Optical dating of St. Lawrence Iroquoian ceramics from the Mailhot-Curran site,southern Québec

Infrared-stimulated luminescence dating (IRSL) was applied on pottery sherds from the Mailhot-Curran archaeological site (BgFn-2), a Late Woodland period village located in the Saint-Anicet region, southern Québec (Canada). This site witnessed the presence of St. Lawrence Iroquoians and is thought to have been occupied roughly 500 years ago. The main goal of this study is to chronologically constrain the occupation of this site in order to establish a fine chronological sequence for the occupations of various sites found in this area. The IRSL is used to complement previous dating methods used for Mailhot-Curran (radiocarbon and seriation of the archaeological remains).Dating results are reported for ten samples of local ceramics made from Quaternary Champlain Sea clay. Luminescence dating on fine grains was preferentially used, since feldspar luminescence typically emit a decent luminescence signal. Although the latter was rather dim, anomalous fading was measured and luminescence ages were successfully corrected. It is therefore possible to validate the occupation age of this site.     

Sources of particulate and dissolved organic carbon in the St Lawrence River: isotopic approach

We investigate sources of both dissolved and particulate organic carbon in the St Lawrence River from its source (the Great Lakes outlet) to its estuary, as well as in two of its tributaries. Special attention is given to seasonal interannual patterns by using data collected on a bi‐monthly basis from mid‐1998 to mid‐2003. δ¹³C measurements in dissolved inorganic carbon, dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as molar C : N in particulate organic matter (POM), are used to bring insight into the dynamic between aquatic versus terrigenous sources. In addition, ¹⁴C activities of DOC were measured at the outlet of the St Lawrence River to its estuary to assess a mean age of the DOC exported to the estuary. In the St Lawrence River itself, aquatically produced POC dominates terrestrially derived POC and is depleted in ¹³C by approximately 12‰ versus dissolved CO₂. In the Ottawa River, the St Lawrence River's most important tributary, the present dataset did not allow for convincing deciphering of POC sources. In a small tributary of the St Lawrence River, aquatically produced POC dominates in summer and terrestrially derived POC dominates in winter. DOC seems to be dominated by terrestrially derived organic matter at all sampling sites, with some influence of DOC derived from aquatically produced POC in summer in the St Lawrence River at the outlet of the Great Lakes and in one of its small tributaries. The overall bulk DOC is relatively recent (¹⁴C generally exceeding 100% modern carbon) in the St Lawrence River at its outlet to the estuary, suggesting that it derives mainly from recent organic matter from topsoils in the watershed. Copyright © 2005 John Wiley & Sons, Ltd.     

Distribution and fluxes of metals in the St. Lawrence River from the outflow of Lake Ontario to Québec City

Six anchor stations in the St. Lawrence River from the outflow of Lake Ontario to Quebec City, were occupied for ca. 24 hours in June 1987 during low flow conditions. Samples of water and suspended particulate matter were separated by continuous-flow centrifugation, and were collected every two hours.During this sampling period, fluxes of dissolved forms of zinc, copper and nickel increased in Lac St. François and downstream relative to the sum of the fluxes for the two upstream stations at the outflow of Lake Ontario. Increases in the flux of dissolved zinc and copper were pronounced below Montreal and above Lac St. Pierre. For particulate forms of metals, all five metals show that there are significant inputs in the section of the St. Lawrence River between Lac St. François and the station just above the entrance to Lac St. Pierre.The average concentrations of dissolved cadmium, lead, zinc, copper and nickel ranged from 7–23 ng/l; 9–35 ng/l; 0.434–0.939 g/l; 0.15–0.89 g/l and 0.58–1.12 g/l respectively.Regression analysis of the dissolved and particulate metal concentrations suggests that the concentration of dissolved cadmium, lead, zinc and nickel can be predicted from the regression equation and the determination of particulate metal concentration. This prediction appears to be independent of the suspended particulate matter concentration which varies from ca. 1 to 10 mg/l from the outflow of Lake Ontario to Québec City.     

K-feldspar IRSL dating of a Pleistocene river terrace staircase sequence of the Lower Tejo River (Portugal,western Iberia)

We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs, geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced by faults (known from previous studies). Integration of these new ages with those obtained previously in the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley. However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces (with uncorrected D_e values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the natural signals close to luminescence saturation, giving a minimum corrected D_e of about 1000 Gy, and thus minimum terrace ages; this may even be true for those doses >200 Gy. Luminescence dating results suggest that: T3 is older than 300 ka, probably ca. 420–360 ka (～Marine Isotope Stage [MIS]11); T4 is ca. 340–150 ka (～MIS9-6); T5 is 136–75 ka (～MIS5); T6 is 60–30 ka (MIS3); an aeolian sand unit that blankets T6 and some of the older terraces is 30–≥12 ka. Collectively, the luminescence ages seem to indicate that regional river downcutting events may be coincident with periods of low sea level (associated, respectively, with the MIS10, MIS6, MIS4 and MIS2).     

CHRONOLOGY OF YICHANG GRAVEL LAYER BASED ON QUARTZ TI-LI CENTER ESR DATING AND ITS IMPLICATIONS FOR THE INCISION TIMING OF THE THREE GORGES VALLEY

Uplift of Tibet Plateau and formation of Asian Monsoon greatly affect East Asian geomorphological evolution, climate change and environment systems. Thus, those phenomena also control the origin, size and direction of river systems. The Yangtze River, as the most important linkage between Tibet Plateau and the East Asian marginal seas, delivers large volumes of water, sediment, and associated chemicals from its headwater regions and tributaries to the East China Sea, significantly influencing sedimentary system evolution in its drainage basin. Therefore, the formation of the modern Yangtze River and its geological-time evolution history have been paid more and more attention to since the beginning of the last century. After debated for more than a century, the First Bend in Shigu area and the Three Gorges have been known as the key capture point of the Yangtze River's evolution history. In particularly, the Three Gorges incision period remains greatly controversial, which mainly focuses on Cretaceous period-Neogene period, early Pleistocene period, and late Quaternary period. The Yichang Gravel, just located downstream and outlet of the Three Gorges with an inverted triangle shape, is mainly distributed in western Jianghan Basin with over 1 000km². Because of its wide distribution and key geographical location, many typical profiles of Yichang Gravel have been the critical materials for studies on stratigraphic division, geomorphic evolution, and paleoenvironment change in middle Yangtze River Basin, especially on the Three Gorges incision history. Based on the previous field investigation, the Yichang gravel unconformably overlies the Cretaceous bedrocks and underlies the mid-Pleistocene vermicular red earth. In addition, studies on heavy mineral assemblages, Pb isotopic compositions of detrital K-feldspar grains, magnetic characteristics as well as pollen assemblage characteristics have showed that sediments in Yichang Gravel are mainly derived from upper Yangtze River Basin, such as Jinshangjiang drainage, Minjiang drainage, Jialingjiang drainage and Wujiang drainage. Based on the above comprehensive analysis, researchers demonstrated that the depositing time of Yichang Gravel can best constrain the Three Gorges incising time. The absolute altitude of Yichang Gravel exceeds 110m, and many thick sand lens are developed from top to bottom of the profiles. In this study, we applied the quartz Ti-Li center ESR dating method in Yichang Gravel to determine its absolute formation age, and then to constrain the minimum cutting-through time of Three Gorges. Eight samples(SXY-1, SXY-2, YC-1-4, LJY-1, LJY-2)were collected from the sand lens at depths of 4m, 19m, 40m, 51m, 63m, 75m, 83m and 99m respectively from the top of the profile. At the same time, in order to evaluate the residual dose of Ti-Li center after sunlight bleaching, we also sampled four modern surface Yangtze River sediments near Yichang Gravel for ESR measurement. The result shows that the quartz Ti-Li center ESR signal intensity of the 4 modern fluvial sediments samples are zero, which implies that the Ti-Li center ESR signal intensity of quartz in Yichang Gravel sand lens could be bleached to zero before the last burial. Thus, the above results indicate that the ESR dating results of this paper are reliable. The ESR absolute age from top to bottom of the profile is 0.73Ma BP,0.87Ma BP,0.98Ma BP,1.04Ma BP,1.05Ma BP,1.10Ma BP, 1.11Ma BP, 1.12Ma BP, respectively. The ESR dating results show that the Yichang Gravel began to deposit at about 1. 12Ma BP until 0. 73Ma BP, and the Ti-Li center ESR age indicates that the Yangtze River cut through Three Gorges area no later than 1.12Ma BP.     

Specific features of hydrological and morphological processes in the mouth area of the Mekong River

The geographical and hydrological features of the Mekong River basin and the nearshore zone of the South China Sea, which influence the hydrological regime of the Mekong River mouth, are discussed. Detailed characteristics are provided for the drainage system of the Mekong River mouth area, water flow distribution among the delta branches, processes of the delta submergence during floods, as well as propagation of tidal, surge-induced level fluctuations and seawater intrusion into the river. Regularities of the Holocene evolution of the Mekong River mouth area and recent processes of delta formation are also discussed.     

OSL and IRSL dating of raised beach sand deposits along the southeastern coast of Norway

Raised beach sand deposits along the southeastern coast of Norway were dated by optical (OSL) and infrared stimulated luminescence (IRSL) and the quartz and K-feldspar luminescence characteristics were described. Due to the poor quartz luminescence characteristics, only a limited number of samples were suitable for OSL dating. More promising are the K-feldspar extracts, with typical K-feldspar luminescence characteristics and no sign of fading. For equivalent dose (D_e) determination, sand-size quartz and feldspar extracts were used, applying a single aliquot regenerative (SAR) protocol. Both, OSL and IRSL D_e estimates show a wide distribution, unexpected for beach deposits. The calculated OSL and IRSL age estimates were generally in good agreement and the correctness of the ages was confirmed by independent age control. Because only a limited number of the quartz samples were suitable for OSL dating, IRSL dating of the K-feldspar represents an alternative to OSL quartz dating.     

Concentrations and transport of trace metals in the St. Lawrence River

Samples of raw water were collected in the St. Lawrence River during six sampling trips from August 1990 to April 1992. Water samples were analyzed for both dissolved and particulate phases for five trace metals. Partition coefficients (Kd) and metal fluxes were calculated in order to determine metal transport. A mass balance equation was used for the determination of the major metal sources to the St. Lawrence River and an estimation of metal loadings to the estuary was made. Average dissolved metal concentrations were found to be Cd 10 ng/L, Co 74 ng/L, Cu 64 ng/L, Fe 69 µg/L and Mn 700 ng/L. Particulate concentrations were (in µg/g) 1.68 for Cd, 31 for Co, 73 for Cu, 25 mg/g for Fe and 1.69 mg/g for Mn. Co, Fe and Mn were transported essentially in the particulate phase while Cd and Cu were predominantly found in the dissolved phase at 56% and 48% of the total metal concentration respectively. Log Kd values varied from 5.1 (for Cu) to 6.8 (for Mn). In the dissolved phase the major sources were found to be the Great Lakes and the Ottawa River whereas in the particulate phase Québec tributaries appear to be the most important. Industrial inputs are quite important in both the dissolved and the particulate phases for Cd, whereas other sources are very variable, especially for the dissolved phase.     

Analysis of the interannual variability of annual daily extreme water levels in the St Lawrence River and Lake Ontario from 1918 to 2010

We compared the interannual variability of annual daily maximum and minimum extreme water levels in Lake Ontario and the St Lawrence River (Sorel station) from 1918 to 2010, using several statistical tests. The interannual variability of annual daily maximum extreme water levels in Lake Ontario is characterized by a positive long‐term trend showing two shifts in mean (1929–1930 and 1942–1943) and a single shift in variance (in 1958–1959). In contrast, for the St Lawrence River, this interannual variability is characterized by a negative long‐term trend with a single shift in mean, which occurred in 1955–1956. As for annual daily minimum extreme water levels, their interannual variability shows no significant long‐term change in trend. However, for Lake Ontario, the interannual variability of these water levels shows two shifts in mean, which are synchronous with those for maximum water levels, and a single shift in variance, which occurred in 1965–1966. These changes in trend and stationarity (mean and variance) are thought to be due to factors both climatic (the Great Drought of the 1930s) and human (digging of the Seaway and construction of several dams and locks during the 1950s). Despite this change in means and variance, the four series are clearly described by the generalized extreme value distribution. Finally, annual daily maximum and minimum extreme water levels in the St Lawrence and Lake Ontario are negatively correlated with Atlantic multidecadal oscillation over the period from 1918 to 2010. Copyright © 2013 John Wiley & Sons, Ltd.     

Groundwater recharge and palaeoclimate in the Sirte and Kufra basins, Libya

Stable and radio-isotope results (C, H, O) for groundwaters from the Sirte and northern Kufra basins are used to determine the recharge history during the Holocene and late Pleistocene. Radiocarbon ages have been corrected on the basis of their stable carbon isotope ratios and on environmental samples from the areas, and two groups may be recognised: (1) low ¹⁴C activity groundwaters (13000–34000 yr. BP) with δ ¹³C-5.6 to −11.7‰; and (2) higher ¹⁴C activity groundwaters (5000–7800 yr. BP) enriched in ¹³C up to δ ¹³C = −3.2‰. There is a general correlation of age with depth.

A well defined freshwater (< 50 mg/l Cl⁻) channel can be traced within the aquifer for some 130 km through the region, which is considered to represent recharge from a former wadi. This water with an age of ± 7800 yr. BP is chemically and isotopically distinct from the regional groundwaters and provides direct evidence of a significant recharge event during the Holocene.

The stable isotope (O and H) composition of groundwater from the Kufra and Sirte basins are all related by an evaporative line with slope δ D = 4.5δ ¹³O − 35 with an intercept on the meteoric line of -11‰. This suggests a recharge source continuing into the Holocene from air masses, analogous to current heavy monsoon rain derived from south of the Sahara. The spatial and temporal distribution of groundwaters in relation to the evaporative line suggests a progressive change in character of the recharge which is controlled by a shift towards strongly convective rainfall during the Holocene.

The direct hydrogeological and geochemical evidence supports climatic models proposed by several workers in which discrete humid episodes during the Holocene are inferred.     

Luminescence characteristics of coastal sediments in Langhovde,East Antarctica

The Late Pleistocene sea-level history of Antarctica is key to understanding and predicting the responses of icesheets, which significantly contribute to the global sea level, to changing climates. Coastal sediments at Lützow-Holm Bay, East Antarctica, have yielded radiocarbon ages of Holocene and Marine Oxygen Isotope Stage (MIS) 3 from deposits above the present sea level, suggesting that there have been two episodes of sea-level highstand. However, radiocarbon dating is likely to be less accurate for dating sediments close to or older than 40–50 ka, the upper limit of its application. We thus explored the applicability of luminescence dating to coastal sediments newly sampled from Langhovde on the eastern coast of Lützow-Holm Bay. Samples were collected from a trench <1 m deep and at several meters above the present sea level. Quartz coarse and fine grains, K-feldspar coarse grains, and polymineral fine grains were extracted from the samples. Quartz coarse and fine grains both showed very low optically stimulated luminescence sensitivity and no fast component and thus were not considered further. Dose-recovery tests on post-infrared infrared-stimulated luminescence (post-IR IRSL) signals of K-feldspar and polymineral grains yielded variable results and suggested acceptable measurement conditions for each grain size fraction and expected equivalent dose. Preliminary measurements revealed that the trench section can be divided into the upper and lower layers, corresponding to the Holocene and MIS 7, respectively. Further application of post-IR IRSL dating to coastal sediments, including to previously radiocarbon-dated sections, could refine our understanding of the late Quaternary relative sea-level history in East Antarctica.     

Luminescence dating of shoreline sediments indicates a late deglacial lake-level rise of Selin Co on the central Tibetan Plateau

The palaeo-shorelines around the lakes on the Tibetan Plateau can be used to reconstruct water level variations, which serve as sensitive indicators of hydroclimate change. Extensive studies have been carried out to constrain the Holocene lake level fluctuations by dating shorelines with a variety of methods (e.g., luminescence, ¹⁴C, ¹⁰Be and U–Th series). In comparison, the timing of the lake level variations during the last glacial and subsequent deglaciation periods has been rarely studied. The driving factors of such changes, therefore, remain elusive. In this study, we performed a detailed luminescence dating investigation on six samples taken from a nearshore sedimentary outcrop in the south of Selin Co basin. The post-IR IRSL signals measured at 225 °C (pIRIR₂₂₅) on sand-sized K-feldspar grains demonstrated a generally good behavior and yielded reliable chronologies, while the optically stimulated luminescence (OSL) signals of quartz showed systematical age underestimation, which was attributed to anomalous fading. Six pIRIR₂₂₅ ages ranging from 15 to 10 ka suggested that the lake level of Selin Co during the last deglaciation reached up to 40–45 m high above the modern lake level. In view of the regional precipitation and temperature proxy records, we consider that the glacier meltwater supply has likely been the primary contributor to the lake highstands during the last deglaciation.     

Luminescence dating of the late Quaternary sediments in Hangzhou Bay,China

Coastal plain of Hangzhou Bay, to the south of the present Yangtze Estuary, is closely linked to the evolution of the Yangtze River delta. However, absolute age of Pre-Holocene sediments is limited, which hinders the understanding of this area's environmental evolution. In this study, using optically stimulated luminescence (OSL), single aliquots and single grains of quartz and K-feldspar were used to date the late Quaternary sediments in coastal plain on the southern Hangzhou Bay. The vertical difference in particle size composition render either silt- or sand-sized quartz for dating. Cross-checking of multiple OSL dating methods indicated that the upper ∼65 m recorded the Holocene part of the succession; sediment from a depth of 136.6 m was dated to ∼180 ka. It was found that the single-grain method was more reliable in comparison to single-aliquot age, the former minimized the effect of signal components. Single-grain quartz and K-feldspar luminescence yielded consistent ages at sample depth of 136.6 m (∼160–180 ka), while the latter gave robust age at depth of 115.5 m (∼150 ka). This chronology is in general in accordance with neighbouring cores and can constrain paleomagnetic dating results in those cores. Taking together, the study site has thickest Holocene deposits in comparison to the highland centered around Taihu Lake on the southern Yangtze delta. Moreover, the luminescence characteristics of quartz from different sample depths, behaved differently with respect to luminescence sensitivity, signal components and saturation level, perhaps reflecting varied provenance and weathering characteristics caused by climate change.     

Environmental characteristics of Mid-Holocene recorded by lacustrine sediments from Lake Daihai, north environment sensitive zone, China

One of the goals for paleaoenvironmental research is to predict the tendency of future climate and environmental changes based on the understanding of the past. The key approach is to find similar pictures which happened in the past. By understanding the background and mechanism of the paleaoenvironmen- tal changes, reliable parameters and verifications can be provided for the numerical model to predict the tendency of future climate and environmental changes. The Mid-Holocene as the nearest …     

Sensitivity of coastal waters to anthropogenic trace metal emissions

Trace metal concentrations in the waters of the Gulf of St. Lawrence and the Scotian Shelf and their sensitivity to changes in the composition of the major contributor of fresh water, the St. Lawrence River, are assessed. Comparison of the levels of trace metals in eastern Canadian coastal waters with those in other coastal waters of the world reveals that the former area generally has lower trace metal concentrations and is, therefore, probably less contaminated than elsewhere. The increased anthropogenic activity in the St. Lawrence drainage basin required to increase the levels of trace metals in receiving coastal waters by detectable amounts is discussed. We demonstrate that faster and more reliable warning of changes due to anthropogenic activity will be revealed through river monitoring than through monitoring of coastal waters. Changes in the marine environment can then be assessed through a thorough understanding of the geochemical behaviour of trace metals in nearshore waters.     

Holocene vegetational and climatic variation in westerly-dominated areas of Central Asia inferred from the Sayram Lake in northern Xinjiang,China

Changes in the vegetation and climate of the westerly-dominated areas in Central Asia during the Holocene were interpreted using pollen-assemblages and charcoal data from a 300-cm-long sediment core of the Sayram Lake,northern Xinjiang.Accele-rator Mass Spectrometry(AMS) radiocarbon dating methods were applied to bulk organic matter of the samples.Artemisia spp./Chenopodiaceae ratios and results from principal component analysis were used to infer that the lake basin was dominated by desert vegetation before ca.9.6 cal.ka BP,which suggests a warm and dry climate in the early Holocene.Desert steppe/steppe expanded during 9.6-5.5 cal.ka BP,indicating a remarkable increase both in the precipitation and temperature during the mid-Holocene.Desert vegetation dominated between 6.5 and 5.5 cal.ka BP,marking an extreme warmer and drier interval.The steppe/meadow steppe recovered,and temperatures decreased from 5.5 cal.ka BP in the late Holocene,as indicated by the increased abundance of Artemisia and the development of meadows.Holocene temperatures and moisture variations in the Sayram Lake areas were similar to those of adjacent areas.This consistency implies that solar radiation was the main driving factor for regional temperature changes,and that the effect of temperature variations was significant on regional changes in humidity.The evolution of climate and environment in the Sayram Lake areas,which were characterized as dry in the early Holocene and relatively humid in the middle-late Holocene,are clearly different from those in monsoonal areas.Dry conditions in the early Holocene in the Sayram Lake areas were closely related to decreased water vapor advection.These conditions were a result of reduced westerly wind speeds and less evaporation upstream,which in turn were caused by seasonal changes in solar radiation superimposed by strong evaporation following warming and drying local climate.     

Effects of hypoxia on benthic macrofauna and bioturbation in the Estuary and Gulf of St. Lawrence,Canada

The bottom water in the >300 m deep Lower St. Lawrence Estuary (LSLE) is persistently hypoxic in contrast to the normoxic bottom waters in the Gulf of St. Lawrence (GSL). We photographed the seabed at 11 stations in the Estuary and Gulf of St. Lawrence (EGSL) during the summers 2006 and 2007 and analysed the images to identify bioturbation traces (lebensspuren) and benthic macrofauna. The objective was to identify the environmental variables that influence the density and diversity of benthic macrofauna and bioturbation traces, and the differences that exist among regions with high, medium and low oxygen levels in the bottom water. The bottom water oxygen concentration is the variable that best explains the densities of total-traces as well as surface-traces. However, the density of these traces was higher in hypoxic regions than in well-oxygenated regions. The higher density of traces in the hypoxic region of the LSLE is mainly due to the activities of the surface deposit feeder Ophiura sp., which occurs in large numbers in this region. Possible explanations explored are stress behaviour of the organisms in response to hypoxia and different benthic macrofauna community structures between the hypoxic regions of the LSLE and the normoxic regions of the GSL. In the former, surface deposit feeders and low-oxygen tolerant species dominate over suspension feeders and low-oxygen intolerant species.     

High-resolution peat records for Holocene monsoon history in the eastern Tibetan Plateau

Holocene peat sediment has been attached impor-tance to reconstruct the Holocene climatic variations because it can provide much palaeoclimatic informa-tion with high resolution. Analysis on the plant mac-rofossil[1], pollen[2―4], isotope ratio[5―9], element con-tent[10,11], total organic carbon (TOC)[12], humifica-tion[13,14] and grayscale[12] for peat sediment has shown its superiorities as a recorder medium for reconstruct-ing the Holocene climate. Hong et al. have done great fruitful w…     

Variations in luminescence properties of quartz and feldspar from modern fluvial sediments in three rivers

Studies of modern sediments, their sedimentology and depositional processes are important for understanding the behaviour of the luminescence characteristics of quartz and feldspar in fluvial settings. Previous studies have shown large variations in OSL characteristics of quartz from different fluvial systems, while the IRSL and pIRIR signals from K-feldspar have been understudied. We test the effects of fluvial setting on luminescence characteristics by collecting modern (<1 year old) bedload sediments down the courses of three river systems with very different hydrological characteristics, geologic contexts, and catchment lithologies. The single grain (SG) and multi-grain aliquot (MGA) OSL (quartz) and IRSL and pIRIR (K-feldspar) properties of samples were measured and compared to better understand intra- and inter-fluvial system patterns in sensitivity, bleaching, and equivalent dose (D_e) distribution skewness and kurtosis. The quartz OSL and K-feldspar IRSL and pIRIR signal sensitivities increase with downstream transport distance of sediments, confirming previous studies (quartz) and showing that IRSL signals from K-feldspar also increase in response to reworking cycles. Increasing transport distance also results in better bleaching of the OSL signal from quartz samples (MGA and SG) due to more grains being exposed to sunlight. By contrast, the IRSL and pIRIR signals retain significant residuals in all samples, though 5–15% of grains yield zero-dose D_e values and age modelling of SG data yields accurate burial dose estimates. Additionally, the skewness and kurtosis of SG OSL D_e datasets from one river increase with transport distance, with the best bleached samples exhibiting the highest skewness, thereby questioning the applicability of the skewness-value of a D_e dataset as an accurate indicator for partial-bleaching. Our data shows marked variability between (i) different river systems and (ii) the measured minerals, however consistent use of statistical models allows accurate D_e estimation in all contexts. Age modelling of SG data from K-feldspar, thus, provides a valuable tool for future fluvial research in regions where poor OSL characteristics prevent the use of quartz as a dosimeter.     

Late Quaternary history of the Vakinankaratra volcanic field (central Madagascar): insights from luminescence dating of phreatomagmatic eruption deposits

The Quaternary Vakinankaratra volcanic field in the central Madagascar highlands consists of scoria cones, lava flows, tuff rings, and maars. These volcanic landforms are the result of processes triggered by intracontinental rifting and overlie Precambrian basement or Neogene volcanic rocks. Infrared-stimulated luminescence (IRSL) dating was applied to 13 samples taken from phreatomagmatic eruption deposits in the Antsirabe–Betafo region with the aim of constraining the chronology of the volcanic activity. Establishing such a chronology is important for evaluating volcanic hazards in this densely populated area. Stratigraphic correlations of eruption deposits and IRSL ages suggest at least five phreatomagmatic eruption events in Late Pleistocene times. In the Lake Andraikiba region, two such eruption layers can be clearly distinguished. The older one yields ages between 109?±?15 and 90?±?11 ka and is possibly related to an eruption at the Amboniloha volcanic complex to the north. The younger one gives ages between 58?±?4 and 47?±?7 ka and is clearly related to the phreatomagmatic eruption that formed Lake Andraikiba. IRSL ages of a similar eruption deposit directly overlying basement laterite in the vicinity of the Fizinana and Ampasamihaiky volcanic complexes yield coherent ages of 68?±?7 and 65?±?8 ka. These ages provide the upper age limit for the subsequently developed Iavoko, Antsifotra, and Fizinana scoria cones and their associated lava flows. Two phreatomagmatic deposits, identified near Lake Tritrivakely, yield the youngest IRSL ages in the region, with respective ages of 32?±?3 and 19?±?2 ka. The reported K-feldspar IRSL ages are the first recorded numerical ages of phreatomagmatic eruption deposits in Madagascar, and our results confirm the huge potential of this dating approach for reconstructing the volcanic activity of Late Pleistocene to Holocene volcanic provinces.