A. K. Singhvi

Late Pleistocene and Holocene dune activity and wind regimes in the western Sahara Desert of Mauritania

The western Sahara Desert in Mauritania is dominated by extensive sand seas consisting largely of linear dunes. Analyses of Landsat images, geomorphic and stratigraphic studies, and optically stimulated luminescence dating of dunes in the Azefal, Agneitir, and Akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25–15 ka (centered around the Last Glacial Maximum), 10–13 ka (spanning the Younger Dryas event), and after 5 ka. The wind regimes that occurred during each of these periods were significantly different, leading to the formation of dunes on three distinct superimposed trends—northeast, north-northeast, and north—and the development of the sand seas as composite geomorphic features.

Investigations into the Potential Effects of Pedoturbation on Luminescence Dating

Much effort has been focussed on understanding the luminescence properties of natural minerals to achieve a reliable, accurate and precise dating technique. However, some field related aspects, such as the influence or effect of post-depositional disturbance on luminescence dates, are as yet underexplored. In the case of pedoturbation, depending on its intensity, the rate of sedimentation and unit thicknesses, potentially the whole sedimentary record at a site can be affected. This may lead to distorted OSL chronologies and erroneous sediment burial ages. Pedoturbation can result in sediment mixing and/or exhumation that affect luminescence both at the bulk and single grain level. Effects of these two principle processes on luminescence ages are examined using standard multigrain and single grain protocols. High resolution sampling of surface gopher mounds was used to determine the efficiency of bio-exhumation in resetting luminescence signal. Results show this is an inefficient mechanism for onsite sediment bleaching. The effects on luminescence signal of bio-mixing were explored by comparing a sample collected from within a krotovina (infilled burrow) to an adjacent undisturbed sample. Results show the difficulties in identifying pedoturbated samples at the single aliquot level and the possible inaccuracies in using the lowest palaeodose values to calculate OSL ages. Where pedoturbation of samples is suspected, use of probability plots of palaeodoses data is recommended. From these plots it is proposed that only data falling within a normal distribution centred on the peak probability be used to calculated OSL ages and to mitigate problems arising from pedoturbation.

Luminescence dating of loess–palaeosol sequences and coversands: methodological aspects and palaeoclimatic implications

Abstract Of the terrestrial archives, loess–palaeosol sequences provide a most complete record of climatic change. This may be compared with the Marine oxygen isotope stratigraphy, and may help in the reconstruction of past atmospheric circulation patterns. Numerical chronometry of loess–palaeosol sequences has generally been based on correlation of variations in climatic proxies (such as magnetic susceptibility and particle sizes) with Marine isotopic data. Such chronometric assignments involve implicit assumptions about the constancy of sedimentation rates and particle fluxes through time. This review presents a brief survey of the present status, methodology, outstanding problems and interpretational aspects of luminescence techniques, and discusses the import of luminescence ages on global land–sea correlations. Statistical analysis of the ages suggests episodicity of loess accumulation with extended periods of quiescence. Recent luminescence dating studies on closely spaced samples also lead to a similar inference. Luminescence ages imply high variability in loess sedimentation rates. This conflicts with the assumption, made in some current attempts to correlate loess records with marine records, of almost constant particle fluxes. A review of source-proximal coversand deposits of northwest Europe is also presented. Evidence of the onset of coversand deposition at 15 ka, with a peak in accretion during the Younger Dryas and subsequent minor reactivation episodes, is discussed.

Limits to depletion of blue-green light stimulated luminescence in feldspars: implications for quartz dating

Feldspar contaminants in quartz aliquots, either as micro-inclusions or as remnant grains (due to inadequate etching) can affect the accuracy and precision of paleodose estimates based on blue-green light stimulated luminescence (BGSL). Such contamination could also alter the shape of the BGSL stimulation curve of otherwise pure quartz. In this study, the functional relationship between the infra-red stimulated luminescence (IRSL) and BGSL of feldspars, (1) at different preheats, and (2) with IR bleaching at different stimulation temperatures and durations, is examined. The results suggest two trap populations participate in the feldspar BGSL process. These are: (1) Type (A) trap populations that can be stimulated by both the infra-red and the blue-green light at 125°C and, (2) Type (B) trap populations that respond only to blue-green-light stimulation at 125°C. However, infra-red stimulation at elevated temperature (220°C) (ETIR) permits depletions of charges in Type (A) and Type (B) to the extent that the feldspar BGSL can be reduced by up to 97% in 5 min. These results offer prospects for (1) improved precision in paleodose estimates based on quartz; (2) BGSL dating of quartz in a polyminerallic fine grain samples; (3) age estimates based on both quartz and feldspars from the same aliquots, and (4) dating based on feldspar micro-inclusions.

Luminescence chronometry and Late Quaternary geomorphic history of the Ganga Plain, India

Abstract Flexing of the Indian lithosphere due to the continent–continent collision and thrust fold loading produced the Gangetic Foreland Basin during the Middle Miocene. The basin attained its present configuration during the Late Quaternary. Neotectonics and climate change since the last interglacial influenced the evolution of the present landscape of the Gangetic (Ganga) plains. The regional geomorphic surfaces in these plains are: (1) Upland Interfluve Surface (T2); (2) Marginal Plain Upland Surface (MP); (3) Megafan Surface (MF); (4) Piedmont Fan Surface (PF); (5) River Valley Terrace Surface (T1); (6) Active Flood Plain Surface (T0). Minor geomorphic features comprise ponds, alluvial ridges, and small channels, which occur on the Upland Interfluve Surfaces. The present contribution reviews geomorphic processes that sculpted the basin landscape and, then, using the blue green-stimulated luminescence (BGSL) and infrared-stimulated luminescence (IRSL) ages provides a first order chronological framework for the relationship of major and minor geomorphic features to climate and tectonics. Luminescence dating of the Upland Interfluve Surface (T2) shows that formation of this surface started before 51 ka and continued at least up to 7 ka. Deposition in the Marginal Plain (MP) began at least at 76 ka and continued after 32 ka bracketing the evidence of a more humid climate during 50–40 ka. Sedimentation on the T2 and MP surfaces was synchronous. The top of these surfaces gives variable ages ranging from 48 to 7 ka, due to differential erosion of an undulatory topography caused by tectonic activity. Age upper bound for the initiation and termination of the final building phase of Ganga Megafan can be placed at ∼26 ka and ∼22 ka respectively. Deposition on Piedmont Fan Surface continued between 8 and 3 ka. The River Valley Terrace Surface (T1) of Ganga River shows the activity of Yazoo type channels during 3–1.1 ka. The last phase of active accretion on the narrow, poorly developed youngest Active Flood Plain Surface (T0) was after 1.5 ka. The ponds of T2 surface formed due to cessation of fluvial activity sometime during 8–6 ka. A prominent tectonic activity at 7–5 ka and dry climate at 5 ka produced undulatory topography and high siltation rates in the lakes and ponds. The peripheral bulge, southern Ganga Plain has been tectonically active causing deep incision of rivers and a 40-ka seismic event, and Mid–Late Holocene tectonic activity can be inferred.

Sedimentary records and luminescence chronology of Late Holocene palaeofloods in the Luni River, Thar Desert, Northwest India

River Luni is the only well-integrated river system in the Thar Desert of India. This river catastrophically flooded due to unusually heavy rainfall in the catchment area during July 1979. In order to establish whether floods of this magnitude have occurred in the recent geological past, sedimentary records of palaeofloods occurring in the Sindari Gorge, in the lower Luni Basin were investigated. The principal slackwater flood deposits were observed in a back-flooded tributary near Bhuka. Analysis of the slackwater flood deposits preserved in the back-flooded tributary and their luminescence dating suggests that the Luni River has experienced at least 17 extreme floods during the past millennium. Evidence from the Bhuka site also suggests that no floods comparable in magnitude to the July 1979 megaflood have occurred during this period. This observation is in conformity with the palaeoflood record of central India. Comparison of the long-term monsoon rainfall series for the Luni Basin and the Indian region reveals a clear link between the two, and indicates that the clustering of large floods in the last few decades and during the Medieval warming period is a regional phenomenon associated with wetter conditions. This correlates with a regionally extended episode of landscape stability denoted by stabilization of dunes in this region.

Luminescence chronology of river adjustment and incision of Quaternary sediments in the alluvial plain of the Sabarmati River, north Gujarat, India

River adjustment and incision in the Sabarmati basin, Gujarat, India have been examined at a site near Mahudi. Towards this, the morphostratigraphy and depositional chronometry of the middle alluvial plains were investigated. The upper fluvial sequence, along with the overlying aeolian sand and riverbed scroll plains, provide clues to the evolution of the present Sabarmati River. Sedimentological analyses of the upper fluvial sequence indicate its deposition by a meandering river system during what is believed to be a persistent wetter phase. Luminescence chronology bracketed this sequence to between 54 and 30 ka, which corresponds to Oxygen Isotope Stage-3, during which the SW monsoon was enhanced. The overlying aeolian sand has been dated to 12 ka, indicating that dune accretion occurred simultaneously with the strengthening of the SW monsoon during the Early Holocene. Adjustment of the Sabarmati along a N-S transect is placed around 12 ka and the incision is bracketed between 12 and 4.5 ka. River adjustment could have been tectonic; however, the incision was facilitated by the availability of continuous flow in the river caused by the SW monsoon. The basin experienced two tectonic events at about 3 and 0.3 ka, as demonstrated by the morphology of the scroll plains.

Evolution of the Lower Gangetic Plain landforms and soils in West Bengal, India

Abstract Three major landforms, Uplands, Old Fluvial/Deltaic Plains and Young Fluvial Plains are identified from the Lower Gangetic Plains of West Bengal, India, on the basis of remote sensing and field studies. Morphologic, quantitative and thermoluminescence studies of soils of the study area have been conducted. Two types of Uplands are recognised: Upland in the west overlain by red soils (autochthonous and allochthonous types) and upland in the north (Barind Tract), characterised by three topographic levels. Aspects of the autochthonous Upland Red Soils (Lower to Middle Pleistocene age) show the ferrugination (ferrisol) phase of development. Soils of the Old Fluvial/Deltaic Plains (6–3 ka) and Lower Level of the Barind Tract have argillic horizons and exhibit the fersiallitisation phase of development. Development of ferrugination and fersiallitisation phases was favored due to the pre-weathered nature of the parent material. Soils of the Old Fluvial/Deltaic Plains, Barind Tract (Lower Level) and Young Fluvial Plains show effects of hydromorphism due to waterlogging in the form of segregations of Fe–Mn oxides/gleying and chloritisation, probably due to ferrolysis in the upper horizons of some of these soils. Neotectonics seems to have affected development of landforms and soils significantly. Due to reactivation of some basement faults in the western region, some tectonic blocks subsided, causing transgression during the Early Pleistocene and at ca. 7 ka. Subsequent uplift of these blocks caused regressions and development of soils on the exposed landscapes. Episodic uplift of the Barind Tract in the northern region may have given rise to three topographic levels. Some faults confine the courses of the Damodar, Ganga and Bhagirathi rivers.

Thermoluminescence chronology of sand profiles in the Thar desert and their implications

Thermoluminescence chronology of three sand profiles in Western Rajasthan indicates a peak in sand accretion rate at ca 14 ka and a period of dormancy during 13-6 ka BP. This observation shows a general correlation with pollen and geochemical records from lakes in Thar and demonstrates the potential of desert sands in paleoclimatic reconstruction. Thermoluminescence ages of 36 ka and 40 ka on two samples suggest that the aeolian sand mobilization activity in this regions is at least as old.

Contributions of floodplain stratigraphy and evolution to the spatial patterns of groundwater arsenic in Araihazar, Bangladesh

Extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fluviodeltaic environments. Here we examine patterns of arsenic heterogeneity in the shallow ( 1 −10 3 m in the 25 km 2 area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size, thickness, and distribution of fine-grained ( < 63 µm) sediments that overlie buried aquifer sands. The overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface, whereas higher arsenic levels typically underlie, or are adjacent to thicker, fine-grained deposits. Furthermore, chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution, and consequently the patterning of dissolved arsenic, is readily explained in the context of local river history and flood-plain development within the past 1000 yr. An important implication is that complex patterns of groundwater arsenic in afflicted fluviodeltaic settings can be better understood through reconstructions of local aquifer history. This finding is especially relevant because the village and tube-well locations are closely linked with surface landforms such as former levees and bars. An additional and worrisome finding is that the artificial filling of villages to protect from flooding can mimic the natural fine-grained stratigraphy commonly associated with high concentrations of arsenic.