Radiocarbon reservoir age of high latitude North Atlantic surface water during the last deglacial

The radiocarbon reservoir age of high latitude North Atlantic Ocean surface water is essential for linking the continental and marine climate records, and is expected to vary according to changes in North Atlantic deep water (NADW) production. Measurements from this region also provide important input and/or tests of oceanic radiocarbon using 3-D global ocean circulation models. Here, we present a surface water radiocarbon reservoir age record of the high latitude western North Atlantic for the deglacial period via the use of fossil cold-water corals growing in waters that are rapidly exchanged with nearby surface waters. The reservoir age of high latitude North Atlantic surface waters was computed from the radiocarbon age difference between our radiocarbon calibration record (http://radiocarbon.LDEO.columbia.edu) and our marine radiocarbon data. ²³⁰Th/²³⁴U/²³⁸U dates provide the absolute coral ages. Our high latitude North Atlantic Ocean reservoir age data combined with recalculated reservoir ages based on published coexisting terrestrial and marine material and Vedde ash radiocarbon dates from central and eastern North Atlantic show modern values (380±140 year, n=14) during the Bolling and Allerod warm period and a 200 year increase in reservoir age (590±130 year, n=10) during the entire Younger Dryas (YD) cold episode. The reservoir age then decreased to 270±20 year (n=2) at the Preboreal/YD transition, although the dates are too sparse for us to be confident in this estimate. We are not able to resolve the timing of the transition to increased reservoir ages from the mid-Allerod to the YD due to the relatively small change and correspondingly large uncertainty in the estimates. The atmospheric Δ¹⁴C record derived from our atmospheric radiocarbon record displays a 40 per mil increase from 12,900 to 12,650 cal years BP, coincident with the shift to high reservoir ages in the early YD cold event. Intrusion of ¹⁴C depleted Antarctic Intermediate Water (AAIW) to the high latitude North Atlantic and reduction of NADW formation are possible causes for the coincident shift to high reservoir ages in the North Atlantic surface ocean and increased atmospheric Δ¹⁴C during the beginning of the YD event.     

Sea level changes in the holocene

Over the last 30 years the emphasis in studies of the recovery from low last glaciation sea levels has changed significantly. The search for a eustatic sea level curve having global relevance has ended. Studies into the rheology of the earth's crust, and recognition that the geoid has not remained stable over time, have resulted in the recognition that there must have been regional differences in eustatic response to deglaciation. As a part of this re-appraisal there has been a growing appreciation that crustal isostatic response to the removal of the weight of ice sheets has been accompanied by a consequential hydro-isostatic response, particularly in the areas of the shelf seas. In the later part of the post-war period attention has additionally been focussed on the much greater potential for error over the whole field of palaeoenvironmental reconstruction in the Holocene, including not only errors in dating, but also a large number of possible sources of errors in heighting. As a result of this, an increasing number of scientists are withholding judgement on the nature of sea level rise and, more particularly, on the problem of higher than present late Holocene eustatic sea levels. The problems outstanding in the early 1960s have not yet been resolved but the bases of uncertainty have changed.     

Climatic changes during the last 35,000 years as indicated by land, sea, and air data

Detailed climatic records from land (glaciation curves, fossil records, etc.), sea (eustatic changes, deep-ses date), and air (Greenland 0¹⁸ curve) are almost identical for the last 35,000 years. This cannot be a mere coincidence: it indicates that even minor fluctuations are caused by global climatic changes. The various records seem to be easily correlated with each other. The Last Ice Age is characterized by drastic changes between colder and warmer periods. The Present (Flandrian) Interglacial (Holocene Epoch) is also characterized by climatic fluctuations, although of minor amplitude. Analysis of peaks and bottoms in six Atlantic deep-sea cores gives a climatic sequence identical to the eustatic transgression/regression sequence, indicating the recording of global short-term warm/cold fluctuations. From these fluctuations, the climatic cyclicity was calculated. Two drastically frequency-changing cycles were found, one varying from 230 to 1,000 years and one from 1,000 to 3,600 years. A third cycle of 21,000 years was also estabished. The transition from the Last Ice Age to the Present Interglacial is marked by three major steps towards interglacial conditions; viz. at 12,7000, 10,000 and 9,300 radiocarbon years B. P. The 10,000 boundary has earler been suggested as the Pleistocene/Holocene boundary.     

青藏高原末次冰消期以来人类活动的时空演化特征及其原因探讨

青藏高原处于欧亚文明交汇区, 拥有古丝绸之路的高原支线, 其严酷的极端环境对人类生存形成巨大挑战, 研究高原自然环境与过去人类活动之间的关系, 对认识和理解人类应对极端环境的适应模式与机制具有重要价值。文章选用了海拔、地形起伏度、地被指数、水网密度指数、温湿指数、风寒指数、人体舒适度及绝对含氧量自然因子指标, 采用地理加权回归模型, 构建以1 km×1 km栅格为研究单元的青藏高原极端环境指数(EEI)分区, 探讨末次冰消期以来人类活动时空演化及原因。结果表明: 高原EEI变化趋势由东南向西北递减, 根据EEI数值高低将评价结果依次分为低极端区、较低极端区、中极端区、较高极端区和高极端区, 其中高极端区分布在高原腹地和西部少量高大山脉, 较高极端区面积广大且高山横亘, 中极端区包括柴达木盆地、川西高原、青海南部及藏南谷地等地区, 较低极端区以高原边缘河谷和横断山区为主体, 低极端区为面积占比最小的藏东南地区。末次冰消期以来气候条件的转变、东西方文化交流引起的生存技术革新和生业模式的转变, 使人类活动分布重心先后经历了较高极端区均衡散布型(旧-中石器时代)、较低极端区丛簇集聚型(新石器时代)、较低极端区连片集聚型(青铜时代)、中极端区边缘集聚-腹地均衡型(汉-元代)、较低极端区边缘集聚型(明清时期)的空间调整。

     

Palaeoenvironments and changes in relative sea level during the last interglaciation at Langelandselv, Jameson Land, East Greenland

Coastal cliffs and stream cut sections at Langelandselv on Jameson Land show a 22 m thick sedimentary succession reflecting the development of shallow marine and fluvial environments during the last interglaciation. The shallow marine sediments were deposited in upper shoreface, back-barrier, and delta environments during a rise in the relative sea level from 0 to 18 m. The interglacial succession ends with glaciotectonically dislocated fluvial sand, and is capped by alternating beds of lodgement till and fluvial sand, deposited during the Early Weichselian. The age is determined by palacoceanographic correlation of molluse and foraminifer faunas with isotopic substage 5e in the deep sea record, supported by luminescencs and U/Th dates and amino acid analysis.     

Paleohydrological changes during the last deglaciation in Northern Brazil

We here report a reconstruction of hydrological balance variations in Northern Brazil for the last 20 ka deduced from the δD values of aquatic and land plant molecules extracted from the sediment infill of Lake Caçó. Our reconstructed precipitation, lake water isotope ratio and evaporation–evapotranspiration isotope effect allows us to obtain an estimate of moisture balance, and, to a lesser extent, precipitation amount and seasonality changes. During the end of the Last Glacial Maximum (LGM, between ca 20 and 17.3 ka), high δD values and smaller fractionation of leaf waxes indicate an arid to semi-arid climate with a long lasting dry season. An abrupt change towards much wetter conditions occurred within ca 500 years from 17.3 to 16.8 ka, as shown by a 50‰ decrease in D/H ratios and a marked increase in H isotopic fractionation of leaf waxes. This abrupt isotopic change coincides with a major transformation from savanna-dominated vegetation to humid rain forest around the lake, based on pollen data. Comparisons with other paleo-precipitation records from South American sites indicate that Lateglacial humid conditions were controlled by intensification of the ITCZ and/or a southward shift of its mean position across our study site. Our isotope data show only a small rise in aridity during Younger Dryas event (13–11.5 ka). Although the Holocene was not screened in details, D/H ratios of terrestrial and aquatic compounds show near constant offsets, suggesting stable and relatively humid climate conditions during this period.     

末次冰消期以来泰国湾沉积物物源变迁的元素地球化学证据

本文对泰国湾西部T93柱状沉积物样品进行了常微量元素测试,探讨了沉积物物源和沉积环境变迁。T93柱状样全长381 cm,底层年代约14640 cal.a B. P.,间隔10 cm取样测试,样品平均年代间隔375 a。测试结果表明,末次冰消期以来泰国湾沉积物来源主要经历了4个阶段的变迁：1)14640~ 11700 cal.a B. P.期间,该阶段为低海平面时期的陆相沉积,沉积物为粘土质粉砂,Na、K、Ca等活跃的碱金属和碱土金属元素含量较低,Ti、Zr、Ba等含量高,表现出强烈的化学风化特征;δEu平均为0.60,(La/Yb)_PAAS平均0.92,物质来源以马来半岛为主,但分异程度高,成分较复杂。2)11700~8000 cal.a B. P.期间,该阶段为海平面快速上升期的滨海沉积,沉积物主要为粘土质粉砂,粉砂组分含量开始增加,Mg和Mn元素含量大幅升高,Fe、Ti含量减少;δEu平均为0.58,(La/Yb)_PAAS平均0.99,物质来源主要为马来半岛。3)8000~4000 cal.a B. P.期间,该阶段为高海平面时期的陆架沉积,沉积物包括粉砂和粘土质粉砂,Mg、Na、Ca含量明显增大,化学风化程度减弱;δEu平均为0.60,(La/Yb)_PAAS平均1.08,沉积物主要来自中南半岛。4)4000 cal.a B. P.至今,该阶段为海平面稍有下降的稳定陆架沉积,沉积物主要为粉砂,沉积物元素总体稳定,现代沉积体系基本形成;δEu平均为0.62,(La/Yb)_PAAS平均0.97,沉积物以马来半岛物质为主,中南半岛物质为辅,物源与现代一致。

     

Evidence of reworking and resuspension of carbonates during last glacial maximum and early deglacial period along the southwest coast of India

A gravity core collected from the upper slope of southwest of Quilon at a water depth of 776 m (Lat: 8°12′263″N, Long: 76°28′281″E) was analysed for texture (carbonate free), calcium carbonate and organic carbon. Variation in silicic fraction seems to be controlled by silt, i.e., enrichment from 15 ka BP to 10 ka BP and then constant in Holocene. Below 15 ka BP, the silicic fraction gets depleted compared to the Holocene section with a minimum around 21 ka BP. Clay content remains nearly constant except in the Holocene where it shows an enrichment. Carbonate content of less than 63 micron when computed by subtracting coarse fraction content from the total carbonate suggests that the total carbonates are mainly concentrated in the finer fraction. All these carbonate phases show an inverse relationship with silicic fraction except in Holocene. Below 15 ka BP, CaCO₃ dominates in sediments comprising more than 65%, such an increase is also seen in the coarse fraction. Coarse fraction from these sections contains abundant nodular type aggregates encrusting small forams. This period is marked by a high sedimentation rate comparable to Holocene. These parameters suggest that the productivity and precipitation have increased in the Holocene due to the intensification of the southwest monsoon. During the last glacial maximum and early deglacial period the high sedimentation rate indicates redeposition of the carbonates from the existing carbonate lithofacies situated between Quilon and Cape Comorin probably due to the slope instability.     

Controls on deglacial changes in biogenic fluxes in the North Pacific Ocean

The subarctic North Pacific Ocean holds a large CO₂ reservoir that is currently isolated from the atmosphere by a low-salinity layer. It has recently been hypothesized that the reorganization of these high-CO₂ waters may have played a crucial role in the degassing of carbon dioxide to the atmosphere during the last deglaciation. This reorganization would leave some imprint on paleo-productivity records. Here we present ²³⁰Th-normalized biogenic fluxes from an intermediate depth sediment core in the Northwest Pacific (RC10-196, 54.7°N, 177.1°E, 1007 m) and place them within the context of a synthesis of previously-published biogenic flux data from 49 deep-sea cores north of 20°N, ranging from 420 to 3968 m water depth. The ²³⁰Th-normalized opal, carbonate, and organic carbon fluxes from RC10-196 peak approximately 13,000 calendar years BP during the Bølling/Allerød (B/A) period. Our data synthesis suggests that biogenic fluxes were in general lowest during the last glacial period, increased somewhat in the Northwest Pacific during Heinrich Event 1, and reached a maximum across the entire North Pacific during the B/A period. We evaluate several mechanisms as possible drivers of deglacial change in biogenic fluxes in the North Pacific, including changes in preservation, sediment focusing, sea ice extent, iron inputs, stratification, and circulation shifts initiated in the North Atlantic and North Pacific. Our analysis suggests that while micronutrient sources likely contributed to some of the observed changes, the heterogeneity in timing of glaciogenic retreat and sea level make these mechanisms unlikely causes of region-wide contemporaneous peaks in export production. We argue that paleo-observations are most consistent with ventilation increases in both the North Pacific (during H1) and North Atlantic (during B/A) being the primary drivers of increases in biogenic flux during the deglaciation, as respectively they were likely to bring nutrients to the surface via increased vertical mixing and shoaling of the global thermocline.     

3万年以来日本海西部海冰活动变化

海冰是现代气候系统的一个基本组成要素,对气候变化有着重要的调控作用。现今日本海西部仍发育有限的季节性海冰,也是深层水形成的关键区域。由于缺乏沉积岩芯,长期以来对日本海西部海冰范围变化的历史还缺乏深入的认识。本文对日本海西部LV53-18-2岩芯中长393 cm的沉积物进行了粒度分析,采用粒径端元分析方法,重建了过去30.0~5.0 ka海冰和底流活动历史。粒径端元(EM)分析显示,端元3(40~160 μm)是指示海冰活动的有效指标;EM1(2~20 μm)与可分选粉砂平均粒径呈现一致的变化趋势,是指示底流活动的有效指标,但在15.0~7.8 ka受到冰筏碎屑影响。日本海西部海冰活动由冬季海冰形成和春/夏季海冰融化条件的共同制约。末次冰期-冰盛期(30.0~18.0 ka),在强盛东亚冬季风(EAWM)和低太阳辐射量控制下,日本海西部季节性海冰活动保持微弱;在末次冰消期晚期-中全新世早期(15.0~7.8 ka),EAWM仍然强盛,同时北半球高纬日射量和海平面升高,导致海冰活动十分活跃;7.8~5.0 ka以来,随着EAWM减弱和对马暖流入侵,日本海西部季节性海冰活动显著减弱。在海因里希冰阶(HS)1时段(17.5~15.0 ka),大西洋经向翻转流减弱诱发EAWM增强,同时北半球夏季太阳辐射量和海平面都相对较低,可能导致日本海西部海冰持续覆盖。在季节性海冰形成时期,盐析过程增强了水体垂向对流和底流活动,但是在HS1,深层水体形成源区被常年海冰覆盖,抑制了深层水形成,减弱了研究区底流活动。

     

Late holocene climate changes in the Sea of Azov region

The results of paleoclimatic reconstructions made with the help of the information-statistical method developed by V.A. Klimanov based on palynological data from the Sea of Azov bottom sediments. For the period of the last 3000 years, four phases of warm and dry climates and three phases of relatively cool and humid climates were identified. The latter phases were characterized by wider expansion of tree vegetation in the region around the Sea of Azov. The range of mean annual temperatures between warmer and cooler intervals was about 4°C.     

长白山老里克泥炭地过去2000年以来古水位定量重建

地下水位作为影响泥炭地形成和发育的关键因素之一,在泥炭地古环境研究中广受关注。本文以采自长白山老里克泥炭地（42°28'49.75"N,128°39'9.02"E;1470 m a.s.l.）100 cm的泥炭岩芯作为研究对象,通过²¹⁰Pb/¹³⁷Cs和AMS¹⁴C定年方法,建立剖面年代框架,以有壳变形虫为代用指标,定量重建该泥炭地古水位变化。重建结果表明,过去2000年老里克泥炭地整体湿润,经历了比较湿润→非常湿润→比较湿润的变化过程。根据有壳变形虫组合的约束聚类结果,具体可划分为以下3个阶段：1）5~150 A.D.（100~91 cm）,泥炭地比较湿润,有壳变形虫主要以喜湿物种茄壳虫（Hyalosphenia sp.）、齿口阿吉尼亚虫（Argynnia dentistoma）、Archerella flavum为主,喜干物种苔藓鳞盖虫（Assulina muscorum）、小匣三角嘴虫（Trigonopyxis arcula）等较少,水位埋深保持在1.2~2.0 cm。2）150~1796 A.D.（91~28 cm）,泥炭地非常湿润,地表长期保持1~4 cm积水,但积水深度呈不断变浅趋势。喜湿的茄壳虫（H. sp.）、盘状表壳虫（Arcella discoides type）占优势。3）1796~2017 A.D.（28~0 cm）,泥炭地虽较前阶段已显著变干,但仍比较湿润,水位埋深自0.2 cm持续降至5.2 cm。适合中等湿润条件的针棘匣壳虫（Centropyxis aculeata type）、片口匣壳虫（Centropyxis platystoma type）占优势,喜湿的茄壳虫（H. sp.）大幅减少。老里克泥炭地古水位变化受内生过程和气候变化共同影响。

     

Paleoceanographic conditions in the western Bering Sea as a response to global sea level changes and remote climatic signals during the last 180 kyr

We present results from the sediment core SO201-2-85KL retrieved from the western Bering Sea that recovered the past 180 000 years. For the first time, the intense dissolution of calcareous microfossils has been established when the Bering Strait was open during the glacioeustatic sea level rise. Possible mechanisms of climatic teleconnections between remote regions are considered.     

River inflow and salinity changes in the Caspian Sea during the last 5500 years

Pollen, spores and dinoflagellate cysts have been analysed on three sediment cores (1.8–1.4 m-long) taken from the south and middle basins of the Caspian Sea. A chronology available for one of the cores is based on calibrated radiocarbon dates (ca 5.5–0.8 cal. ka BP). The pollen and spores assemblages indicate fluctuations between steppe and desert. In addition there are some outstanding zones with a bias introduced by strong river inflow. The dinocyst assemblages change between slightly brackish (abundance of Pyxidinopsis psilata and Spiniferites cruciformis) and more brackish (dominance of Impagidinium caspienense) conditions. During the second part of the Holocene, important flow modifications of the Uzboy River and the Volga River as well as salinity changes of the Caspian Sea, causing sea-level fluctuations, have been reconstructed. A major change is suggested at ca 4 cal. ka BP with the end of a high level phase in the south basin. Amongst other hypotheses, this could be caused by the end of a late and abundant flow of the Uzboy River (now defunct), carrying to the Caspian Sea either meltwater from higher latitudes or water from the Amu-Daria. A similar, later clear phase of water inflow has also been observed from 2.1 to 1.7 cal. ka BP in the south basin and probably also in the north of the middle basin.     

Climate change in lowland Central America during the late deglacial and early Holocene

The transition from arid glacial to moist early Holocene conditions represented a profound change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition (～11 250 to 7500 cal. yr BP) inferred from sediment cores retrieved in Lake Petén Itzá, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by ～11 250 cal. yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage. From 11 250 to 10 350 cal. yr BP, during the Preboreal period, lithologic changes in sediments from deep‐water cores (>50 m below modern water level) indicate several wet–dry cycles that suggest distinct changes in effective moisture. Four dry events (designated PBE1‐4) occurred centred at 11 200, 10 900, 10 700 and 10 400 cal. yr BP and correlate with similar variability observed in the Cariaco Basin titanium record and glacial meltwater pulses into the Gulf of Mexico. After 10 350 cal. yr BP, multiple sediment proxies suggest a shift to a more persistently moist early Holocene climate. Comparison of results from Lake Petén Itzá with other records from the circum‐Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period appears to be tightly linked to climate change in the high‐latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores–Bermuda high‐pressure system. This mechanism operated on millennial‐to‐submillennial timescales and may have responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and the Atlantic meridional overturning circulation (MOC). Copyright © 2005 John Wiley & Sons, Ltd.     

Relative sea level fluctuations in aegean coastal areas from middle to late holocene

The current study presents and combines data concerning relative sea level fluctuations from six coastal areas in the Aegean Sea, from Thrace to North-Eastern Crete. These data have been derived from analyses and geo-chronologies on coastal sediments obtained from boreholes in coastal lagoons and from beachrock bends. Trends of relative sea level change at the study areas are presented for the last 6000 years and are compared with K. Lambeck's and A. Purcell [1] estimations for the same areas. It is concluded that sea level is rising faster in Skyros and Mykonos-Delos insular complex of Cyclades islands, mainly due to tectonic movements or lithosphere elastic deformations.