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.     

Quartz OSL sensitivity from dating data for provenance analysis of pleistocene and holocene fluvial sediments from lowland Amazonia

The properties of the quartz luminescence signal have been shown to be a useful tool for sediment provenance analysis. These provenance studies are based on the sensitivity of the fast optically stimulated luminescence (OSL) component, which is also used for sediment dating. Besides the widespread occurrence of quartz in terrigenous sediments, OSL sensitivity can be acquired using relatively fast and low-cost measurements compared to sediment provenance analysis methods based on accessory minerals or isotopes. Additionally, laboratories worldwide already have an extensive database of recorded quartz OSL signals primarily measured for dating studies, and these data could potentially be repurposed for provenance analysis of Quaternary sedimentary systems through OSL sensitivity calculation. Here, we investigate the use of OSL quartz signals measured in sediment dating surveys for OSL sensitivity calculation and evaluation of changes in sediment sources. The OSL sensitivity was calculated and expressed as %BOSL_F, which corresponds to the percentage of the fast OSL component signal (blue stimulation) to the total OSL curve; such approach is advantageous as it does not require any normalisation of the measured signal intensity to dose or aliquot size (weight). Three sets of samples from Amazonian fluvial sediments are investigated: two sets of Holocene floodplain sediments representing different sediment sources to the Amazonian fluvial system, i.e. the Amazon craton and the Andes Mountain belt, and a set of samples from the Içá Formation, a paleo-fluvial system active during the Pleistocene whose provenance is not fully known. Results show that the quartz OSL signal derived from the first test doses (T_n) applied in dating protocols had the best performance for %BOSL_F calculation when compared to results from a measurement protocol designed specifically for sediment provenance analysis. There is significant correlation (R² = 88) between sensitivities derived from T_n and a specific OSL provenance analysis protocol. The proposed approach indicates to be appropriate for sediment provenance analysis since it is able to discriminate signal differences among samples from known sources: Brazilian cratonic quartz yield high sensitivity values (mean %BOSL_F >70), in contrast to the relatively lower values from Andean quartz (mean %BOSL_F <50). In general, quartz OSL sensitivities from the Içá Formation samples fall into the same range of modern sediments transported by the Içá and Japurá rivers draining the Andean Eastern Cordillera of Colombia and Ecuador. We also observe a decrease in quartz OSL sensitivity during the Holocene, notably after 4 ka, with younger deposits showing lower sensitivity. Sediment provenance variations are discussed in terms of watershed rearrangement and/or precipitation-driven changes during the Late Pleistocene and Holocene across Amazonia.     

Allogenic and autogenic effects on mangrove dynamics from the Ceará Mirim River,north‐eastern Brazil,during the middle and late Holocene

It is possible that climate changes and sea level fluctuations (allogenic processes) are and will cause major changes in mangrove dynamics. However, other driving forces may be significantly affecting this system. Distinguishing allogenic and autogenic influence on mangroves is a challenging question, because mechanisms related to the natural dynamics of depositional environments (autogenic processes) have strong influences on the establishment and degradation of mangroves. Thus, impacts on mangroves caused by autogenic processes may be erroneously attributed to allogenic mechanisms. Therefore, it is imperative to identify the ‘fingerprint’ of global changes in modern mangrove dynamics. In order to characterize the influence of these forces on mangroves, this work has used geomorphology and vegetation maps integrated with sedimentological and palynological data, radiocarbon dating, as well as δ¹³C, δ¹⁵N and C/N from sedimentary organic matter. The inter‐proxy analyses reveal an estuarine influence with mangrove development along the Ceará Mirim River, north‐eastern Brazil, since ~6920 cal yr bp , after the post‐glacial sea level rise. Relative sea level (RSL) has been stable during the middle and late Holocene. Mangrove establishment along this fluvial valley begins at about 6920 cal yr bp , caused by the sea‐level stabilization, an allogenic influence. However, after its establishment, wetland dynamics were mainly controlled by autogenic factors, related to channel migrations, instead of allogenic process. Some influence of sea‐level and climate changes on mangrove dynamics in this estuarine channel have been weakened by more intense tidal channels activities. Therefore, the expansion and contraction of mangrove areas along the estuary of the Ceará Mirim River since 6920 cal yr bp has been mainly influenced by channel dynamics that regulate the accretion and erosion of mangrove substrates. Copyright © 2018 John Wiley & Sons, Ltd.     

Rapid climate change-induced collapse of hunter-gatherer societies in the lower Mississippi River valley between ca. 3300 and 2780 cal yr BP

Hunter-gatherer communities in the American Southeast reached an apogee of social and political complexity in the period between ca. 4200 and 3000 cal yr BP. In the lower Mississippi Valley(LMV) the Poverty Point culture defined this period of socio-political elaboration. However, following a significant period of climate change that led to exceptional flooding and a major reorganization of the course of the Mississippi River, this culture collapsed beginning ca. 3300–3200 cal yr BP and the LMV was abandoned for the subsequent 500 years. In this study, we use data from the Jaketown site in the Yazoo Basin of west-central Mississippi to refine the chronology of the climate event that caused the collapse of the Poverty Point culture. A large flood buried Poverty Point-era occupation deposits at Jaketown around 3310 cal yr BP. Lateral migration of the Mississippi River during flooding led to inundation of the Yazoo Basin and re-occupation of ancient river courses. A coarse sand stratum topped by a more than a meter-thick fining upward sediment package marks a crevasse deposit caused by a rupture of the natural levee at Jaketown. This levee breach was part of a larger pattern of erratic flooding throughout the LMV and is associated with major landscape evolution and the abandonment of Poverty Point sites within the valley. Early Woodland peoples re-colonized the crevasse surface after ca. 2780 cal yr BP. Following this event, the Jaketown site and the eastern Yazoo Basin witnessed a period of landscape stability that lasts to this day. These archaeological data demonstrate how climate change and natural disasters can lead to socio-political dissolution and reorganization even in relatively small-scale hunter-gatherer populations.     

Prograded foredunes of Western Australia's macro‐tidal coast – implications for Holocene sea‐level change and high‐energy wave impacts

The Holocene evolution of the Canning Coast of Western Australia has largely been overlooked so far mainly due to its remoteness and low population density. We report on new data from a sequence of foredunes inside the macro‐tidal Admiral Bay, 110 km southwest of Broome. Based on sediment cores, differential global positioning system (dGPS)‐based elevation transects, and stratigraphical analyses on outcrops of the relict foredunes, we aim at reconstructing Holocene coastal changes and relative sea levels (RSLs), as well as identifying and dating imprints of extreme‐wave events. Sedimentary analyses comprise the documentation of bedding structures, foraminiferal content and macrofaunal remains, grain size distribution, and organic matter. The chronological framework is based on 26 carbon‐14 accelerator mass spectrometry (¹⁴C‐AMS) datings. Marine flooding of the pre‐Holocene surface landward of the 2.5 km‐wide foredune barriers occurred 7400–7200 cal bp , when mangroves colonized the area. After only 200–400 years, a high‐energy inter‐tidal environment established and prevailed until c. 4000 cal bp , before turning into the present supralittoral mudflat. During that time, coastal regression led to beach progradation and the formation of aligned foredunes. Drivers of progradation were a stable RSL or gradual RSL fall after the mid‐Holocene and a positive sand budget. The foredunes overlie upper beach deposits located up to >2 m above the present upper beach level and provide evidence for a higher mid‐Holocene RSL. Discontinuous layers of coarse shells and sand are intercalated in the foredunes, indicating massive coastal flooding events. One such layer was traced over three dune ridges and dated to c. 1700–1550 cal bp . However, it seems that most tropical cyclones induce net erosion rather than deposition at aligned foredunes and thus, they are only suitable for reconstructing temporal variability if erosional features or sedimentation reliably tied to these events can be identified and dated accurately. Copyright © 2014 John Wiley & Sons, Ltd.     

Sediment accumulation rates in subarctic lakes: Insights into age-depth modeling from 22 dated lake records from the Northwest Territories,Canada

Age-depth modeling using Bayesian statistics requires well-informed prior information about the behavior of sediment accumulation. Here we present average sediment accumulation rates (represented as deposition times, DT, in yr/cm) for lakes in an Arctic setting, and we examine the variability across space (intra- and inter-lake) and time (late Holocene). The dataset includes over 100 radiocarbon dates, primarily on bulk sediment, from 22 sediment cores obtained from 18 lakes spanning the boreal to tundra ecotone gradients in subarctic Canada. There are four to twenty-five radiocarbon dates per core, depending on the length and character of the sediment records. Deposition times were calculated at 100-year intervals from age-depth models constructed using the ‘classical’ age-depth modeling software Clam. Lakes in boreal settings have the most rapid accumulation (mean DT 20 ± 10 yr/cm), whereas lakes in tundra settings accumulate at moderate (mean DT 70 ± 10 yr/cm) to very slow rates, (>100 yr/cm). Many of the age-depth models demonstrate fluctuations in accumulation that coincide with lake evolution and post-glacial climate change. Ten of our sediment cores yielded sediments as old as c. 9000 cal BP (BP = years before AD 1950). From between c. 9000 cal BP and c. 6000 cal BP, sediment accumulation was relatively rapid (DT of 20–60 yr/cm). Accumulation slowed between c. 5500 and c. 4000 cal BP as vegetation expanded northward in response to warming. A short period of rapid accumulation occurred near 1200 cal BP at three lakes. Our research will help inform priors in Bayesian age modeling.     

Channel and floodplain sediment dynamics in a reach of the tropical meandering Rio Beni (Bolivian Amazonia)

The study investigates interactions, water and sediment exchanges, between a rapidly migrating meander and its associated floodplain at fine temporal and spatial scales. The Beni River, an Amazonian free meandering river, makes the transition between Andean ranges and Amazonian lowlands. For the period 2002–2006, an assemblage of tools and methods (water and sediment discharges, topometric and bathymetric surveys, sedimentation rate estimations from unsupported ²¹⁰Pb and sediment trapping system) was used to jointly analyse the influence on the sediment budget of external factors (mainly water and sediment discharge) and the inherent behaviour of the system. The main issue addressed is the investigation of the complex relationship between ‘morphological conditioning’ of fluvial landform and process. The first part of the study was undertaken with the aim of linking erosion–deposition in an active meander with water and sediment fluxes. The three inter‐annual evolutions are characterized by very unequal sediment budgets; the first two intervals underwent predominant erosion, and the latter slight accumulation. Digital elevation models, evaluated for the active meander, demonstrate that sedimentation on the point bar depends more on external factors than erosion of the concave bank, which fluctuates slightly. The second part of the study, focusing on water and sediment exchanges between active bend and floodplain, examines the respective parts played by overbank flow and by an abandoned channel on the diffusion and sequestration of sediment. The association of short‐ and long‐term estimation of sedimentation rates suggests that floodplain construction is associated with two different processes and rhythms of sediment transportation. Finally, a sediment budget is proposed for the Beni River in the upper part of the Amazonian lowlands. Copyright © 2010 John Wiley & Sons, Ltd.     

Late Holocene mangrove dynamics dominated by autogenic processes

Generally, palaeoenvironmental interpretations consider only allogenic processes, when autogenic factors may have a strong influence on proxies of stratigraphic sequences. For instance, the Holocene history of the vegetation along the southern littoral of the State of Bahia in north‐eastern Brazil is characterized by mangrove dynamics controlled by allogenic processes. However, over smaller timescales (~700 years), autogenic processes may have controlled vegetation dynamics and hence observed pollen distribution. This work proposes tidal channel dynamics as one of the main cause for changes in pollen assemblage along the studied stratigraphic profiles during the last centuries, based on sedimentology, pollen and elemental analysis (δ¹³C, δ¹⁵N and C/N) and radiocarbon dating of sedimentary organic matter from two cores sampled from an abandoned meander and a tidal flat at the mouth of the Jucuruçu River. One core was sampled from a mangrove formed during the past ~550 cal yr bp . Another core recorded sediments in a várzea forest (swamp seasonally and permanently inundated by freshwater) located ~2.7 km from the current shoreline, which displayed a maximum age of ~680 cal yr bp . Two facies associations were identified: tidal channel (A) and tidal flat/oxbow lake (B). This work proposes allogenic processes as the main driving forces controlling the wetlands dynamics at the studied site during the Holocene. However, our data also reveal that part of the changes in vegetation over the last ~700 years reflect tidal channels and tidal flats development, which represent autogenic processes. The change in timescale analysis from the Holocene to recent centuries may have weakened the influence of allogenic factors. However, this needs interpretation with reference to the spatial scale of the depositional environment as the larger the depositional system analyzed, the stronger the influence of autogenic processes on stratigraphic sequences over longer timescales. Copyright © 2017 John Wiley & Sons, Ltd.     

Composition and origin of a lithalsa related to lake‐level recession and Holocene terrestrial emergence,Northwest Territories,Canada

Lithalsas of the Great Slave Lowland, Northwest Territories, occur within fine‐grained glaciolacustrine, lacustrine, and alluvial deposits. Detailed investigations of a lithalsa revealed that it is composed of ice‐rich sediments with ice lenses up to 0.2 m thick below 4 m depth. The observed ice accounted for about 2 m of the 4 m between the top of the lithalsa and adjacent terrain. The ice is isotopically similar to modern surface water, but enriched in δ¹⁸O relative to local precipitation. Total soluble cation concentrations are low in the basal, Shield‐derived and unweathered glaciolacustrine sediments of the lithalsa. Higher concentrations in the overlying Holocene‐aged lacustrine and alluvial deposits may be due to greater ion availability in Holocene surface waters. Increasing Cl^‐ and Na⁺ concentrations in clays at depth likely relate to exclusion and migration of these dissolved ions in pore water during ice lens formation though total soluble cations remain comparatively low. The lithalsa developed 700 to 300 cal yr BP. A conceptual model of lithalsa formation and landscape evolution illustrates that this feature and more than 1800 other lithalsas in the region have developed in association with Holocene terrestrial emergence following lake‐level recession. Copyright © 2017 John Wiley & Sons, Ltd.     

Using in situ cosmogenic 10Be to identify the source of sediment leaving Greenland

We use the concentration of in situ ¹⁰Be in quartz isolated from fluvial and morainal sand to trace sediment sources and to determine the relative contribution of glacerized and deglaciated terrain to Greenland's sediment budget. We sampled along the western, eastern, and southern margins of the Greenland Ice Sheet, and collected sediment sourced from glacerized (n = 19) and non‐glacerized terrain (n = 10), from channels where sediment from glacerized and non‐glacerized terrain is mixed (n = 28), from Holocene glacial‐fluvial terraces (n = 4), and from one sand dune. In situ ¹⁰Be concentrations in sediment range from 1600 to 34 000 atoms g^‐1. The concentration of in situ ¹⁰Be in sediment sourced from non‐glacerized terrain is significantly higher than in sediment sourced from glacerized areas, in mixed channel sediment, and in terrace sediment that was deposited during the Holocene. To constrain the timing of landscape exposure for the deglaciated portion of the Narsarsuaq field area in southern Greenland, we measured in situ ¹⁰Be concentration in bedrock (n = 5) and boulder (n = 6) samples. Paired bedrock and boulder ages are indistinguishable at 1σ uncertainty and indicate rapid exposure of the upland slopes at ~10.5 ka. The isotope concentration in sediment sourced from non‐glacerized terrain is higher than in sediment sourced from glacerized terrain because the non‐glacerized landscape has been exposed to cosmic radiation since early Holocene deglaciation. Sediment from glacerized areas contains a low, but measurable concentration of ¹⁰Be that probably accumulated at depth during a prolonged period of exposure, probably before the establishment of the Greenland Ice Sheet. The concentration of ¹⁰Be in mixed fluvial sediment and in terrace sediment is low, and similar to the concentration in sediment from glacerized areas, which indicates that the Greenland Ice Sheet is the dominant source of sediment moving through the landscape outside the glacial margin in the areas we sampled. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Natural and historical variability in fluvial processes,beaver activity,and climate in the Greater Yellowstone Ecosystem

Two centuries of human activities in the Greater Yellowstone Ecosystem (GYE) have strongly influenced beaver activity on small streams, raising questions about the suitability of the historical (Euro‐American) period for establishing stream reference conditions. We used beaver‐pond deposits as proxy records of beaver occupation to compare historical beaver activity to that throughout the Holocene. Forty‐nine carbon‐14 (¹⁴C) ages on beaver‐pond deposits from Grand Teton National Park indicate that beaver activity was episodic, where multi‐century periods lacking dated beaver‐pond deposits have similar timing to those previously documented in Yellowstone National Park. These gaps in the sequence of dated deposits coincide with episodes of severe, prolonged drought, e.g. within the Medieval Climatic Anomaly 1000–600 cal yr bp , when small streams likely became ephemeral. In contrast, many beaver‐pond deposits date to 500–100 cal yr bp , corresponding to the colder, effectively wetter Little Ice Age. Abundant historical beaver activity in the early 1900s is coincident with a climate cooler and wetter than present and more abundant willow and aspen, but also regulation of beaver trapping and the removal of wolves (the beaver's main predator), all favorable for expanded beaver populations. Reduced beaver populations after the 1920s, particularly in the northern Yellowstone winter range, are in part a response to elk overbrowsing of willow and aspen that later stemmed from wolf extirpation. Beaver populations on small streams were also impacted by low streamflows during severe droughts in the 1930s and late 1980s to present. Thus, both abundant beaver in the 1920s and reduced beaver activity at present reflect the combined influence of management practices and climate, and underscore the limitations of the early historical period for defining reference conditions. The Holocene record of beaver activity prior to Euro‐American activities provides a better indication of the natural range of variability in beaver‐influenced small stream systems of the GYE. Copyright © 2012 John Wiley & Sons, Ltd.     

A palaeovalley system in the floodplain of the Mark River (Central Belgium)

A borehole programme on the floodplain of the River Mark has revealed an important fossilized fluvial system, now invisible in the present landscape. The palaeovalley meanders gently and is up to 200 m wide and 8 m deep. It is eroded in older fluvial sands, probably of Pleniglacial age, overlain by an aeolian cover of varying thickness. As indicated by several radiocarbon dates of the valley fill deposits, the erosion of the system took place in the pre-Holocene period. The filling proceeded in three phases. The morphological implications of each phase and their spatial extension are demonstrated. Most of the vertical fill consists of a sandy loam to loam. Around 9000 yr BP, the accumulation of an organic facies (mainly woodpeat) started, followed by the deposition of a weak fluvial clay. At 1400 yr BP the filling of the palaeovalley was complete.     

Holocene flood histories in south‐western USA

River basins in south‐western USA are some of the most extensively studied arid land fluvial systems in the world. Since the early 1960s their hydro‐climatic histories have been reconstructed from the analysis of alluvial cut‐and‐fill cycles, while from the late 1970s there have been investigations of slackwater deposits and palaeostage indicators for large floods in stable‐boundary bedrock reaches. However, no studies have regionally integrated Holocene fluvial histories from these two different types of fluvial environments. The current study combines the alluvial archive with flood records from bedrock reaches to generate a probability‐based 12,000 year record of flooding in south‐western USA. Using more than 700 ¹⁴C‐dated fluvial units, the analysis produces a high resolution (centennial) flood record. Seven episodes of increased flooding occurred at 11,250–10,400, 8800–8350, 8230–7600, 6700–5700, 5600–4820, 4550–3320 and 2000–0 cal. BP. Bedrock reaches are found to record more frequent floods during the middle to late Holocene, while in alluvial rivers more flood units are dated to the early and middle Holocene. These differences are primarily the result of selective preservation with alluvial reaches tending to erode during periods characterised by very large floods. Episodes of major Holocene flooding recorded in slackwater deposits within bedrock systems correspond with periods of increased precipitation in the region and lower temperatures. In contrast, within alluvial rivers above‐average flooding probabilities, as well as regionally extensive channel entrenchment episodes, match with reduced annual precipitation and lower temperatures. The results of this study clearly demonstrate the value of the Holocene fluvial archive for reconstructing regional, short‐term hydro‐climatic change in south‐western USA. Copyright © 2010 John Wiley & Sons, Ltd.     

Impacts of Holocene and modern sea-level changes on estuarine mangroves from northeastern Brazil

Projections of the impacts of modern Relative Sea Level (RSL) rise on estuarine mangroves should be supported by coastal topographic data and records of mangrove dynamics under past RSL change. This work identified inland and seaward mangrove migrations along the Jucuruçu River (Bahia, Northeastern Brazil), during the Holocene based on sedimentary features, palynological and geochemical (δ¹³C, δ¹⁵N, C/N) data integrated with digital elevation models. During the Middle Holocene, in response to RSL rise, the estuary saw mangrove forest establish up to ~37 km inland. RSL stood between -1.4 (+0.36/-2.2 m) and +1 (2.19/0.2 m) around 7400 cal yr BP, and rose to a highest position of +3.25 (4.22/2.45 m) reached around 5350 cal yr BP. That marine incursion caused the inland replacement of freshwater vegetation by mangroves on tidal flats. Since then, the estuary experienced RSL fall, reducing inland tidal water salinity towards the Late Holocene, making that the mangroves were replaced by freshwater floodplain vegetation. Today, in the seaward part of the estuary near its mouth, mangroves occupy an area of ~10 km² along tidal channels. Considering a RSL rise of 98 cm up to the end of the 21^st century, at a rate significantly higher than that of Middle Holocene RSL rise (1.5 mm/yr) and fall (0.6 mm/yr), the current mangrove substrates are expected to drown and/or eroded near the coast, while new mangroves may establish inland, at topographically higher tidal flats in nowadays freshwater-tidal zones. Mangrove area could expand over 13 km² of coastal and flood plain. Following the same interaction between RSL/climate changes and Holocene mangrove dynamics, such upstream mangrove migration may be attenuated or intensified by changes in fluvial discharge. © 2019 John Wiley & Sons, Ltd.     

Sedimentary evolution of a foreland basin from the northeastern Qilian Mountains based on the integration of geomorphological and luminescence dating of alluvial conglomerates and fluvial terrace sediments

Sedimentary deposits in the foreland basin of the northeastern Qilian Mountains are crucial documents recording tectonic activity and climate changes on the Tibetan Plateau. In this study, luminescence dating was used to date alluvial conglomerates and fluvial terrace sediments collected from the Beida River in the Jiuquan Basin, a foreland basin in the Hexi Corridor, northeastern Qilian Mountains. Detailed sedimentology and luminescence ages reveal that alluvial conglomerates accumulated from before 620 ka to 12 ka and that sediment accumulation rates increased at ∼330 ka and ∼35 ka, coinciding with the dates of two tectonic events (∼350 and ∼50 ka) and followed by climate cooling (from marine isotope stage (MIS) 9 to MIS 8 and from MIS 3 to MIS 2). This reveals that variations in the sediment accumulation rates are controlled by the coupling of tectonic uplift and climate cooling. The highest terrace (T7) that developed on the alluvial conglomerate base formed at ∼ 12 ka. The incision rate in the early Holocene was ∼2.1 mm/yr and increased to ∼14.6 mm/yr during the middle and late Holocene. The variations in the river incision rate provide geomorphic evidence for Holocene climate patterns in arid and semiarid areas. Luminescence dating offers a credible temporal framework for the deposits and reveals climate and tectonic effects on the evolution of the foreland basin, northeastern Qilian Mountains.     

Sediment transmission and storage: the implications for reconstructing landform development

The late Holocene (last 3000 years) development of the lower Ribble valley (northwest England) displays evidence for a complex response to a sediment recharge event forced by land‐use change induced increases in erosion and sediment delivery. The deposition of fluvial sediments during the late Holocene was restricted to a series of reaches or depocenters separated by zones with no sediment accumulation constrained by older glacial and fluvial terrain. Apparent reach‐wide correlations of fluvial terraces break down under the scrutiny applied by comprehensive and extensive radiocarbon control. Bayesian testing of relative order models show that large‐scale geomorphological changes, e.g. the progression from one terrace level to another, were time transgressive between different depocenters. The different histories of sediment delivery and storage are probably a function of local‐ and process‐scale variations in these depocenters, and reflect (dis)connectivity relationships within a reach in propagating a basin‐scale change (superslug) in the sediment regime. Disconnectivity in the depositional regime through a fluvial reach limits what we can reconstruct in terms of sediment budgets, but radiocarbon dating of multiple palaeochannels offers considerable potential for landform‐based research to uncover rates of change within individual depocenters. Copyright © 2009 John Wiley & Sons, Ltd.     

Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12 000 cal yr: long-term North Atlantic oceanographic variability and solar forcing

Giant piston core MD99-2269 recovered 25 m of sediment in Hunáfloáall, a deep trough on the North Iceland margin fronting the Iceland Sea, and the site of a shelf sediment drift. The rate of sediment accumulation is 2 m/kyr (5 yr/cm); the core terminated in the Vedde tephra (12 cal ka). The sediment was sampled at between 5 and 50 yr/sample, including rock magnetic, grain-size, and sediment properties. Data reduction was carried out using principal component analysis. Two PC axes for the 5-yr/sample magnetic data are strongly correlated with measures of coercivity (ARM_{20 mT}/ARM) and magnetic concentrations (kARM). In turn ARM_{20 mT}/ARM is highly correlated (negatively) with grain-size and the mean size of the sortable silt fraction. Analyses of the two PC axes with MTM spectral methods indicate a series of significant (>99%) periodicities at millennial to multidecadal scales, including those at 200, 125, and 88 yr which are associated with solar variability. We also document a strong correlation between the sediment magnetic properties and the ∂¹⁸O on benthic foraminifera on the North Iceland inner shelf. We hypothesize that the links between variations in grain-size, magnetic concentrations, and solar forcing are controlled by atmospheric and oceanographic changes linked to changes in the relative advection of Atlantic and polar waters along the North Iceland margin. Today these changes are associated with variations in the deep convection in the Greenland and Iceland Seas. The precise linkages are, however, presently elusive although a combination of coarser sediments and low ∂¹⁸O values define a Holocene thermal maximum between 8 and 6 cal ka.     

湖泊沉积物磁学特征指示的埃及Faiyum盆地全新世以来的环境演变

利用埃及北部Faiyum盆地获得的高取芯率沉积物岩芯,进行沉积物多种磁性参数的测量,结合有机碳、介形虫、粒度等分析,在AMS14C加速器测年的基础上,建立全新世以来湖泊沉积物磁性特征变化的时间序列.结果表明,粒度效应以及沉积后的各种次生作用对沉积物的磁性特征没有明显的影响,磁性变化主要反映了沉积物不同来源组成的相对变化.全新世前沉积物磁性较弱,主要含不完全反铁磁性矿物,与周边沙漠的物质相似,结合其粒度特征,沉积物来源应以近源物质为主.全新世早中期(约10 5.4 ka BP)沉积物磁性变化相对稳定,有机质含量也较高,指示了来自尼罗河较为稳定的物质供应;而大约5.4 ka BP尤其最近约4.2 ka BP以来,磁性的明显变化反映了流域降水减少情况下,来自青尼河物质贡献的相对增加;最近约2.0 ka BP以来沉积物的磁性变化,则更多地与盆地人类活动的强化有关.总体而言,Faiyum盆地全新世以来的环境演变主要受控于全新世以来尼罗河与盆地的水力学联系.即:全新世前盆地未与尼罗河连通时,沉积物主要来源于周边沙漠的风成物质;而受全新世早-中期来流域季风降水增加的影响,泛滥的尼罗河为盆地提供了相对稳定的物质供应,湖泊也处于高湖面;全新世晚期以来,随着流域干旱化的加剧,尼罗河与盆地的连通性开始减弱,来自高磁性的青尼罗河物质贡献开始相对增加.最近约2.0 ka BP以来,虽然仍有人工运河连接尼罗河与盆地,但沉积物磁性的显著变化更多地反映了盆地人类活动的不断强化.