Organic carbon isotopic composition(δ^13C) is one of the important proxies in paleoenvironment studies.In this paper modern plant δ^13C in the arid areas of China and Tibetan Plateau is studied.It is found that most terrestrial plant species in western China are C3 plants with δ^13C values ranging from -32.6‰ to -23.2‰ and only few species are C4 plants with δ^13C values from -16.8‰ to -13.3‰.The δ^13C is closely related to precipitation (or humidity),i.e., light δ^13C is related to high precipitation(or humid climate),while heavy δ^13C to low precipitation (or dry climate),but there is almost no relation between plant δ^13C and temperature.Submerged plants have δ^13C values ranging from -22.0‰ to -12.7‰,like C4 plants,while merged plants have δ^13C values ranging from -28.1‰ to -24.5‰,like C3 C4 plants,while marged plants have δ^13C values ranging from -28.1‰ to -24.5‰,like C3 plants.It can then be concluded that organic δ^13C variations in terrestrial sediments such as loeas and soil in western China can indicate precipitation changes,but those in lake sediments can reflect organic sources and the productivity of different types of aquatic plants. 相似文献
Deep-sea corals hold a great potential as a key to important aspects of paleoceanography for at least two reasons, 1) they offer temporal high resolution records of deep-sea environment, because they have growth banding structures, 2) and they are well suited for studying vital effects, because the deep-sea environment does not change over short time scales. However, the relationship between the chemical composition of deep-sea coral skeletons and environmental factors is not well understood. In this study, the chemical composition of deep-sea corals was measured in bulk individuals and along skeletal micro-structures. Among the bulk individuals, δ18O value and Sr / Ca ratio show a negative but weak correlation with ambient temperature. On the other hand, the Mg / Ca ratio has a positive, weak correlation with the temperature. Large variations were found among samples collected from similar temperature. The variation is up to 3.8‰ for δ18O, 0.9 mmol/mol for Sr / Ca ratios, and 0.78 mmol/mol for Mg / Ca ratios among samples with ambient average temperature within 1 °C. This variation may be due to a large vital effect. The centers of calcification (COCs), which were formed at high calcification rate, have lower Sr / Ca, U / Ca and higher Mg / Ca ratios than surrounding fasciculi. This chemical distribution supports the model that elemental incorporation depends on calcification rate. This suggests that calcification rate is a very important factor for the chemical composition in deep-sea corals and is one of the most significant mechanisms of the vital effect. Because of this large vital effect, further investigations are essential to use the deep-sea coral as a temperature proxy. 相似文献
The earthquake cycles that characterize continental-interior areas that are far from active plate boundaries have proven highly cryptic and difficult to resolve. We used a novel paleoseismic proxy to address this issue. Namely, we reconstructed Holocene Mississippi River channels from maps of floodplain strata in order to identify channel perturbations reflective of major displacement events on the high-hazard and mid-plate Reelfoot thrust fault, New Madrid seismic zone, U.S.A. Only three discrete slip events are currently documented for the Reelfoot fault ( AD 900, AD 1450, and AD 1812). This study extends this record and, thus, illustrates the utility of stratigraphic proxies as paleoseismic tools. We concurrently offer here some of the first quantified response times for tectonically induced channel pattern changes in large alluvial rivers.
We identified at least two cycles of pervasive meandering that were interrupted by channel-straightening responses occurring upstream of the Reelfoot fault scarp. These straightening responses initiated at 2244 BC +/− 269 to 1620 BC +/− 220 and AD 900, respectively, and each records initiation of a period of Reelfoot fault slip after millennia of relative tectonic quiescence. The second (or New Madrid) straightening response was triggered by the previously known AD 900 fault slip event, and this initial low sinuosity has been protracted until the modern day by the latter AD 1450 and AD 1812 events. The first (or Bondurant) straightening response began a period of several hundred to 1400 years of low river sinuosity which evidences a similar period of multiple recurrent displacement events on the Reelfoot fault. These Bondurant events predate the existing paleoseismic record for the Reelfoot fault.
These data offer initial evidence that slip events on the Reelfoot fault were temporally clustered on millennial scales and, thus, offers the first direct evidence for millennial-scale clustering of earthquakes on a continental-interior fault. This carries additional ramifications. Namely, faults that have been quiescent and non-hazardous for millennia could re-enter an enduring period of recurrent hazardous earthquakes with little warning. Likewise, the Reelfoot fault also reveals evidence of temporal clustering of earthquakes on short-term cycles (months), as well as evidence for longer-term reactivation cycles (104–106 years). This introduces the possibility that temporal clustering could be hierarchical on some continental-interior faults. 相似文献
Here we report a detailed trace element study of the cherts from Liuchapo Formation, which is a terminal Ediacaran (551-542
Ma) succession in South China deposited in deep-water basinal setting. The REE of Liuchapo cherts shows similar features as
observed for anoxic modern seawater (but not for hydrothermal fluids), characterized by positive La anomaly (LaN/CeN = 0.83–1.91, average 1.37), moderately negative Ce anomaly (0.53–1.1, average 0.73), positive Gd anomaly (average 1.08),
positive Y anomaly (average 1.21), and depleted LREE and MREE. In addition, the Liuchapo cherts have low ΣREE (3.36–56.13
ppm, average 20.6 ppm), low Al2O3, Ti, Th and Zr concentrations, and high Y/Ho ratios (up to 43.9). The redox-sensitive trace elements concentrations in the
cherts do not correlate with detrital input proxies. All of these features suggest that the redox-sensitive trace elements
in the cherts were authigenically concentrated in water column and their concentrations thus are excellent indicators of ancient
redox conditions. Very low Th/U ratios, high V/(V+Ni) and Fe⊤/Al ratios, enrichments of redox-sensitive trace elements (U, V, Mo), and low concentration of Mn in the cherts imply anoxia
in the deep seawater. Our data reveal that the terminal Ediacaran ocean was not completely oxidized and the deep ocean was
still anoxic, at least in South China. We propose that although the oxidative events existed in the terminal Ediacaran oceans,
decomposition of organic matter prolonged anoxia in the deep ocean.
Supported by National Natural Science Foundation of China (Grants Nos. 40532012, 40873007, 40603021) and Chinese Academy of
Sciences (Grant No. KZCX3-SW-141) 相似文献
An overview of basic research on climate change in recent years in China is presented. In the past 100 years in China, average annual mean surface air temperature (SAT) has increased at a rate ranging from 0.03℃ (10 yr)-1 to 0.12℃ (10 yr)-1 . This warming is more evident in northern China and is more significant in winter and spring. In the past 50 years in China, at least 27% of the average annual warming has been caused by urbanization. Overall, no significant trends have been detected in annual and/or summer precipitation in China on a whole for the past 100 years or 50 years. Both increases and decreases in frequencies of major extreme climate events have been observed for the past 50 years. The frequencies of extreme temperature events have generally displayed a consistent pattern of change across the country, while the frequencies of extreme precipitation events have shown only regionally and seasonally significant trends. The frequency of tropical cyclone landfall decreased slightly, but the frequency of sand/dust storms decreased significantly. Proxy records indicate that the annual mean SAT in the past a few decades is the highest in the past 400-500 years in China, but it may not have exceeded the highest level of the Medieval Warm Period (1000-1300 AD). Proxy records also indicate that droughts and floods in eastern China have been characterized by continuously abnormal rainfall periods, with the frequencies of extreme droughts and floods in the 20th century most likely being near the average levels of the past 2000 years. The attribution studies suggest that increasing greenhouse gas (GHG) concentrations in the atmosphere are likely to be a main factor for the observed surface warming nationwide. The Yangtze River and Huaihe River basins underwent a cooling trend in summer over the past 50 years, which might have been caused by increased aerosol concentrations and cloud cover. However, natural climate variability might have been a main driver for the mean and extreme precipitation variations observed over the past century. Climate models generally perform well in simulating the variations of annual mean SAT in China. They have also been used to project future changes in SAT under varied GHG emission scenarios. Large uncertainties have remained in these model-based projections, however, especially for the projected trends of regional precipitation and extreme climate events. 相似文献
We review here proxy records of temperatare and precipitation in China during the Holocene,especially the last two millennia.The quality of proxy data,methodology of reconstruction,and uncertainties in reconstruction were emphasized in comparing different temperatare and precipitation reconstruction and clarilying temporal and spatial patterns of temperature and precipitation during the Holocene.The Holocene climate was generally warm and wet.The warmest period occurred in 9.6-6.2 cal ka BP,whereas a period of maximum monsoon precipitation started at about 11.0 cal ka BP and lasted until about 8.O-5.0 cal ka BP.There were a series of millennial-scale cold or dry events superimposed on the general trend of climate changes.During past two millennia,a warming trend in the 20th century was clearly detected,but the warming magnitude was smaller than the maximum level of the Medieval Warm Period and the Middle Holocene.Cold conditions occurred over the whole of China during the Little Ice Age (AD 1400-AD 1900),but the warming of the Medieval Warm Period(AD 900-AD 1300)was not distinct in China,especially west China.The spatial pattern of precipitation showed significant regional differences in China,especially east China.The modern warm period has lasted 20、years from 1987 to 2006.Bi-decadal oscillation in precipitation variability was apparent over China during the 20th century. Solar activity and volcanic eruptions both were major forcings governing the climate variability during the last millennium. 相似文献