首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The fossils of Chaoxian hominin, widely accepted as representing archaic Homo sapiens in eastern China, were recovered from the middle or slightly higher levels of Layer 2 deposits of a collapsed cave at Yinshan, Anhui Province. Results of mass spectrometric U-series dating of intercalated speleothem calcites are presented. Based mainly on four broadly coeval calcite samples, the hominin fossils should be bracketed in the range of 310–360 ka or somewhat older. These ages are much older than the previous estimate at 160–200 ka based on the U-series dating of fossil teeth and bones, and may be cited as supporting evidence for an earlier H. erectus–archaic H. sapiens interface in China.  相似文献   

2.
We assessed the potential of drosophilids as indicators of the response of tropical ecosystems to climatic factors over an annual cycle in the montane forest of Mt Oku, by collecting samples at seven sites evenly spaced from 2200 m to 2800 m asl. Only 0.1% of the 11,000 specimens collected belonged to invading species or those commensal with humans, showing the weakness of anthropogenic factors. Species abundance was highly skewed. One species, Zaprionus vittiger, made up 81% of the sample, whereas 42 of the 62 morphological species found were represented by fewer than 20 individuals. Many of the most abundant species occurred over a narrow period, in the dry or in the rainy season, others also occurred at intervening periods. These different patterns of population dynamics, determined a succession of species over the annual cycle. Abundant species departed significantly from each other in their distribution over collection sites. The drosophilid populations from the central African montane forests are highly dependent on climatic factors, either directly, or indirectly through climatic effects on the biotic environment of the insects.  相似文献   

3.
This study focuses on the morphometry and taxonomy of the Late Cretaceous coccolith genus Arkhangelskiella. Sixty samples from the Campanian–Maastrichtian interval of DSDP Hole 390A (Blake Nose) were investigated for their contents of Arkhangelskiella spp. In each sample one hundred specimens of Arkhangelskiella spp. were examined by measuring the coccolith length and width, as well as the length and width of the central area. In the samples investigated the Arkhangelskiella group exhibits a large size variation, specimens length varies from 4.95 μm to 14.52 μm. Former taxonomic concepts, based on morphometry, subdivided the Arkhangelskiella group into three species: Arkhangelskiella maastrichtiana, Arkhangelskiella confusa and Arkhangelskiella cymbiformis. Our data show a large variability of the morphometric data (coccolith length, width of the outer rim). There is no indication for three independant species; two of the quoted taxa (1. thick outer rim = Arkhangelskiella maastrichtiana; 2. very thin outer rim = Arkkhangelskiella cymbiformis) seem to be extreme forms of a continuous morphometric lineage. The lower part of the investigated succession (139.92–133.42 mbsf) is dominated by small specimens with an average length of 6.8 μm whereas the upper part (132.86–126.15 mbsf) is characterized by larger specimens (mean 8.7 μm). In DSDP Hole 390A the size increase appears to be very abrupt, within two samples (samples 133.42 mbsf, 132.86 mbsf) the mean size increases by 1.51 μm. Previous morphometric studies of Arkhangelskiella indicate a more continuous size increase throughout the late Campanian–Maastrichtian. The abrupt size increase observed here hints toward a minor hiatus in DSDP Hole 390A separating upper Campanian from lower Maastrichtian sediments. It seems likely that the size increase of Arkhangelskiella reflects changes of various environmental factors like nutrient supply and sea water chemistry (Mg/Ca ratio; Ca concentration). A comparison of morphometric results with previous palaeoecological studies documents a nutrient control for the growth of Arkhangelskiella. Small specimens can be related to more mesotrophic conditions whereas large specimens are linked to oligotrophic surface waters.  相似文献   

4.
146Sm–142Nd and 147Sm–143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe 207Pb/206Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ?142Nd = ? 6 ± 12 ppm, ?143Nd = ? 32.5, and 147Sm/144Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142Nd excess in South African cratonic mantle. The lack of a 142Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ?142Nd = 8 ± 10 ppm, ?143Nd = 45 ± 1, and 147Sm/144Nd = 0.5891. Back-calculation of this present-day ?143Nd yields an unrealistic estimate for the initial ?143Nd of ? 160 ?-units, clearly indicating post-crystallization disturbance of the 147Sm–143Nd system. Examination of 146,147Sm–142,143Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of 146Sm–142Nd systematics in Jack Hills zircons.  相似文献   

5.
《Cretaceous Research》2008,29(1):115-130
The diminutive (2.5–3.0 cm long), Cretaceous dinosaur track ichnogenus Minisauripus, previously known only from the type ichnospecies, M. chuanzhuensis, from a single locality in Sichuan Province China, is here reported from two new localities in South Korea and one in China. Material from the new Chinese locality is assigned to the new ichnospecies M. zhenshuonani on the basis of its distinctive morphology. Most of the new material is well-preserved, revealing narrow asymmetric tracks with claw traces, long step and phalangeal formula (2-3-4 for digits II, III and IV, respectively), suggesting a theropod track maker rather than an ornithischian, as originally inferred for the Chinese type material.The South Korean samples (eight tracks), from two localities in the Haman Formation, are considered Early Cretaceous (Aptian-Albian) in age, whereas the Chinese type material (21 specimens) has been assigned both an Early and Late Cretaceous age. The former age is probably correct as suggested by a new Minisauripus locality (5 specimens) from the Early Cretaceous (Barremian-Albian) of Shandong Province, China.Other diminutive tracks from the Sichuan fauna include Aquatilavipes sinensis (2.5 cm long, a possible junior synonym of Koreanoris hamanensis), Grallator emeiensis (2 cm long) and Velociraptorichnus sichuanensis (10–11 cm), which occur, in various combinations, with Minisauripus at both the new Korean and Chinese localities.In Minisauripus, digit III is very short in comparison with other theropods and provides a striking contrast to G. emeinsis. This difference has significant implications for standard assumptions about theropod track allometry. Based on the classic Early Jurassic forms Grallator and Eubrontes, it has long been inferred that relative digit III length shrank with increasing size (up to foot lengths of 30–40 cm). The reiteration of reduction in relative length of digit III in specimens in the size range of 2–3 cm indicates that the allometric or morphodynamic ‘program’ that influenced development in large theropod clades reiterated fractally in theropod clades a full order of magnitude smaller. This shows that a given allometry can be size-dependent in one clade and size-independent in another. Thus, the developmental program appears ‘contracted’ or morphologically miniaturized by heterochrony to manifest paedomorphically in some clades and peramorphically in others. This strongly suggests that ‘formal’ developmental ‘programs operated’ along similar morphodynamic lines in quite different clades.  相似文献   

6.
Three models were examined to predict C aromaticity (fa) of biochars based on either their elemental composition (C, H, N and O) or fixed C (FC) content. Values of fa from solid state 13C nuclear magnetic resonance (NMR) analysis with Bloch-decay (BD) or direct polarisation (DP) techniques, concentrations of total C, H, N, and organic O, and contents of FC of 60 biochars were either compiled from the literature (dataset 1, n = 52) or generated in this study (dataset 2, n = 8). Models were first calibrated with dataset 1 and then validated with dataset 2. All models were able to fit dataset 1 when atomic H to C ratio (H/C) < 1 (except two ash rich biochars) and to estimate fa of HF treated biochars (H/C < 1). Model 1, which was based on values of H/C only and calibrated with a root mean square of error (RMSE) of 0.04 fa-unit (n = 41), could predict the experimental data with a RMSE = 0.02 fa-unit (n = 6). Model 2, which was based on biochar elemental composition data, showed the most accurate prediction, with a RMSE of 0.03 fa-unit (n = 41) for the calibration data, and of 0.02 fa-unit (n = 6, H/C < 1) for the validation data. Model 3, which was based on contents of FC and C, and modified with a correction factor of 0.96, displayed the highest RMSE (0.06 fa-unit, n = 19) among the three models. Models 1 and 2 did not work properly for samples having either an H/C ratio > 1, high concentrations of carbonate or high inorganic H. These models need to be further tested with a wider range of biochars before they can be recommended for classification of biochar stability.  相似文献   

7.
A ～6.35 m core (06SD) was retrieved from Lake Shudu, Yunnan Province, China. The sediments spanning the period ～22.6–10.5 kcal. yr BP (6.35–1.44 m) were analysed using a combination of variables including pollen, charcoal, particle size, magnetic susceptibility and loss-on-ignition. The resulting palaeorecord provides a high-resolution reconstruction of Late Pleistocene to Early Holocene climatic and environmental changes in southwestern China. Our findings indicate that from c. 22.6 to 17.7 kcal. yr BP, vegetation assemblages were primarily aligned to sparse xerophytic grassland/tundra or cold-tolerant boreal Pinus forest, indicating that climatic conditions in southwestern China were cold and dry. However, from c. 17.7 to 17.4 kcal. yr BP, the Lake Shudu record is punctuated by marked environmental changes. These include the establishment of denser vegetation cover, a marked expansion of boreal Pinus forest and enhanced hydrological activity in the catchment over centennial timescales, perhaps suggesting that stepwise variations in the Asian Monsoon were triggering fundamental environmental changes over sub-millennial timescales. Thereafter, the pollen record captures a period of environmental instability reflected in fluctuations across all of the variables, which persists until c. 17.1 kcal. yr BP. After c. 17.1 kcal. yr BP, the expansion of steppe vegetation cover and cold–cool mixed forest consisting of mesophilous vegetation such as Tsuga and Picea, thermophilous trees including Ulmus and deciduous Quercus inferred from the Lake Shudu pollen record point to the establishment of warmer, wetter and perhaps more seasonal conditions associated with a strengthening Asian Summer Monsoon during the shift from Pleistocene to Holocene climatic conditions.  相似文献   

8.
We present the results of a palaeogenetic analysis of two Late Pleistocene camelids originating in southern Chile. Our analysis of two mitochondrial DNA fragments (control region and cytochrome b gene) reveals that these specimens do not belong to an extinct taxon, but rather to extant vicuña (Vicugna vicugna), whose modern distribution is restricted to the extreme elevations of the Andes, more than 3500 km to the north of where these specimens originated. Our results also suggest fossil specimens from Patagonia that are currently assigned to the extinct taxon Lama gracilis, may actually belong to V. vicugna, implying a continuous distribution of the latter from the southern tip of South America to the Andes during the Final Pleistocene. The haplotypes of both specimens are not present in modern populations, suggesting a loss of genetic diversity concomitant with the contraction of the vicuña geographical distribution during the Final Pleistocene or early Holocene.  相似文献   

9.
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   

10.
The role of ocean feedback on monsoon variations at 6 and 9.5 kyr Before Present (BP) compared to present-day is investigated by using sets of simulations computed with the IPSL–CM4 ocean–atmosphere coupled model and simulations with the atmospheric model only with the SST prescribed to the present-day simulation for the coupled model. This work is complementary to the study by Marzin and Braconnot (2009) who have analyzed in detail the response of Indian and African monsoons to changes in insolation at 6 and 9.5 kyr BP using the IPSL–CM4 coupled model. The monsoon rainfall was intensified at 6 and 9.5 kyr BP compared to 0 kyr BP as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. In this paper, the impact of the ocean feedback is analysed for the Indian, East-Asian and African monsoons. The response of the ocean to the 6 and 9.5 kyr BP insolation forcing shares similarities between the two periods, but we highlight local differences and a delay in the response of the surface ocean between 6 and 9.5 kyr BP. The ocean feedback is shown to be positive for the early stage of the African monsoon. A dipole of SST in the tropical Atlantic favouring the earlier build-up of the monsoon in the 6 and 9.5 kyr BP coupled simulations. However, it is strongly negative for the Indian and East Asian monsoons, and of stronger amplitude at 9.5 than at 6 kyr BP over India. In these Asian regions, the convection is more active over the ocean than over the continent during the late monsoon season due to the ocean feedback. The results are consistent with previous studies about 6 kyr BP climate. In addition, it is shown that the ocean feedback is not sufficient to explain the relative amplifications of the different monsoon systems within the three periods of the Holocene, but that the mechanisms such as the effect of the precession on the seasonal cycle of monsoons as discussed in Marzin and Braconnot (2009) are more plausible.  相似文献   

11.
The normal and shear strains obtained in torsion shear tests may be interpreted in two different ways to gain insight into (1) the type of plastic potential to be employed in hardening plasticity stress–strain models, and (2) the coincidence in physical space of the plastic strain increment direction with the stress direction during principal stress rotation. Thirty-four drained torsion shear tests were performed on hollow cylinder specimens of Santa Monica Beach sand deposited by dry pluviation. Twenty-six tests were performed on tall specimens with height of 40 cm, and eight tests were performed on short specimens with height of 25 cm to investigate the effect of the specimen height on the soil behavior in hollow cylinder specimens. Each test was conducted with the same, constant inside and outside confining pressure, σr, thus tying the value of b = (σ2–σ3)/(σ1– σ3) to the inclination, β, of the major principal stress. The directions of strain increment vectors at failure are compared with the directions corresponding to associated and non-associated flow. The relation between the directions of major principal strain increment and major principal stress during rotation of principal stress axes in physical space are investigated.  相似文献   

12.
A paleomagnetic study has been conducted on a formation dated as Autunian in the Nekheila area (31.4°N, 1.5°W) in the Mezarif basin. ChRM was thermally isolated in 117 samples from seven sites. This ChRM (D = 131.8°, I = 15.7°, k = 196, α95 = 3.8° after dip correction; corresponding pole 29.3°S, 56.4°E) is very similar to that obtained in the neighboring Abadla basin from a formation of the same age. Fold tests associated with progressive unfolding applied to the full merged data from the dated formations of these two basins clearly indicate that the magnetization acquisition predates the deformation, which is attributed to the last phase of the late-Hercynian. The magnetization in these basins is therefore primary or acquired just after deposition. For the African Apparent Polar Wander Path, the age of the paleomagnetic poles of the Autunian part is now confirmed by paleomagnetic test.  相似文献   

13.
Jacobsoniidae is a small but perplexing beetle family, with unknown phylogenetic relationships to other polyphagan Coleoptera. To date, only a single fossil jacobsoniid has been described, from Eocene Baltic amber (~ 40 Ma). Here, we push back the oldest definitive record of Jacobsoniidae by approximately 60 million years with a new fossil species recovered from mid-Cretaceous (~ 99 Ma) Burmese amber from Myanmar. Remarkably, exploration of the fossil's morphology with confocal laser scanning microscopy revealed that it belongs to an extant genus, Derolathrus. The similarity of the new taxon, Derolathrus abyssus n. sp., to modern congeners provides a striking example of morphological stability over deep evolutionary time—a possible outcome of long-term persistence of mesic microhabitats, a hypothesis we argue is supported by a variety of other Recent, litter-inhabiting arthropod taxa now known to be largely unchanged since the Mesozoic. Many such examples belong to the Staphylinoidea—a hyperdiverse beetle superfamily that dominates contemporary mesic habitats, and with which Jacobsoniidae may have a close phylogenetic relationship.This published work has been registered in ZooBank, http://zoobank.org/urn:lsid:zoobank.org:pub:7C65ACAF-456E-4301-BDD7-0A801768EEB9.  相似文献   

14.
The Linzizong Group (64–44 Ma) of the Lhasa Terrane in Tibet is critically positioned for establishing the paleoposition of the southern leading edge of the Asian continent during Paleogene times and constraining onset of the India–Asia collision. Here we report paleomagnetic results from a collection comprising 384 drill-core samples from 34 sites embracing all three formations of this group. Comprehensive demagnetization and field tests isolate characteristic remanent magnetizations (ChRM) summarized by overall tilt-corrected formation-mean directions of D = 183.6°, I = −12.4° (α95 = 8.1°) for the Dianzhong (64–60 Ma), D = 1.0°, I = 18.1° (α95 = 8.1°) for the Nianbo (60–50 Ma), and D = 12.4°, I = 23.2° (α95 = 7.3°) for the Pana (50–44 Ma). Fold tests are positive in each formation suggesting a pre-folding origin and we interpret the magnetizations as quasi-primary and acquired at, or slightly later than, formation of the Linzizong Group. Revised Paleogene paleopoles with Ar–Ar age constraints for the Lhasa Terrane indicate that onset of the India–Asia collision occurred no later than ∼60.5 ± 1.5 Ma at a low paleolatitude of ∼10°N. Analysis of 60 site-mean observations from a range of studies of the Pana Formation in the higher part of the succession highlight a large dispersion of ChRM directions; a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue.  相似文献   

15.
The polymetallic Mykonos vein system in the Cyclades, Greece, consists of 15 tension-gashes filled with barite, quartz, pyrite, sphalerite, chalcopyrite and galena in ca. 13.5 Ma, I-type, Mykonos monzogranite. Zones of silica and chlorite–muscovite alteration are associated with the veins and overprint pervasive silicification, phyllic and argillic alteration that affected large parts of the monzogranite. The mineralization cements breccias and consists of an early barite–silica–pyrite–sphalerite–chalcopyrite assemblage followed by later argentiferous galena. A combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17 wt.% NaCl equivalent, at temperatures of ~ 225° to 370 °C, under a hydrostatic pressure of ≤ 100 bars. The mineralizing fluids boiled and were saturated in H2S and SO2.Calculated δ18OH2O and δDH2O, initial 87Sr/86Sr isotope compositions and the trace and REEs elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the Mykonos intrusion and subsequently mixed with Miocene seawater. Heterogeneities in the calculated δ34SSO4− 2 and δ34SH2S compositions of the ore fluids indicate two distinct sources for sulfur, namely of magmatic and seawater origin, and precipitation due to reduction of the SO4− 2 during fluid mixing. The physicochemical conditions of the fluids were pH = 5.0 to 6.2, logfS2 = − 13.8 to − 12.5, logfO2 = − 31.9 to − 30.9, logfH2S(g) = − 1.9 to − 1.7, logfTe2 = − 7.9 and logα(SO4− 2(aq)/H2S(aq)) = + 2.6 to + 5.5. We propose that retrograde mesothermal hydrothermal alteration of the Mykonos monzogranite released barium and silica from the alkali feldspars. Barite was precipitated due to mixing of SO4− 2-rich Miocene seawater with the ascending Ba-rich magmatic fluid venting upwards in the pluton.  相似文献   

16.
Devonian magmatism was very intensive in the tectonic evolutionary history of the Chinese Altai, a key part of the Central Asian Orogenic Belt (CAOB). The Devonian Keketuohai mafic–ultramafic complex in the Chinese Altai is a zoned intrusion consisting of dunite, olivine gabbro, hornblende gabbro and pyroxene diorite. The pyroxene diorite gives a zircon U–Pb age of 409 ± 5 Ma. Variations in mineral assemblage and chemical composition suggest that the petrogenesis of the Keketuohai Complex was chiefly governed by fractional crystallization from a common magma chamber. Low SiO2, K2O and Na2O contents, negative covariations between P2O5, TiO2 and Mg# value suggest insignificant crustal assimilation/contamination. Thus the positive εNd(t) values (0 to + 2.7) and slight enrichments in light rare earth elements (e.g., La/YbN = 0.98–3.64) suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle. Model calculation suggests that their parental magma was high-Mg (Mg# = 66) tholeiitic basaltic melt. The Keketuohai intrusion was coeval with diverse magmatism, high temperature metamorphism and hydrothermal mineralization, which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the Chinese Altai.  相似文献   

17.
A series of methane (CH4) adsorption experiments on bulk organic rich shales and their isolated kerogens were conducted at 35 °C, 50 °C and 65 °C and CH4 pressure of up to 15 MPa under dry conditions. Samples from the Eocene Green River Formation, Devonian–Mississippian Woodford Shale and Upper Cretaceous Cameo coal were studied to examine how differences in organic matter type affect natural gas adsorption. Vitrinite reflectance values of these samples ranged from 0.56–0.58 %Ro. In addition, thermal maturity effects were determined on three Mississippian Barnett Shale samples with measured vitrinite reflectance values of 0.58, 0.81 and 2.01 %Ro.For all bulk and isolated kerogen samples, the total amount of methane adsorbed was directly proportional to the total organic carbon (TOC) content of the sample and the average maximum amount of gas sorption was 1.36 mmol of methane per gram of TOC. These results indicate that sorption on organic matter plays a critical role in shale-gas storage. Under the experimental conditions, differences in thermal maturity showed no significant effect on the total amount of gas sorbed. Experimental sorption isotherms could be fitted with good accuracy by the Langmuir function by adjusting the Langmuir pressure (PL) and maximum sorption capacity (Γmax). The lowest maturity sample (%Ro = 0.56) displayed a Langmuir pressure (PL) of 5.15 MPa, significantly larger than the 2.33 MPa observed for the highest maturity (%Ro > 2.01) sample at 50 °C.The value of the Langmuir pressure (PL) changes with kerogen type in the following sequence: type I > type II > type III. The thermodynamic parameters of CH4 adsorption on organic rich shales were determined based on the experimental CH4 isotherms. For the adsorption of CH4 on organic rich shales and their isolated kerogen, the heat of adsorption (q) and the standard entropy (Δs0) range from 7.3–28.0 kJ/mol and from −36.2 to −92.2 J/mol/K, respectively.  相似文献   

18.
《Quaternary Science Reviews》2007,26(19-21):2586-2597
Recent paleoclimatic work on terrestrial and marine deposits from Asia and the Indian Ocean has indicated abrupt changes in the strength of the Asian monsoon during the last deglaciation. Comparison of marine paleoclimate records that track salinity changes from Asian rivers can help evaluate the coherence of the Indian Ocean monsoon (IOM) with the larger Asian monsoon. Here we present paired Mg/Ca and δ18O data on the planktic foraminifer Globigerinoides ruber (white) from Andaman Sea core RC12-344 that provide records of sea-surface temperature (SST) and δ18O of seawater (δ18Osw) over the past 25,000 years (ka) before present (BP). Age control is based on nine accelerator mass spectrometry (AMS) dates on mixed planktic foraminifera. Mg/Ca-SST data indicate that SST was ∼3 °C cooler during the last glacial maximum (LGM) than the late Holocene. Andaman Sea δ18Osw exhibited higher than present values during the Lateglacial interval ca 19–15 ka BP and briefly during the Younger Dryas ca 12 ka BP. Lower than present δ18Osw values during the BØlling/AllerØd ca 14.5–12.6 ka BP and during the early Holocene ca 10.8–5.5 ka BP are interpreted to indicate lower salinity, reflect some combination of decreased evaporation–precipitation (E–P) over the Andaman Sea and increased Irrawaddy River outflow. Our results are consistent with the suggestion that IOM intensity was stronger than present during the BØlling/AllerØd and early Holocene, and weaker during the late glaciation, Younger Dryas, and the late Holocene. These findings support the hypothesis that rapid climate change during the last deglaciation and Holocene included substantial hydrologic changes in the IOM system that were coherent with the larger Asian monsoon.  相似文献   

19.
Over 300 samples for paleomagnetic analysis and K–Ar dating were collected from 27 sites at NW–SE and NE–SW trending dike swarms (herein, NW dikes and NE dikes, respectively) in the Koshikijima Islands, northern Ryukyu Arc. The NW dikes are Middle Miocene in age and have directions (D = ? 37.7°, I = 51.8°, α95 = 9.6°, and κ = 40.8) that are deflected westward relative to the stable eastern Asian continent. Conversely, the NE dikes, of Late Miocene age, have directions (D = 16.1°, I = 57.7°, α95 = 7.1°, and κ = 41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40° of counter-clockwise rotation during the Middle to Late Miocene. A synthesis of the paleomagnetic and structural data suggests a three-stage history of extensional deformation: (1) displacement upon normal faults (F1 faults) without vertical-axis block rotation, (2) strike-slip reactivation of F1 faults and oblique-normal displacement on NE–SW-trending faults (F2 faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F2 faults without vertical-axis block rotation. Regional differences in the timing and amount of counter-clockwise vertical-axis block rotations indicate that the northern Ryukyu Arc rotated as several distinct rigid blocks.  相似文献   

20.
The latest hydraulic fracturing and stress relief measurement data in the Chinese mainland were collected. The total of 3856 data entries are measured at 1474 locations. The measured area covers 75–130°E and 18–47°N, and the depth range varies from surface to 4000 meters depth, which generally includes each active tectonic block of China and each segment of North–South seismic belt. We investigated the tectonic stress field by removing the effect of gravity. For this, we assume lateral constraints and Heim’s rule. The gravity contribution is removed by using the assumption of lateral constraint and Heim’s rule. Our results show: (1) the maximum and the minimum horizontal principal stress σH, σh and the vertical stress σV in the shallow crust of China all increase linearly with depth: σH = 0.0229D + 4.738, σh = 0.0171D + 1.829, σV = 0.0272D. Maximum and minimum horizontal tectonic stress varies as a function of depth D linearly 4.738 < σT < 0.0139D + 4.738 and 1.829 < σt < 0.0162D + 1.829. The horizontal tectonic differential stress is σT − σt = 0.0058D + 2.912. (2) The intermediate value of σT1 (regression value of tectonic stress inferred from the assumption of lateral constraint at 2000 m depth) changes in different areas, the maximum value of which is 45.6 MPa, while the minimum value of which is 26.8 MPa. Horizontal tectonic differential stress σT − σt increases linearly with depth and the maximum and minimum of σT − σt is 25.3 MPa and 13.0 MPa, respectively. In general, the stress magnitude is much higher in western than in eastern China. This indicates that the strong Indo-Eurasian collision dominates the present tectonic stress field in Chinese mainland. (3) Compared with other study regions, the northward crustal compression to the Qinghai-Tibet block is relatively lower in magnitude in the shallow subsurface and higher at deeper depth. (4) The orientations of σT in China mainland generally form a radial scattering pattern centered in Tibetan Plateau. From western to eastern China, they rotate gradually clockwise from NS to NNE, NE, NEE, and SE, which is consistent with the result of focal mechanism solutions.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号