Asynchronous Holocene climatic change across China

A review of Holocene climatic variations in different parts of China shows that they were asynchronous. Proxy data from ice cores, pollen, loess, lacustrine sediments, and changes of sea and lake levels demonstrate that many warm and cold oscillations have occurred in China during the Holocene, including a most important climatic event known as the “Holocene optimum,” a milder and wetter period, and that the duration and amplitude of the optimum period, as well as its start and end times, differed in different parts of China. Uplift of the Tibetan plateau over the past millions of years led to the development of the monsoon climate and to complex atmospheric circulation over continental China during the Holocene. As a result, the Holocene optimum began and terminated earlier in high-altitude regions of western China than at lower elevations in eastern China, and the amplitude of the variations was lower in the east. This suggests that the western higher-altitude areas were more sensitive to climatic change than were the eastern lower-altitude areas. Holocene climatic records in the Dunde and Guliya ice cores do not correspond. Inverse δ¹⁸O variations between the two cores indicate that the effects of climate and atmospheric processes on the stable isotopes at the two sites differed. The correlation between the isotopic composition of carbonates in lake deposits in western China and climatic variations is similar to that in the ice cores. The climatic resolution in ice cores and lake sediments is higher than that in other media. The lack of precise correspondence of climatic records constructed on the basis of proxy data from different parts of China is a result of the different locations and elevations of the sampling sites, the different resolutions of the source material, and the varied climatic conditions within China. Further work is needed to confirm both the conclusions and the inferences presented here.     

Holocene climatic change in the northern Midwest: Pollen-derived estimates

Mapping of Holocene pollen data in the midwestern United States has revealed several broadscale vegetational changes that can be interpreted in climatic terms. These changes include (1) the early Holocene northward movement of the spruce-dominated forest and its later southward movement after 3000 yr B.P. and (2) the eastward movement of the prairie/forest border into southwestern Wisconsin by 8000 yr B.P. and its subsequent westward retreat after 6000 yr B.P. When certain basic assumptions are met, multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation. To maximize the reliability of the regression equations, we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis. Reconstructions of precipitation during the Holocene indicated that from 9000 to 6000 yr B.P. precipitation decreased by 10 to 25% over much of the Midwest, while mean July temperature increased by 0.5° to 2.0°C. At 6000 yr B.P. precipitation was less than 80% of its modern values over parts of Wisconsin and Minnesota. After 6000 yr B.P. precipitation generally increased, while mean July temperature decreased in the north, and increased in the south. The time of the maximum temperature varies within the Midwest and is earlier in the north and later in the south.     

Responses of floods to Holocene climatic change in the upper Mississippi Valley

Dimensions of Holocene relict channels and sedimentological characteristics of point bars associated with these relict channels were used to reconstruct a Holocene history of long-term changes in magnitudes of 1.58-yr floods in Upper Mississippi Valley watersheds of southwestern Wisconsin. The reconstructed record of floods shows relatively large and persistent (nonrandom) departures from contemporary long-term average flood magnitudes. The flood history indicates climatic changes that are broadly similar to climatic changes indicated from fossil pollen in the same region. The Holocene floods ranged from about 10–15% larger to 20–30% smaller than contemporary floods of the same recurrence frequency. Large floods were characteristic between about 6000 – 4500 and 3000 – 2000 yr B.P., and during a brief interval after 1200 yr B.P. Small floods were common between about 8000 – 6500, 4500 – 3000, and 2000 – 1200 yr B.P. These fluvial responses were found to be closely associated with a long-term episodic mobility and storage of sediments in the Wisconsin watersheds. During periods of relatively large floods, relatively rapid lateral channel migration either reworked or removed extensive tracts of valley bottom alluvium. In contrast, during periods of relatively small floods, relatively slow lateral channel migration is apparent and the channel and floodplain system appear to have been relatively stable.     

Isotopic evidence of Holocene climatic change in the San Juan Mountains, Colorado

The δD of cellulose from ¹⁴C-dated wood, collected in the San Juan Mountains of southwestern Colorado, decreased by about 45‰ from 9600 to 3100 yr B.P. and an additional 25‰ to the present. The wood samples are from trees that grew above present-day tree line and reflect a time of warmer average summer temperatures. These changes in δD are interpreted to indicate a major change during the Holocene in the sources of moisture, in the seasonality of precipitation, or in both.     

Evidence of stream response to Holocene climatic change in a small Wisconsin watershed

During the Holocene, moderate climatic and vegetational changes triggered several episodes of adjustment in the Brush Creek fluvial system. The alluvial chronology includes an episode of erosion at 7800 – 5700 yr B.P. corresponding to the mid-Holocene precipitation minimum and an episode of floodplain construction at 5700 – 5000 yr B.P. corresponding to a rapid increase in precipitation. Holocene climatic changes have influenced the sedimentology of the alluvial deposits and soil development on them. Fluvial adjustment is caused primarily by hydrologic and hydraulic changes related to climatic change, but there is no simple model for fluvial response to climatic change. The relationship between the direction of climatic change and the type of fluvial response is complex.     

Climate changes: impacts on geomorphic processes

Climate is one of the conditions which permits, inhibits, retards or enhances certain responses in the geomorphic environment. Components of climate important to geomorphic processes include (1) the duration and character of climate episodes, i.e., the variation of the various climate parameters during any such period, and (2) the nature of discontinuities at both a climate episode's beginning and end, i.e., the magnitude of change and the rate of change occurring at the time of discontinuity. We will evaluate changes in the climate character over the Holocene, compare that to that observed during the instrumental period and, in turn, compare to predicted changes for an atmosphere with twice today's carbon dioxide concentration.     

Marine records of Holocene climatic variations

Holocene millennial climate variability is smaller than that of the last glaciation, due to the disappearance of large unstable ice sheets. Marine records show that the sea-surface temperature (SST) exhibited small variations, mainly in the high and low latitudes. They may be interpreted as a linear response to the mean annual insolation. Major changes in the hydrological cycle have been evidenced in the Asian and African monsoon area, resulting in enhanced precipitation and large river outflow in the Bay of Bengal and the Gulf of Niger. Enhanced rainfall over the Mediterranean Basin resulted in a weak circulation and sapropel formation below 800-m water depth in the eastern Mediterranean Sea. Finally small changes in the thermohaline circulation and the warm North Atlantic Drift have been detected in the Nordic Seas. The Holocene climatic variability is therefore similar to that of the Quaternary, but with small amplitude, while continents experienced large rainfall variations. To cite this article: J.-C. Duplessy et al., C. R. Geoscience 337 (2005).     

Dating climatic episodes of the Holocene

Monitoring evidence indicates that the Holocene embraced a sequence of rather discrete climatic episodes. The transitions between these environmental episodes apparently were abrupt and globally synchronous. This paper reports on statistical analyses of radiocarbon dates associated with environmental change and cultural change.Over 800 ¹⁴C dates associated with pollen maxima and minima, sea level maxima and minima, and top and bottom surfaces of peat beds were simultaneously analyzed to identify times of globally synchronous environmental discontinuities.Some 3700 ¹⁴C dates associated with 155 cultural continua of the world were collectively analyzed to identify worldwide synchroneities in appearance and termination of the cultures.Significant globally synchronous discontinuities were identified in each independent analysis. The dates of environmental and cultural discontinuities are rather similar, particularly during the recent half of the Holocene. The fact that the cultural discontinuities mostly follow rather closely those of the paleobotanical record suggests that there has been a distinct climatic impact on the cultural history of man.     

Sensitivity of the Eastern Mediterranean geomorphic system towards environmental change during the Late Holocene: a chronological perspective

Alluvial and colluvial sediment deposition provide a vital record of environmental change during the Holocene. Firm chronological control on these archives is necessary to enable us to relate sediment dynamics to human activity and climate variability. In the Eastern Mediterranean, such relationships are hard to establish due to the lack of spatially well‐distributed sediment archives with good chronological control. This scarcity is problematic with respect to regional‐scale reconstructions of the temporal variation of sediment dynamics. Here, we present a radiocarbon database (n = 178) of geomorphological activity collected from multiple distinct sediment archives within the territory of Sagalassos in south‐western Turkey. The data were grouped according to their sedimentary facies for analysis using cumulative probability distributions (CPDs) and sedimentation rate (SR) modelling. Two small‐scale colluvial valleys, where chronological information was abundant, were investigated in more detail. Results show that sedimentation chronology differs between individual, nearby cores, as it depends strongly on the local geomorphic situation. A generalizing approach combining multiple core results yields more widely valid conclusions. High sedimentation rates coincided with the initial major anthropogenic disturbance of the landscape and decreased afterwards, probably due to hillslope soil depletion. CPD and SR analysis indicates that in general colluvial sedimentation rates did not change much from 2000 BC onwards. River floodplain sedimentation, in contrast, increased markedly during the first millennium BC and during recent times, and a significant time lag in enhanced sediment deposition between the upper and lower reaches of the river valleys was observed. Copyright © 2012 John Wiley & Sons, Ltd.     

Holocene fluctuations of Lake Ngami,Middle Kalahari: chronology and responses to climatic change

Lake Ngami, a 3000 km² basin at the distal end of the Okavango Delta, Botswana, has a complex suite of shorelines ranging from the lake sump at 919 m asl up to 945 m asl, linked to topographic thresholds and inflows, current and fossil, in the Okavango system. The lake has been ephemeral throughout the 20th century and completely dry since 1982, yet held a substantial body of water up to a level of 930 m asl when visited by European travellers in the mid-nineteenth century. The historical decline in the lake has been well documented and used as evidence for regional desiccation, but the Holocene record, based on archaeological studies and radiocarbon dating of inorganic carbonates, is more speculative.A study of a 30 km² diatomite at the eastern end of the basin indicates that a substantial freshwater lake at a level of at least 932 m asl was present throughout the Holocene, linked to a now abandoned inflow of the Kunyere River. A programme of 18 luminescence age determinations of the peripheral Dautsa and Magotlwanen shoreline sequences suggests that they are old features that have been constructed on a lacustrine substrate dating back to 90–100 ka or earlier, and have been modified by Holocene fluctuations in lake level and subsequent aeolian reworking. The water level peaked at 936 m asl between 4 and 3 ka, returned to 934 m at 2.8–2 ka, ponding water into the Phatane Gap, and fell rapidly thereafter to 932 m and thence to historical levels. The 936 m level can be correlated with increased throughflow in the Okavango Delta, hence increased rainfall in the catchment. The subsequent decline can be attributed to episodic closure and rerouting of Okavango distributaries, in particular the Thaoghe, perhaps aided by anthropogenic activities. As such the desiccation of the lake is not indicative of regional climatic change.     

滇西北格贡错那卡湖沉积记录揭示的晚全新世气候变化

云南省西北部高山湖泊格贡错那卡湖（海拔4214 m）地处西南季风区,对气候变化十分敏感。文章以在格贡错那卡湖湖心钻取的1.84 m长的沉积岩芯为研究对象,对粒度、磁化率和总有机碳（TOC）含量等环境指标进行了高分辨率的分析,在精确测年的基础上（²¹⁰Pb测年结合陆生植物残体的AMS ¹⁴C测年建立年代序列）,重建了研究区距今约3570年来的气候与环境变化。结果表明,格贡错那卡湖所在地区近3570年来的气候与环境变化经历了5个较明显的演化阶段：1）在3570~3490 cal.a B.P.期间,气候明显降温、减湿;2）3490~1710 cal.a B.P.期间,气候总体偏冷、较干,其中3490~2860 cal.a B.P.期间气候处于较稳定的最冷干时期,随后在2860~1710 cal.a B.P.期间气候波动地有所增温、增湿;3）1710~930 cal.a B.P.（即240~1020A.D.）期间,为较稳定的暖湿期,年均温较高,温差较小;4）930~80 cal.a B.P.（即1020~1870A.D.）期间,气候总体偏冷较湿,但存在多次短暂变暖;降水总量或冰雪融水可能有所减少,但降水强度或冰雪融水强度变化较大;5）自80 cal.a B.P.（即1870 A.D.）以来,温度明显增加,可能是过去3570年以来温度最高的时期;降水有所减少,气候总体为暖偏湿。该气候特征与相邻地区泸沽湖的乔木花粉含量反映的气候变化非常一致,揭示了研究区气候变化的区域性。

     

Beijing international symposium on climatic change

Reports

Beijing international symposium on climatic change     

Palynological data on major Holocene climatic events in NW Iberia

Three NW Iberia Cantabrian Mountain pollen records are presented. They reflect the main Holocene climatic shifts in the North Atlantic region as recorded in the isotopic data from Greenland ice, Irish speleothems and reconstructed sea surface temperatures. Two brief forest regression episodes reconstructed from pollen may be synchronous with GH-11.2 and GH-8.2 events. At mid-altitude, two woodland expansion phases (7000-6000 ¹⁴C yr BP and 4000-2500 ¹⁴C yr BP) are separated by a phase of heaths and peat deposits. Major woodland declines occurred during the Galician-Roman Period (which includes the Bronze Age, the Iron Age and the Roman occupation) and from the end of the Medieval Period. The pollen data, backed up by archaeological and historical sources, suggest climatic impact of the Iron Age Cold Period, but are indecisive concerning the Little Ice Age. However, the pollen records do not support any significant 'Neoglacial' period (4000-3000 ¹⁴C yr BP) influence on NW Iberia.     

Lateglacial–Holocene shoreface progradation offshore eastern Scotland: a response to climatic and coastal hydrographic change

Seismic facies, provenance and marine faunal associations of a nearshore prograding sediment wedge offshore eastern Scotland are studied to investigate environmental changes in the adjacent North Sea during Lateglacial–Holocene time. The sediments form part of the St. Andrews Bay Member (Forth Formation), which is divided into four lithozones (L1–L4) that represent distinct pulses of sedimentation during the sequential growth of the sediment wedge. Radiocarbon dates, combined with the local curve of relative sea level change, indicate that progradation was initiated as a fluvio-deltaic deposit (L1) during the Younger Dryas Stadial. Further construction of the sediment package took place during the mid- to late Holocene by sublittoral tidal processes that deposited three discrete, highstand shoreface wedges (L2–L4), which display both progradation and longshore migration (to the NE), and may have experienced episodic brackish marine conditions. A depositional cyclicity of about 1000 years is proposed for lithozones L2–L4, separated by hiatuses of 1000–2000 years. We tentatively suggest that the Holocene development of the prograding wedge offshore eastern Scotland was a response to phases of strong westerly winds driving an enhanced influx of Atlantic Water into the North Sea. A concomitant increase in rainfall may account for the freshening of the coastal zone at this time. However, correlation with the recently postulated global periods of Holocene rapid climate change remains unclear.     

Nondeterministic theories of climatic change

A basic assumption in some climatic theories is that, given the physical properties of the atmosphere and the underlying ocean and land, specified environmental parameters (amount of solar heating, etc.) would determine a unique climate and that climatic changes therefore result from changes in the environment. The possibility that no such unique climate exists and that nondeterministic factors are wholly or partly responsible for long-period fluctuations of the atmosphere-ocean-earth system, is considered. A simple difference equation is used to illustrate the phenomena of transitivity, intransitivity, and almost-intransitivity. Numerical models of moderate size suggest that almost-intransitivity might lead to persistence of atmospheric anomalies for a whole season. The effect of this persistence could be to allow substantial anomalies to build up in the underlying ocean or land, perhaps as abnormal temperatures or excessive snow or ice. These anomalies could subsequently influence the atmosphere, leading to long-period fluctuations. The implications of this possibility for the numerical modeling of climate, and for the interpretation of the output of numerical models, are discussed.     

Impact of monsoonal climatic change on Holocene overbank flooding along Sushui River,middle reach of the Yellow River,China

In the semiarid loess regions, slackwater deposition of overbank flooding over the piedmont alluvial plains was episodic and alternated with dust accumulation and soil formation throughout the Holocene. The records of past hydrological events are therefore preserved within the architecture of loess and soils and are protected from subsequent erosion and destruction. Several Holocene loess–soil sequences with the deposits of overbank flooding over the semiarid piedmont alluvial plains in the southeast part of the middle reaches of the Yellow River drainage basin were investigated by field observation, OSL and C¹⁴ dating, measurement of magnetic susceptibility, particle-size distribution and chemical elements. This enables the reconstruction of a complete catalog of Holocene overbank flooding events at a watershed scale and an investigation of hydrological response to monsoonal climatic change as well. During the Holocene, there are six episodes of overbank flooding recorded over the alluvial plain. The first occurred at 11,500–11,000 a BP, i.e. the onset of the Holocene. The second took place at 9500–8500 a BP, immediately before the mid-Holocene Climatic Optimum. After an extended geomorphic stability and soil formation, the third overbank flooding episode came at about 3620–3520 a BP, i.e. the late stage of the mid-Holocene Climatic Optimum, and the floodwater inundated and devastated a Bronze-age town of the Xia Culture built on the alluvial plain, and therefore the town was abandoned for a period of ca 100 years. During the late Holocene, the alluvial plain experienced three episodes of overbank flooding at 2420–2170, 1860–1700 and 680–100 a BP, respectively. The occurrence of these overbank flooding episodes corresponds to the anomalous change in monsoonal climate in the middle reaches of the Yellow River drainage basin when rapid climate change or climatic decline occurs. During at least the last four episodes, both extreme floods and droughts occurred and climate departed from its normal condition, which was defined as a balanced change between the northwestern continental monsoon and southeastern maritime monsoon over time. Great floods occurred as a result of extreme rainstorms in summers caused by rare intensive meridianal airflows involving northwestward moving tropical cyclone systems from the Pacific. These results could be applied to improve our understanding of high-resolution climatic change, and of hydrological response to climatic change in the semiarid zones.     

Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves

Activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U, and ²³⁰Th/²³²Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of ²³⁰Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.