Dating the recent past

The dramatic environmental changes of the last 500 years are likely to continue into the future and to have an increasing impact on both the Earth and human society. Any understanding of future environmental change is thus critically dependent on our capacity to reconstruct the environmental changes of the past. Fundamental to this is an ability to place the environmental responses of the last half millennium within a reliable chronological framework. Unfortunately, this most recent part of the geological timescale presents us with some of the greatest challenges for dating. With the exception of ²³⁰Th/²³⁴U methods, whose use is restricted to the rather specific depositional environments of shallow marine and terrestrial carbonates, there is no established geochronometric tool capable of dating more than a fraction of the recent past at a resolution adequate to tackle the environmental issues of this period. This challenge has been met by refining existing procedures (including ¹⁴C, ⁴⁰Ar/³⁹Ar, event chronostratigraphy and optically stimulated luminescence) and developing new ones (such as ³²Si). These offer a means of calibrating the high-resolution environmental records of the last 500 years and answering the critical environmental questions presented by this period.     

The role of aridification in constraining the elevation range of Holocene solifluction processes and associated landforms in the periglacial belt of the Sierra Nevada (southern Spain)

Climate variability during the Mid‐Late Holocene has influenced the activity of geomorphic processes in the current periglacial belt of the Sierra Nevada. We studied two types of sedimentary records that reveal a synchronous timing for slope instability in this high semi‐arid massif: solifluction landforms and mountain lake sediments. Lithological and sedimentological properties of both records have recorded numerous cycles of different magnitude of slope processes in the massif. Solifluction deposits record seven phases of solifluction activity and soil development during the last 7 ka bp and lake sediments show evidence of eight periods with increased geomorphic activity in the catchments over the last 6 ka bp . Although present‐day climate conditions do not promote active solifluction processes in the Sierra Nevada, colder and wetter periods during the Holocene triggered solifluction and transported coarse‐grained sediments into the lakes. By contrast, warm phases favoured soil formation and spread an incipient vegetation cover over the headwaters of the highest valleys, diminishing the grain size of the particles reaching the lakes. Lake sediments record an aridification trend in the massif intensifying since 4·2 ka bp that has conditioned solifluction activity to shift gradually to higher elevations. During major cooler phases such as the Little Ice Age active solifluction was recorded back down to 2500 m altitude. Copyright © 2010 John Wiley & Sons, Ltd.     

Age of glacial and periglacial landforms in northern Hokkaido, Japan, using OSL dating of fine grain quartz

The timing of glacial advances, periglacial phenomena, and the ages of two marker tephras in northern Hokkaido were estimated by OSL dating. It appears that the glacier of Yamunai 2 stage on Rishiri Island expanded between 24 and 15 ka. In northern Hokkaido, OSL ages indicate ice wedge formation during the period 24–18 ka. These results indicate that both the glacial advance and the development of ice wedges were synchronous phenomena relating to the Last Glacial Maximum.     

Dating of megaflood deposits in the Russian Altai using rock surface luminescence

Catastrophic drainage of ice-dammed lakes in the Altai Mountains has been inferred from geomorphological evidence in the Katun Valley (Russia), and is presumed to have occurred during the Pleistocene. The sedimentary features have been difficult to date directly, due to the absence of organic carbon, and the improbability that luminescence signals in sand grains would be reset during transport. However, the development of rock-surface luminescence dating provides a new opportunity to date the features: clasts have a different transport history to sand grains, and their luminescence depth profiles can be inspected for evidence of bleaching before burial. Here we investigate two sites in the Altai Mountains, and use rock-surface luminescence burial dating to constrain the age of the megaflood deposits. In the Katun Valley, we sampled granite cobbles from a frozen sediment clast emplaced as a dropstone within a massive megaflood gravel terrace. Burial ages for the clasts range from 16.7 to 21.4 ka, with a mean age of 19.8 ± 1.5 ka. This represents the depositional age of the fluvial sediments that preceded the lake outburst flood, (and hence places a maximum age on the catastrophic flood). Clasts sampled from mega-ripples in the Kurai Basin are shown to have a mid-to-late Holocene burial age, which is not consistent with the possible origin of these features during a catastrophic drainage of a glacier-dammed lake. Instead, the burial age of the Kurai Basin sediments may reflect local-scale periglacial or seismic processes along the Kurai Fault Zone.     

断层泥中石英碎砾溶蚀形貌的测年研究

考虑到同一条断层的同一个位置所取的断层泥中,不同碎砾的溶蚀形貌并不相同,即使是同一个碎砾,其不同的表面,溶蚀形貌也可能不同,将统计学参数估计的方法引入结果处理中,并把在金川水电站坝区F1断层一次所取得断层泥样品碎砾的各个表面的总体当作一个整体进行处理,得出此断层的活动时间可能为早更新世的结果。与ESR法测得的结果基本一致。     

镜泊湖全新世火山研究成果概述

本文从火山口的形态、分布、火山喷发时代、火山基浪堆积物及火山成因类型等方面概述镜泊湖全新世火山研究成果，指出该火山活动时间为距今5200～5500年，在该火山碎屑物中发现有火山基浪堆积物，该火山成因类型属于单成因火山。     

Influence of bedrock structures on the spatial pattern of erosional landforms in small alpine catchments

Structural settings and lithological characteristics are traditionally assumed to influence the development of erosional landforms, such as gully networks and rock couloirs, in steep mountain rock basins. The structural control of erosion of two small alpine catchments of distinctive rock types is evaluated by comparing the correspondences between the orientations of their gullies and rock couloirs with (1) the sliding orientations of potential slope failures mechanisms, and (2) the orientation of the maximum joint frequency, this latter being considered as the direction exploited primarily by erosion and mass wasting processes. These characteristic orientations can be interpreted as structural weaknesses contributing to the initiation and propagation of erosion. The morphostructural analysis was performed using digital elevation models and field observations. The catchment comprised of magmatic intrusive rocks shows a clear structural control, mostly expressed through potential wedges failure. Such joint configurations have a particular geometry that encourages the development of gullies in hard rock, e.g. through enhanced gravitational and hydrological erosional processes. In the catchment underlain by sedimentary rocks, penetrative joints that act as structural weaknesses seem to be exploited by gullies and rock couloirs. However, the lithological setting and bedding configuration prominently control the development of erosional landforms, and influence not only the local pattern of geomorphic features, but the general morphology of the catchment. The orientations of the maximum joint frequency are clearly associated with the gully network, suggesting that its development is governed by anisotropy in rock strength. These two catchments are typical of bedrock‐dominated basins prone to intense processes of debris supply. This study suggests a quantitative approach for describing the relationship between bedrock jointing and geomorphic features geometry. Incorporation of bedrock structure can be relevant when studying processes governing the transfer of clastic material, for the assessment of sediment yields and in landforms evolution models. Copyright © 2012 John Wiley & Sons, Ltd.     

内蒙古湖相沉积14C年代测定中“硬水”影响的发现

对内蒙古科尔沁沙地巴克窑泡子湖相沉积物分别做了＾２１０Ｐｂ和＾１４Ｃ测年,比较这两种方法测定结果发现,＾１４Ｃ年龄比＾２１０Ｐｂ估计年龄偏老约２０００年,内蒙古其它湖泊的沉积物＾１４Ｃ测年资料一般也都存在年龄偏老问题,作者认为,这一现象主要是由干燥和半干燥地区湖泊“硬水”影响引起的。     

Assessment of long‐term Holocene soil erosion rates in Polish loess areas using sedimentary archives from closed depressions

Soil erosion is a particularly important problem in the loess areas of Central Europe. Numerous studies of past and present soil erosion based on colluvial sediments have so far been conducted. The main problem is the fact that colluvia usually do not represent the complete sedimentation record. Closed depressions (CDs) collect all colluvial sediments from their catchment, therefore, constitute sediment stores enabling the calculation of soil erosion rates. Colluvial sediments and fossil soils, infilling four CDs in the Polish loess belt, were OSL and C‐14 dated. Human settlements near the studied CDs were analyzed. Phases of soil erosion and colluviation from the Neolithic (5400–2900 bc ), from the Middle Bronze Age to the Early Iron Age (1600–0 bc ), and from the Early Middle Ages to Modern Times (500 AD until today) were documented within the CDs studied. Phases of low soil erosion rate and pedogenesis occurred from the Late Vistulian to the Early Neolithic and from the Iron Age to Early Middle Ages. This study reveals that these phases are not synchronous with the soil erosion phases in Central Europe, as the latter mainly occurred in the Bronze Age, Roman Period and Middle Ages. The obtained soil erosion rates were compared with erosion rates in different areas of Central Europe. This study indicates that in loess regions with long‐term agricultural land use, mean erosion rates (i.e. 3.7–5.9 t ha^‐1 yr^‐1) from the Middle Ages to Modern Times were ten times higher than during the entire prehistoric period (0.39–0.67 t ha^‐1 yr^‐1). The mean soil erosion rates for forested CDs was 0.24–0.74 t ha^‐1 yr^‐1. Soil erosion phases are most probably caused by human activities (i.e. land use change) but the early Holocene erosion phase (7.96 +/‐ 0.67 kyr) could have been induced by a climatic fluctuation (e.g. a 8.2 kyr Bond event). Copyright © 2018 John Wiley & Sons, Ltd.     

The chronology and long term dynamics of a low energy river system: the Kelvin Valley,central Scotland

The long term (Holocene) channel and floodplain dynamics of a low gradient, low energy, fine grained aggradational fluvial system within a formerly glaciated landscape in central Scotland, the Kelvin Valley, are described from a series of sediment stratigraphic transects and 12 ¹⁴C assays in a headwater reach between Kirkintilloch and Kilsyth. The ¹⁴C assays and dated archaeological sites on the floodplain together suggest that the River Kelvin ceased to aggrade more than 2000 years ago, probably much more, so the 4–6 m of channel and floodplain deposits are almost entirely of early to mid‐Holocene age. The Kelvin Valley is characterized, despite its low flow characteristics, by a highly variable floodplain architecture, in which some transects suggest long term channel stability and strong partitioning of floodplain sedimentation and others indicate high channel mobility. This variation makes the application of general models of fluvial evolution difficult. Copyright © 2007 John Wiley & Sons, Ltd.     

14C dating and palaeoenvironment of the historic ‘little ice age’ glacier advance of Nigardsbreen Southwest Norway

Moss and grass remains associated with a well-developed in situ palaeosol buried beneath a moraine ridge in front of Nigardsbreen (Jostedalsbreen ice cap, southern Norway) have been ¹⁴C-dated. Pollen preserved with the plant remains suggests the existence of an agricultural landscape prior to the deposition of the moraine. The calibrated dates and the pollen spectra are in close accord with historically-documented evidence for the timing and palaeoenvironment of the ‘Little Ice Age’ advance of the glacier. Considerable potential is indicated for estimating the maximum ages of moraine ridges and for palaeoenvironmental reconstruction where such documentary evidence does not exist.     

Relationships between magnetic susceptibility and heavy metals in urban topsoils in the arid region of Isfahan, central Iran

Electromagnetic induction (EMI) method results are shown for vertical magnetic dipole (VMD) configuration by using the EM38 equipment. Performance in the location of metallic pipes and electrical cables is compared as a function of instrumental drift correction by linear and quadratic adjusting under controlled conditions. Metallic pipes and electrical cables are buried at the IAG/USP shallow geophysical test site in São Paulo City, Brazil. Results show that apparent electrical conductivity and magnetic susceptibility data were affected by ambient temperature variation. In order to obtain better contrast between background and metallic targets it was necessary to correct the drift. This correction was accomplished by using linear and quadratic relation between conductivity/susceptibility and temperature intending comparative studies. The correction of temperature drift by using a quadratic relation was effective, showing that all metallic targets were located as well deeper targets were also improved.     

A quantitative soil-stratigraphic approach to the correlation and dating of post-glacial river terraces in Glen Feshie,western Cairngorms

Little is known about Holocene river terrace development in upland Scottish valleys. Interpretation of many of the terrace sequences, previously suggested to have been formed by meltwaters either from the last Scottish ice sheet or the Loch Lomond Advance, has generally been based on morphological data. In this paper an alternative approach to the traditional height-based method of terrace correlation and dating is presented using data from Glen Feshie, western Cairngorms. Terrace fragments are numerically classified and objectively grouped using quantitative soil-stratigraphic data. Principal Components Analysis is used to both quantify pedological data and separate temporal trends in the data from variance due to local site factors. The scores on the temporal component are used to derive soil-stratigraphic units developed on the surficial sediments of the Glen Feshie terraces by grouping soil sites using a hierarchical clustering technique. This provides evidence for at least five soil-stratigraphic units developed on the fluvial surfaces. Various methods of absolute dating control permits association of these surfaces with five phases of terrace development. These are placed tentatively at 13 000, 10 000, 3600, 1000, and 80 radiocarbon years B.P., suggesting at least three phases of valley floor incision in Glen Feshie during the late Holocene.     

Soil formation and weathering in a permafrost environment of the Swiss Alps: a multi‐parameter and non‐steady‐state approach

Spatially discontinuous permafrost conditions frequently occur in the European Alps. How soils under such conditions have evolved and how they may react to climate warming is largely unknown. This study focuses on the comparison of nearby soils that are characterised by the presence or absence of permafrost (active‐layer thickness: 2–3 m) in the alpine (tundra) and subalpine (forest) range of the Eastern Swiss Alps using a multi‐method (geochemical and mineralogical) approach. Moreover, a new non‐steady‐state concept was applied to determine rates of chemical weathering, soil erosion, soil formation, soil denudation, and soil production. Long‐term chemical weathering rates, soil formation and erosion rates were assessed by using immobile elements, fine‐earth stocks and meteoric ¹⁰Be. In addition, the weathering index (K + Ca)/Ti, the amount of Fe‐ and Al‐oxyhydroxides and clay minerals characteristics were considered. All methods indicated that the differences between permafrost‐affected and non‐permafrost‐affected soils were small. Furthermore, the soils did not uniformly differ in their weathering behaviour. A tendency towards less intense weathering in soils that were affected by permafrost was noted: at most sites, weathering rates, the proportion of oxyhydroxides and the weathering stage of clay minerals were lower in permafrost soils. In part, erosion rates were higher at the permafrost sites and accounted for 79–97% of the denudation rates. In general, soil formation rates (8.8–86.7 t/km²/yr) were in the expected range for Alpine soils. Independent of permafrost conditions, it seems that the local microenvironment (particularly vegetation and subsequently soil organic matter) has strongly influenced denudation rates. As the climate has varied since the beginning of soil evolution, the conditions for soil formation and weathering were not stable over time. Soil evolution in high Alpine settings is complex owing to, among others, spatio‐temporal variations of permafrost conditions and thus climate. This makes predictions of future behaviour very difficult. Copyright © 2016 John Wiley & Sons, Ltd.     

Detecting soil disturbance/restoration effects on stream sediment loading in the Lake Tahoe Basin,USA—modelling predictions

Establishment and ‘crediting’ for total maximum daily loads (TMDL) of sediment require development of stream monitoring programs capable of detecting changes in land use and erosion ‘connectivity’ conditions across the watershed. As a ‘proof of concept’ directed at developing such an effective stream monitoring program considering only the effects of soil disturbances or restoration in the Lake Tahoe Basin, variability in daily stream sediment load predictions from a local‐scale, field data–based distributed runoff and erosion model developed previously is analysed for the west‐shore watersheds of Homewood (HMR) and Madden Creeks. The areal extent effects of forest fuel reductions (slight soil disturbances in Madden) and soil restoration efforts (e.g. dirt road removal and ski‐run rehabilitation in HMR) on watershed daily sediment loads for the 1994–2005 period are considered. Based on model predictions, forest fuel management in the Madden Creek watershed must occur across more than 30% of the basin area to result in a detectable increase in daily sediment loads at the >95% confidence level. Similarly, a daily load reduction that could be assessed with >95% confidence within the HMR basin required substantial dirt road removal (50% by roaded area) and restoration of 20% of the ski‐run area (combined for ~5% of the basin area) for the 11‐year record but was also possible within 2–3 years following restoration. These modelling results suggest that despite considerable flow–load variability, it may be possible to detect cumulative changing land‐use conditions within several years of project completion such that quantitative TMDL ‘crediting’ may be developed. Copyright © 2012 John Wiley & Sons, Ltd.     

Sediment Yield During Late Glacial and Holocene periods in the Lac Chambon Watershed,Massif Central,France

A sediment budget for the Late Glacial and Holocene periods was calculated for the Lac Chambon watershed which is located in a formerly glaciated temperate crystalline mountain area. It appears that over 15 500 years: (1) 69 per cent of eroded particles have been displaced by gravity processes and then stored within the watershed, compared to 31 per cent that have been displaced by running water and evacuated outward; (2) the mean mechanical erosion due to gravity processes on the slopes amounted to 16·1 ±6 m and only developed on a quarter of the watershed surface, whereas the mean mechanical erosion due to running water amounted 1·24 ± 0·37 m and involved the whole watershed surface. The mean sediment yields due to gravity processes on slopes were 2300 ± 1360, 1770 ± 960 and 380 ± 100 m³ km⁻³ a⁻¹, respectively, for basalts, and basic and acidic trachyandesites. Values of sediment yield due to running water were 49±15, 120±36 and 79±24 m³ km⁻² a⁻¹, respectively, during the Bôlling–Allerôd, the Younger Dryas and the Pre-Boreal–Boreal periods. They were 56±17 and 166±50 m³ km⁻² a⁻¹ during the Sub-Atlantic period before and after 1360 a BP , respectively. These values reflect variations in the natural environment and the impact of human-induced deforestation. © 1997 by John Wiley & Sons, Ltd.     

Dating of erosion surface and terraces in the eastern Qilian Shan,northwest China

The actively deformed foreland of eastern Qilian Shan (mountains) contains well‐preserved geomorphic features such as erosion surfaces, river terraces and tectonically uplifted alluvial fans, providing suitable archives for research on regional tectonic activities and palaeoclimatic changes. These geomorphic surfaces are well dated by using a combination of magnetostratigraphy, electron spin resonance, thermoluminescence, infra‐red stimulated luminescence, radiocarbon dating, and correlation with the well‐established loess–palaeosol sequences of China. Our results show that the erosion surface formed about 1·4 Ma ago, and the age of river terraces is 1·24 Ma, 820–860 ka, 780 ka, 420–440 ka, 230–250 ka, 140 ka, 60 ka and 10 ka, respectively. Valley incision rates of c. 0·09–0·25 m ka^?1 have been identified. The repetitive stratigraphic and geomorphic pattern of these terraces indicates the fluvial sedimentation–incision cycles are tightly associated with the 100‐ka glacial–interglacial climatic cycles. Copyright © 2006 John Wiley & Sons, Ltd.     

结合粒度和137Cs对小流域水库沉积物的定年——以黔中喀斯特地区克酬水库为例

对取自黔中喀斯特地区克酬水库32cm长沉积物柱样进行^137Cs比活度以及粒度测定,结合气象站点降雨数据进行比对分析,对沉积物不同层位的年代进行了划分,并以此结合质量深度推算了1960-2004年间该水库的沉积速率。结合实例分析了^137Cs沉降、运移和沉积,改进了仅依靠^137Cs峰值定年方法,认为在流域面积较小、^137Cs比活度测试绝对数值较小的流域,降水和其他条件造成的土壤侵蚀程度差异可能是造成土壤中^137Cs进入湖泊沉积物多少的更主要原因,并表现为沉积物中^137Cs不同层位的差异。     

The chronology of Late Quaternary fluvial activity in part of the Milfield Basin,northeast England

The sediment stratigraphy of a 4 m thick intercalated Holocene alluvial fill and valley floor peat at a site in the Milfield Basin, Northumberland, has been dated by a series of eight ¹⁴C assays, and related to a previously analysed pollen record. The sequence extends from the earliest Holocene until c. 2800 cal. BP . Prior to the onset of peat inception, substantial amounts of channel-trenching can be demonstrated to have occurred in the Milfield Basin during the Loch Lomond Stadial. There is no measurable early Holocene accelerated fluvial activity, but a major flooding event occurred at c. 7500 cal. BP , much earlier than recorded elsewhere in the region. The explanation for this is not clear. However, the cessation of mid-Holocene overbank sedimentation at c. 4000–3500 cal. BP is tentatively correlated with slope stability associated with woodland regeneration. © 1998 John Wiley & Sons, Ltd.     

华北盆地北缘香河—皇庄断裂晚更新世以来活动性研究

香河—皇庄断裂位于华北盆地北缘,是京津唐地区一条重要的NE向断裂构造。浅层人工地震反射剖面探测发现,香河—皇庄断裂在廊坊市香河县西马家窝地区表现为一条NE－SW走向的NW倾正断层,剖面上呈铲状,上部略陡,向下逐渐变缓;其对大厂凹陷东缘没有明显的控制作用,为一条新近纪以来形成的断裂,切穿了前中生界基底,且第四纪以来仍有活动。通过部署于断层两侧的构造观察孔岩芯对比及测年数据判断,(6.69±5.6) ka以来香河—皇庄断裂无活动,其上断点很可能位于距地表15～30 m范围内,大致相当于晚更新世早期;在中更新世上部地层中,香河—皇庄断裂存在1.5～1.8 m的垂直落差。香河—皇庄断裂现阶段处于不活动状态,未来发生中强地震的可能性较低。