罗布泊不是游移湖--从罗布泊的演化讨论罗布泊的游移问题

从构造、沉积学的观点出发,讨论了罗布泊的发展演化,明确提出罗布泊不是游移湖.罗布泊从中更新世以来,从淡水湖演化到盐湖直至1972年干涸,其沉积中心基本是固定的.在新构造运动、气候和水源补给等的共同影响下,尽管其水域大小有一定程度的变化,其沉积中心也有过一定距离的迁移,但从总体来说并没有变化,因此,它不是“游移”湖.     

Some new progress in scientific research on the Lop Nur Lake region,Xinjiang, China

The Lop Nur Lake, a famous lake in the arid areas of China, is located in the communications center of the ancient Silk Road which facilitated the flow of culture between China and Western countries and enjoyed the great reputation in the ancient history of China. Since the recent 100 years, some Chinese and foreign scholars have explored and investigated here, they, however, could not get to the hinterland of the lake basin due to the hard and dangerous roads. Therefore, there are many divergences and controversies about the Lop Nur Lake region[1,2]. In order to clarify these issues, know the usable natural resources and sum up the historical experiences and lessons of dry-up of the Lop Nur Lake, a scientific investigation team was organized by Xinjiang Branch of the Chinese Academy of Sciences to carry out the investigation and research about the Lop Nur Lake region in the 1980s. Recently, some personnel from CCTV and other institutions went to the lower reaches of the Tarim River and the Lop Nur Lake region to undertake the research for coordinating the strategic decision of the Great Development of Western China and the synthetic harnessing of the Tarim River, and a series of achievements in scientific research have been achieved.

     

Some new progress in scientific research on the Lop Nur Lake region, Xinjiang, China

The Lop Nur Lake, a famous lake in the arid areas of China, is located in the communi-cations center of the ancient Silk Road which facilitated the flow of culture between China andWestern countries and enjoyed the great reputation in the ancient history of China. Since the re-cent 100 years, some Chinese and foreign scholars have explored and investigated here, they,however, could not get to the hinterland of the lake basin due to the hard and dangerous roads.Therefore, there are many divergences and controversies about the Lop Nur Lake region[1'2]. Inorder to clarify these issues, know the usable natural resources and sum up the historical experi-ences and lessons of dry-up of the Lop Nur Lake, a scientific investigation team was organized byXinjiang Branch of the Chinese Academy of Sciences to carry out the investigation and researchabout the Lop Nur Lake region in the 1980s. Recently, some personnel from CCTV and other in-stitutions went to the lower reaches of the Tarim River and the Lop Nur Lake region to undertakethe research for coordinating the strategic decision of the Great Development of Western Chinaand the synthetic harnessing of the Tarim River, and a series of achievements in scientific researchhave been achieved.     

Possible resetting of quartz OSL signals during earthquakes—Evidence from late Pleistocene injection dikes, Dead Sea basin, Israel

Clastic dikes are formed either by passive deposition of clastic material into pre-existing fissures (depositional dikes), or by fracturing and injection of clastics during earthquakes (injection dikes). We proposed to use optically stimulated luminescence (OSL) dating to distinguish between the two modes of formation and hypothesized that (1) depositional dikes filled from above show OSL ages younger than the host rock; and (2) injection dikes filled from below show the same OSL ages as that of the host rock. We studied the mechanisms of clastic-dike formation and their ages within the seismically active Dead Sea basin, where hundreds of dikes crosscut the late Pleistocene (70–15 ka) lacustrine sediments of the Lisan Formation. Field observations and analysis of magnetic tensors show unequivocally that most of these dikes were emplaced by injection, inferred to be due to seismically triggered fluidization–liquefaction during earthquakes. Twenty-eight samples were collected from the Lisan source material and dikes that, based on field observations, are unmistakably either depositional dikes or injection dikes.

Quartz single aliquot OSL ages of the source Lisan layers are between 43 and 34 ka, and are typical for the Lisan Formation. The ages of both depositional and injection dikes are between 15 and 17 ka, younger than the Lisan host rock. Depositional dikes show a highly scattered distribution of single grain ages, suggesting several episodes of infill. Single grain ages of injection dikes are of latest Pleistocene to Holocene, and do not contain recently bleached grains that infiltrated from above. These results imply that the OSL signals were reset at the time of fluidization–liquefaction and buildup of fluid pressure within the injection dikes. If this resetting mechanism has a physical ground, then OSL dating is an important tool for constraining the ages of earthquake-induced injection dikes and recovering paleoseismic data from them.     

Stratification and mixing of a post-glacial Neoproterozoic ocean: Evidence from carbon and sulfur isotopes in a cap dolostone from northwest China

To improve our knowledge about the geochemical and environmental aftermath of Neoproterozoic global glaciations, we analyzed stable isotopes (δ¹³C, δ¹⁸O, δ³⁴S) and elemental concentrations (Ca, Mg, S, Sr, Fe, and Mn) of the ～ 10-m-thick Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Available chemostratigraphic data suggest that the Tereeken diamictite is probably equivalent to the Marinoan glaciation. Our new data indicate that organic and carbonate carbon isotopes of the Zhamoketi cap dolostone show little stratigraphic variations, averaging ? 28.2‰ and ? 4.6‰, respectively. In contrast, sulfur isotopes show significant stratigraphic variations. Carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and δ³⁴S_CAS composition varies between + 9‰ and + 15‰ in the lower 2.5 m. In the overlying interval, CAS abundance remains low while δ³⁴S_CAS rises ～ 5‰ and varies more widely between + 10‰ and + 21‰. The range of δ³⁴S_py of the cap dolostone overlaps with that of δ³⁴S_CAS, but direct comparison shows that δ³⁴S_py is typically greater than δ³⁴S_CAS measured from the same samples. Hypotheses to explain the observations must account for both the remarkable sulfur isotope enrichment of pyrites and the inverse fractionation. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin or a basin with a sulfate minimum zone. In this model, CAS was derived from shallow, oxic surface waters with moderate sulfate concentration and depleted in ³⁴S due to the post-glacial influx of sulfur from continental weathering. In contrast, pyrite was derived from anoxic bottom waters (or a sulfate minimum zone) with low sulfate concentration and ³⁴S enrichment due to long-term syn-glacial sulfate reduction. The rapid shift in CAS abundance and sulfur isotope composition within the cap dolostone is interpreted to reflect the mixing of the two reservoirs after initial deglaciation. Comparison with other post-Marinoan cap carbonates shows significant spatial heterogeneity in δ³⁴S_CAS, which together with strong temporal variation in δ³⁴S_CAS, points to generally low sulfate concentrations in post-Marinoan oceans.     

Neoglacial gelifluction in the Jostedalsbreen Region,Western Norway: Evidence from dated buried palaeopodsols

Radiocarbon dating of thin palaeopodsols buried beneath turf-banked gelifluction lobes at four localities in the low alpine mountain zone in the Jostedalsbreen region, western Norway, show that gelifluction processes were initiated subsequent to the late Subboreal Chronozone. Although large age-depth gradients have been demonstrated from buried palaeosols in southern Norway, evidence is presented that the palaeosols in this study show only moderate age-depth gradients. The age estimates from these buried palaeosols give maximum dates of burial, but the error is not thought to be large. Gelifluction processes were probably initiated close to the time of the climatic deterioration, which led to the formation of the present glaciers during the Subatlantic Chronozone. The processes may have been most active during the peak of the Little Ice Age, during which a periglacial climate was established to low levels in this mountainous region.     

Surface-rupturing history of the Bree fault scarp, Roer Valley graben: Evidence for six events since the late Pleistocene

Since 1996 paleoseismological investigations have been used to develop the surface- rupturing history of the Bree fault scarp, the morphologically best-defined segment of the southwestern border fault of the Roer Valley graben in northeastern Belgium. The first studies determined that the escarpment is associated with a surface fault, and they exposed evidence for three surface displacements since about 40 ka BP. The most recent eventprobably occurred between 1000 and 1350 yr cal BP. Geophysical and trenching studies at a new site near the southeastern end of the fault scarp reconfirmed the coincidence of the frontal escarpment with a shallow normal fault, which displaces the Middle Pleistocene `Main Terrace' of the Maas River, as well as overlying coversands of Saalian to late Weichselian age. Different amounts of displacement shown by the two youngest coversand units indicate two discrete faulting events, but primary evidence for the coseismic nature of these events is sparse. Radiocarbon and optically stimulated luminescence dating constrainthe age of these events to the Holocene and between 14.0 ± 2.3 ka BP and 15.8 ± 2.9 ka BP, respectively. In addition, four older surface-rupturing events are inferred from the presence of four wedge-shaped units of reworked Main Terrace deposits that are interbedded with coversand units in the hanging wall of the trench and in shallow boreholes. These wedges are interpreted as colluvial wedges, produced by accelerated slope processes in response torejuvenation of the fault scarp, most probably in a periglacial environment. Luminescence dating indicates that five out of a total of six identified faulting events are younger than 136.6 ± 17.6 ka. The antepenultimate event was the largest faulting event, associated with a total fault displacement in excess of 1 m. Thus, the newly investigated trench site represents the longest and most complete record of surface rupturing recovered so far along the Bree fault scarp. This study also demonstrates the viability of the paleoseismological approach to identify past large earthquakes in areas of present-day moderate to low seismic activity.     

Late Paleozoic adakites and Nb-enriched basalts from northern Xinjiang, northwest China: Evidence for the southward subduction of the Paleo-Asian Oceanic Plate

Abstract   Volcanic rocks consisting of adakite and Nb-enriched basalt are found in the early Devonian Tuoranggekuduke Group in the northern margin of the Kazakhstan-Junggar Plate, northern Xinjiang, northwest China. The geochemical characteristics of the andesitic and dacitic rocks in this area resemble that of adakites. The relatively high Al₂O₃, Na₂O and MgO content and Mg^♯ values indicate that the adakites were generated in relation to oceanic slab subduction rather than the partial melting of basaltic crust. A slightly higher SrI and a lower ɛ _Nd( t  = 375 Ma) compared to adakites of mid-oceanic ridge basalt (MORB) imply that slab sediments were incorporated into these adakites during slab melting. The Nb-enriched basalt lavas, which are intercalated in adakite lava suite, are silica saturated and are distinguished from the typical arc basalts by their higher Nb and Ti content (high field strength element enrichment). They are derived from the partial melting of the slab melt-metasomatized mantle wedge peridotite. Apparently, positive Sr anomalies and a slightly higher heavy rare earth element content in these adakites compared to their Cenozoic counterparts indicate that the geothermal gradient in the Paleo-Asian Oceanic subduction zone and the depth of the Paleo-Asian Oceanic slab melting are between those of their Archean and Cenozoic counterparts. The distribution of the adakites and Nb-enriched basalts in the northern margin of the Kazakhstan-Junggar Plate, northern Xinjiang, indicates that the Paleo-Asian Oceanic Plate subducted southward beneath the Kazakhstan-Junggar Plate in the early Devonian period.     

Human exploitation of carnivores in Pleistocene China: A case study of the faunal remains from Shuidonggou Locality 7

Hominin-animal interactions at Pleistocene archaeological sites have been the main subject of research during human evolution. However, in contrast to that of ungulates, little is known about the evidence of human processing of carnivore species in this time interval of China. In this paper, we present the first case report of an interactive relationship between carnivore species and hunter-gathers in Pleistocene China, based mainly on a detailed taphonomic study of the faunal remains from Shuidonggou Locality 7. It seems highly probable that hominins butchered some individuals of carnivores and roasted bone elements with adhering meat at the site;whereas it’s equally feasible that these animals were exploited by hominins mainly for their pelt or fur. In either way, evidence from the site indicates novel strategies in adaptive behaviors adopted by Late Pleistocene foragers in harsh environments of Northwest China.     

Spatial distribution and temporal trends of mean precipitation and extremes in the arid region,northwest of China,during 1960–2010

On the basis of daily precipitation records at 76 meteorological stations in the arid region, northwest of China, the spatial and temporal distribution of mean precipitation and extremes were analysed during 1960–2010. The Mann–Kendall trend test and linear least square method were utilized to detect monotonic trends and magnitudes in annual and seasonal mean precipitation and extremes. The results obtained indicate that both the mean precipitation and the extremes have increased except in consecutive dry days, which showed the opposite trend. The changes in amplitude of both mean precipitation and extremes show seasonal variability. On an annual basis, the number of rain days (R0.1) has significantly increased. Meanwhile, the precipitation intensity as reflected by simple daily intensity index (SDII), number of heavy precipitation days (R10), very wet days (R95p), max 1‐day precipitation amount (RX1day) and max 5‐day precipitation amount (RX5day) has also significantly increased. This suggests that the precipitation increase in the arid region is due to the increase in both precipitation frequency and intensity. Trends in extremes are very highly correlated with mean trends of precipitation. The spatial correlation between trends in extremes and trends in the mean is stronger for winter (DJF) than for annual and other seasons. The regional annual and seasonal precipitation and extremes are observed the step jump in mean in the late 1980s. Overall, the results of this study are good indicators of local climate change, which will definitely enhance human mitigation to natural hazards caused by precipitation extremes. Copyright © 2012 John Wiley & Sons, Ltd.     

Estimation of the shortening rate since late Pleistocene in the Aksu area on the southern flank of the Tianshan, China

This research focused on the Aksu area in the central part of the southern Tianshan. Along the 60 km wide Aksu fold-and-thrust belt, active thrusts reach the surface and offset the youngest sediments. Our research was based on the geomorphologic study that examined the advance and retreat of glaciers cut by thrusts in the Tomur area in the north of Aksu. Our fieldwork revealed that two fault scarps were clearest across three different moraines that represent the maximum of advance of glaciers during three glacial periods along the Tailan River in the Tomur area. The measured heights of the fault scarps that cut the moraines, together with the moraines-inferred age, imply a shortening rate of 1.85 mm/a on the Aksu area since late Pleistocene. This rate, similar to that of the Korla area on its east side and of the Kaping area on its west side, but lower than that of the Kashgar area farther west and of the Manas area in the northern margin of the belt, implies that the distribution of shortening across the Tianshan changed markedly along the mountain.     

Evolution and variation of the Tsushima warm current during the late Quaternary: Evidence from planktonic foraminifera, oxygen and carbon isotopes

The evolution and variation history of the Tsushima warm current during the late Quaternary was reconstructed based on the quantitative census data of planktonic foraminiferal fauna, together with oxygen and carbon isotope records of mixed layer dweller G. ruber and thermocline dweller N. dutertrei in piston core CSH1 and core DGKS9603 collected separately from the Tsushima warm current and the Kuroshio dominated area. The result showed that the Tsushima warm current vanished in the lowstand period during 40―24 cal ka BP, while the Kuroshio still flowed across the Okinawa Trough, arousing strong upwelling in the northern Trough. Meanwhile, the influence of freshwater greatly increased in the northern Okinawa Trough, as the broad East China Sea continental shelf emerged. The freshwater reached its maximum during the last glacial maximum (LGM), when the upwelling obviously weakened for the lowest sea-level and the depression of the Kuroshio. The modern Tsushima warm current began its development since 16 cal ka BP, and the impact of the Kuroshio increased in the middle and north- ern Okinawa Trough synchronously during the deglaciation and gradually evolved as the main water source of the Tsushima current. The modern Tsushima current finally formed at about 8.5 cal ka BP, since then the circulation structure has been relatively stable. The water of the modern Tsushima cur- rent primarily came from the Kuroshio axis. A short-term wiggle of the current occurred at about 3 cal ka BP, probably for the influences from the enhancement of the winter monsoon and the depression of the Kuroshio. The cold water masses greatly strengthened during the wiggle.     

The Quetzalapa Pumice: a voluminous late Pleistocene rhyolite deposit in the eastern Trans-Mexican Volcanic Belt

The study area is located in the east part of the Trans-Mexican Volcanic Belt, in the Las Cumbres Volcanic Complex (LCVC) which lies between two large stratovolcanoes: Pico de Orizaba (5700 m a.s.l.) to the south, and Cofre de Perote (4200 m a.s.l.) to the NNE. The most conspicuous structure of the LCVC is a 4-km-diameter circular crater with a dacitic dome in the center, which constitutes the remains of a destroyed stratovolcano.The Quetzalapa Pumice (QP) was produced by a plinian eruption that was dated by the ¹⁴C method at 20 000 yr. BP. The eruptive sequence consists predominantly of pumice fall deposits and scarce intra-plinian pyroclastic flow deposits, which crop out on the west flank of the LCVC. The absence of post-plinian ignimbrite deposits is striking.The deposits are well sorted, clast-supported with reverse grading at the base, with a medium to high accessory lithics content. The maximum average thickness of the deposit in the proximal areas is about 15 m and has been divided into three members: the Basal Member (BM), 2 m thick with four submembers (BMf₁, BMf₂, BMf₃, and BMafl), the Intermediate Member (IM), 10 m thick with two submembers (IMpf and IMaf), and the Upper Member (UM), 3 m thick with four submembers (UMpl, UMsdf, UMwaf, and UMpls).The predominant component of the fall deposits is a white, highly vesiculated pumice with 71% SiO₂ content. Plagioclase is the most abundant mineral followed by 1–3-mm-long biotite phenocrysts. The accessory lithics are lavas mostly of andesitic composition. Their abundance increases toward the uppermost levels of the sequence.We calculate a minimum volume of 8.4 km³ (2.22 km³ dense rock equivalent), for the entire QP deposit. Isopach and isopleth maps show that the IM deposit has an elongated distribution with a NNE–SSW direction, whereas the UM deposit has a circular distribution.We estimate a maximum eruptive column height for the IM of 20 km. Field studies and isopach and isopleth maps indicate that the eruptive column was affected by a strong wind.Previous studies located the QP source in the Las Cumbres crater. However, based on the isopach and isopleth distribution, and the lack of pumice fall deposits inside the Las Cumbres crater, we consider that the QP emission center is located on the west flank of the LCVC, and was buried by its own pumice fall deposits. It coincides with an explosion crater called La Capilla formed during the closing phase of the QP eruption.A ‘pumice fountain’ model is proposed to explain the observed sequence of deposits. According to this model, the material was emitted through a ‘hose-type’ conduit during a monogenetic eruption of rhyolitic composition. This kind of volcanic activity is not extensively reported in the literature.     

Occurrence of Alaskan-type mafic-ultramafic intrusions in the North Qilian Mountains, northwest China: Evidence of Cambrian arc magmatism on the Qilian Block

Field relationships, mineralogy and petrology, whole‐rock chemistry, and age of the Zhamashi mafic–ultramafic intrusion in the North Qilian Mountains, northwest China, have been studied in the present work. The Zhamashi intrusive body consists of ultramafic, gabbroic, and dioritic rocks in a crudely concentrically zoned structure. The ultramafic rocks are layered cumulates with rock types varying continuously from dunite through wehrlite and olivine clinopyroxenite to clinopyroxenite. The gabbroic and dioritic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to diorite. The ultramafic and adjoining gabbroic rocks are discontinuous in lithology and discordant in structure across the interface. The interface is steep, sharp, and fractured. Contact metamorphic zones are well developed between the Zhamashi intrusive body and the country rock. The concentrically zoned structure of the intrusive body and the intrusion into the continental crust are the two main pieces of evidence for considering that the Zhamashi intrusion is Alaskan‐type. The mineral chemistry of the chromian spinels (Cr‐spinels) and clinopyroxenes, and the variation trend of the whole‐rock compositional plot in the (Na₂O + K₂O)–FeO–MgO (AFM) diagram are also supportive of this consideration. The age of the Zhamashi intrusive body, determined with sensitive high mass‐resolution ion microprobe on the zircon grains, is 513.0 ± 4.5 Ma. Parental magma of the Zhamashi intrusion is compositionally close to the primitive magma produced by partial melting of the mantle peridotite. It was differentiated by fractional crystallization at low total pressure and under H₂O‐rich conditions in an arc environment to form all the major rock types. The concentrically zoned structure of the Zhamashi intrusive body was constructed in two stages: formation of a stratiform‐type layered sequence, followed by diapiric re‐emplacement. The occurrence of the Alaskan‐type intrusion suggests an active continental margin and Cambrian arc magmatism for the northern margin of the Qilian Block.     

Palynological record during the Pleistocene (between 1.05 Ma and 0.36 Ma) from ODP1144, northern South China Sea

This paper presents the pollen record from the lower section of ODP1144 (depth 501.3-225.7 m, ca. 1.05-0.36 Ma). Two pollen zones (PA and PB) and eleven pollen subzones are recognized. Within zone PB, the 11 pollen subzones (PB21-11) are defined according to the pine, fern and herb variations, and are equivalent to the marine isotope stages 21-11 (MISs 21-11). The interglacial periods are typified by an increase in pine pollen and fern spores, and a decrease in herbaceous pollen, while the patterning during the glacial periods is just the opposite. During the interglacial periods, pollen assemblages were dominated by pine similar to those of the present day, suggesting that the paleoenvironment of the interglacial periods was similar to that of the present day, whereas the glacial periods are marked by an increase in herbaceous pollen, mainly Gramineae and Cyperaceae, indicating that grassland covered the exposed continental shelf when sea level declined. Increased Artemisia percentages and the highest     

Underestimated ~(14)C-based chronology of late Pleistocene high lake-level events over the Tibetan Plateau and adjacent areas: Evidence from the Qaidam Basin and Tengger Desert

The palaeolake evolution across the Tibetan Plateau and adjacent areas has been extensively studied, but the timing of late Pleistocene lake highstands remains controversial. Robust dating of lacustrine deposits is of importance in resolving this issue. This paper presents 14 C or optically stimulated luminescence(OSL) age estimates from two sets of late Quaternary lacustrine sequences in the Qaidam Basin and Tengger Desert(northeastern Tibetan Plateau). The updated dating results show:(1) the radiocarbon dating technique apparently underestimated the age of the strata of >30 ka BP in Qaidam Basin;(2) although OSL and 14 C dating agreed with each other for Holocene age samples in the Tengger Desert area, there was a significant offset in dating results of sediments older than ~30 ka BP, largely resulting from radiocarbon dating underestimation;(3) both cases imply that most of the published radiocarbon ages(e.g., older than ~30 ka BP) should be treated with caution and perhaps its geological implication should be revaluated; and(4) the high lake events on the Tibetan Plateau and adjacent areas, traditionally assigned to MIS 3a based on 14 C dating, are likely older than ~80 ka based on OSL chronology.     

A discontinuity in the late Pleistocene alluvial deposits,Hwacheon‐ri,Gyeongju, Korea: Occurrences and paleoenvironmental implication

A sedimentary discontinuity is present in the late Pleistocene alluvial deposits exposed along the cliff in stream side, Gyeongju, Korea. Sedimentological study, Optically Stimulated Luminescence (OSL) age dating, and X‐ray diffraction (XRD), and X‐ray fluorescence spectrometer (XRF) analyses were conducted in order to interpret the origin of the discontinuity surface. Based on the sedimentological study, these deposits can be divided into three distinct stages, in ascending order: (Stage 1) development of a braided stream flowing from south to north and deposition by debris flows from the mountain aside the stream during and/or after deposition of the braided stream; (Stage 2) deposition by intermittent events during a state of stagnation after the abandonment or migration of the previously existed braided stream; and (Stage 3) deposition of gravels from debris flows from the mountain aside the stream. Hornfelsic gravels are common in Stage 1, whereas andesitic gravels are predominant in Stage 3, suggesting the provenance change from Stage 1 to Stage 3. The discontinuity surface is laterally extensive and marked by a distinct carbonaceous dark grey horizon between Stage 2 and Stage 3. It is characteristic that rootlets mineralized by vivianite are present, and iron‐oxide crusts are cutting across irregularly below the discontinuity surface. It is thus interpreted that the shift of depositional environment from an alluvial plain (Stage 2) (125 ka) to an alluvial fan (Stage 3) (94–55 ka) was an alluvial response to sea level change from the interglacial to the glacial. The development of iron‐oxide crusts and diagenetic vivianite in the discontinuity surface suggests humid condition persisted during the paleoclimatic shift from the last interglacial to the last glacial stages.     

Evolution of the oceanic calcium cycle during the late Mesozoic: Evidence from δ44/40Ca of marine skeletal carbonates

The Ca isotope compositions of 37 late Mesozoic skeletal carbonates, belemnites and brachiopods, from the Tethyan realm were analyzed by thermal (TIMS) and plasma (MC-ICP-MS) ionization mass spectrometry. A poor correlation between δ^44/40Ca and δ¹⁸O values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02‰/°C. The δ^44/40Ca record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater (δ^44/40Ca_SW), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite (α_CC–SW) of ∼ 0.9986. The inferred δ^44/40Ca_SW record, with an average stratigraphic resolution of 1 Ma, shows systematic temporal variation of ∼ 0.5‰ with the Middle/Late Jurassic (∼ 154 Ma) minimum of ∼ 1.4‰ and a subsequent general increase to the Early Cretaceous (∼ 124 Ma) maximum of ∼ 1.9‰. The global nature of the δ^44/40Ca_SW record is supported by identical Ca isotope compositions of coeval (Kimmeridgian) belemnites collected from two distinct paleogeographic regions, the southern (New Zealand) and northern (Germany) margin of the Tethys Ocean. The observed late Mesozoic δ^44/40Ca_SW record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in δ^44/40Ca_SW record can be explained by changes in oceanic input fluxes of Ca that were independent of the carbonate ion fluxes, such as the hydrothermal Ca flux or the release of Ca to the oceans via dolomitization of marine carbonates.     

西北太平洋俯冲带日本本州至中国东北段应力场反演

本研究基于Global CMT提供的1196个1976年11月-2017年1月M_W>4.6地震矩心矩张量解,对西北太平洋俯冲带日本本州至中国东北段的应力场进行反演计算,得到了从浅表到深部俯冲带应力状态的完整分布.结果显示：俯冲带浅表陆壳一侧应力场呈现水平挤压、垂向拉伸状态,洋壳一侧的应力状态则相反,即近水平拉张、近垂向压缩.沿着俯冲板片向下,应力主轴逐渐向俯冲板片轮廓靠拢,其中位于双地震层（120 km深度附近）之上的部分,主张应力轴沿俯冲板片轮廓展布而又比其更为陡倾;双地震层内的应力模式同典型Ⅰ型双层地震带内的应力模式一致,即上层沿俯冲板片轮廓压缩、下层沿俯冲板片轮廓拉伸;双地震层之下,应力模式逐步转变为主压应力轴平行于俯冲板片轮廓.通观所研究的整个俯冲系统,水平面内主压和主张应力轴基本保持了与西北太平洋板片俯冲方向上的一致性,同经典俯冲板片的应力导管模型所预言的俯冲带应力模式相符;而主张应力轴在俯冲板片表面之下的中源地震深度范围内转向海沟走向,或许同研究区域横跨日本海沟与千岛海沟结合带,改变的浅部海沟形态致使完整俯冲板片下部产生横向变形有关.

     

Thermal history of the Sichuan Basin,SW China:Evidence from deep boreholes

The Sichuan Basin is a superimposition basin composed of terrestrial and marine sediments that is well known for its abundant petroleum resources. Thermal history reconstruction using paleogeothermal indicators, including vitrinite reflectance and thermochronological data, shows that different structural subsections of the Sichuan Basin have experienced various paleogeothermal episodes since the Paleozoic. The lower structural subsection comprising the Lower Paleozoic to Middle Permian(Pz–P_2) successions experienced a high paleogeothermal gradient(23.0–42.6°C/km) at the end of the Middle Permian(P_2), whereas the upper structural subsection comprising Late Permian to Mesozoic strata underwent a relatively lower paleogeothermal gradient(13.2–26.9°C/km) at the beginning of the denudation(Late Cretaceous or Paleocene in the different regions). During the denudation period, the Sichuan Basin experienced a successive cooling episode. The high paleogeothermal gradient resulted from an intensive thermal event correlated to the Emeishan mantle plume. The heat flow value reached 124.0 m W/m~2 in the southwestern basin near the center of the Emeishan large igneous province. The low geothermal gradient episode with heat flow ranging from 31.2 to 70.0 m W/m~2 may be related to the foreland basin evolution. The cooling event is a result of the continuous uplift and denudation of the basin.