基于地磁场相对古强度变化的下蜀黄土年代序列及其古气候意义

分布于宁镇地区的下蜀黄土年代标尺研究薄弱.我们以镇江大港钻孔岩芯的下蜀黄土为例,探讨了下蜀黄土记录地磁场相对古强度的可靠性.岩石磁学的实验显示,下蜀黄土大体上符合估计相对古强度对沉积物的要求.以低频磁化率()作为天然剩磁(NRM300)的归一化因子,我们获得了下蜀黄土记录的地磁场相对古强度变化.通过与邻区和全球的单个及合成曲线的对比,我们发现大港钻孔岩芯的相对古强度记录展现出主要的全球偶极场的变化特征,因而可用于建立下蜀黄土的年代标尺.新的年代标尺表明,大港钻孔的磁化率年代序列与北方黄土无法直接对比,证实了该地点的下蜀黄土磁化率变化机制与北方的不同.大港钻孔下蜀黄土的沉积速率与磁化率相关,低风尘沉积速率对应低磁化率,是降水增加所导致的结果.沉积速率与磁化率年代序列显示,下蜀黄土记录的本区季风变化过程可分为4个阶段.阶段Ⅳ(819～700ka)对应中更新世转型阶段,东亚季风降水较少.阶段Ⅲ(700～412ka)对应中更新世的大间冰期,东亚季风降水最多.阶段Ⅱ(412～197ka)时东亚季风降水减少,较阶段Ⅳ略少.阶段Ⅰ(197～34ka)东亚季风降水最少.因此,中更新世气候转型后,长时间尺度东亚季风降水持续减少,可能受全球温度阶段性降低驱动.     

丁村旧石器遗址群的黄土地层研究

通过测量黄土的磁化率 ,并结合传统的黄土地层学研究方法 ,对丁村旧石器文化遗址群中两个典型的剖面 ,即代表丁村旧石器中期文化的 10 0地点和代表丁村旧石器早期文化的 79:0 2地点文化层上部的黄土 -古土壤序列进行划分 ,并将其结果与位于丁村附近黄土塬上的敬村黄土剖面和典型洛川黄土剖面的地层和磁化率曲线进行对比。结果表明 ,丁村 10 0地点地层上部黄土 -古土壤序列为S0 ,L1,S1;而 79:0 2地点上部的黄土 -古土壤序列则包括S0 ,L1,S1,L2 ,S2 。因此 ,丁村 10 0地点文化层的年代大于S1的形成年代即 0 13MaB .P .,79:0 2地点文化层的年代大于S2 的轨道调谐年龄 ,即 0 2 4 7MaB .P     

伊犁黄土的磁学性质及其与黄土高原对比

近20多年来黄土高原的黄土磁学性质与古气候研究取得了重要进展, 极大地推动了东亚古季风演化、亚洲内陆干旱化及过去全球变化的研究, 但对亚洲内陆天山地区的黄土磁学性质知之甚少. 选择天山伊犁盆地黄土剖面进行了系统的磁性测量, 包括磁化率、无磁滞磁化率、高低温磁化率和磁滞参数等, 并与黄土高原进行了对比, 开展了伊犁黄土的X射线衍射矿物学分析, 初步探讨了伊犁黄土磁化率增强的机制. 结果表明, 伊犁黄土磁性矿物的总含量要远远低于黄土高原, 但磁性矿物类型与黄土高原黄土基本类似, 以磁铁矿、磁赤铁矿和赤铁矿为主, 部分样品还有钛铁矿等. 在磁性矿物中磁赤铁矿占的比例总体上要比黄土高原黄土低, 而磁铁矿和赤铁矿占的比例比黄土高原黄土要高. 磁性矿物的粒度要比黄土高原粗, 以准单畴(PSD)和多畴(MD)为主. 成壤作用产生的细粒超顺磁体对磁化率的贡献非常有限, 粒径为PSD和MD的磁铁矿和磁赤铁矿为伊犁黄土磁化率的主要贡献者. 伊犁黄土磁化率增强既有风速论模式(阿拉斯加或西伯利亚黄土成土模式), 又有黄土高原超细颗粒成壤模式, 但以前一种模式为主导. 伊犁黄土磁化率增强除与源区的原生磁性矿物有关以外, 还与当地的地形气候环境和地质背景有关. 伊犁黄土磁化率增强的机制十分复杂, 将磁化率应用古气候的解释时要慎重.     

捷克黄土的磁学性质及古气候意义

对捷克Znojmo剖面末次间冰期以来黄土-古土壤序列进行了较为详细的岩石磁学和X射线衍射实验研究, 结果表明捷克黄土-古土壤磁化率变化与中国黄土高原风成沉积物具有相似的特征, 即土壤化作用使磁化率升高. 磁化率随温度的变化特征和等温剩磁测定以及X射线衍射实验揭示出Znojmo剖面黄土和古土壤的主要磁性矿物是磁铁矿, 并含有少量的磁赤铁矿、赤铁矿和黄铁矿或磁黄铁矿以及针铁矿. 磁化率各向异性研究表明, Znojmo剖面黄土和古土壤的磁线理小于磁面理, 这说明磁化率椭球体为压扁形, 最大轴的方向呈现随机分布特征, 不能指示大气传输尘埃沉积时的古风向.     

黄土地区粒度与磁化率分层对古地震研究的意义——以山西洞峪沟黄土剖面为例

探槽技术虽然已经发展成为古地震研究的重要手段,结合定年技术可以识别出古地震事件与地震重复间隔,但是仍然存在较大的不确定性和局限性。例如,目视解译难以区分黄土沉积内部的细层理,严重影响了古地震事件发生位置或时间的判定。如何提高古地震研究的精度和准确性,降低古地震事件判定的不确定性,是目前面临的一个迫切问题。山西洞峪沟剖面位于山西临汾盆地东北角,横跨霍山山前断裂带,不仅揭示了较好的黄土沉积序列,还揭露了明显的地层错断事件。因此,该剖面是一个开展高精度探槽古地震研究,降低判定古地震事件不确定性的理想场所。根据高精度的粒度与磁化率变化曲线,结合目视分层解译结果,对洞峪沟黄土剖面进行了精细分层,界定了各层的厚度和边界。依据细分层和断层两盘地层的对应关系,将u6层沉积以来的3次断错事件的发生位置和时间作了再限定,它们分别发生在u5-7、u4以及u2的顶部,对应断层上盘埋深7.1m、4.7m与2.9m。根据释光测年结果以及断层上盘地层的平均沉积速率,推测3次断错事件发生的时间分别在(48.1±1.5)~(43.2±2.5)ka BP接近45.8ka BP、(35.0±2.4)~(30.6±1.3)ka BP接近32.8ka BP、(26.4±0.8)~(20.9±0.7)ka BP接近23.3ka BP。根据3次黄土-古土壤沉积旋回的厚度差,判定3次地震的同震垂直位移分别为0.5m、0.4m和1.3m,累计位移2.2m。依据高精度粒度与磁化率变化曲线的地层分层方法,为有效降低黄土地区古地震研究的不确定性提供了一种很好的参考。     

华南二叠系卡匹敦阶高分辨率浮点年代标尺

基于详细的生物地层学研究,以磁化率为古气候替代指标,对广西来宾铁桥剖面卡匹敦阶地层开展时间序列分析,建立高分辨率浮点年代标尺(FPTS).结果表明,磁化率记录了铁桥剖面中二叠世晚期沉积序列中的米兰科维奇旋回,卡匹敦阶上部磁化率突然增加与峨眉山玄武岩喷发和卡匹敦晚期全球性海退有关,这些事件导致同期沉积物中碎屑物质增加.铁桥剖面瓜德鲁普—乐平统界线附近磁化率和蓬莱滩剖面(乐平统底界GSSP)表现出一致的变化趋势,具可对比性.利用多窗谱法(MTM)和傅里叶变换(FT)从磁化率序列中识别出五个米兰科维奇周期:长偏心率周期(E2,405ka)、短偏心率周期(E1,100ka)、长地轴斜率周期(O2,44.1ka)、长岁差周期(P2,20.95ka)和短岁差周期(P1,17.7ka).对比基于E2周期建立的磁性地层磁化率(MSS)带和标准参考带(SRZ),建立整个沉积序列的高分辨率(200ka)FPTS,提出卡匹敦阶的时限为3.85 Ma(存在+0~0.28 Ma误差),整段沉积序列的平均沉积速率为2.91cm·ka-1.同时计算出卡匹敦阶内部七个牙形石带的时限,从最短26.6ka到最长2.3 Ma.另外,估算出峨眉山大火成岩省喷发启动时间为262.67 Ma,位于瓜德鲁普—乐平统界线之下1.42 Ma.     

黄土沉积物的岩石磁学特征 与土壤化作用的关系

对黄土高原由北向南分布的靖边、宜川和段家坡3个采样点上,黄土-古土壤序列黄土层L8和古土壤层S8详细的岩石磁学实验表明1)从北到南,高场磁化率逐渐增高,反映了粘土矿物含量增加,土壤化作用加强的趋势;2)3个采样点黄土层的低场磁化率基本相同,而古土壤层S8的低场磁化率则表现为宜川高于靖边,而段家坡剖面磁化率低于宜川剖面磁化率的特征,表明低场磁化率值的大小与土壤化作用、古降雨量之间的关系是很复杂的;3)随着磁化率的升高,超顺磁性颗粒的含量明显增加,但磁性矿物含量的增加对磁化率增强的影响可能大于磁性矿物粒度变化的影响;4)3个剖面黄土层L8的磁化率-温度曲线形状基本相同,磁赤铁矿相对含量与土壤化作用成正相关.对古土壤层S8而言,段家坡剖面的温度磁化率曲线稍微不同于其他两个采样点,可能是由于磁性矿物粒度的差异造成的.     

风成沉积物磁组构与中国黄土区第四纪风向变化

通过对黄土高原几个剖面黄土样品磁化率各向异性的初步研究，发现了风成沉积物中的磁组构特征，且它的形成受沉积作用控制并与黄土高原形成时的古风场有关，即风成沉积物磁化率椭球体主轴方向及各轴比值与磁性颗粒分布排列方式亦即与空气动力条件相关．阐明了风成沉积物磁组构形成机制及其与古风向的关系，提出一种能够定量研究黄土高原形成时古风场的方法，为研究黄土高原形成演化和第四纪以来气候变化提供了基础数据．     

巴基斯坦Bahawalpur黄土岩石磁学特征及磁化率变化机制研究

风成黄土是陆地上分布最广泛的沉积物之一,记载了各种古气候演化信息.目前巴基斯坦的黄土研究甚少,磁化率与气候对应的变化机制研究尚未开展.本文对位于巴基斯坦印度河平原Bahawalpur地区新发现的黄土-古土壤剖面进行系统的岩石磁学研究,结合粒度和漫反射光谱(DRS)数据,讨论巴基斯坦黄土的磁化率变化机制.实验结果显示:Bahawalpur(BH)剖面黄土层主要的载磁矿物为磁铁矿,同时含有少量磁赤铁矿和针铁矿,磁性颗粒以原生的MD和PSD颗粒为主.相对于黄土层,古土壤层则是以针铁矿为主,含有顺磁性矿物和少量磁铁矿.BH剖面磁化率与成土作用关系和中国黄土高原典型剖面相反,磁化率的变化可能存在一个阈值12.8×10^-8 m³·kg^-1,在阈值之上,强磁性矿物(磁铁矿、磁赤铁矿)占主导;阈值之下,以弱磁性矿物(主要是针铁矿)为主,这种磁性矿物的转变可能导致磁化率降低.本文可为今后利用磁化率解读该地区地层蕴含的古气候信息提供新线索.     

天山北麓黄土环境磁学特征及其古气候意义

新疆黄土-古土壤序列环境磁学参数的变化机理及其气候意义仍存在争议.本文选择天山北麓的中梁黄土剖面,系统开展了低温和常温下环境磁学参数的测试与研究,测量包括室温的磁化率与饱和磁化强度,以及磁化率与饱和剩余磁化强度的低温变化.结果发现,该剖面黄土和古土壤样品的磁性矿物主要由磁铁矿与磁赤铁矿组成,不含任何粒级成壤形成的超顺磁矿物颗粒,其磁化率信号主要记录了粉尘磁性矿物含量变化,较高的磁化率指示较强的风动力状况或者较近的风尘源区,新疆黄土的这种环境磁学"风尘输入模式"可用来重建干旱区的风动力强弱变化.     

A geomagnetic field reversal time scale back to 13.0 million years before present

The ages of reversals of the Earth's magnetic field have been dated accurately back to 3.4 m.y. ago. Between this time and the age of the Cretaceous-Tertiary boundary, dates for reversals have been calculated assuming a constant rate of sea-floor spreading in the South Atlantic Ocean. The presence of thick piles of lava flows in Iceland allows us to produce independent evidence for the ages of reversals back to 13.0 m.y. B.P. Because of the extreme regularity of extrusion of these lava flows, the measurement of their magnetic polarity allows us to correlate the lava flows which were extruded during the polarity intervals associated with sea-floor spreading anomalies. The measurement of many K-Ar ages on these lava flows also allows us to compare the ages of reversals assumed by the linear interpolation between the ages of 3.4 m.y. and the Cretaceous-Tertiary boundary at 66.5 m.y., with those suggested by the radiometric dates. We find that in general the assumption of constant spreading has been a good one, but suggest a small change in the ages of reversals, amounting to an increase of about 0.27 m.y. in ages of reversals between 8.5 and 13.0 m.y. ago.     

A magnetic polarity time scale for the Early Cretaceous and Late Jurassic

The ages of polarity chrons in previous M-sequence magnetic polarity time scales were interpolated using basal sediment ages in suitably drilled DSDP holes. This method is subject to several sources of error, including often large paleontological age ranges. Magnetostratigraphic results have now tied the Early Cretaceous and Late Jurassic paleontological stage boundaries to the M-sequence of magnetic polarity. The numeric ages of most of these boundaries are inadequately known and some have been determined largely by intuition. An examination of relevant data suggests that 114 Ma, 136 Ma and 146 Ma are optimum estimates for the ages of the Aptian/Barremian, Cretaceous/Jurassic and Kimmeridgian/Oxfordian stage boundaries, respectively. Each of these boundaries has a good correlation to the M-sequence of magnetic reversals. The magnetostratigraphic tie-level ages are linearly related to the spreading distance and have been used to calculate a new magnetic polarity time scale for the Early Cretaceous and Late Jurassic. All stage boundaries in this time interval were correlated by magnetic stratigraphy to the proposed new time scale which was then used to estimate their numeric ages. These are, with the approximate relative errors of placement within the M-sequence:The absolute errors of these interpolated stage boundary ages depend on the accuracy of the tie-level ages.     

Evaluating OSL-SAR protocols for dating quartz grains from the loess in Ili Basin,Central Asia

Late Pleistocene records of loess deposition are a critical archive for understanding terrestrial paleoenvironment changes in Central Asia. The age of loess is not well known for the deserts regions and surrounding high plateaus in Central Asia. Previous studies have shown that there remains a disparity between ages for loess deposition by luminescence and ¹⁴C dating. This study evaluates the potential of optically stimulated luminescence (OSL) to date a loess sequence resting on fluvial sands in the east Ili Basin, Central Asia. The single-aliquot regenerative-dose (SAR) protocol on coarse grain quartz was employed for equivalent dose determinations. The basal fluvial sand returned a secure OSL age, with low overdispersion value in equivalent doses (19 ± 2%) of ca. 36 ka and provides a close, but maximum age estimate (within 5 ka) on the initiation of loess deposition. However, the loess yielded high overdispersion values for equivalent doses and age reversals, coincident with diffuse paleosols; indicating that pedoturbation with loess deposition may be a dominant process. OSL ages between ca. 45 and 14 ka calculated using a maximum age model and OSL ages from other sites in the Basin suggests that the latest major period of loess deposition was between 70 and 10 ka ago. A future hypothesis to test based on these analyses is that there may be three periods of heightened loess deposition at ca. 45, 35 to 19 and 14 ka, when desert source areas to the west were particularly dry.     

High resolution OSL and post-IR IRSL dating of the last interglacial–glacial cycle at the Sanbahuo loess site (northeastern China)

Northeastern China is located in the East Asian monsoon region; it is sensitive to both high and low latitude global climate systems. Loess deposits in the region have considerable potential as sensitive archives of past climate changes. However, research into loess deposition and climate change in this region is restricted by the lack of independent age control. In this study, coarse-grained quartz SAR OSL and K-feldspar post-IR infrared (IR) stimulated luminescence (post-IR IRSL; pIRIR₂₉₀) methods have been used to date the Sanbahuo loess site in northeastern China. The quartz OSL characteristics are satisfactory. The measured pIRIR₂₉₀ D_e's do not vary significantly with IR stimulation temperatures between 50 °C and 260 °C; a first IR stimulation temperature of 200 °C was adopted. Dose recovery tests were performed by adding different laboratory doses to both laboratory bleached (300 h SOL2) samples and natural samples; the results are satisfactory up to ∼800 Gy. Resulting quartz OSL and feldspar pIRIR₂₉₀ ages are in good agreement at least back to ∼44 ka; beyond this feldspar pIRIR₂₉₀ ages are older. The feldspar ages are consistent with the expected age of the S1 palaeosol (MIS 5). There appears to have been a period of fast loess deposition at ∼62 ka, perhaps indicative of winter monsoon intensification with a very cold and dry climate that lead to a serious desertification of dunefields in northeastern China.     

Optical dating of Romanian loess using fine-grained quartz

Romanian loess -palaeosol sequences are amongst the thickest and most complete available in Europe. These deposits represent an extended continental record of environmental and climatic change during at least five glacial/interglacial cycles. Their chronology, however, is mainly based on relative methods. In this paper, we investigate whether SAR–OSL dating of fine-grained quartz can be used to establish a reliable chronology for Romanian loess.The samples were collected from the loess–palaeosol sequence near Mircea Vod? (Dobrogea, SE Romania). The luminescence characteristics of the fine-grained quartz extracts are investigated to some extent, and indicate that the applied laboratory measurement procedure (SAR) is reliable. An internally consistent set of optical ages is obtained for the loess deposited up to ～70 ka, and evidence is presented for a varying loess accumulation rate during the Last Glacial. Comparison with independent age control (pedostratigraphy and a newly-developed palaeomagnetic time–depth model) indicates that the optical dating procedure underestimates the true burial age from the penultimate glacial period onwards (i.e. for samples below the last interglacial S1 palaeosol). These results indicate that an apparently reliable laboratory measurement procedure not necessarily yields accurate sedimentation ages. We suggest that quartz-based SAR–OSL ages obtained using the high dose linear region of the growth curve are interpreted with caution.     

The upper Pleistocene loess-palaeosol sequence at solonovka on the Cis-Altai plain,west Siberia – First luminescence dating results

Loess-palaeosol deposits of the Upper Pleistocene cover an extensive territory in the south of Western Siberia. Previously, most studies of loess-palaeosol sequences were carried out on river bank sections of the Ob river around Novosibirsk and upstream of this location (Ob Loess plateau); more recently, the focus of research has shifted towards the inner part of the Cis-Altai plain. Despite a good knowledge of the structure of regional loess-palaeosol sequences, there is a considerable lack of absolute dating beyond the radiocarbon limit. However, recent high resolution luminescence dating of a key Marine Isotope Stage 5 (MIS 5) stratotype at Lozhok has identified the presence of a long hiatus in the deposit. As a result, the published ages of the main units have been underestimated, because the existing chronology is largely based on palaeosol counting. This new observation argues for an urgent re-evaluation of the accepted chronostratigraphy of Western Siberian Late Quaternary. Here we present the first luminescence data for a loess-palaeosol sequence of the Cis-Altai plain, at the Solonovka key section, using both quartz optically stimulated luminescence and feldspar post-infrared infrared stimulated luminescence from 25 samples. The results show that the deposits were formed during the Late Pleistocene. The ∼1 m thick well-developed upper pedocomplex (PC1) has cryoturbation cracks filled with upper loess; this a characteristic regional benchmark for stratigraphic correlation. Two distinct hiatuses in sedimentation are found in the section: at the boundary of MIS 3/2, and after the formation of the MIS 5 Berdsk pedocomplex (PC3 and 2) until the beginning of the formation of MIS 3 deposits. The age of the thick PC3 palaeosol at the base of the section is determined as MIS 5, confirming evidence for the pronounced Kazantsevo (Eemian) interglacial in the loess-palaeosol record of Western Siberia. The results of our study emphasize the importance of understanding the palaeogeomorphological background to soil development, and the necessity of absolute chronology; we consider that the position of MIS 5 in the regional chronostratigraphic chart of South Western Siberia requires further study.     

No apparent lock-in depth of the Laschamp geomagnetic excursion: Evidence from the Malan loess

Developing an accurate chronological framework is always a key issue in paleoclimatic studies. Magneto- stratigraphy has been a routine tool for such purposes. However, complexities arise for inter-profile correla- tions of magnetostratigraphy due to effects of the lock-in process. One good example is the “mystery” of the mismatching of stratigraphic locations of the Matuyama/Brunhes boundary (MBB) (occurred at ~780 ka) recorded in Chinese loess and marine sedi- ments. Tauxe et al.[1] con…     

黄土高原北部晚新近纪“吴起古湖”的古地磁年代学与古环境记录

晚新近纪以来中国北方堆积的风成红粘土-黄土序列是古气候信息的良好载体, 在黄土高原北部的吴起地区存在一套湖相沉积物, 层位上介于第四纪黄土与晚新近纪红粘土之间, 但该古湖存在的具体时限尚不清楚. 我们利用岩石磁学及古地磁年代学方法并结合多种气候指标对代表“吴起古湖”的土佛寺剖面进行了初步研究, 结果表明湖相沉积物中特征剩磁的载体主要为磁铁矿和赤铁矿, 在此基础上得到的磁极性序列能很好地与国际地磁极性年表对比, 剖面起始于4.28 Ma B. P.左右, 湖相沉积层起始于3.0 Ma B. P.左右, 结束于1.2 Ma B. P.左右. 岩性、磁化率、粒度和沉积速率变化均反映出这一时期该地区环境变化的总体趋势是由暖湿向冷干发展, 期间“吴起古湖”的演化大致经历了三个阶段: 3.0~2.5 Ma B. P.为古湖形成时期, 水体相对较深; 2.50～2.05 Ma B. P.期间受古气候变化的影响湖水明显变浅; 2.05~1.20 Ma B. P.时古湖中仍有一定量的水体存在, 但随后逐步缩小并最终消亡. 该地区气候条件在1.2 Ma B. P.左右的恶化是导致吴起古湖消亡的主要因素, 区域构造活动引起的北洛河溯源侵蚀并切穿湖盆可能也是原因之一.     

浅议中国黄土磁化率的物理意义

中国的陆相黄土地层连续地记录了第四纪以来的古气候变化。基于中国黄土磁化率曲线与深海氧同位素记录具有可比性这一事实，磁化率作为一种气候替代性指标在黄土研究中得到广泛的应用。但是，磁化率的物理意义仍不清楚。本文作者对陕西榆林蔡家沟剖面末次间冰期以来的风成沉积作了研究，指出黄土、古土壤的磁化率差异不仅与前人提出的碳酸盐淋失、空隙度变化、“就地”新生成磁性矿物等作用有关，而且还可能与有机质有重要关系。有机     

中更新世以来青藏高原RH孔和洛川黄土记录的比较研究

根据若尔盖盆地ＲＨ孔的研究成果,具体地将该孔的２１个阶段和洛川黄土记录进行对比,分析其异同点,探讨了高原不同降升阶段自身环境特点及其对邻近西北干旱区的影响。