查看更多>>摘要：Poor understanding of the differential evolution of interglacial climate over various regions in Eurasia greatly limits our ability to predict the specific local impacts of future climate change. Here we demonstrate starkly opposing trends in interglacial intensities in Asia and Europe over the early Middle Pleistocene and Mid-Brunhes Transitions based on the study of various climate proxies in the Paks loess record (Hungary), a key profile in the European Loess Belt. These contrasting climate trends imply major but unexplained differences in the regional response of Quaternary climate to shifts in forcing mechanisms during two major reorganizations of Earth's climate across Eurasia. Using new rock magnetic datasets from the Paks record we suggest that the changing dominance of Mediterranean, Atlantic (Westerlies) and continental air masses under the influence of the quasi permanent high pressure centres over the Fennoscandian ice sheet played a key role in the observed differences in the evolution of early Middle Pleistocene climate. We also propose that the intensification of the early Middle Pleistocene glaciation of Central Asian mountains may have strengthened and shifted the Siberian High west-ward on multi-millennial/orbital timescales, which in turn forced geographically contrasting expressions of the MIS 19 to 11 interglacials in Eurasia. This is the first coherent explanation for the geographically diverse response of regional climate to the early Middle Pleistocene climate transitions and points to the clear role of global cooling and expanded mid-latitude glaciers in driving these events.
查看更多>>摘要：The early Mesozoic greenhouse period was characterized by variable climate conditions. The driving mechanisms of this long-term variability are unclear due to a lack of full-period climate reconstructions. In particular, reconstructions of the terrestrial environment are rare, especially reconstructions representing South China during the early Mesozoic period. We focused on the major, trace and rare earth element compositions and clay mineralogy of 173 mudstones from the northern margin of the South China Plate. These data were used as proxies to evaluate the weathering intensity and reconstruct the climate variability from the Middle Triassic to the Late Jurassic after considering the potential effects of provenance, diagenesis and sedimentary sorting and recycling. From the latest part of the Triassic to the early Middle Jurassic, five well-correlated chemical weathering indices, the chemical index of alteration (CIA), chemical index of weathering (CIW), plagioclase index of alteration (PIA), sodium depletion index (tau Na) and weathering index of Parker (WIP), imply an advanced degree of chemical weathering (e.g., CIAcorr, the corrected CIA, ranging from 71.1 to 88.0) and a clay mineral assemblage zone with abundant kaolinite. Together, these findings indicate a dominant humid climate. In the pre-Late Triassic and post-Middle Jurassic, the dominant seasonally arid climate was characterized by low and widely variable chemical weathering index values (e.g., CIAcorr values of 59.4-85.4) and clay mineral assemblage zones containing chlorite, smectite and mixed-layer illite-smectite but less kaolinite. We suggest that PCO2 variations, the megamonsoon effect, plate motion and regional topography all contributed to the observed climate changes during the early Mesozoic.
查看更多>>摘要：The Early Permian is recognized as the peak of the Late Paleozoic Ice Age (LPIA), and ice sheet covered most of Gondwana. During this time interval, the North China Craton (NCC) developed abundant economic coal seams within the Taiyuan Formation and the Lower Shanxi Formation. However, coal seams are absent in the Upper Shanxi Formation. In order to explore whether a change in paleoclimatic conditions may have been a factor in this coal-forming episode termination, we conducted a chemical weathering study on the Shanxi and Xiashihezi formations in the Ordos Basin of the NCC. Multiple chemical weathering indices (CIA, CIW, CIX, PIA and WIP) calculated from geochemical ratios in mudstones and derived paleotemperature estimates indicate that the chemical weathering intensity underwent a rapid decrease near the base of the Sakmarian Stage, remained low through the lower Sakmarian, then fluctuated at relatively high levels through the upper Sakmarian. A second drop in weathering intensity occurred near the base of the Artinskian Stage. This pattern was produced by cool climates during the Early Sakmarian associated with the P1 glacial interval (the first stage of the Permian glaciation), by a high temperature climate corresponding to the interglacial interval during Late Sakmarian, and then by another cooling into the P2 glacial at the beginning of the Artinskian. Similar changes in chemical weathering indices occur in the Paran acute accent a Basin of South America in southern high latitudes and in the mid-latitude Junggar Basin that is now in Northwest China. This chemical weathering trend mirrors reconstructed paleotemperature from global sea-level changes, from fusulinids diversity on North American shelves, and from atmospheric CO2 concentrations. A sharp decline of coal seams on the NCC is related to the mass extinction of wetland plants. The Late Sakmarian high-temperature event played an important role, and the continuous drift of the NCC toward higher latitudes and the regression associated with orographic uplift also exacerbated regional aridification.
查看更多>>摘要：We estimate global surface albedo from the areal proportion of land to sea in climatically-significant latitudinal belts at ten million-year intervals for the Late Cretaceous and Cenozoic (120 million years ago to Present) using modern plate tectonic reconstructions and a composite apparent polar path designed to minimize known biases in the determination of paleolatitude. We find that global surface albedo stayed almost constant until it shifted 30% higher to the modern value of around 0.15 with the inception of the Late Cenozoic Ice Age 34 million years ago, reflecting polar ice-albedo amplification of global cooling resulting from the reduction of greenhouse gases below a critical threshold, most probably as the culmination of enhanced CO2 weathering consumption of continental mafic rocks in the tropical humid belt. The contribution from cloud cover toward a planetary albedo is unclear in the absence of measurable proxies but might eventually be gauged from the role cloudiness evidently plays in maintaining radiative balance with the increasing land bias between northern and southern hemispheres over the Cenozoic.
查看更多>>摘要：It has been demonstrated that the sedimentary deposits of Eocene succession in Bohai Bay Basin in China responded to astronomically forced climate changes. Several sections have been evaluated for cyclostratigraphy and many astronomical time scales (ATSs) have been constructed in Paleogene Bohai Bay Basin. However, the calibrations of age model and the constructed ATSs failed to reach an agreement and many of them relied on totally different orbital interpretations within the same section/borehole. In this work, we generated a new ATS for the middle Eocene lacustrine strata of middle to lower Member 3 (Es3m-l) and upper Member 4 (Es4u) of Shahejie Formation in Bohai Bay Basin. In this research, high resolution natural gamma-ray (GR) data were used for the cyclostratigraphy analysis of Es3m-l and Es4u Members of Well Fanye-1. We filtered -11.11 m cycles of t Es3m-l and -9.26 m cycles of Es4u which were directly correlated to the short eccentricity cycles (-100-kyr) of La2004 solution. The geomagnetic polarity ages of C18r/C19n, C19n/C19r and C19r/C20n in Geological Time Scale 2020 were adopted as age markers for the tuning in this study. This astrochronology provided astronomical time scale (ATS) in the range of 39.268 +/- 0.050 Ma to 43.621 +/- 0.050 Ma in Es3m-l and Es4u Member. The astrochronology of Well Fanye-1 of the same interval had been studied before. However, the ATS built in this paper used correlation coefficient (COCO) analysis and average spectral misfit (ASM) approach to provide quantitative measurement for fitting the observed sedimentary cycles to astronomical periods and given a range of plausible sedimentation rate. In this research, a sedimentary noise model was applied for the detection of highresolution lake-level variations controlled by orbital forcing. The lake-level variations generated from the sedimentary noise model were supported by previous sequence stratigraphic interpretations. Variable sedimentation could be affected by lake-level changes which have been demonstrated by the evolutionary correlation coefficient analysis of GR series. In addition, from the ATS and thin section observations, it was concluded that carbonate-clay laminae in lacustrine shale were generated annually. Seasonal carbonate-clay laminated shale formations are controlled by orbital scale variability of monsoon and are always developed in a relatively high eccentricity amplitude under deep water environment conditions. Finally, the lake-level rise at -40.500 Ma to -40.050 Ma was demonstrated to be the response to Middle Eocene Climate Optimal (MECO) event in lacustrine records.
查看更多>>摘要：Lajishan suture in the Qilian Orogen records subduction process of the southern branch of the Proto-Tethyan Ocean. However, when and where the subduction started remain controversial issues. We present a study of the Cambrian volcanic rocks from the Lajishan suture. These volcanic rocks are coherent and composed of basalt, andesite, and volcanic breccia with minor interlayers or veins of dacite and rhyolite. Zircons from the dacite and rhyolite yield concordant Early-Middle Cambrian ages ranging from 521 to 510 Ma. Basalt and andesite exhibit subduction-related geochemical signatures with enrichment in large ion lithophile elements and depletion in high field strength elements. They have evolved REE, LREE, epsilon(Nd)(t) (+2.6 to +8.2), and (La/Sm)N ratios, sug-gesting derivation from depleted mantle wedge with addition of hydrous fluid or various amounts of sediment-derived melts. Dacite and rhyolite have positive zircon epsilon(Hf)(t) values (+6.1 to +11.6) and whole-rock epsilon(Nd)(t) values (+1.7 to +4.2). They were generated by partial melting of juvenile mafic crust resulting from magma under -plating. These Cambrian volcanic rocks in the Lajishan suture are exposed to the south of the accretionary complex and therefore are likely to be relicts of Cambrian oceanic arc built on the oceanic crust during south-ward subduction of the South Qilian Ocean. Considering the Cambrian-Ordovician intra-oceanic trench-arc-basin system reconstructed in the North Qilian suture, it is suggested that both southern and northern branches of the Proto-Tethyan Ocean have experienced intra-oceanic subduction process which initiated at ca. 530 Ma.
查看更多>>摘要：Lacustrine organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. These fine-grained sediments have been proven to have good hydrocarbon generation potential; therefore, understanding the sedimentary palaeoenvironment of these shales and organic matter enrichment mechanism is essential to increase prospects for shale oil and gas resources. The mineralogy, elemental geochemistry, and organic geochemical data combined with lithology of ZK23-1 well from the Yuka Depression, Qaidam Basin have been comprehensively analyzed to determine the hydrocarbon potential, palae-oenvironmental changes, and tectonic setting of the lacustrine organic-rich shales in the seventh member of Middle Jurassic Dameigou Formation (J(2)d(7)). The profile of the ZK23-1 well is composed of three stages (Stages I to III) based on variations in bulk geochemical parameters and lithological observations. Mineral compositions of studied samples are dominated by clay minerals and quartz, with small amount of pyrite, calcite, dolomite, and plagioclase. Interestingly, all samples contain siderite, which is closely related to palaeoenvironment changes. Total organic carbon (TOC) content and hydrogen index (HI) vary significantly in the various lithologies, sug-gesting that oil shale holds the best hydrocarbon potential, followed by shales and mudstones. A warm-humid palaeoclimate prevailed in the Qaidam Basin during the Middle Jurassic; however, significant palaeoclimatic fluctuations occurred from Stages I to III in the J(2)d(7), especially in Stage II. The climate varies from hot-arid to warm-humid in Stage II, likely resulting in changes in palaeowater column, chemical weathering intensity, as well as palaeoproductivity. The results of this study suggest that the Yuka Depression in the Middle Jurassic could be located at an active continental margin. Warm and humid conditions, as well as the tectonically quiescent setting with stable subsidence rate, are considered as two primary drivers for the enrichment and preservation of OM in the sediments of the J(2)d(7). Ultimately, a comprehensive depositional model is proposed to interpret the evolution of deposition of the J(2)d(7), providing new insights into the deposition of lacustrine organic-rich sediments linked to palaeoenvironment conditions and tectonic setting.
查看更多>>摘要：An increase in the atmospheric pCO2 and temperature is expected to reduce ocean CO32- concentration, make oceans warmer and alter ocean circulation patterns. This will also affect the production and biogeographic distribution of marine calcifying organisms including coccolithophores. The lowering of oceanic CO32- is expected to interfere with the coccolithophore calcification process and cause malformation of coccoliths, whereas changes in the oceanic temperature and circulation patterns may shift their biogeographic boundaries. In this study, we have investigated Emiliania huxleyi coccolith and coccosphere size response to the wide-ranging physico-chemical conditions of the Indian sector of the Southern Ocean between latitudes 38oS and 58oS during the austral summer of 2010 (January-February). This study helps to understand the response of E. huxleyi coccolith/coccosphere morphometry and mass changes to the fluctuating temperature, salinity, CO32-, pCO2, and nutrient values. Our results show that in the Indian sector of the Southern Ocean, E. huxleyi coccoliths are larger and coccospheres are smaller in the Subtropical Zone (STZ). In contrast, coccoliths size is smaller and coccospheres size is larger in the Subantarctic Zone (SAZ), which is due to the decrease in Sea Surface Temperature, Sea Surface Salinity and increase in nutrient concentrations. In the Indian sector of the Southern Ocean, E. huxleyi shows a north-to-south morphotype shift from the heavily calcified 'Group A' (E. huxleyi morphotype A) to the weakly calcified 'Group B' (E. huxleyi morphotypes B/C, C) forms. We demonstrate that although weakly calcified E. huxleyi morphotypes (morphotypes B/C and C) comprise less mass than that of the E. huxleyi morphotype A, due to the large-sized coccospheres and numerous coccoliths per coccosphere, 'Group B' coccospheres precipitate large amount of CaCO3 in the SAZ compared to 'Group A' coccospheres located in the STZ. We have documented the presence of large E. huxleyi overcalcified coccospheres with large-sized coccoliths in the southernmost cold, high pCO2, and nutrient-rich waters which show extracellular calcite precipitation. The energy dispersive spectrometry analysis indicates the presence of a large amount of Mg in the overcalcified E. huxleyi specimens. We suspect that E. huxleyi in the colder nutrient-rich waters, with future projected changes in the carbonate chemistry, may adapt to low pH, high pCO2 conditions through extracellular Ca and Mg mineralization.
查看更多>>摘要：The East Asian summer monsoon (EASM) system plays a crucial role in regulating climatic and hydrological variability in East Asia. The hydrological effects of the EASM in eastern China during the Little Ice Age (LIA) remains controversial, however. Here, we established a relatively high-resolution palaeohydrological record over the last 700 years in East China via analyzing n-alkanes from a peat core. Mid- and long-chain (C-21-C-25 and C-27-C-31) n-alkanes account for more than 95% of the total odd-carbon-numbered n-alkanes, with short-chain n-alkanes (C-17-C-19) accounting for only 2.3% on average, suggesting that organic matter in the peat core originated primarily from vascular plants. Organic geochemical indices, including P-aq (an indicator of contributions from aquatic plants), the average chain length (ACL) of long-chain n-alkanes, and C-31/C-27 and C-23/C-29 n-alkane ratios, suggest a transition from dry conditions during the middle LIA to wet conditions in the late LIA. Comparing P-aq records from the study profile with previous records from the EASM region, we found that the northern and southern parts of eastern China showed contrasting changes in humidity during the LIA. Specifically, the southern part shifted from mid-LIA dry conditions to late-LIA wet conditions, whereas the northern part shifted from mid-LIA wet conditions to late-LIA dry conditions. This spatial heterogeneity in hydroclimate could be attributed to La Nina-like conditions and intense summer monsoons during the middle LIA and El Nino-like conditions and weak summer monsoons during the late LIA. Our study demonstrates the importance of understanding spatial heterogeneity of hydroclimatic changes to probe the mechanisms whereby EASM modulates regional climatic changes during the LIA.
Tian, LiBai, RuoyuSong, HaijunBenton, Michael J....
9页
查看更多>>摘要：Conodonts are some of the most important biostratigraphic index fossils in the Paleozoic and Triassic because of their rapid evolution and extensive distribution. Here we present a cladistic analysis of Early Triassic conodonts to reexamine the systematic classification and evolutionary relationships of the three related families Anchignathodontidae, Ellisonidae, and Gondolellidae. The tree consists of 38 Early Triassic conodont species with multielement apparatus documented, based on parsimony analysis of 73 discrete morphological characters including 40 P1 element characters, 4 P2 element characters, 27 S element characters and 2 M element characters. Based on the phylogenetic time tree, a series of evolutionary lineages are supported: Parafurnishius xuanhanensis-Furnishius triserratus-Platyvillosus regularis; Scythogondolella mosheri-Scythogondolella milleri; Discretella-Conservatella-Paullella meeki; Triassospathodus homeri-Chiosella; and Triassospathodus symmetricus-Triassospathodus chioensis. Reassignment of Ellisonia agordina to be Hadrodontina agordina is supported while Neospathodus planus should be resigned to the genus Triassospathodus. Meanwhile, Parafurnishius and Furnishius may belong to Gondolellidae rather than Ellisonidae or Ellisonidae may be a polyphyletic group rather than a monophyletic group. The cladogram suggests that Hindeodus, Hadrodontina and Pachycladina are monophyletic, whilst Novispathodus and Triassospathodus are paraphyletic and polyphyletic, respectively. In addition, a P1 element with a round basal cavity is an important trait to distinguish Novispathodus from Triassospathodus. The cladogram and revised classification provide a robust phylogenetic foundation for further investigations on systematic classification and evolutionary rates of conodonts during the Early Triassic.
NSTL
Elsevier