洞穴石笋的^14C年代学研究——石花洞研究系列之二

由于受“死碳”的影响,年轻洞穴碳酸盐的＾１４Ｃ测年始终受到限制,为此,建立了＾１４Ｃ法测定石笋生长速率及校正“死碳”影响的理论模式,从而将石笋的测年时限和精度大大提高。这一理论成功地运用于北京石花洞年轻石笋的定年。利用＾１４Ｃ法与石笋生长光性年际纹层推算的平均生长速率十分吻合,表明石笋Ｓ３１２的生长速率为０．０４２ｍｍ／ａ。＾２１０Ｐｂ法测定石笋顶部的年龄小于１００ａ,与＾１４Ｃ结果的外推年龄基本     

13kaBP以来滇池地区古环境演

根据ＤＣ９３－１孔孢粉组合、总有机碳（ＴＯＣ）、总氮（ＴＮ）、碳氮比（Ｃ／Ｎ）、有机碳同位素δ＾１３Ｃｒｏｇ、磁化率（ｘ）、频率磁化率（ｘｆｄ）等资料,结合＾１４Ｃ、＾２１０Ｐｂ和＾１３７Ｃｓ测年,汗池地区１３ｋａ以来的古环境演化历史经历了以下几个阶段：１３－１０．２ｋａＢＰ,气候偏凉湿,湖水深度不大;１０．２－７．５ｋａＢＰ,气候向暖湿过渡,湖水渐深：７．５－４．０ｋａＢＰ,气候暖湿,水热条件达     

女山湖现代沉积速率和环境解释

为探讨本世纪以来女山湖生态环境的演化历史,以＾１３７Ｃｓ和＾２１０Ｃｂ放射性同位素为测试手段,分析该湖近期沉积物中＾１３７Ｃｓ和＾２１０Ｐｂ的含量分布,并根据其沉降特征和衰减规律,对沉积物做了年代定位,分段推导出平均沉积速率,建立柱状沉积年代序列。在此基础上,利用沉积记录,对沉积速率的变化进行环境解释,探讨了本世纪以来湖泊环境的演化过程。结果表明,１９２９￣１９３８年沉积速率达４．８ｍｍ／ａ,是淮     

137Cs湖泊沉积年代学方法应用的局限——以Crawford湖为例

＾１３７Ｃｓ湖泊沉积年代学方法是测定现代湖泊沉积物沉积年代和沉积速率的重要同位素年代学方法之一。对加拿大Ｃｒａｗｆｏｒｄ湖采集的沉积孔柱内＾１３７Ｃｓ垂直分布的研究发现,该方法给出时的标有明显偏差。比较＾２１０Ｐｂ和纹层等年代方法的结果,其１９６３年时标蓄积峰值所在位置明显移向表层,而作为１９５４年时标的该核素出现蓄积的层位则远早于该年沉积物蓄积层位。研究还表明,孔柱中较高的间隙水含量、缺少足以吸     

云南省泸沽湖、洱海现代沉积物中210Pb,137Cs的垂直分布及其计年

云南省泸沽湖,洱海＾２１０Ｐｂｅｘ的垂直分布受Ｆｅ,Ｍｎ循环下＾２１０Ｐ;ｏ和＾２１０Ｐｂ沉积后再迁移的影响,分别表现为三种不同的剖面分布特征,＾２１０Ｐｂｅｘ计年的没积物堆积速率分别为０．０５０－０．０５１ｇ．ｃｍ＾－２ａ＾－１和０．０４４－０．０４６ｇ．ｃｍ＾－２ａ＾－１,反映百年时间尺度上的堆积结果,泸沽湖,洱海＾１３７Ｃｓ的垂直剖面分布虽然受分子扩散作用的影响,并没有改变其峰值位置。     

山东南四湖成湖时代浅析

对南四湖两个沉积岩心进行了多环境代用指标分析,利用１４Ｃ方法测定了沉积剖面下部年代,采用２１０Ｐｂ和１３７Ｃｓ方法测定了剖面上部沉积速率,进而确定了整个沉积剖面的时代根据沉积岩心色素指标特征、有机碳氮比值和有机碳同位素特征分析,初步确定南四湖的成湖时代为２４５０ａＢＰ     

独山子地区荒漠土的^14C年龄测定及意义

通过对独山子地区荒漠土碳酸盐淀积层的＾１４Ｃ年龄测定，探讨了碳酸盐淀积层碳的来源及其＾１４Ｃ年龄的可靠性问题，提出了根据碳酸盐淀积层的无机碳年龄，对确定土壤的年龄是有意义的。     

淮河中游本世纪气候与环境波动的湖泊记录证据

为研究本世纪淮河中游气候与环境波支的湖泊沉积响应,１９９１年秋季在女山湖湖心提取了４．８ｍ沉积物。对顶部２０ｃｍ沉积物运用＾１２６Ｐｂ、＾１３７Ｃｓ的年代定位,获得１９０４－１９９１年湖泊沉积物的年序。以此通过高分辨沉积记录的代用标志提取,重建了本女山湖气候与环境的变化过程。结果表明,本世纪最初２０－３０年女山湖地区温度偏低、湿度偏大;４０、５０年代为本世纪温度最高的时段,５０至６０年代中期为延结     

湖泊沉积物有机质δ13C所揭示的环境气候信息

根据我国不同纬度和高海拔地区青藏高原东部地区湖泊沉积物有机质δ＾１３Ｃ组成特征的分析,就其揭示的环境气候意义作了探讨。研究表明,湖泊沉积物有机质δ＾１３Ｃ波动间接地反映气候冷暖的波动,但高原与平原地区湖泊沉积物有机质δ＾１３Ｃ值变化具有不同的环境气候意义。这与陆生植物Ｃ３及Ｃ４植物的分布密切相关,据此初步划分为三种有机质δ＾１３Ｃ古气候类型。     

内蒙古岱海湖泊沉积记录的小冰期气候环境

通过对岱海深水区沉积柱样有机碳同位素,总有机碳、碳酸盐含量、频率磁化率等环境代用指标的分析,结合剖面的岩性特征和＾２１０Ｐｂ定年,初步探讨了岱海小冰期以来气候环境演化过程,结果表明：在小冰期初期７４０－５４０ａＢＰ,以冷湿气候为主;在５４０－４８０ａＢＰ为寒冷干旱气候,可能为小冰期盛期;４８０－３７０ａＢＰ,气候转温和,以凉偏湿气候为主;３７０－２２０ａＢＰ,凉偏干气候为主;２２０－１３０ａＢＰ,     

Identifying a reliable target fraction for radiocarbon dating sedimentary records from lakes

Lake basins that experience rapid rates of deposition act as high-resolution environmental archives because they produce sedimentary records that have centennial or even decadal resolution. However, identifying target fractions for radiocarbon dating of lake sediments remains problematic because reworked organic material from fluvial catchments can produce anomalously old radiocarbon ages. This study determines the extent to which reworked material from catchment soils impacts radiocarbon dates on pollen and other organic concentrates by comparing radiocarbon dates produced by these techniques against a chronostratigraphic marker in cores from Lake Mapourika, New Zealand. Pollen preferentially preserved and reworked from catchment soils was identified using soil palynology. A technique was then developed to remove reworked pollen types from pollen concentrates extracted from lake sediment. Identification and removal of reworked pollen from pollen concentrates produced ages that were consistently closer to the age of the chronostratigraphic horizon than other organic concentrates. However, these dates were still between 736 and 366 calendar years older than expected. The only organic fractions that reliably reproduced the age of the chronostratigraphic horizon were terrestrial leaf macrofossils, although terrestrial leaf macrofossils isolated from megaturbidite deposits, which are formed by high-energy depositional events, also provided anomalously old ages. The results indicate that leaf material extracted from hemipelagite, which accumulates gradually, is likely to be the only organic fraction to produce reliable chronology in lakes where a component of sedimentation is driven by the fluvial system. The results also demonstrate the importance of conducting a detailed investigation of physical sedimentology before selecting material for radiocarbon dating lake sediments.     

Characterization of organic matter in marine sediments to estimate age offset of bulk radiocarbon dating

Radiocarbon dating of Arctic marine sediment is often challenging due to the low availability of calcareous fossils. Consequently, bulk organic matter dating has at times been used to establish sediment core chronologies. Yet, radiocarbon dates based on bulk organic matter often appear to deviate vastly from dates based on fossils, mainly caused by input of allochthounous carbon, including terrigenous organic matter. In this study, we aim to examine the link between the composition of the bulk organic matter and the age offsets between the bulk radiocarbon dates and those obtained from calcareous foraminiferal tests. All samples are taken from the marine sediment core AMD14-204C from offshore Upernavik (eastern Baffin Bay). The radiocarbon dates for bulk organic matter are on average ∼3000 years older than the radiocarbon dates based on foraminifera, but with changing age offsets throughout the record. To investigate the cause of this age offset and its variations over time, we applied core scanning, X-ray Fluorescence analysis, stable isotopes, organic pyrolysis and microscopic organic petrology to examine the distribution and characterization of the organic matter. The results show that the older organic matter includes clastic input of reworked sedimentary rocks potentially originating from West Greenland and/or the Canadian Arctic Archipelago. Changes in the input of contemporary marine algal produced organic matter versus both terrigenous input and reworked ancient organic matter appear to control the age offsets between the bulk and foraminifera dates. A low Hydrogen Index and low δ¹³C_org values together with a high Oxygen Index, indicative of high influence of terrigenous organic matter, seem to correspond to samples with the largest age offsets; 1000–2000 years greater than in other samples. To examine the cause of the variations in the age offsets, a new quantification of the autochthonous organic matter as a fraction of the TOC was calculated. This shows that samples with the largest age offsets contained the lowest fraction (as low as ∼12%) of autochthonous organic matter in the TOC.     

Using the N/C ratio to correct bulk radiocarbon ages from lake sediments: Insights from Chilean Patagonia

The offset between AMS radiocarbon ages obtained on bulk lake sediments and the true age of deposition was evaluated at four sites in Northern Chilean Patagonia. Our results show that the bulk radiocarbon ages are systematically older by 300 to 1100 years. In this region free of carbonate and carbonaceous rocks, we argue that this difference results from variable inputs of terrestrial organic carbon from the Holocene soils that cover the lake watersheds. For the four studied lakes, the age offset is clearly related to the fraction of terrestrial carbon preserved in the lake sediments, which was estimated using the N/C ratio of the bulk organic matter. We propose that N/C measurements can be used to significantly improve chronologies based on radiocarbon dating of bulk lake sediments.     

Sudden enhancement of sedimentation flux of~(210)Pb_(ex) as anindicator of lake productivity as exemplified by Lake Chenghai

Recent sediments in lakes and gulfs are a sensi-tive recorder of the information about environmentalchanges in the catchment areas during recent geologi-cal history. Precise determination of the ages of sedi-ments is the key to deciphering the environmental re-cords. The 210Pb dating technique and the markertechnique based on fallout radionuclide 137Cs havefound wide applications in sedimentation rate on atime scale of several tens to one hundred years, as wellas the varve chron…     

Improving age-depth models of fluvio-lacustrine deposits using sedimentary proxies for accumulation rates

Lacustrine fills, including those of oxbow lakes in river floodplains, often hold valuable sedimentary and biological proxy records of palaeo-environmental change. Precise dating of accumulated sediments at levels throughout these records is crucial for interpretation and correlation of (proxy) data existing within the fills. Typically, dates are gathered from multiple sampled levels and their results are combined in age-depth models to estimate the ages of events identified between the datings. In this paper, a method of age-depth modelling is presented that varies the vertical accumulation rate of the lake fill based on continuous sedimentary data. In between Bayesian calibrated radiocarbon dates, this produces a modified non-linear age-depth relation based on sedimentology rather than linear or spline interpolation.The method is showcased on a core of an infilled palaeomeander at the floodplain edge of the river Rhine near Rheinberg (Germany). The sequence spans from ∼4.7 to 2.9 ka cal BP and consists of 5.5 m of laminated lacustrine, organo-clastic mud, covered by ∼1 m of peaty clay. Four radiocarbon dates provide direct dating control, mapping and dating in the wider surroundings provide additional control. The laminated, organo-clastic facies of the oxbow fill contains a record of nearby fluvial-geomorphological activity, including meander reconfiguration events and passage of rare large floods, recognized as fluctuations in coarseness and amount of allochthonous clastic sediment input. Continuous along-core sampling and measurement of loss-on-ignition (LOI) provided a fast way of expressing the variation in clastic sedimentation influx from the nearby river versus autochthonous organic deposition derived from biogenic production in the lake itself. This low-cost sedimentary proxy data feeds into the age-depth modelling. The sedimentology-modelled age-depth relation (re)produces the distinct lithological boundaries in the fill as marked changes in sedimentation rate. Especially the organo-clastic muddy facies subdivides in centennial intervals of relative faster and slower accumulation. For such intervals, sedimentation rates are produced that deviate 10–20% from that in simpler stepped linear age-models. For irregularly laminated muddy intervals of the oxbow fill – from which meaningful sampling for radiocarbon dating is more difficult than from peaty or slowly accumulating organic lake sediments – supplementing spotty radiocarbon sampling with continuous sedimentary proxy data creates more realistic age-depth modelling results.     

Establishing Holocene sediment core chronologies for northern Ungava lakes, Canada, using humic acids (AMS C) and Pb

The absence of datable macrofossils in six sediment cores recovered from northern Ungava (Canada) lakes constituted a major challenge for the establishment of reliable lacustrine chronologies for the Holocene. Consequently, AMS radiocarbon dating of humic acids was used to assess age–depth in the cores. The reliability of the radiocarbon results near some of the core tops was evaluated through ²¹⁰Pb dating. The offset of sediment radiocarbon ages with their most probable time of formation and deposition in the lakes was found to be in the order of about 1000 years for recently deposited sediments. However, the basal dates in one core covering the entire postglacial period yielded a remarkable fit with previously established dates performed on marine shells at the maximum marine limit. Hence, the aim of this study was to describe how the two dating methods can be combined to address some of the problems paleolimnologists face when trying to assign ages to high-latitude lake sediment records. Suggestions are made for improving the quality of age–depth models developed in future studies for northernmost Québec and other comparable regions where paleolimnologists must deal with the combined challenges of very slow sediment accumulation rates in lakes, an extreme paucity of datable material and the sequestration of old carbon in the watersheds.     

Chronometry of pedogenic and stratigraphic events from calcite produced by earthworms

Dating phases of pedogenesis, soil carbonate deposition or even the burial of whole soil profiles using ¹⁴C is a valuable goal in archaeology and pedology, but one that has been consistently hampered by the presence of old carbon skewing the measurements to produce apparent dates older than the true formation date. Calcite produced by earthworms could be a useful alternative source of datable carbon. Since earthworms both inhabit and ingest soils with an old carbon content, however, the granules could yield a ¹⁴C date older than the date of their formation. In this study, by examining granules from two sites of known-age stratigraphy, we show that the radiocarbon date derived from the granules' calcite closely reflects their true formation date, opening up the possibility of using the granules either individually or as distributions of dates to understand soil processes and date sealed archaeological layers.     

Underestimated ~(14)C-based chronology of late Pleistocene high lake-level events over the Tibetan Plateau and adjacent areas: Evidence from the Qaidam Basin and Tengger Desert

The palaeolake evolution across the Tibetan Plateau and adjacent areas has been extensively studied, but the timing of late Pleistocene lake highstands remains controversial. Robust dating of lacustrine deposits is of importance in resolving this issue. This paper presents 14 C or optically stimulated luminescence(OSL) age estimates from two sets of late Quaternary lacustrine sequences in the Qaidam Basin and Tengger Desert(northeastern Tibetan Plateau). The updated dating results show:(1) the radiocarbon dating technique apparently underestimated the age of the strata of >30 ka BP in Qaidam Basin;(2) although OSL and 14 C dating agreed with each other for Holocene age samples in the Tengger Desert area, there was a significant offset in dating results of sediments older than ~30 ka BP, largely resulting from radiocarbon dating underestimation;(3) both cases imply that most of the published radiocarbon ages(e.g., older than ~30 ka BP) should be treated with caution and perhaps its geological implication should be revaluated; and(4) the high lake events on the Tibetan Plateau and adjacent areas, traditionally assigned to MIS 3a based on 14 C dating, are likely older than ~80 ka based on OSL chronology.     

Problems in the dating of volcanic rocks by the potassium-argon method

The potassium-argon method is attractive for dating volcanics since it can be applied to rocks of Pleistocene age and older, thus encompassing important periods of general volcanic activity. However it has been found that dates obtained on whole rocks and on included minerals frequently show gross discordances. In order to establish this dating method in this application an attempt has been made to trace the sources of the anomalies. To illustrate these efforts, dating results from a rhyodacite of Mauna Kuwale, Oahu, Hawaii, are reported. Determinations on several minerals and the whole rock of this ridge give a concordant age of 2.3 ± 0.3 million years, excluding some few results on minerals which show old age anomalies. It has been noted that xenoliths in certain Hawaiian volcanics contain fluid inclusions which show evidence of formation at depth. We have found that gas released from such inclusions by crushing contains radiogenic argon, and that the constituent minerals give very old potassium-argon ages (circa 800 million years). Similar gaseous inclusions have been noted in a variety of other lava phenocrysts, and their presence in a dated sample may produce an anomalous old age. In the minerals from Mauna Kuwale sporadic occurrences of inclusions have been noted in biotites and hornblendes, and crushing of the mineral releases the excess radiogenic argon. The determination of the age of such a material would give an old age, and thus account for the anomalies found. For meaningful dating of volcanics by the potassium argon method it is concluded that phenocryst-containing materials should be examined for fluid inclusion content, and samples which contain these should be rejected.     

Optical dating of fluvio-deltaic clastic lake-fill sediments – A feasibility study in the Holocene Rhine delta (western Netherlands)

We test the applicability of quartz optically stimulated luminescence (OSL) dating on clastic lake sediments to investigate whether this dating method can be applied to study the timing and rate of deposition in Holocene fluvio-deltaic lakes. Our study concerns the filling of a lake by the Angstel-Vecht system, part of the Rhine delta in the western Netherlands. Age constraints are provided by radiocarbon dates on the development and abandonment of the fluvial channels debouching into the lake. Results indicate that light exposure prior to deposition and burial was sufficient to reset the OSL signal of the vast majority of the quartz grains. Special attention was given to accurate estimation of the dose rate in the laminated and bedded deposits. The OSL ages obtained are in good agreement with the age constraints, especially for the relatively coarse sediments. OSL results indicate that the filling of the lake took about 700 years, with a sedimentation rate of ～3 mm per year. This study is a demonstration of the use of OSL dating of sand-sized quartz to determine the timing and rate of sedimentation in a Holocene fluvio-deltaic environment.