring the global temperature decrease. At 40Ma, during the MECO, large-sized reticulofenestrids with a peak of Reticulofenestra umbilicus (>14μm) occurred, whereas smaller but heavily calcified Dictyococcites spp. peak in the aftermath of the MECO. Calcareous nannofossil fluxes show a significant increase with respect to background values during the CAEs (Carbonate Accumulation Event), although they are low during the MECO. Calcareous nannofossil fluxes seem to have had important impact on the oceanic carbon cycles, by creating a carbonate flux to the ocean interior, likely contributing to the CCD deepening.

Mitochondrial Phylogenomics Resolves the Global Spread of Higher Termites, Ecosystem Engineers of the Tropics.

The higher termites (Termitidae) are keystone species and ecosystem engineers. They have exceptional biomass and play important roles in decomposition of dead plant matter, in soil manipulation, and as the primary food for many animals, especially in the tropics. Higher termites are most diverse in rainforests, with estimated origins in the late Eocene (∼54 Ma), postdating the breakup of Pangaea and Gondwana when most continents became separated. Since termites are poor fliers, their origin and spread across the globe requires alternative explanation. Here, we show that higher termites originated 42-54 Ma in Africa and subsequently underwent at least 24 dispersal events between the continents in two main periods. Using phylogenetic analyses of mitochondrial genomes from 415 species, including all higher termite taxonomic and feeding groups, we inferred 10 dispersal events to South America and Asia 35-23 Ma, coinciding with the sharp decrease in global temperature, sea level, and rainforest cover in the Oligocene. After global temperatures increased, 23-5 Ma, there was only one more dispersal to South America but 11 to Asia and Australia, and one dispersal back to Africa. Most of these dispersal events were transoceanic and might have occurred via floating logs. The spread of higher termites across oceans was helped by the novel ecological opportunities brought about by environmental and ecosystem change, and led termites to become one of the few insect groups with specialized mammal predators. This has parallels with modern invasive species that have been able to thrive in human-impacted ecosystems.

Ancient DNA from the extinct South American giant glyptodont Doedicurus sp. (Xenarthra: Glyptodontidae) reveals that glyptodonts evolved from Eocene armadillos.

Glyptodonts were giant (some of them up to ~2400kg), heavily armoured relatives of living armadillos, which became extinct during the Late Pleistocene/early Holocene alongside much of the South American megafauna. Although glyptodonts were an important component of Cenozoic South American faunas, their early evolution and phylogenetic affinities within the order Cingulata (armoured New World placental mammals) remain controversial. In this study, we used hybridization enrichment and high-throughput sequencing to obtain a partial mitochondrial genome from Doedicurus sp., the largest (1.5m tall, and 4m long) and one of the last surviving glyptodonts. Our molecular phylogenetic analyses revealed that glyptodonts fall within the diversity of living armadillos. Reanalysis of morphological data using a molecular 'backbone constraint' revealed several morphological characters that supported a close relationship between glyptodonts and the tiny extant fairy armadillos (Chlamyphorinae). This is surprising as these taxa are among the most derived cingulates: glyptodonts were generally large-bodied and heavily armoured, while the fairy armadillos are tiny (~9-17cm) and adapted for burrowing. Calibration of our phylogeny with the first appearance of glyptodonts in the Eocene resulted in a more precise timeline for xenarthran evolution. The osteological novelties of glyptodonts and their specialization for grazing appear to have evolved rapidly during the Late Eocene to Early Miocene, coincident with global temperature decreases and a shift from wet closed forest towards drier open woodland and grassland across much of South America. This environmental change may have driven the evolution of glyptodonts, culminating in the bizarre giant forms of the Pleistocene.

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

Abstract. Magnesium is an element critically involved in the carbon cycle, because weathering of Ca-Mg silicates removes atmospheric CO2 into rivers, and formation of Ca-Mg carbonates in the oceans removes carbon from the ocean-atmosphere system. Hence the Mg cycle holds the potential to provide valuable insights into Cenozoic climate-system history, and the shift during this time from a greenhouse to icehouse state. We present Mg isotope ratios for the past 40 Myr using planktic foraminifers as an archive. Modern foraminifera, which discriminate against elemental and isotopically heavy Mg during calcification, show no correlation between the Mg isotope composition (δ26Mg, relative to DSM-3) and temperature, Mg / Ca or other parameters such as carbonate saturation (ΔCO3). However, inter-species isotopic differences imply that only well-calibrated single species should be used for reconstruction of past seawater. Seawater δ26Mg inferred from the foraminiferal record decreased from ~0‰ at 15 Ma, to −0.83‰ at the present day, which coincides with increases in seawater lithium and oxygen isotope ratios. It strongly suggests that neither Mg concentrations nor isotope ratios are at steady state in modern oceans, given its ~10 Myr residence time. From these data, we have developed a dynamic box model to understand and constrain changes in Mg sources to the oceans (rivers) and Mg sinks (dolomitisation and hydrothermal alteration). Our estimates of seawater Mg concentrations through time are similar to those independently determined by pore waters and fluid inclusions. Modelling suggests that dolomite formation and the riverine Mg flux are the primary controls on the δ26Mg of seawater, while hydrothermal Mg removal and the δ26Mg of rivers are more minor controls. Using Mg riverine flux and isotope ratios inferred from the 87Sr / 86Sr record, the modelled Mg removal by dolomite formation shows minima in the Oligocene and at the present day (with decreasing trends from 15 Ma), both coinciding with rapid decreases in global temperatures.

Experimental investigation of direct injection charge cooling in optical GDI engine using tracer-based PLIF technique

Investigation of direct injection charge cooling effects is indispensable in design and development of new combustion systems for Gasoline Direct Injection (GDI) engines. The charge cooling can be utilized to increase engine volumetric efficiency or compression ratio. It can be employed to suppress pre-ignition of highly boosted downsized engines or knocking combustion of naturally aspirated engines. The main purpose of this work was to develop an experimental setup for quantitative measurements of charge cooling during fuel injection process inside the combustion chamber of a GDI engine with optical access. For this purpose a tracer-based two-line Planar Laser Induced Fluorescence (PLIF) technique was implemented for the measurements. A specially designed Constant Volume Chamber (CVC) was utilized for quasi in situ calibration measurement so in-cylinder charge temperature measurements can be achieved independent of the photophysical model of dopant tracer. The thermometry technique was evaluated by measurements of average in-cylinder charge temperature during compression stroke for both motoring and firing cycles and comparing the results with temperature values calculated from in-cylinder pressure data assuming a polytropic compression. The PLIF technique was successfully utilized to quantify the extend of global temperature decrease as a result of direct injection charge cooling of two injection timings of 90 and 250°CA ATDC and two injection quantities of 10 and 30mg/cycle. Test results demonstrated the capability of the two-line PLIF thermometry technique in quantitative study of direct injection charge cooling effects.

Assessment of forced responses of the Australian Community Climate and Earth System Simulator (ACCESS) 1.3 in CMIP5 historical detection and attribution experiments.

The Australian Community Climate and Earth System Simulator (ACCESS) coupled climate model version 1.3 participated in phase five of the Coupled Model Intercomparison Project (CMIP5) with an initial contribution of high priority experiments. Further standard experiments have since been conducted with ACCESS1.3, including an ensemble of three simulations for the historical period (1850-2005) forced with time-evolving natural and anthropogenic forcings. Additional ensembles of simulations have been conducted for the same period with subsets of known forcings, including with natural forcings only ('historicalNat') and with greenhouse gas forcings only ('historicalGHG'). In this study, we describe this ACCESS1.3 contribution to CMIP5 and assess several key aspects of ACCESS1.3 forced responses in these experiments against observations and an ensemble of participating CMIP5 models, consisting of 40 realisations from ten models. Overall, ACCESS1.3 historical experiments demonstrate skill in simulating the global and regional metrics assessed that is comparable to the CMIP5 multi-model ensemble utilised. Global annual average temperature and precipitation trends simulated with ACCESS1.3 (0.05-0.07 K/decade; -0.007 to -0.0004 mm day /decade) largely lie within the CMIP5 ensemble window (0.06 - 0.18 K/decade; -0.01 to 0.009 mm day /decade) and near those observed (0.10 K/decade; -0.0007 to -0.001 mm day-1/decade) over the 1950-2005 period. For the ACCESS1.3 historicalNat and historicalGHG experiments, simulated temperature trends are also predominately within the CMIP5 multi-model ensemble range. Similarly, ACCESS1.3 (-0.07 to -0.12 K) and the CMIP5 models (-0.03 to -0.21 K) largely capture the composited observed decrease in global temperature (-0.04 K) following three major late 20th century volcanic eruptions. However, like all global climate models, ACCESS1.3 has deficiencies that should be considered. In particular, one of most notable features of ACCESS1.3 historical simulations is the reduced warming trend over the period 1950-2005 that is evident in all ACCESS1.3 realisations at the global-scale for Australia, relative to both observations and the CMIP5 multi-model mean. This appears to be related to the overly strong response to increases in anthropogenic aerosols. Overall, these historical period experiments using ACCESS1.3 with various forcings are useful for inclusion with other CMIP5 models for studies aimed at detecting and attributing climatic changes. -1 -1

Past Climate Change and Plant Evolution in Western North America: A Case Study in Rosaceae

Species in the ivesioid clade of Potentilla (Rosaceae) are endemic to western North America, an area that underwent widespread aridification during the global temperature decrease following the Mid-Miocene Climatic Optimum. Several morphological features interpreted as adaptations to drought are found in the clade, and many species occupy extremely dry habitats. Recent phylogenetic analyses have shown that the sister group of this clade is Potentilla section Rivales, a group with distinct moist habitat preferences. This has led to the hypothesis that the ivesioids (genera Ivesia, Horkelia and Horkeliella) diversified in response to the late Tertiary aridification of western North America. We used phyloclimatic modeling and a fossil-calibrated dated phylogeny of the family Rosaceae to investigate the evolution of the ivesioid clade. We have combined occurrence- and climate data from extant species, and used ancestral state reconstruction to model past climate preferences. These models have been projected into paleo-climatic scenarios in order to identify areas where the ivesioids may have occurred. Our analysis suggests a split between the ivesioids and Potentilla sect. Rivales around Late Oligocene/Early Miocene (∼23 million years ago, Ma), and that the ivesioids then diversified at a time when summer drought started to appear in the region. The clade is inferred to have originated on the western slopes of the Rocky Mountains from where a westward range expansion to the Sierra Nevada and the coast of California took place between ∼12-2 Ma. Our results support the idea that climatic changes in southwestern North America have played an important role in the evolution of the local flora, by means of in situ adaptation followed by diversification.

Cooling of a reciprocating compressor through oil atomization in the cylinder

An analysis of the influence of oil atomization in the cylinder of a reciprocating compressor is presented in this paper. During compression, oil atomization enhances heat removal from the refrigerant gas. This cooling effect, which eventually results in a global temperature decrease of the compressor parts, aims primarily at reducing the refrigerant superheating in the suction system and inside the cylinder, which is largely responsible for overall energy losses and a decrease of the volumetric efficiency. A prototype was constructed and tested with R-134a in a hot gas-cycle calorimeter. A significant reduction of the compressor thermal profile has been achieved, with the largest variations of 30.9°C (55.6°F) and 23.6°C (42.5°F) in the discharge chamber and cylinder wall, respectively. A major dependence of the compressor efficiency parameters on the refrigerant solubility in the oil has been observed. A simulation model using an integral control volume formulation for mass and energy conservation in the cylinder and in other compressor components is proposed. Based on this model, the effect of oil atomization on the compressor performance is presented and discussed in terms of oil injection parameters, such as nozzle position, oil temperature and flow rate.

Dust storms over the Arabian Gulf: a possible indicator of climate changes consequences

Dust storm frequencies and strengths were monitored during 2009 at various locations along the coast of the United Arab Emirates (UAE), as representative sites of the Arabian Gulf marine environment. The results have been compared with a pre-2009 five-year data set. Mineralogical components of dust samples collected during the 2009 study period were analyzed using both X-ray diffraction (XRD) and X-ray Fluorescence (XRF) techniques. The comparison of the 2009 and pre-2009 data revealed a shift in the characteristics of dust storms in the Arabian Gulf, which may constitute a sensitive indicator of climate change affecting the region. The range of mineral compositions of the collected dust samples is consistent with temporally and spatially variable dust sources, associated with changing wind patterns over the Arabian Gulf. From a positive perspective, the dust may deliver mineralogical nutrients enhancing microbial productivity in the marine ecosystem. Increasing productivity and liberation of iron from sediments will lead to an increase of Dimethyl Sulfide (DMS) in the atmosphere, which by oxidation will scatter solar radiation effectively with a consequent decrease in global temperature as a climate feedback.

A multiproxy palaeoenvironmental reconstruction of Loch Sunart (NW Scotland) since the Last Glacial Maximum

Abstract Loch Sunart is located on the NW coast of Scotland and contains a sedimentary sequence that records Lateglacial to Holocene climatic variations. A 12 m core MD04-2833 was acquired in the main basin of the loch sampling this sequence. We present the palaeoenvironmental data and palaeoclimatical scenario based on a multiproxy approach using pollen concentrations, sortable silt variation, lithic fraction and marine benthic foraminifera assemblages. These analyses allow the identification of major climate fluctuations such as cooling events. Global temperature decreases are discriminated from local water temperature decreases due to ice-melting processes by the presence of Elphidium subarcticum and the assemblage of Cassidulina obtusa and Haynesina germanica . Two meltwater pulses (MWP) are distinguished, which correspond to the MWP-Ia (15.5–13 cal ka BP) and MWP-Ib (12.2–10.1 cal ka BP). After the maximum water stratification occurred at 7.5 cal ka BP, full marine conditions were established around 6 cal ka BP, which correspond to the highest relative sea-level reached in the loch.

The possible pivotal role of the eastward dust transport from Central Asia in the global temperature decrease

It is generally considered that the occurrence of glacial-interglacial gyrations can be explained with the Milankovitch theory. However, the solutions of some problems in the theory are currently not derived yet. After researching the eastward dust transport from Central Asia, we have found that there is a positive feedback mechanism between the eastward dust transport and the global temperature decrease. This magnifies the effect of solar radiation change in the high-latitude area of the Northern Hemisphere, and results in the occurrence of the global glacial epoch. The positive feedback mechanism starts with the reduction of solar radiation in the high-latitude area of the Northern Hemisphere. Subsequently, the global temperature decreases and global glacial epoch occurs, in which the pivotal factor is the eastward dust transport from Central Asia. With the theory of the positive feedback mechanism, some problems in the Milankovitch theory can be solved well.

The closing of a seaway: ocean water masses and global climate change

The Late Neogene witnessed various major paleoceanographic changes that culminated in intense Northern Hemisphere Glaciation (NHG). The cause and effects of these changes are still debated. We use a multiproxy approach to determine the relative timing of the closure of the Panama gateway, changes in Atlantic circulation, global cooling and ice sheet growth. Benthic foraminiferal Mg/Ca records from a Pacific and an Atlantic Site have been produced and are interpreted in terms of bottom water temperatures. These Mg-temperature records are combined with published benthic δ 13C, δ 18O and erosion records to reconstruct the flow of proto-North Atlantic Deep Water (proto-NADW) over the past 12 Ma. The results suggest that between 12.5 and 10.5 Ma, and again between about 8.5 and 6 Ma, a nutrient-depleted water mass that was colder (by 1–2°C) and fresher than the intervening deep water mass filled the Atlantic basin. This proto-NADW became warmer (by ∼1°C) and saltier between 6 and 5 Ma, coincident with the restriction of surface water flow through the Central American Seaway. The Mg-temperature records define a subsequent global cooling trend of ∼3.5°C between 5 Ma and today. Early NHG in the late Miocene was perhaps related to the formation of the relatively cold, fresh proto-NADW. The formation of the warmer and saltier proto-NADW in the early Pliocene may have initially limited Northern Hemisphere ice growth. However, the increased moisture released at high northern latitudes associated with formation of ‘warm’ proto-NADW, coupled with the global temperature decrease of deep (and hence polar surface) waters, likely helped initiate the intense NHG of the Plio–Pleistocene.

R/S analyses of some geochemical indexes for Tianshuihai Lake core in West Kunlun Mountain and their environmental implications

With the decrease of global temperature, glacial epoch came over the earth and global climate fluctuated over a great range since the beginning of Quaternary. Paleoclimotologists of various countries have focussed attention to the periodic characteristics and dynamics of climatic fluctuation in the past many years (Berger, 1977; Imbrie and Hays, 1984; Ding Zhongli et al., 1990; Yu Zhiwei et al., 1992; Liu Youmei et al., 1996). Although some of the workers have paid their attention to the nonlinear characteristics of the global Quaternary environmental evolution (Nicolis and Nicolis, 1984; Lu Houyuan et al., 1993), it is worth while to do this kind of work in some special areas in the world, for example the Qinghai-Tibet Plateau.

Possible climatic impacts of land cover transformations, with particular emphasis on tropical deforestation

The climatic impact of albedo changes associated with land-surface alterations has been examined. The total surface global albedo change resulting from major land-cover transformations (i.e. deforestation, desertification, irrigation, dam-building, urbanization) has been recalculated, modifying the estimates of Sagan et al., (1979). Tropical deforestation (11.1 million ha yr-1, or 0.6% yr-1, Lanly, 1982) ranks as a major cause of albedo change, although uncertainties in the areal extent of desertification could conceivably render this latter process of similar significance. The maximum total global albedo change over the last 30 yr for the various processes lies between 0.000 33 and 0.000 64, corresponding to a global temperature decrease of between 0.06 K and 0.09 K (scaled from the 1-D radiative convective model of Hansen et al., 1981), which falls well below the interannual and longer period variability.