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1.
Coral reef terraces are one of the best recorders of relative sea-level changes during the last glacial cycle. Thus far, knowledge of relative sea-level record based on coral reefs during the marine Oxygen Isotope Stage (OIS) 3 has been limited to studies of the Huon Peninsula, Papua New Guinea. High-precision a α-spectrometric 230Th/234U dating demonstrated an offlapping sequence of five coral reef complexes, ages of which are 66, 64, 62, 55 and 52 ka, in the northern part of Kikai Island, central Ryukyus of Japan. Interstadial reefs, characterized by deepening-upward sequences of coral assemblages, recorded three hemicycles from transgression to highstand at 52, 62, and 66 ka, during which these reefs were drowned. These highstands in the relative sea-level record can be correlated with the eustatic record reconstructed from the Huon reef terraces and with the interstadials 14, 18, and 19 of the GISP 2 oxygen isotope record. This consistency confirms the Huon sea-level record of OIS 3 and implies that the eustatic sea level responded to the millennial-scale climate changes even during the glacial period of OIS 4.  相似文献   

2.
Analyses of the Nd isotope composition of conodonts from the Anti-Atlas (Morocco), the Montagne Noire (France) and the Rhenohercynian domain (Germany) provide insight into the temporal and lateral variations in seawater geochemistry in the western part of the Variscan Sea during the Late Devonian. Most of the isotopic excursions accurately record changes in sea level. The εNd values decreased during regression phases when erosion accelerated supply of unradiogenic Nd from old continental sources. A rise in sea level generated a positive shift in εNd values due to input of more radiogenic oceanic water into the seawater reservoir of the shelf areas. The method has great potential for sea-level research, paleoceanography and correlation because sea-level changes can be precisely recognized even in successions with uniform, monotonous lithologies. However, it has a significant limitation being valid only in epeiric seas and shelf areas. We have used the Nd isotope record to reconstruct the eustatic sea-level changes in late Frasnian–early Famennian time. The onset of the organic-rich Kellwasser units coincides with negative trends in εNd values. This accords with oxygen deficiency at the sea bottom being initiated by short-term regressive events, which in turn would be consistent with cooling periods of glacial origin, as previous authors have suggested.  相似文献   

3.
Interhemispheric anti-phasing of rainfall during the last glacial period   总被引：1，自引：0，他引：1  
We have obtained a high-resolution oxygen isotopic record of cave calcite from Caverna Botuverá (27°13′S, 49°09′W), southern Brazil, which covers most of the last 36 thousand years (ka), with an average resolution of a few to several decades. The chronology was determined with 46 U/Th ages from two stalagmites. Tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change. We interpret our record in terms of meteoric precipitation changes, hence the variability of South American Monsoon (SAM) intensity. The oxygen isotopic profile broadly follows local insolation changes and shows clear millennial-scale variations during the last glacial period with amplitudes as large as 3‰ but with smaller centennial-scale shifts (<1‰) during the Holocene. The overall record is strikingly similar to, but strongly anti-correlated with, a number of records from the Northern Hemisphere.We compared our record to other precisely dated contemporaneous records from Hulu Cave eastern China. Minima in δ18O (wet periods, intense SAM) at our site are synchronous with maxima in δ18O (dry periods, weak East Asian Monsoon, EAM) in eastern China (within precise dating errors) and vice versa. This anti-phased precipitation relationship between two low-latitude locations may be interhemispheric in extent, based on comparison with records from other sites. Precipitation anti-phasing may be related to north–south shifts in the mean position of the intertropical convergence zone (ITCZ) and asymmetry in Hadley circulation in two hemispheres, associated not with seasonal changes as observed today, but with millennial-scale climate shifts. The millennial-scale atmospheric see-saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor's greenhouse properties.  相似文献   

4.
A radiometric calibration of the SPECMAP timescale   总被引：1，自引：0，他引：1  
The astronomical theory of climate change asserts that Earth's climate is affected by changes in its orbit, which vary the seasonal and latitudinal distribution of solar radiation. This theory is the basis of the orbitally tuned SPECMAP timescale. A key constraint for this important chronology was the mid-point of the Penultimate Deglaciation, initially dated to 127,000 years ago. Recent work suggests this event may be considerably older, casting doubt on the astronomical theory, the SPECMAP timescale, and the accuracy of orbitally tuned chronologies. Difficulties with U/Th coral dating of sea-level events have impeded progress on this problem, because most corals are not closed systems. Here, we use a new approach to U/Th dating that corrects for open-system behavior and produces a sea-level curve of sufficient resolution to confidently correlate with SPECMAP over the last 240,000 years, permitting a reassessment of both this critical chronology and a central tenet of climate change theory. High-precision ages for 24 oxygen isotope events provide a 240,000-year chronology for marine δ18O records that is independent of orbital tuning assumptions. Although there appear to be significant differences between the radiometric and orbitally tuned timescales near the lastglacial maximum and at the Marine Isotope Stage 7/6 boundary, a comparison of radiometric and SPECMAP ages for identical isotope events suggest that the SPECMAP timescale is quite accurate and that its errors were, in general, overestimated. Despite suborbital complexity, orbital cyclicity is clearly evident in our record. High-amplitude sea-level oscillations at periods greater than 20,000 years are very close in phase to summer insolation in the Northern Hemisphere. Although sea-level changes cannot be uniquely tied to a specific season or latitude of insolation forcing, the simplest explanation is that long-period, high-amplitude sea-level change is linked to Northern Hemisphere insolation forcing. These results validate the principles of orbital tuning and suggest such timescales are generally robust.  相似文献   

5.
Many studies have documented the existence of millennial-scale variability in the Earth system during the last glacial period. An increasing number of studies document the occurrence of similar millennial variability during glacial periods previous to the last one. Here we use the simplest possible thermal-bipolar seesaw model to consider this variability for the last four glacial periods. We invert this model and use the high-pass filtered Vostok stable isotope records to make a first, tentative, attempt to estimate high-latitude N. Hemisphere temperature variability over the last four glacial periods, beyond the reach of Greenland ice-core records. The model result is compared against the Vostok methane record, which shows rapid variations in parallel to Greenland temperature records during the last glacial period. A further comparison is carried out against the planktonic oxygen isotope of north Atlantic core ODP 980. There is agreement between the records on the existence of similar millennial-scale variability during the last three glacial periods with very similar characteristics to the variability during the last glacial cycle.  相似文献   

6.
It is well established that orbital scale sea-level changes generated larger transport of sediments into the deep-sea during the last glacial maximum than the Holocene. However, the response of sedimentary processes to abrupt millennial-scale climate variability is rather unknown. Frequency of distal turbidites and amounts of advected detrital carbonate are estimated off the Lisbon–Setúbal canyons (core MD03-2698, at 4602 mwd), within a chronostratigraphy based on radiometric ages, oxygen isotopes and paleomagnetic key global anomalies. We found that: 1) higher frequency of turbidites concurred with Northern Hemisphere coldest temperatures (Greenland Stadials [GS], including Heinrich [H] events). But more than that, an escalating frequency of turbidites starts with the onset of global sea-level rising (and warming in Antarctica) and culminates during H events, at the time when rising is still in its early-mid stage, and the Atlantic Meridional Overturning Circulation (AMOC) is re-starting. This short time span coincides with maximum gradients of ocean surface and bottom temperatures between GS and Antarctic warmings (Antarctic Isotope Maximum; AIM 17, 14, 12, 8, 4, 2) and rapid sea-level rises. 2) Trigger of turbidity currents is not the only sedimentary process responding to millennial variability; land-detrital carbonate (with a very negative bulk δ18O signature) enters the deep-sea by density-driven slope lateral advection, accordingly during GS. 3) Possible mechanisms to create slope instability on the Portuguese continental margin are sea-level variations as small as 20 m, and slope friction by rapid deep and intermediate re-accommodation of water masses circulation. 4) Common forcing mechanisms appear to drive slope instability at both millennial and orbital scales.  相似文献   

7.
Abstract Based on a comparison between the oxygen isotope records of benthic and planktonic foraminifers from core 8KL of the South China Sea and sea-level change records derived from the Huon Peninsula, New Guinea, it is found that both records are very similar from 72 Ka B.P. to the present, especially for the benthic oxygen isotope record. The linear regression shows that δ<sup>18</sup>O changes (0.9995‰ for benthic foraminifers and 1.022‰ for planktonic foraminifers) are equal to 100 m in sea-level fluctuation. After making temperature correction in the δ<sup>18</sup>O record of benthic foraminifers from 72 to 120 Ka B.P., the curve of sea-level oscillation of the South China Sea since 186 Ka B.P. has been reconstructed. The lowermost sea-level that occurred in the last glacial maximum and oxygen isotope stage 6 is approximately - 130 m.  相似文献   

8.
《Quaternary Science Reviews》2005,24(14-15):1637-1653
Pollen and oceanographic data from deep ocean core MD95-2039 provide a centennial to millennial scale record of conditions offshore and of the vegetation of north-west Iberia for the period 10–65 ka. The planktonic oxygen isotope record of this core, reflecting predominantly sea surface temperature (SST), shows a pattern of millennial-scale oscillations that is very similar to climatic changes recorded by the Greenland ice core records over the same interval. In turn, tree populations show a pattern of rapid expansions and contractions that follow the pronounced and abrupt isotopic shifts recorded offshore. Through Marine Isotope Stage (MIS) 3, this millennial-scale pattern of vegetation change, alternating between steppe and open woodland, is superimposed on a longer-term pattern of shrinking ericaceous heathland and decreasing size of successive interstadial tree populations. Trees persisted during the Last Glacial Maximum (LGM), at greater abundance than during many of the coldest episodes of MIS 3. This agrees with the marine data which indicate that LGM sea surface temperatures here were significantly warmer than the minima recorded in MIS 3. Our combined marine-terrestrial record, together with data from nearby sequences, provides a stepping stone between terrestrial sequences and the Greenland ice core and North Atlantic marine records. This will permit a better understanding of the behaviour of vegetation across different regions at several scales of climatic forcing.  相似文献   

9.
Sea level reconstruction is an important proxy for climate change. It can be compared with other reconstructed climatic factors as well as the results of the computer model-based reconstruction to offer a better recognition of the relationship between sea-level changes and other factors in the climate system. Oxygen isotope record obtained from calcium foraminiferal shell in deep-sea sediments contains global continental ice volume signal which can be used to reconstruct the evolution of paleo-sea level. Researchers reconstructed many long time scale and continuous records of sea level change by using foraminiferal oxygen isotope ratio data. In the future of paleo-sea level reconstruction, however, researchers need to contrast the sea level reconstruction with other climate proxy results to validate its accuracy and employ the interdisciplinary study method to acquire further development.  相似文献   

10.
A 50-yr resolution reconstruction of climate and environment variability during the period 43–14 ka was developed using 26 high-precision U/Th dates and 390 oxygen isotope (δ18O) data of a stalagmite (SJ1) collected from Songjia Cave in central China, which is close to the northwestern boundary of the Asian summer monsoon (ASM). The δ18O record in SJ1 displays significant millennial-scale changes that correlate well in timing and duration with Dansgaard/Oeschger (D/O) events 5–10 and Heinrich event 4 (H4) identified in high-latitude regions of the Northern Hemisphere. Four 230Th dates constrain the H4 event precisely to the period of 39.7 to 38.3 ka. Notable centennial variations of the ASM activity could be observed within the H4 event. The magnitude and duration of D/O event 4.1 recorded in SJ1 are similar to those archived in east China but different from those documented in southwest China, suggesting that the manifestation of this event may be regionally different. The timing, duration and structure of D/O events 5–10 and Heinrich event 4 suggest that temperature changes in both hemispheres have exerted significant influences on the ASM variations in central China.  相似文献   

11.
Sepkoski’s compendium of 28,730 marine genera was analyzed with a view toward comparing taxonomic diversity, strontium isotope stratigraphy and sea-level changes. The fossil record analyses discriminated between origination, extinction and pass through genera and collectively delineated changes in number of genera with time at the largely stratigraphic stage level. With notable exceptions of the Cretaceous and Tertiary, taxonomic diversity generally paralleled both strontium isotope stratigraphy and sea-level changes. Sampling bias was suggested in the Tertiary where an increase in genera numbers toward the Recent opposed the downward trends of both strontium isotope and sea level curves. The removal of all Recent genera from the Sepkoski database confirmed the bias in producing complimentary downward trends in all curves. Regression analysis of taxonomic diversity with respect to sea level extent and time duration explained 82% of the variability of the remaining data. Sea-level fluctuations are not always accompanied by significant changes in taxonomic diversity but strontium isotope analyses offers the potential of constraining the extent to which the magnitude of changes are associated with time duration.  相似文献   

12.
In this study, we use records of nitrogen isotope ratios (δ15N), UK’37 temperature estimates, organic carbon and opal percentages from high-resolution sediment cores located in the eastern equatorial Pacific (EEP) to explore the mechanisms linking millennial-scale changes in low-latitude sea surface temperature, water column denitrification and surface productivity to the timing of northern or southern polar climate during the last 100,000 yr. Our results support a hypothesis that the Southern Hemisphere, and its connection to the low latitudes via shallow subsurface ocean circulation, has a primary influence on the biogeochemistry of the EEP. In addition, our results suggest that, during the last glacial stage, denitrification rates fluctuated on millennial timescales in response to water-column ventilation rather than upstream oxidant demand in intermediate-depth waters.However, due to the poor age constraints available for Marine Isotopic Stage (MIS) 3, the EEP sedimentary data presented here could support two conflicting mechanisms, one driven by enhanced intermediate overturning circulation in the Southern Ocean during Heinrich Events/Antarctic Warm Events, implying that subsurface flow rates control thermocline ventilation, and a second one consistent with more sluggish intermediate circulation during Antarctic Warm Events and giving a central role to the temperature control on oxygen solubility in Southern Ocean surface waters.  相似文献   

13.
Core Vema 28-238 preserves an excellent oxygen isotope and magnetic stratigraphy and is shown to contain undisturbed sediments deposited continuously through the past 870,000 yr. Detailed correlation with sequences described by Emiliani in the Caribbean and Atlantic Ocean is demonstrated. The boundaries of 22 stages representing alternating times of high and low Northern Hemisphere ice volume are recognized and dated. The record is interpreted in terms of Northern Hemisphere ice accumulation, and is used to estimate the range of temperature variation in the Caribbean.  相似文献   

14.
Reconstructing the environmental and hydrological response to millennial-scale fluctuations of climate-sensitive areas of mid-latitudes is crucial to understanding the Late Pleistocene climate in terrestrial inland regions. We use sedimentological and geochemical proxies (e.g. lithology, gain size, loss-on-ignition, soluble salts, X-ray fluorescence elements) from Balikun Lake in the eastern Tienshan Mountains, to elucidate variations in sedimentation, water chemistry and watershed weathering intensity of the Late Pleistocene interval (~20−60 ka). The record documents high-frequency oscillations and alternating hydrological patterns on (multi-) millennial timescales. Balikun Lake changes from nearly fresh to brackish and finally to hypersaline conditions during MIS3. The warm and wet climate during the early MIS3 facilitated regional vegetation and bioproductivity. Halite sedimentation took place from the middle to late MIS3, indicating a gradual drying trend. This reduced the lake area and vegetation cover, as well as weakened the chemical weathering rates of the watershed. These new interpretations challenge the idea of climate amelioration with the highest lake level and wettest conditions prevailing in the arid areas of north-west China during the late MIS3, indicating a possible westerlies-dominated Late Pleistocene climate in these areas. The evolution of the palaeohydrological regime and climate change in the Balikun Basin correlated well with the millennial-scale high-latitude Atlantic climate superimposed on the Northern Hemisphere summer insolation.  相似文献   

15.
High sedimentation rate (SR) cores retrieved from the South China Sea (SCS) form the basis for studying the marine components of millennial-scale Asian monsoon (AM) variability and for comparison with the AM reconstructions from cave records on land (e.g. Dongge and Hulu). However, carefully correlating the SCS sedimentary records to the cave records that are precisely dated by U/Th methods with resolution of decadal-scale, has not been completed. Such a correlation is essential when comparing AM influences expressed over land and sea, but requires a construction of marine AMS 14C age models that are precise enough to be compared to the cave U/Th age models. For the purpose of establishing such a correlation, this study presents new data from intensive AMS 14C dated marine cores retrieved from the northern SCS (MD972146, MD972148). The discrepancy of marine and cave δ18O record for the interval of ∼18–30 ka might be due to the change of marine 14C reservoir age in SCS surface water during the glacial period, and to the change in interhemispheric dominance of the AM systems. With the new AMS 14C dating on MD972146 and MD972148, we examined the millennial-scale records of planktonic foraminifer δ18O and carbonate contents of MD972146, MD97248, and SONNE 17940-2 and compared those records with Dongge–Hulu δ18O record of the past 30 ka. Our results show that in the intervals corresponding to the high-latitude Northern Hemisphere (NH) Younger Dryas (YD) and Heinrich I event (H1), the AMS 14C dated millennial-scale oscillations show relatively heavy δ18O and low carbonate contents, but H2 and H3 are in the opposite direction. Our results indicate the complexity for the marine cores which were used in interpreting the millennial-scale AM variability.  相似文献   

16.
Predictions of global changes in relative sea level caused by retreat of the Antarctic Ice Sheet from its 18,000 yr B.P. maximum to its present size are calculated numerically. When combined with the global predictions of relative sea-level change resulting from retreat of the Northern Hemisphere ice sheets, the results may be compared directly to observations of sea-level change on the Antarctic continent as well as at distant localities. The comparison of predictions to the few observations of sea-level change on Antarctica supports the view that the Antarctic Ice Sheet was larger 18,000 years ago than at present. The contribution of the Antarctic Ice Sheet to the total eustatic sea-level rise is assumed to be 25 m (25% of the assumed total eustatic rise). If as little as 0.7 m of this 25-m rise occurred between 5000 yr B.P. and the present, few mid-oceanic islands would emerge. If the Antarctic Ice Sheet attained its present dimensions by 6000 yr B.P., however, and if the volume of the ocean has remained constant for the past 5000 years, numerous islands throughout the Southern Hemisphere would emerge. It is suggested that a thorough study of Pacific islands, believed by some to have slightly emerged shorelines of Holocene age, would yield useful information about ocean volume changes during the past 5000 years, and hence on the glacial history of the Antarctic Ice Sheet.  相似文献   

17.
对全球海面变化均衡模式的改进   总被引：5，自引：0，他引：5       下载免费PDF全文
杨学祥 《地质科学》1992,(4):404-408
以J.A.Clark为代表的全球海面变化均衡模式为基础,讨论冰盖形或对地核运动的影响。数值计算表明,由于地核偏离地心的运动,不同海区大约有±1.54m的海面波动。把这个结果叠加在均衡模式的结果之上,会使预测的结果与所观察结果更为符合。  相似文献   

18.
谭明 《第四纪研究》2009,29(5):851-862
根据最近的研究结果,中国季风区同一洞穴或同一区域的晚全新世石笋氧同位素序列具有较好的重复性,表明中国季风区的石笋氧同位素短尺度(10～100年尺度,后同)变率所具有的信号强于噪音。通过进一步比较分析发现,一些石笋氧同位素记录虽然来自距离上千公里之遥的不同区域,但它们的波动形式在年代际至世纪尺度极其相似,说明它们是一致的大区域信号,但多数石笋氧同位素序列未能校准到本区器测的降水或温度记录上。通过与中国气候学家建立的各类季风指数比较,发现这些具有一致大区信号的中国石笋氧同位素序列与海平面气压差指数或海陆温差指数关系最好。再通过与印度洋－太平洋海域海平面气压差、以及许多海、气环流观测记录对比,发现中国季风区石笋氧同位素序列记录了20世纪后期最强的一次全球海、气环流年代际均值突变,由此初步确认了中国季风区石笋氧同位素短尺度变化的环流意义： 当印度洋海水和中东太平洋海水温度偏低时,西太平洋副热带高压偏北东缩而弱,中国季风区内来自印度洋的水汽份额增大,由于这些水汽的输送路程很远,导致中国季风区的雨水氧同位素及石笋氧同位素较轻; 而当印度洋海水和中东太平洋海水温度偏高时,西太平洋副热带高压偏南西伸而强,中国季风区内来自印度洋的远源水汽份额减少,而来自西太平洋的水汽份额增大,由于后者输送路程较近,导致中国季风区的雨水氧同位素及石笋氧同位素较重。由此命名瑞利分馏原理所决定、反映印度洋/太平洋海、气环流转变且大区域一致的中国季风区石笋氧同位素10～100年尺度变化为“环流效应”。  相似文献   

19.
High-resolution clay mineralogical investigations and their comparison with other sedimentological data from ODP Sites 908 and 909 (central Fram Strait) were used to reconstruct the paleoclimate and paleoceanography in the high northern latitudes since the Middle Miocene. Ice rafting has probably occurred since 15 Ma. The comparison of sand-sized components and clay mineral distribution demonstrate that both were not delivered by the same transport process. The input of the clay fraction is related to transport through sea ice and/or oceanic currents. A provenance change at 11.2 Ma is indicated by variations within clay mineral distribution and increased accumulation rates. This is interpreted as a result of an increase in water mass exchange through the Fram Strait. Decreases of the smectite to illite and chlorite ratio at Site 909 suggest a Middle Miocene cooling phase between 14.8 and 14.6 Ma, and a further cooling phase between 10 and 9 Ma. The intensification of glaciation in the Northern Hemisphere is documented by an increase of illite and chlorite from 3.4 to 3.3 Ma, which is synchronous to the onset of major Northern Hemisphere glaciation as indicated by oxygen isotope data.  相似文献   

20.
主要研究了意大利上新统3个剖面Punta Piccola、Cape Spertivento以及Punta Rossello蛋白石含量的变化，综合其它古环境指标对非洲季风和地中海表层生产力加以分析，研究表明，Punta Piccola剖面和Cape Spertivento剖面各古环境指标对岁差周期影响的响应明显而单纯，即岁差低值，对应氧同位素偏负，有机碳和蛋白石高值以及碳酸盐低值，说明地中海表层生产力明显受到岁差的驱动。岁差驱动下的北非夏季风导致尼罗河泛滥，带来的丰富营养元素使得生产力剧增，同时淡水注入，阻碍了水体的垂向交换，直接导致了海底的缺氧环境，硅藻利用深层营养产生勃发，并组成“藻席”，快速沉降到海底，使得有机碳迅速埋葬和保存，从而引发了腐泥层的形成。Punta Rossello剖面硅藻层证实了“藻席”的存在，但是蛋白石含量并不高。 [HT5H]关 键 词:[HT5K] [HT5H]中图分类号： 文献标识码：A[HT5SS][HK]  相似文献   

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