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1.
《Quaternary Science Reviews》2007,26(5-6):759-772
Quantitative reconstruction of the climatic history of the Chinese Loess Plateau is important for understanding present and past environment and climate changes in the Northern Hemisphere. Here, we reconstructed mean annual temperature (MAT) and mean annual precipitation (MAP) trends during the last 136 ka based on the analysis of phytoliths from the Weinan loess section (34°24′N, 109°30′E) near the southern part of the Loess Plateau in northern China. The reconstructions have been carried out using a Chinese phytolith–climate calibration model based on weighted averaging partial least-squares regression. A series of cold and dry events, as indicated by the reconstructed MAT and MAP, are documented in the loess during the last glacial periods, which can be temporally correlated with the North Atlantic Heinrich events. Our MAT and MAP estimations show that the coldest and/or driest period occurred at the upper part of L2 unit (Late MIS 6), where MAT dropped to ca 4.4 °C and MAP to ca 100 mm. Two other prominent cold-dry periods occurred at lower Ll-5 (ca 77–62 ka) and L1-1 (ca 23–10.5 ka) where the MAT and MAP decreased to about 6.1–6.5 °C and 150–370 mm, respectively, ca 6.6–6.2 °C and 400–200 mm lower than today. However, the highest MAT (average 14.6 °C, max. 18.1 °C) and MAP (average 757 mm, max. 1000 mm) occurred at Sl interval (MIS 5). During the interstadial of L1-4–L1-2 (MIS 3) and during the Holocene warm-wet period, the MAT was about 1–2 °C and MAP 100–150 mm higher than today in the Weinan region. The well-dated MAT and MAP reconstructions from the Chinese Loess Plateau presented in this paper are the first quantitatively reconstructed proxy record of climatic changes at the glacial–interglacial timescale that is based on phytolith data. This study also reveals a causal link between climatic instability in the Atlantic Ocean and climate variability in the Chinese Loess Plateau.  相似文献   

2.
Chironomids and pollen were studied in a radiocarbon-dated sediment sequence obtained from a former lake near the Maloja Pass in the Central Swiss Alps (1865 m a.s.l.) to reconstruct the Lateglacial environment. Pollen assemblages imply a vegetation development around the Maloja Pass from shrub tundra at the beginning of the Allerød to coniferous forest during the early Holocene with a lowering of the timberline during the Younger Dryas. Chironomid assemblages are characterized by several abrupt shifts in dominant taxa through the Lateglacial. The occurrence of taxa able to survive hypoxia in the second part of the Allerød and during the Preboreal, and their disappearance at the onset of the Younger Dryas cold phase suggest summer thermal stratification and unfavourable hypolimnetic oxygen conditions in the palaeo-lake during the warmer periods of the Lateglacial interstadial and early Holocene. Mean July air temperatures were reconstructed using a chironomid-temperature transfer function from the Alpine region. The pattern of reconstructed temperature changes agrees well with the Greenland δ18O record and other Lateglacial temperature inferences from Central Europe. The inferred July temperatures of ca 10.0 °C during most of the Allerød were slightly lower than modern values (10.8 °C) and increased up to ca 11.7 °C (i.e., above present-day values) at the end of the Allerød. The first part of the Younger Dryas was colder (ca 8.8 °C) than the second part (ca 9.8 °C). During most of the Preboreal, the temperatures persisted within the limits of 13.5–14.5 °C (i.e., ca 3 °C above present-day values). The amplitudes of temperature changes at the Allerød–Younger Dryas–Preboreal transitions were ca 3.5–4.0 °C. The temperature reconstruction also shows three short-lived cooling events of ca 1.5–2.0 °C, which may be attributed to the centennial-scale Greenland Interstadial events GI-1d and GI-1b, and the Preboreal Oscillation.  相似文献   

3.
《Quaternary Science Reviews》2007,26(19-21):2420-2437
Lateglacial environments at Hijkermeer, northwest Netherlands, were reconstructed by means of chironomid, diatom and pollen analyses. Diatom assemblages indicate that Hijkermeer was a shallow, oligo- to mesotrophic lake during this period. Pollen assemblages reflect the typical northwest European Lateglacial vegetation development and provide an age assessment for the record from the beginning of the Older Dryas (ca 14 000 calibrated 14C yr BP) into the early Holocene (to ca 10 700 calibrated 14C yr BP). The chironomid record is characterized by several abrupt shifts between assemblages typically found in mid-latitude subalpine to alpine lakes and assemblages typical for lowland environments. Based on the chironomid record, July air temperatures were reconstructed using a chironomid-temperature transfer-function from central Europe. Mean July air temperatures of ca 14.0–16.0 °C are inferred before the Older Dryas, of ca 16.0–16.5 °C during most of the Allerød, of ca 13.5–14.0 °C during the Younger Dryas, and of ca 15.5–16.0 °C during the early Holocene. Two centennial-scale decreases in July air temperature were reconstructed during the Lateglacial interstadial which are correlated with Greenland Interstadial events (GI)-1d and -1b. The results suggest that vegetation changes in the Netherlands may have been promoted by the cooler climate during GI-1d, immediately preceding the Older Dryas biozone, and GI-1b. The Hijkermeer chironomid-inferred temperature record shows a similar temperature development as the Greenland ice core oxygen isotope records for most of the Lateglacial and a good agreement with other temperature reconstructions available from the Netherlands. This suggests that chironomid-based temperature reconstruction can be successfully implemented in the Northwest European lowlands and that chironomids may provide a useful alternative to oxygen isotopes for correlating European lake sediment records during the Lateglacial.  相似文献   

4.
The large landmass of northern Russia has the potential to influence global climate through amplification of climate change. Reconstructing climate in this region over millennial timescales is crucial for understanding the processes that affect the global climate system. Chironomids, preserved in lake sediments, have the potential to produce high resolution, low error, quantitative summer air temperature reconstructions. Canonical correspondence analysis of modern surface sediments from high-latitude lakes, located in northern European Russia and central Siberia, suggests that mean July air temperature is the most significant variable explaining chironomid distribution and abundance. This strong relationship enabled the development of a chironomid-based mean July air temperature-inference model based on 81 lakes and 89 taxa which has a rjack2 = 0.92 and RMSEP = 0.89 °C. Comparison of taxon responses to July temperature between this Russian and existing Norwegian data-sets shows that the temperature optima of individual taxa were between 1 and 3 °C higher in the Russian data regardless of modelling technique. Reconstructions based on fossil assemblages from a Russian tundra lake core (VORK5) using a Norwegian chironomid-based inference model provide mean July air temperature estimates that are 1.0–2.7 °C colder than from the 81-lake Russian model and are also lower than the instrumental record from a nearby meteorological station. The Norwegian model also did not reconstruct decadal-scale fluctuations in temperature seen in the instrumental record. These observations suggest that chironomid-based inference models should only be applied to sediment cores which have similar climate regimes to the geographic area of the training set. In addition a 149 lake, 120 taxa chironomid-based continentality inference model was also developed from the modern Norwegian and Russian training sets. A 2-component WA-PLS model was the minimal adequate model with rjack2 = 0.73 and RMSEP = 9.9 using the Gorczynski continentality index. Comparison of reconstructed continentality indices from the tundra lake, VORK5, show close agreement with local instrumental records over the past 70 years and suggest that the model is reliable. Recent warming in the Arctic has been spatially and seasonally heterogeneous; in many areas warming is more pronounced in the spring and autumn leading to a lengthening of the summer, while summer temperatures have remained relatively stable. A continentality inference model has the potential to detect these seasonal changes in climate.  相似文献   

5.
《Quaternary Science Reviews》2003,22(5-7):541-554
The ecotone between the boreo-nemoral (hemiboreal) and the southern boreal vegetation zones constitutes the northern distributional limit of a number of thermophilous tree species in northern Europe and is, to a large extent, controlled by climatic conditions. We present a quantitative annual mean temperature reconstruction from a high-resolution pollen stratigraphy in southern boreal Finland, using a pollen-climate calibration model with a cross-validated prediction error of 0.9°C. Our model reconstructs low but steadily rising annual mean temperature from 10,700 to 9000 cal yr BP. At 8000–4500 cal yr BP reconstructed annual mean temperature reaches a period of highest values (Holocene thermal maximum) with particularly high temperatures (2.0–1.5°C higher than at present) at 8000–5800 cal yr BP. From 4500 cal yr BP to the present-day, reconstructed annual mean temperature gradually decreases by ca 1.5°C. Comparison of present results with palaeotemperature records from the Greenland ice cores, notably with the NorthGRIP δ18O record, shows marked similarities, suggesting parallel large-scale Holocene temperature trends between the North Atlantic and North European regions. The verification of the occurrence, timing, and nature of the short-term temperature fluctuations during the Holocene in the southern boreal zone in Europe requires replicate, high-resolution climate reconstructions from the region.  相似文献   

6.
Forty-four boulders from moraines in two glacial valleys of Mount Erciyes (38.53°N, 35.45°E, 3917 m), central Turkey, dated with cosmogenic chlorine-36 (36Cl), indicate four periods of glacial activity in the past 22 ka (1 ka = 1000 calendar years). Last Glacial Maximum (LGM) glaciers were the most extensive, reaching 6 km in length and descending to an altitude of 2150 m above sea level. These glaciers started retreating 21.3 ± 0.9 ka (1σ) ago. They readvanced and retreated by 14.6 ± 1.2 ka ago (Lateglacial), and again by 9.3 ± 0.5 ka ago (Early Holocene). The latest advance took place 3.8 ± 0.4 ka ago (Late Holocene). Using glacier modeling together with paleoclimate proxy data from the region, we reconstructed the paleoclimate at these four discrete times. The results show that LGM climate was 8–11 °C colder than today and moisture levels were somewhat similar to modern values, with a range between 20% more and 25% less than today. The analysis of Lateglacial advance suggests that the climate was colder by 4.5–6.4 °C based on up to 1.5 times wetter conditions. The Early Holocene was 2.1–4.9 °C colder and up to twice as wet as today, while the Late Holocene was 2.4–3 °C colder and its precipitation amounts approached to similar conditions as today. Our paleoclimate reconstructions show a general trend of warming for the last 22 ka, and an increase of moisture until Early Holocene, and a decrease after that time. The recent glacier terminates at 3450 m on the northwest side of the mountain. It is a remnant from the last advance (possibly during the Little Ice Age). Repeated measurements of glacier length between 1902 and 2008 reveal a retreat rate of 4.2 m per year, which corresponds to a warming rate of 0.9–1.2 °C per century.  相似文献   

7.
《Quaternary Science Reviews》2007,26(1-2):142-154
We present chironomid-based temperature reconstructions from lake sediments deposited between ca 26,600 cal yr BP and 24,500 cal yr BP from Lyndon Stream, South Island, New Zealand. Summer (February mean) temperatures averaged 1 °C cooler, with a maximum inferred cooling of 3.7 °C. These estimates corroborate macrofossil and beetle-based temperature inferences from the same site and suggest climate amelioration (an interstadial) at this time. Other records from the New Zealand region also show a large degree of variability during the late Otiran glacial sequence (34,000–18,000 cal yr BP) including a phase of warming at the MIS 2/3 transition and a maximum cooling that did not occur until the global LGM (ca 20,000 cal yr BP).The very moderate cooling identified here at the MIS 2/3 transition confirms and enhances the long-standing discrepancy in New Zealand records between pollen and other proxies. Low abundances (<20%) of canopy tree pollen in records from late MIS 3 to the end of MIS 2 cannot be explained by the minor (<5 °C) cooling inferred from this and other studies unless other environmental parameters are considered. Further work is required to address this critical issue.  相似文献   

8.
《Quaternary Science Reviews》2007,26(7-8):954-957
The world's definite southernmost woolly mammoth record is a molar from Ji’nan (around 36°N), Shandong Province, China. AMS 14C dating of the specimen, gave a conventional 14C age of 33,150±250 BP. The period of 40–30 ka BP corresponds to the later phase of the Marine Oxygen Isotope Stage 3 (MIS 3a), recognized as the global interstadial of the last glacial period. However, it is known that the winter monsoon strengthened in Asia during the period 35–33 ka BP, and the age of the woolly mammoth specimen from Ji’nan corresponds to that age. The specimen suggests that this area became cold and dry in 33 ka BP, and grassland or open forest, suitable habitat for woolly mammoths, developed during this short time span. This age is similar to the age of the southernmost woolly mammoth record in Europe, therefore supporting a hypothesis by Porter and An [1995. Correlation between climate events in the North Atlantic and China during the last glaciation. Nature 375, 305–308] that an important component of Chinese palaeoclimates may be linked to changes in North Atlantic oceanic conditions.  相似文献   

9.
《Quaternary Science Reviews》2007,26(17-18):2152-2166
High-resolution modern climate data for the Southern and Central Rocky Mountains provide a starting point for analysis of climate changes necessary to produce regional Late Pleistocene glaciation. A GIS-based model was used to assess the sensitivity of glaciation in different ranges to climatic change and the combinations of temperature and precipitation change necessary to sustain the last glacial maximum (LGM) extent of glaciation in each range. Extensive glaciation initiates first in the Wind River Range of Wyoming under every climate change scenario tested. In absence of precipitation change a summer temperature depression of 6–8 °C would be necessary to maintain LGM ice extents in Colorado and Wyoming. If precipitation was halved, necessary summer temperature depression would be 8–10 °C; if precipitation was doubled, a depression of 3.5–5.5 °C would suffice. Given model uncertainties, these values may underestimate necessary temperature depression by as much as 2 °C. Under all scenarios tested, LGM glaciation in Utah, particularly the Wasatch Range, requires either more temperature depression for given precipitation change, or more precipitation for a given temperature depression than is required in the Colorado/Wyoming Rockies. A summer temperature depression of 7 °C, which would suffice to sustain LGM ice extent with little change from modern precipitation in the Colorado/Wyoming Rockies, would need to be coupled with a near doubling of precipitation to maintain LGM ice extent in the Wasatch Range. This difference appears to reflect LGM precipitation enhancement in the Wasatch Range, and to a lesser degree the Uinta Mountains, resulting from the presence of Lake Bonneville immediately upwind of these ranges.  相似文献   

10.
Shells of the helicid Cepaea nemoralis were studied using taphonomic, isotopic and morphometric measurements to estimate late glacial–Holocene (~ 12.1–6.3 cal ka BP) environmental conditions in northern Spain. Higher taphonomic alteration among Holocene shells suggests lower sedimentation rates or higher shell-destruction rates than during glacial conditions. Shells preserved the aragonitic composition despite differing degree of skeleton damage. Shell δ13C values were ? 10.3 ± 1.1‰, ? 8.2 ± 2.3‰, and ? 7.3 ± 1.6‰ for modern, Holocene and late-glacial individuals, respectively. Higher δ13C values during the late-glacial and some Holocene periods imply higher water stress of C3 plants and/or higher limestone contribution than today. Intrashell δ13C values were higher during juvenile stages suggesting higher limestone ingestion to promote shell growth. Shell δ18O values were ? 1.1 ± 0.7‰, ? 0.9 ± 0.8‰ and ? 0.1 ± 0.7‰ for modern, Holocene and late-glacial specimens, respectively. A snail flux-balance model suggests that during ~ 12.1 ? 10.9 cal ka BP conditions were drier and became wetter at ~ 8.4 ? 6.3 cal ka BP and today. Intrashell δ18O profiles reveal that glacial individuals experienced more extreme seasonality than interglacial shells, despite possible larger hibernation periods. Shell size correlated positively with δ18O values, suggesting that growth rates and ultimate adult size of C. nemoralis may respond to climate fluctuation in northern Spain.  相似文献   

11.
Late glacial and early Holocene summer temperatures were reconstructed based on fossil chironomid assemblages at Lake Brazi (Retezat Mountains) with a joint Norwegian–Swiss transfer function, providing an important addition to the late glacial quantitative climate reconstructions from Europe. The pattern of the late glacial temperature changes in Lake Brazi show both similarities and some differences from the NGRIP δ18O record and other European chironomid-based reconstructions. Our reconstruction indicates that at Lake Brazi (1740 m a.s.l.) summer air temperature increased by ~ 2.8°C at the Oldest Dryas/Bølling transition (GS-2/GI-1) and reached 8.1–8.7°C during the late glacial interstade. The onset of the Younger Dryas (GS-1) was characterized by a weak (< 1°C) decrease in chironomid-inferred temperatures. Similarly, at the GS-1/Holocene transition no major changes in summer temperature were recorded. In the early Holocene, summer temperature increased in two steps and reached ~ 12.0–13.3°C during the Preboreal. Two short-term cold events were detected during the early Holocene between 11,480–11,390 and 10,350–10,190 cal yr BP. The first cooling coincides with the Preboreal oscillation and shows a weak (0.7°C) temperature decrease, while the second is characterized by 1°C cooling. Both cold events coincide with cooling events in the Greenland ice core records and other European temperature reconstructions.  相似文献   

12.
Botanical macrofossil analysis of a more than 9000 years old, radiocarbon dated peat sequence of a moss peat bank from South Georgia, shows a clear evolution in the vegetation. Seven ecological phases could be distinguished and they can be interpreted in terms of climate development during the Holocene. Until 2200 years ago, Warnstorfia fontinaliopsis was the dominant moss species pointing to a wet environment. Lower numbers of this species in association with the presence of drier species are assumed to indicate drier periods, such as occurring between ca 6000–5200 and 4400–3400 cal yr BP. The most prominent and definitive vegetation change took place around 2200 cal yr BP. A Polytrichum–Chorisodontium moss peat bank was formed, which is still growing there today. The forcing mechanism for this vegetation change is thought to be a temperature decrease, rather than a precipitation decrease. This conclusion is mainly based on the fact that, today, moss peat banks have their optimal occurrence range in the maritime Antarctic, a region were the mean annual temperature is ca 4 °C lower than on South Georgia. The remarkable change in the moss bank vegetation at 2200 cal yr BP raises the question whether this moment was only a short climatic deterioration, or a definitive change to a cooler and wetter climate after a Holocene climatic optimum period.  相似文献   

13.
《Journal of Structural Geology》2002,24(6-7):1195-1214
Penetrative deformation occurred ca. 70 Ma ago throughout the northern Valhalla complex in Valhalla and Passmore domes and in the Gwillim Creek shear zone, exposed at the deepest structural levels in both domes. Intense strain (ST) in the Gwillim Creek shear zone (domain II) was synchronous with and outlasted deformation (D2) throughout the northern complex (domain I). Upper-amphibolite facies peak mineral assemblages define the predominant foliation. Temperature and pressure results, determined from microdomains with established relationships to reaction textures and microstructures, provide constraints on conditions under which deformation occurred. Deformation was synchronous with and outlasted peak metamorphic conditions at all structural levels. Peak conditions of 825°C and 730 MPa and 850°C and 840 MPa were determined for domains I and II, respectively. This was followed by cooling and retrograde garnet breakdown at conditions of 715°C and 490 MPa and 765°C and 730 MPa in domains I and II, respectively. The faster cooling rate per kilometer of exhumation for domain II relative to domain I is consistent with a model of conductive cooling via thrusting of domain II on to a cold footwall. Metamorphism is interpreted to have resulted from crustal thickening and burial to depths of ca. 25 km based on an inferred clockwise P–T path and the paucity of Late Cretaceous intrusions. Lack of retrograde metamorphism throughout the complex and the high degree of annealing of microstructures indicates that the rocks remained above greenschist-facies conditions until they were exhumed in the Early Tertiary on the Valkyr–Slocan Lake extensional shear zone system.Previous workers have determined that the peak of metamorphism occurred at 72–67 Ma in a restricted locality in the core of Passmore dome, near Vallican. Our study links this dated metamorphism with the structural evolution and metamorphic history throughout the area, and shows that supracrustal rocks at all structural levels in Valhalla and Passmore domes underwent the same metamorphic and deformation event as those near Vallican. Therefore, we assign a ca. 70 Ma age to the penetrative, high-temperature deformation in northern Valhalla complex and the Gwillim Creek shear zone. This coincides with a major period of shortening in the Rocky Mountains of the Foreland belt. Strain in northern Valhalla complex may represent a local transient shear zone that accommodated crustal thickening in the hinterland during orogen-scale compression, or it may be an exhumed part of the basal detachment of the Rocky Mountains.  相似文献   

14.
Western tropical Pacific sea surface temperatures and Pacific Deep Water temperatures during Marine Isotope Stage 3 have been reconstructed from the δ18O and Mg/Ca of planktonic and benthic foraminifera from Marion Dufresne core MD98-2181. This 36 m marine core was collected at 6.3°N from a water depth of 2114 m. With sediment accumulation rates of up to 80 cm/ky, it provides a decadally resolved history of ocean variability during the Last Glacial period. Surface temperatures and salinities at this site varied in close association with millennial-scale atmospheric temperature swings at high northern latitudes as reflected in the GISP2 ice core. At times of colder atmospheric temperatures over Greenland, the western Pacific was more saline and summer season SSTs were ～2 °C colder. These millennial-scale changes within the tropics are attributed to a southward displacement of the summer season ITCZ in response to steeper meridional temperature gradients within the Pacific. The benthic δ18O record from MD98-2181 documents upper Pacific Deep Water temperature and salinity variability. Benthic δ18O variations of 0.3–0.5‰ during MIS 3 indicate deep waters within the Pacific were varying by ～1–1.5 °C, with the possibility that some of the variability was due to changing salinity and minor glacial–eustatic changes. The observed deep-water variability correlates to changes in Antarctic surface temperatures and thus reflects changes in Southern Ocean temperatures at the site of Pacific Deep Water formation. The combined planktonic and benthic records from MD98-2181 thus provide a northern and southern hemispheric climate record of anti-phased variability during MIS 3 as has been inferred previously from ice core records. Furthermore, the deep sea temperature excursions appear to have led millennial variations in atmospheric CO2 as recorded in the EDML ice core by ～1 kyr.  相似文献   

15.
The recently discovered polymetallic Shazigou Mo–W–Pb–Zn ore field is located at the northern margin of the North China Craton. This integrated metallogenic system is comprised of quartz vein mineralization in three deposits: Shazigou Mo–W, Jindouzishan Pb–Zn and Mantougou Pb–Zn. The total reserves are estimated to be 50 kt Mo, 626 t WO3, 244 kt Pb and 150 kt Zn. Molybdenite Re–Os dating of five quartz vein-type ores yielded a mean model age of 243.8 ± 1.6 Ma (MSWD = 0.81) and hydrothermal zircons yielded a concordant U–Pb age of 245 ± 2.6 Ma (MSWD = 0.65). These results suggest that the mineralization was formed in the early Triassic and could be related to Paleo-Asian Ocean subduction. Microthermometry and quartz fluid inclusion compositions indicate that fluids related to the Mo–W mineralization were mainly derived from magmatic sources and precipitated under relatively high temperature (280–340 °C) and salinity conditions (6–9 wt% NaCl equiv.), whereas subsequent Pb–Zn mineralization-related fluids may have been modified by metamorphic and meteoric waters. The discovery of the Shazigou ore field suggests conditions may be favourable for more extensive mineralization in the western Xilamulun Mo metallogenic belt at the northern margin of the North China Craton.  相似文献   

16.
Even casual observations of continental hot springs reveal that photosynthesis has its limits. In an effort to explore the transition to photosynthesis, field measurements of temperature and pH were made at 996 hot spring locations at Yellowstone National Park ranging from 14° to 94 °C and pH from 0.8 to 9.7. In addition, sulfide measurements were made in 426 of these locations showing concentrations up to 8820 μg L? 1 total sulfide. These data indicate that the previously established upper temperature (73–75 °C) for the transition to photosynthesis is reached in many basic hot springs, but that the transition occurs at lower temperature with decreasing pH below ~ 6.5. As an example, no strong evidence for photosynthesis was found above 45 °C at pH ~ 2. In several locations, photosynthesis appears to be suppressed despite temperatures and pH values that permit photosynthesis elsewhere. Sulfide concentrations may be responsible for the suppression of photosynthesis at these sites. Total sulfide concentrations were observed to decrease downstream in hot spring outflow channels. Abiotic processes (degassing, oxidation, mineral precipitation, etc.) are too slow to account for these decreases, suggesting an explanation from microbial sulfide oxidation that is supported by field experiments. Microbial sulfide oxidation may determine the ultimate suitability of some hot springs for microbial photosynthesis.  相似文献   

17.
Black carbon (BC) is an important residue of wildfires in boreal forests, but its characteristics depend on its formation conditions. The objective of this study was to characterize the chemical and physical properties of BC produced under controlled laboratory conditions, while mimicking a gradient of wildfire severity. We used fuels originating from mosses, ericaceous shrubs and spruce trees, as they constitute the major types available in boreal forests. We varied the maximum temperature (MT) from 75 to 800 °C, the duration of charring from 0.5 to 24 h and the abundance of O2. BC properties were analyzed using elemental analysis and proximate analysis, solid state 13C nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM) and surface area (SA) analysis. MT was the most significant factor affecting both chemical and physical changes. Results from 13C NMR spectroscopy indicated that 350 °C was a threshold temperature, above which spectra became dominated by aromatic structures for all fuels. Charring duration affected BC composition at both low (250 °C for 12 h vs. 24 h) and high (600 °C for 0.5 h vs. 6 h) MT. The presence of O2 influenced BC composition mainly at low MT (250 °C) and resulted in accelerated alkyl C degradation, accompanied with a distinct decrease in yield. Results from the SA analysis and the SEM micrographs showed that fuel type influenced BC physical properties, with moss-derived BC presenting higher surface area and microporosity than BC of woody origin.  相似文献   

18.
The environmental conditions of the Szczecin Bay, which existed prior to Szczecin Lagoon, have been reconstructed on the basis of the stable carbon and oxygen isotope (18O and 13C) analysis and radiocarbon dates obtained for subfossil shells of Cerastoderma (Cardium) glaucum. The shells in the collected core were well preserved in their life positions, representing a geochemical record of past temperature variation over the middle Holocene. Three major periods with different thermal conditions have been distinguished in the interval ~ 6000–4300 cal yr BP, when the important Littorina regional transgression took place. During the first period, 6000–5250 cal yr BP, water temperature decreased by 1.4°C, and then remained constant over the second period (5250–4750 cal yr BP). In contrast, during the third period (4750–4300 cal yr BP) both δ-values were highly variable and the mean summer temperature (March–November) increased by about 3.5°C. During first two periods, δ18O and δ13C were significantly correlated, indicating stability of the environmental conditions.  相似文献   

19.
The Kozbudaklar scheelite skarn deposit in the Tavşanlı Zone, located approximately 22 km southeast of Bursa, is hosted by the Triassic calcic İnönü Marble and Eocene Topuk Pluton. At least four stages have been recognized through skarn evolution. Scheelite skarn distributed close to the Topuk Pluton occurred during the early (stage 1) and late (stage 2) prograde substages. The early prograde endo and exoskarn are composed of hedenbergite (Hd96Joh4)–plagioclase (An55–64) and hedenbergite (Hd61–94Joh4–7), accompanied by calcic garnet (Grs38–94Sps1–5Alm0) and scheelite (Pow1–6). The second stage represents a relatively oxidized mineralogy dominated by diopside (Hd16–48Joh0–9), subcalcic garnet (Grs24–92Sps0–11Alm0–31) and scheelite (Pow7–32). The stage 3 and 4 mineral assemblages are characterized by few hydrous minerals in the retrograde stage and intense fracturing.Fluid inclusions from skarn rocks are indicative of multiple fluid events: (1) low-moderate salinity (5–16 wt.%NaCl equiv.) inclusions homogenized dominantly by a high-temperature (308 °C to > 600 °C) liquid phase in stage 1. Fluid inclusions in an early garnet homogenized over a similar temperature range (440 °C and 459 °C) into both liquid and vapor phases. Eutectic temperatures ranging from − 61.7 °C to − 35.0 °C that indicate the presence of H2O–NaCl–(± MgCl2 ± CaCl2)–CO2 solutions; (2) coexisting daughter mineral-bearing high salinity (29.5 ≥ 70 wt.%NaCl equiv.) and vapor-rich moderate salinity (11.5–16.7 wt.%NaCl equiv.) inclusions that homogenized in the liquid phase by the disappearance of the vapor phase at a similar temperature range (308 °C to > 600 °C) in stage 2. Eutectic temperatures range from − 67.9°C to − 51.8°C that shows the presence of H2O–NaCl–CO2–(± CH4/N2) solutions; (3) low-moderate salinity (12.5–7.6 wt.%NaCl equiv.) and temperature (320 °C to 215 °C) inclusions homogenized by the liquid phase in stage 3. Eutectic temperatures range from − 59.5 °C to − 44.2 °C indicating the presence of H2O–NaCl–(± MgCl2 ± CaCl2)–CO2 solutions; (4) inclusions of low salinity (9.9–0.9 wt.%NaCl equiv.) and homogenization temperature (183 °C to 101 °C) in stage 4.These data show that the Kozbudaklar skarn deposit was formed in a magmatic–hydrothermal system. In this model, carbonaceous fluids may have been exsolved from the plutonic rock during its emplacement and crystallization. Fluid inclusion data indicate that fluid boiling and immiscibility occurred at temperatures between 440 °C and 459 °C and pressures ranging from 50 MPa to 60 MPa based on hydrostatic considerations. Early scheelite was precipitated with relatively reduced mineral compositions. As a result of depressurization, Mo-rich scheelite with oxidized minerals formed via high salinity and vapor-rich inclusions. The second scheelite mineralization occurred in a normal hydrothermal system by an infiltration mechanism at pressures between approximately 40 and 1.5 MPa. At shallow depths (< 1.5 MPa) with increasing permeability, sulfide and oxide minerals were deposited in the retrograde stage, greatly assisted by meteoric water. Finally, as a result of the diminishing of ore-forming fluids, post-depositional barren quartz and calcite veins were formed.  相似文献   

20.
《Journal of Structural Geology》2001,23(6-7):1007-1013
The phenomenon of shear-heating is generally difficult to recognise from petrologic evidence alone. Establishing that shear zones attain higher temperatures than the surrounding country rocks requires independent evidence for temperature gradients. In the Musgrave Block, central Australia, there is a clear spatial association between shear zones and interpreted elevated temperatures. Eclogite facies shear zones that formed at ∼550 Ma record temperatures of ∼650–700°C. Outside the high-pressure shear zones, minerals with low closure temperatures such as biotite (∼450°C in the 40Ar–39Ar and Rb–Sr systems), preserve ages >800 Ma, suggesting that these rocks did not experience temperatures greater than about 450°C at ∼550 Ma for any extended period. Thus, the shear zones record temperatures that are ∼200°C higher than the surrounding country rocks. Simple calculations show that the combination of relatively high shear stresses (∼100 MPa) and high strain rates (∼10−11 s−1) for short durations (<1 Ma) can account for the observed apparent temperature variations. The evidence indicates that shear heating is the dominant mechanism for localised temperature increases in the shear zones, while the country rock remained at relatively lower temperatures.  相似文献   

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