Late Holocene mangrove development and response to sea level change in the northwestern South China Sea

Mangroves,widely distributed along the coasts of tropical China,are influenced by Asia monsoon,relative sea level change and enhanced human activity.To predict the impacts of future climate change on mangrove ecosystems,it can be understood by reconstructing past mangrove dynamics using proxies preserved in coastal sediments.In this study,we quantitatively partitioned buried organic matter(OM)sources,collected from a vulnerable mangrove swamp in the Qinzhou Bay of northwestern South China Sea,using a ternary end-member mixing model of δ^13C and C:N values.Mangrove-derived OM(MOM)contribution was used as a tracer for mangrove development since 2.34 cal ka BP.This information,together with paleoclimate records(i.e.,speleothem δ^18O values,sea level change,grain size parameters)and human activity,was used to divide mangrove development into three stages during the late Holocene:relative flourish(2.34-1.13 cal ka BP),relative degradation(1.13-0.15 cal ka BP)and further degradation(0.15-0 cal ka BP).Before 1.13 cal ka BP,mangroves flourished with a high MOM contribution((88.9±10.6)%),corresponding to stable and high sea level under a warm and humid climate.After 1.13 cal ka BP,rapid fall in relative sea level coupled with the strengthening of the Asian winter monsoon,resulted in mangrove degradation and MOM reduction((62.4±18.9)%).Compared with air temperature and precipitation,the relative sea level fall was the main controlling factor in mangrove development before entering the Anthropocene(the time of the Industrial Revolution).After^150 cal a BP,reclamation of mangrove swamps to shrimp ponds is the main factor causing mangrove degradation and MOM reduction.

Regional Moisture Source Changes Inferred from Late Holocene Stable Isotope Records

Qinghai Lake, China, is located near the northern limit of the East Asian summer monsoon （EASM） and thus is an ideal region for studies of past monsoonal changes. However, isotope records from this region reflect the combined effects of multiple climatic factors, and make climatic interpretations difficult. The authors use multi-proxy records, generated from the same sediment core from Qinghai Lake, to disentangle these multiple effects in isotope records and to infer EASM variability during the late Holocene. Records of leaf wax （C2s） δD, lake carbonate 5180 and the Dunde ice core δ18O all indicate a millennial-scale depletion of mean isotopic values at -1500-1250 years before present. Compared with independent lake temperature and salinity records, the authors suggest that this depletion of long-term mean isotopic values must have resulted from changes in moisture sources in this region. In contrast, the authors attribute high-frequency （centennial timescale） C2s δD and ice core δ18O variability dominantly to a temperature effect. The multiproxy records provide a coherent picture in that many aspects of this regional climate （temperature, dryness, and moisture source） are strongly linked to the EASM variability.

A late Holocene winter monsoon record inferred from the palaeo-aeolian sand dune in the southeastern Mu Us Desert, northern China

The variation of the Asian winter monsoonal strength has seriously affected the climate and environmental conditions in the Asian monsoonal region, and even in marginal islands and the ocean in the East Asian region. However, relevant under-standing remains unclear due to the lack of suitable geological materials and effective proxies in the key study areas. Here, we present a grain-size record derived from the palaeo-aeolian sand dune in the southeastern Mu Us Desert, together with other proxies and OSL dating, which reflect a relatively detailed history of the winter monsoon and abrupt environmental events during the past 4.2 ka. Our grain-size standard deviation model indicated that 〉224 μm content can be considered as an indicator of the intensity of Asian winter monsoon, and it shows declined around 4.2–2.1 ka, enhanced but unstable in 2.1–0.9 ka, and obviously stronger since then. In addition, several typical climate events were also documented, forced by the periodic variation of winter monsoonal intensity. These include the cold intervals of 4.2, 2.8, 1.4 ka, and the Little Ice Age （LIA）, and relatively warm sub-phases around 3.0, 2.1, 1.8 ka, and the Medieval Warm Period （MWP）, which were roughly accordant with the records of the aeolian materials, peat, stalagmites, ice cores, and sea sediments in various latitudes of the Northern Hemisphere. Combined with the previous progresses of the Asian summer monsoon, we prelimi-narily confirmed a millennial-scale anti-correlation of Asian winter and summer monsoons in the Late Holocene epoch. This study suggests that the evolution of the palaeo-aeolian sand dune has the potential for comprehending the history of Asian monsoon across the desert regions of the modern Asian monsoonal margin in northern China.

A New High-resolution Late Glacial-Holocene Climatic Record from Eastern Nanling Mountains in South China

A 350-cm-long sediment core sequence from Dahu Swamp situated in the eastern Nanling Mountains was selected for high-resolution paleoclimatic reconstruction since the Late Glacial period. The multi-proxy records of this paper reveal several evidently dry and cold events that may coincide with the Oldest Dryas, the Older Dryas, the Younger Dryas in the late deglacial period. Two relatively wetter and warmer phases occurred in ca. 15,000-14,400 cal yr B.P. and 13,500-12,800 cal yr B.P. respectively may correspond to the Boiling and Allerod warming events. The Younger Dryas event （ca. 12,800-11,500 cal yr B.P.） revealed by multi-proxies was characterized by relatively colder and drier climate. A warmer and wetter climate, occurred in ca. 10,000~5000 cal yr B.P., was consistent with the Holoeene Optimum, which coincided with the maximum Northern Hemisphere insolation. The ＂8.2kyr cool event＂ and even the ＂8.8kyr cool event＂ were indicated as well from our sediment core. A dry mid-Holocene period （ca. 60000 3000 cal yr B.P.） indicated by multi-proxies does not follow the traditional concept of the wet mid-Holocene conditions observed in other regions in China.

Geochemical weathering of aeolian sand and its palaeoclimatic implications in the Mu Us Desert,northern China,since the Late Holocene

In the semi-arid and arid regions of northern China,geochemical behavior of the aeolian deposit is closely related to climatic and environmental changes,which was used to reconstruct the past history of environmental evolution and possibly forcing mechanisms.However,the related result was still scarce due to the lack of detailed geochemical analysis results in the desert sediments.In the present study,we systematically analyzed the geochemical components and parameters of the paleo-aeolian sand dune and modern mobile sand deposits in the Mu Us Desert and discuss the climatic variation inferred from the paleo-aeolian sand dune during the past 4.2 ka BP.The results indicated that （1） geochemical composition of the sandy deposits were dominated by SiO_2,Al_2O_3 and Na_2O and the deposits probably originated from the widespread upper continental crust （UCC） and were formed by long-term weathering,transport and re-deposition;（2） these sandy deposits were subjected to weaker weathering or uneven weathering under cold and dry conditions,and had highly similar material sources and degrees of weathering and leaching in general;and （3） the direct OSL （Optically Stimulated Luminescence） dating ages and geochemical parameters from the palaeosol-aeolian sand dune indicated that the regional climate change experienced several typically cold and warm intervals.These intervals are 4.2,2.8 ka BP and Little Ice Age and Medieval Warm Period,which probably attributed to periodic variations of the Asian summer monsoonal strength and cold events of the northern Atlantic Ocean in low and high latitudes of the Northern Hemisphere.Our results suggest that the development of the sand dune in the Mu Us Desert provided a suitable archive for understanding the past local climatic change,which is linked to the global climatic change.

Large-Scale Fluidization Features from Late Holocene Coseismic Paleoliquefaction in the Willamette River Forearc Valley, Central Cascadia Subduction Zone, Oregon, USA

A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems.

Coastal landscape evolution tracked from late Holocene wave-cut benches in the Sinan Archipelago,Southwest Korea

Glacio-isostatic adjustment(GIA)and tectonic activity are important factors in the formation of marine terraces.Late Holocene wave-cut benches in the eastern part of the West Sea of Korea,also called the Yellow Sea,can be divided into two steps:531 cm above sea level(ASL)for the upper bench(T2)and 464-481 cm ASL for the lower bench(T1).Sediments on the benches a re classified into four units,and are interpreted to be beach deposits acco rding to gravel shape,textu re,and seaward inclination.The ove rlying sediment indicates that T2 was formed at approximately 530 cm ASL before 2900 yr BP,and T1 at approximately 460-480 cm ASL before 1520 yr BP.Late Holocene(4000-2000 yr BP)relative sea level(RSL)curves based on GIA models are inconsistent with the wave-cut bench elevations.Comparing T1 and T2 benches to the RSL curves of the West Sea,the upper and the lower benches were uplifted by approximately 5-8 m and 4-7 m,respectively.Although the area is several hundred kilometers away from plate boundaries,the high frequency of earthquakes in the West Sea may have induced the uplift of wave-cut benches during the last 2000 years.These indicate that the west coast of the Korean Peninsula(KP)should no longer be considered an area of subsidence,but be assigned to a regime of uplift during the late Holocene.

Late Pleistocene and Holocene Glacier Extent in the Georgian Caucasus

This article presents the information of the Georgian Caucasus glaciation at the Late Pleistocene and Holocene period (~126,000-11,700 calendar years ago). Our primary aim was to numerically reproduce the ice extent deduced from geological and geomorphological mapping. We used the analog method with the 30 m resolution SRTM DEM (Shuttle Radar Topography Mission Digital Elevation Model). In addition, the rates of glaciation of those times are identified based on the stadial moraines and erratic boulders. The current investigation has revealed that in the Late Pleistocene, the central and western Caucasus characterized the highest glaciation, while the eastern Caucasus boasted the lowest glaciated area, and in the southern Georgia’s highland glaciation has almost the same form as there is in the Eastern Caucasus today. The longest glaciers were located in the Enguri (Nenskra ~36 km, Mulkhura ~35.1 km, Dolra ~34.5 km), Kodori (Sakeni ~25 km, Klichi ~20 km, Marukhi ~17.3 km) Rioni (Buba ~23 km, Kirtisho ~20.5 km, Jejora ~17.5 km) and Tergi (Devdoraki ~38.5 km, Suatisi ~32 km) river basins. We found, that topography thresholds related to the elevation and hypsometry of individual catchments controlled the gradient of the rate of glacier expansion in the domain at that time.

Late Holocene glacier variations indicated by theδ18O of ice core enclosed gaseous oxygen in the central Tibetan Plateau

Theδ18O of ice core enclosed gaseous oxygen(δ18Obub)has been widely used for climate reconstruction in polar regions.Yet,less is known about its climatic implication in the mountainous glaciers as the lack of continuous record.Here,we present a long-term,continuousδ18Obub record from the Tanggula glacier in the central Tibetan Plateau(TP).Based on comparisons of its variation with regional climate and glacier changes,we found that there was a good correlation between the variation of theδ18Obub in this alpine ice core and the accumulation and melting of this glacier.The more developed the firn layer on glacier surface,the more positive theδ18Obub.Conversely,the more intense the glacier melting,the more negative theδ18Obub.Combined with the chronology of ice core enclosed gases,the glacier variations since the late Holocene in the central TP were reconstructed.The result showed that there were four accumulation and three deficit periods of glaciers in this region.The strongest glacier accumulation period was 1610-300 B.C.,which corresponds to the Neoglaciation.The most significant melting period was the last 100 years,which corresponds to the recent global warming.The Medieval Warm Period was relatively significant in the central TP.However,during the Little Ice Age,there was no significant glacier accumulation in the central TP,and even short deficit events occurred.Comparisons of the late Holocene glacier variation in the central TP with glacier and climate variations in the TP and the Northern Hemisphere showed that it was closely related to the North Atlantic Oscillation.

Late Holocene Environmental History of Lake Victoria Basin： Evidence from Geochemical Proxies

Sedimentary TOC （total organic carbon）, TN （total nitrogen） and C/N （carbon/nitrogen） at selected sites of Lake Victoria basin have provided evidence of phytoplankton productivity, input of nitrates and allochthonous （land） plant materials/catchment destruction in the basin during the late Holocene period （last 4,000 years to present）. TOC and TN in the sediment cores were determined using EuroEA3000 Series Elemental Analyser and radiocarbon dating done using AMS （accelerator mass spectrometry） standard method. Results showed that TOC （13.45%-20.78%） and TN （1.07%-1.92%） values were higher at Napoleon Gulf from about 4,186 years before present （ca. 4,186 yr. BP） to present than at inlet of river Kagera （TOC： 3.27%-6.32% and TN： 0.30%-0.59%） from about 190 years before present （ca. 190 yr. BP） to present. Generally TOC and TN had a negative correlation （r = -0.37, p = 0.04, n = 32） at Napoleon Gulf in the last 4,186 years to present signifying that C/N ratios were governed mostly by phytoplankton productivity which markedly increased during some periods in the last 370 years to present possibly as a result of increased input of nitrates. However, the periods from about 4,186 years to 1,684 years before present （ca. 4,186 yr. BP to 1,684 yr. BP） and some periods between the last 370 years to present indicated increased input of allochthonous plant materials/catchment destruction. On overall, there was a positive correlation between C/N and TOC at inlet of river Kagera （r = 0.57, p = 0.01, n = 20） in the last 190 years to present possibly signifying that C/N ratios were governed mainly by input of allochthonous plant materials hence increased catchment destruction. Input of land plant materials （catchment destruction） along Kagera basin declined during some period after 1950 AD. However the most recent years have shown increased input of land plant materials （catchment destruction） in Kagera basin.

Late Holocene Isotopic and Molecular Signals in Saline Sediments of Lake Cantara South,Australia

Isotopic,biomarker,and trace elements of a 65 cm long sedimentary core from Lake Cantara south in Adelaide,Australia were measured in an effort to trace environmental change clues in the southern Hemisphere.The geochemical

Environmental anomalies in the northeastern East China Sea during the last 3 000 years:implications for El Nio activity in the Holocene

To reconstruct the productivity changes for the last 10 500 a in the northeastern East China Sea （ECS）, biogenic compounds （such as carbonate, organic carbon and opal）, marine micropaleontological fossils （planktonic foraminifera, benthic foraminifera, radiolarian and silicoflagellate） and the compositional characters of benthic foraminifera fauna analyses were carried out on a sediment core DOC082 obtained from the western slope of Okinawa Trough （29°13.93′N, 128°08.53′E; 1 128 m water depth）. The long-term changes of biogenic and micropaleontological proxies display some similarities through the last 10 500 a, which show three different phases： lower values are recorded during the early and middle ttolocene （before about 4 000 a BP）, followed by an abrupt and remarkable increase at about 4 000 a BP, the late Holocene （after about 3 000 a BP） is characterized by continuously high values. The multi-proxy data of paleoproductivity and percents of benthic foraminifera genera （Uvigerina and Bulimina） show that during the early and middle Holocene （10 500-4 000 a BP） productivity was relatively low with a sudden and distinct increase at about 4 000 a BP, and the late Holocene （3 400-0 a BP） is marked by significantly higher productivity. Also, the radiolarian-based sea surface temperature （SST） records reveal a distinct decline in SST in the late Holocene after 3 200 a BP, very different from the early and middle Holocene. For the last 3 000 a, the enhanced biological productivity and distinctly lower SST indicate a major change of oceanographic conditions in the northeastern ECS. These marine environmental anomalies are consistent with other paleoclimatic records for the late Holocene in the Chinese continent and its surrounding regions. After analyzing the mechanisms of modern productivity and SST changes in the northeastern ECS, and based on the climatic anomalies in the Chinese continent and variations in the Kuroshio Current during modern El Nino periods, we suggest that the anomalous environmental conditions in the northeastern ECS may imply intensified El Nino activity during the late Holocene.

Sediment distribution and provenance since Late Pleistocene in Laizhou Bay,Bohai Sea,China

There are three transgression-regression events and evolutions of the sedimentary environment by sea level changes since the Pleistocene in the southern section of the Bohai Sea, China. It is obvious that a multi-source fluvial delta sedimentary system may be more dominant in a sedimentary environment. Based on previous research and survey or historical data, we carried out studies on the division of sedimentary units, sedimentary facies analysis and strata division and comparison, which aim to establish the sedimentary stratigraph of Laizhou Bay. We focus on the sedimentary procession of the Laizhou Bay since the early Late Pleistocene. It can be divided into two glacial periods and three interglacial periods, corresponding to two regression and three transgression events in Laizhou Bay since Late Pleistocene. In 124.6-72.0 ka BP, 60.0-24.4 ka BP and 10.2-4.0 ka BP, three times warm-wet periods occurred, respectively corresponding to the Cangzhou transgression, Xianxian transgression, and Huanghua transgression. In 72.0-60.0 ka BP and 24.4-10.2 ka BP, two dry and cold periods, it was the continental sedimentary environment, corresponding to Wurm early glacier and Wurm late glacier. The results show:(1) Sediments have the characteristics of phase and stage under the terrestrial input of the Yellow River and middle-small rivers in the southern section of the Bohai Sea.(2) PI moved towards coastal in Cangzhou transgression strata in early Late Pleistocene. PI moved northward from land in Xianxian transgression strata in the late Pleistocene. PI moved further north in the Huanghua transgression strata in Holocene.(3) During the regressive period, the land source input increased and the estuarine or lagoon sedimentation developed, which manifested as progradational superposition.(4) During the transgressive period, it mainly developed shallow coastal sediment and transitionally formed regressive deposition to the south in delta/tidal flat deposition.

Palaeoclimatic evolution recorded by multidicipline in sediments in gahai lake, qaidam basin since late last deglacial

Based on the analyses of Loss-on-Ignition （LOI）, carbonate content and sediment characteristics, this paper reconstructs the climatic and environmental evolution of the Gahai lake area since the late Last Deglacial Period. The results cover the late Last Deglacial Period and the Holocene. The climate was very unstable and rather arid during the late Last Deglacial Period. The Holocene can be divided into three periods： the early Holocene （11360-8240 cal aB. P. ）, which was dry but unstable and featured rising temperatures, the mid-Holocene （8240-3200 cal aB. P. ）, which was warm and wet and the late Holocene （ since 3200 cal aBP）, which was cold and dry. Results also show a warm-wet event around 1500 cal aB. P.

East Asian summer monsoon changes in subtropical China since late Pleistocene:Evidence from the Ailuropoda-Stegodon fauna

Previous studies on the amplitude of East Asian summer monsoon(EASM)changes mainly focused on northern China(represented by the Loess Plateau).However,a rare investigation centered on the subtropical zone of southern China,where the important route for EASM moved northward or southward,especially addressing a lack of the research on mammals.The Ailuropoda-Stegodon fauna is a representative mammal fauna in southern China since the late Pleistocene.It indicates the southern mid-subtropicaltropical forest environment with tropical climate characteristics,and its southward or northward movements in the subtropical zone imply the changes of EASM intensity.Based on previous research,combined with the species characteristics and distribution range of the Ailuropoda-Stegodon fauna with tropical animals during the marine isotope stage 5(MIS5),MIS3,MIS2,and MIS1 Megathermal periods,this paper mainly investigates its evolution and the fluctuations of EASM.The conclusions include:(1)The distribution geometric centers in the MIS5,MIS3,MIS2,and MIS1 Megathermal periods are(26°14′N,111°22′E),(24°35′N,107°30′E),(22°48′N,112°01′E),and(26°19′N,112°25′E),respectively.(2)Compared with the MIS5 period,the EASM of the MIS3 and MIS2 moved 180 km and 380 km southwards,and that of the MIS1 Megathermal period moved about 10 km northwards.The EASM movement indicated by the fauna migration happened synchronous with the climate records of stalagmites in subtropical China,the sporopollen from eastern China,and oxygen isotopes from Sulu Sea.They also correspond to the peaks and valleys of the total solar radiation at 35°N in the winter half-year.This suggests that the migrations of the Ailuropoda-Stegodon fauna are consistent with the changes of the global climate,and the driving force is mainly from the changes of total solar radiation at 35°N in the winter half-year.

Geomorphological Evolution and Palaeoenvironmental Change in the Western Alashan Plateau, China

Although neotectonic activity is considered to be the main factor of the terrain evolution of the Qinghai Tibet Plateau and its surrounding high-altitude areas,further geomorphological analysis and literature analysis are needed for the understanding of the geomorphic evolution and the Quaternary environment change of the western area of the Alxa Plateau near the northern Tibet Plateau.The purpose of this study is to investigate the distribution of site-specific geomorphic units of the landforms developed in the vast topography of Ejina Basin(Western Alxa),in order to identify the geostructural and climatic causes of the geomorphic landscape and its impact on the change of paleoenvironment.At present,the climate and hydrological conditions in Ejina are relatively monotonous and stable.In addition to tectonic dynamic factors,the most widely distributed landform in the basin is climate landform.There are both geomorphological and sedimentological anomalies of Aeolian landforms occurred in the whole basin,indicating that the underlying surface effect(retention effect)of river(Ejina River)and its related uneven ground and weak wind erosion(deflation)process in the nearby area may be the important factors controlling the formation of Ejina dunes,rather than the arid climate.It is believed that the extensive interaction between the aeolian and fluvial processes is the main mechanism of the regional geomorphic difference in Ejina Basin.According to the comparability of regional geomorphology and sedimentology,the period of the formation of relic geomorphology in the edge of Ejina Basin can be reasonably attributed to the local glacial maximum of the last glacial.The geomorphic transformation from quasi plain and desert valley to desert plain,the appearance of widely moving sand dunes and the presence of large ancient lake geomorphology all indicate that the drought index of Ejina Basin is increasing on the scale of geomorphic formation.Paleogeomorphological and chronological evidences show that the climatic and hydrological conditions of the basin in the last glacial period and the early Holocene are much better than those at present.For example,the average annual precipitation in the area before 39-23ka BP is between 60-350 mm(about 36 mm today),but there are large waves in the Holocene.The coexistence of various climates and landforms in Ejina Basin and the resulting geomorphic diversity should be the composite result of various geomorphic processes and surface processes besides glaciation.The low aridity(relative humidity)in the Ejina Region in the late Pleistocene may be the result of the enhancement of the westerly rain belt and the weakening of the Asian Winter Monsoon in the arid region of Central Asia.

孢粉记录的晚全新世以来大兴安岭北部多年冻土泥炭地演化及其影响因素

当前泥炭所揭示的区域古植被、古气候与环境变化相关工作已广泛开展,然而对于泥炭地本身演化的研究却很少涉及,尤其是对于大兴安岭北部多年冻土泥炭地演化及其影响因素仍不清晰,阻碍了人们对这一特殊类型泥炭地历史动态和未来发展趋势的认知。为此基于大兴安岭北部多年冻土泥炭岩芯孢粉证据,利用AMS14C测年技术,重建了区域3500 cal a BP以来植被与气候历史,并与其他古气候指标进行对比,从而揭示区域泥炭地演化及其影响因素。结果表明:3500—2900 cal a BP植被以松属、喜暖乔木及水龙骨科为主,气候温暖湿润成为泥炭孕育期;2900—2250 cal a BP植被以松属、喜暖乔木及蒿属为主,气候温暖潮湿成为泥炭发育启动期;2250—1650 cal a BP植被以松属、桦属及水龙骨科为主,气候寒冷湿润成为泥炭发育旺盛期;1650—1150 cal a BP植被以松属和蒿属为主,气候寒冷干燥成为泥炭发育减缓和停滞期;1150—750 cal a BP阔叶林和湿地植被扩张,气候温暖湿润成为泥炭发育再次启动期,完成由富营养沼泽到中营养沼泽类型的转变;750 cal a BP至今植被以松属、桤木属及莎草科为主,气候寒冷湿润成为泥炭发育再次旺盛期,中营养沼泽开始向贫营养沼泽类型过渡。造成多年冻土泥炭地演化的主导因素并不是地质地貌变动和人类活动,而是来自气候变化及其驱动下多年冻土环境的改变,因此气候变化才是影响多年冻土泥炭地演化的主动力,未来多年冻土泥炭地变化取决于全球气候的发展。

贵州黑洞4750年以来高分辨率石笋δ^(13)C记录

【目的】全球变暖背景下,喀斯特地貌广布的西南地区可能面临石漠化加剧的风险,对该区域石漠化演变历史的重建具有重要意义。【方法】通过采自贵州省黔西县黑洞一支石笋(HD12)的29个230Th年龄和954个δ^(13)C数据,重建了该地区过去4 750年的生态环境演变历史。【结果与结论】发现在4 322~3 526 a B.P.以及803~82 a B.P.时段存在两个显著的δ^(13)C正偏移,说明这两个时段洞穴上方的生态环境出现了恶化。HD12石笋δ^(13)C记录在约803 a B.P.的显著偏正持续了约290 a,其振幅达4.2‰,指示了该区域石漠化的扩张过程。这一时期西南地区多个洞穴石笋δ^(13)C值的一致偏正特征,可能指示了宋末靖康事件(823 a B.P.)后,人口的大量迁入和气候的干旱化导致了该区域石漠化的扩张。HD12石笋δ^(13)C值在4 322~3 526 a B.P.时期的偏正,振幅达4.9‰,其中4 322~3 977 a B.P.偏正过程对应于北半球4.2 ka事件,而3 777~3 526 a B.P.的偏正对应3.7 ka事件,两个时期的干旱事件在西南地区的多个石笋与湖泊记录中均有体现,说明在此期间,亚洲夏季风减弱,降水减少可能引起了该区域植被覆盖度大幅降低和土壤严重退化。

基于钻孔记录探究古云梦泽沉积面貌

古云梦泽是晚全新世时期江汉平原与长江最主要的水沙交换载体,其形成演化特征为沉积物所记录,蕴藏了丰富的古环境、古气候信息。为了探讨古云梦泽沉积演化的规律和模式,基于2014~2019年江汉平原野外钻孔采样数据,对江汉平原钻孔沉积物的理化指标包括粒度、总有机碳、磁化率等进行分析,结合加速器质谱碳十四(AMS^(14)C)精准测年和粒度频率曲线得到晚全新世时期江汉平原整体的沉积环境变化序列,进而探讨古云梦泽沉积演化的规律和模式。结果表明:根据钻孔沉积相记录,古云梦泽形成演化过程中出现了“河流-湖泊-三角洲”复合沉积模式;钻孔横向对比发现,三角洲相沉积最先形成于江汉平原西部,并逐渐向东部扩展,表明这种复合沉积模式不仅仅只影响单个钻孔的沉积序列,还在空间上调控了整个古云梦泽的演化。研究成果揭示了历史时期全球变化背景下江汉平原河湖系统自然演变的特征与规律,对于指导江汉湖群的生态恢复和预测未来长江中下游地区江湖关系演变趋势具有重要的意义。

Late Holocene cooling event in the western Pacific

Cores 255, 170, 17940-2, raised from the Okinawa Trough and South China Sea, have been studied for planktonic foraminifers. Among all the species, Pulleniatina obliquiloculata is shown to be sensitive to winter sea surface temperature in the late Quaternary in the western Pacific. Its relative abundance fluctuations are significant and correlatable between the cores. The most conspicuous change during Holocene is the P. obliquiloculata minimum zone around 4-2ka B. P., which correlates probably to the neoglacial cooling. The widespread occurrence of this cooling event in the western Pacific suggests that P. obliquiloculata is promising as a paleoceanographic and climatic monitor, possibly important for reconstructing sea-land correlation of climate.