Adaptation and human migration, and evidence of agriculture coincident with changes in the Indian summer monsoon during the Holocene

Human societies have evolved through a complex system of climate and ecological interactions. Known records suggest intimate relationship of adaptations, mitigations and migrations to climate extremes leaving their impacts on human societies. The northwestern part of India provides such an example, where human civilizations flourished in the early Holocene along the major fluvial systems when the Indian summer (southwest) monsoon was much stronger and rainfall was higher over the Indian land mass. Summers were thus wetter, conducive to agriculture and ecodiversity. Changes in the early civilizations in the Indian subcontinent had a close relation to changes in the monsoon climate over the past 10,000 years. The summer monsoon has weakened over the last 7000 years since its peak intensification in the early Holocene (10,000-7000 cal yrs BP). Discrete intervals of dry phases in the summer monsoon are visible in the proxy record of the monsoon winds from the marine sediments of the Arabian Sea, which had significant impact on human settlements in South Asia. The strongest aridity in the Indian subcontinent and extended periods of droughts at ca 5000- 4000 cal yrs BP seems to have triggered eastward human migrations towards the Ganga plain. Other times of monsoon weakening during the Holocene are coincident with the initial development of ponds, reservoirs and other rainwater harvesting structures that may have served as an adaptation to climate change

Extending the aridity record of the Southwest Kalahari: current problems and future perspectives

An extensive luminescence-based chronological framework has allowed the reconstruction of expansions and contractions of the Kalahari Desert over the last 50 ka. However, this chronology is largely based on near-surface pits and sediment exposures. These are the points on the landscape most prone to reactivation and resetting of the luminescence dating ‘clock’. This is proving to be a limiting feature for extending palaeoenvironmental reconstructions further back in time. One way to obviate this is to sample desert marginal areas that only become active during significant arid phases. An alternative is to find and sample deep stratigraphic exposures. The Mamatwan manganese mine at Hotazel in the SW Kalahari meets both these criteria. Luminescence dating of this site shows the upper sedimentary unit to span at least the last 60 ka with tentative age estimates from underlying cemented aeolian units dating back to the last interglacial and beyond. Results from Mamatwan are comparable to new and previously published data from linear dunes in the SW Kalahari but extend back much further. Analysis of the entire data set of luminescence ages for the SW Kalahari brings out important inferences that suggest that different aeolian forms (1) have been active over different time scales in the past, (2) have different sensitivities to environmental changes and (3) have different time scales over which they record and preserve the palaeoenvironmental record. This implies that future optically stimulated luminescence work and palaeoenvironmental reconstructions must consider both site location and its relationship to desert margins and sediment depositional styles, so that the resolution and duration of the aridity record can be optimally understood

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

Kinematic analysis of the Pakuashan fault tip fold, west central Taiwan: Shortening rate and age of folding inception

The Pakuashan anticline is an active fault tip fold that constitutes the frontal most zone of deformation along the western piedmont of the Taiwan Range. Assessing seismic hazards associated with this fold and its contribution to crustal shortening across central Taiwan requires some understanding of the fold structure and growth rate. To address this, we surveyed the geometry of several deformed strata and geomorphic surfaces, which recorded different cumulative amounts of shortening. These units were dated to ages ranging from ~19 ka to ~340 ka using optically stimulated luminescence (OSL). We collected shallow seismic profiles and used previously published seismic profiles to constrain the deep structure of the fold. These data show that the anticline has formed as a result of pure shear with subsequent limb rotation. The cumulative shortening along the direction of tectonic transport is estimated to be 1010 ± 160 m. An analytical fold model derived from a sandbox experiment is used to model growth strata. This yields a shortening rate of 16.3 ± 4.1 mm/yr and constrains the time of initiation of deformation to 62.2 ± 9.6 ka. In addition, the kinematic model of Pakuashan is used to assess how uplift, sedimentation, and erosion have sculpted the present-day fold topography and morphology. The fold model, applied here for the first time on a natural example, appears promising in determining the kinematics of fault tip folds in similar contexts and therefore in assessing seismic hazards associated with blind thrust faults

Subsurface signatures and timing of extreme wave events along the Southeast Indian coast

Written history's limitation becomes apparent when attempting to document the predecessors of extreme coastal events in the Indian Ocean, from 550-700 years in Thailand and 1000 years in Indonesia. Detailed ground-penetrating radar (GPR) surveys in Mahabalipuram, southeast India, complemented with sedimentological analyses, magnetic susceptibility measurements, and optical dating provide strong evidence of extreme wave events during the past 3700 years. The diagnostic event signatures include the extent and elevation of the deposits, as well as morphologic similarity of buried erosional scarps to those reported in northern Sumatra region. Optical ages immediately overlying the imaged discontinuities that coincides with high concentration of heavy minerals date the erosional events to 340 ± 35, 350 ± 20, 490 ± 30, 880 ± 40, 1080 ± 60, 1175 ± 188, 2193 ± 266, 2235 ± 881, 2489 ± 293, 2450 ± 130, 2585 ± 609, 3710 ± 200 years ago. These evidences are crucial in reconstructing paleo extreme wave events and will pave the way for regional correlation of erosional horizons along the northern margin of Indian Ocean

Impact of luminescence dating on geomorphological and palaeoclimate research in drylands

Luminescence dating has truly transformed the science of deserts and drylands and has necessitated a revision of conventional interpretation of sedimentary records and stratigraphic correlations. This contribution discusses key questions in palaeoclimatology of global deserts, the meaning of a luminescence age on desert sand and presents a review of luminescence-dated aeolian sand records from the deserts in the two hemispheres. Luminescence dating has in fact transformed dryland palaeoclimatology from a qualitative science into a quantitative one. Processes ranging from past sedimentation rates to dune migration rates and the changes in surface winds can now be quantified. A key inference is that, during the past, aeolian sedimentation occurred episodically and for limited time durations. Accentuated aeolian aggradations occurred during specific climatic regimes, the timing of which was determined by an optimal combination of sediment supply, transport and preservation or erosion. This fact has important implications for global climatology and aspects such as albedo changes through time. The absence of an aeolian record documents climate or erosion and should be interpreted as such. Loess on the fringes of drylands is a still untapped resource for palaeoclimate studies. The review also outlines new developments made using luminescence methods for desert palaeoclimatology

Climate variation in the Thar Desert since the Last Glacial Maximum and evaluation of the Indian Monsoon

Thar Desert is a rainfall deficient (~500-100 mm/year) region in the northwestern India. Previously published information on sediment facies, mineralogy, and radiocarbon chronology helped to reconstruct orbital-scale lake stands and variations in water column salinity of five different lacustrine basins in the desert. We evaluated the hydrological conditions with respect to strength (i.e., amount and geographic coverage) of the southwest summer monsoon since the last glacial maximum (LGM). Between LGM and c.15 cal. ka BP, the eastern basins hosted saline and hypersaline playa lakes and the western part had an intermittent variable lake. A shift from saline-hypersaline playa lakes to perennial deep lakes occurred in the eastern margin at c.15 cal. ka BP as more summer insolation increased sea surface temperature (SST) of the Indian Ocean and strengthened the southwest summer monsoon. During the Pleistocene-Holocene transition, the highest summer insolation as well as warmer SST of the Indian Ocean increased the amount of summer precipitation and expanded the southwest monsoon over the entire desert. However, more winter precipitation and minimal summer rainfall maintained perennial lakes across the desert during the early and middle Holocene. Over the middle-late Holocene, the regional arid conditions were contemporary to intervals of reduced summer insolation, southerly located Inter-Tropical Convergence Zone and frequent El-Niño Southern Oscillation.El desierto de Thar está ubicado en la parte noroccidental de la India y es una región con escasez de lluvia (~500-100 mm/año). Los datos previamente publicados sobre la mineralogía, facies sedimentarias y cronología de radiocarbono han ayudado a reconstruir los cambios en el nivel lacustre y la salinidad de los cuerpos de agua en la escala orbital en cinco diferentes cuencas lacustres del desierto. Se evaluaron las condiciones hidrológicas en términos de la variación en la fuerza (cantidad y cobertura geográfica) del monzón del suroeste desde el último máximo glacial (UMG). Entre el UMG y aproximadamente 15 ka cal AP, las cuencas de la parte oriental  mantuvieron lagos salinos e hipersalinos y la parte occidental tuvo un lago intermitente. El cambio de una playa salina-hipersalina a un lago perenne con condiciones profundas ocurrió en la margen oriental alrededor de los 15 ka cal AP a medida que la insolación de verano aumentó tanto la temperatura superficial del mar (TSM) del Océano Índico como la fuerza del monzón del suroeste. Durante la transición del Pleistoceno al Holoceno, la mayor insolación de verano y la alta TSM  del Océano Índico aumentaron la cantidad de lluvia de verano y facilitaron la expansión del monzón a todo el desierto. Sin embargo, el predominio de las precipitaciones de invierno sobre las lluvias de verano durante el Holoceno temprano y medio mantuvieron lagos perennes a lo largo del desierto. Durante el Holoceno medio y tardío,  las condiciones más secas en la región fueron contemporáneas a los intervalos de reducción en la insolación de verano, la ubicación de la Zona de Convergencia Intertropical en una latitud sureña y el aumento en la actividad de El Niño-Oscilación del Sur

Climate variation in the Thar Desert since the Last Glacial Maximum and evaluation of the Indian Monsoon

El desierto de Thar está ubicado en la parte noroccidental de la India y es una región con escasez de lluvia (~500-100 mm/año). Los datos previamente publicados sobre la mineralogía, facies sedimentarias y cronología de radiocarbono han ayudado a reconstruir los cambios en el nivel lacustre y la salinidad de los cuerpos de agua en la escala orbital en cinco diferentes cuencas lacustres del desierto. Se evaluaron las condiciones hidrológicas en términos de la variación en la fuerza (cantidad y cobertura geográfica) del monzón del suroeste desde el último máximo glacial (UMG). Entre el UMG y aproximadamente 15 ka cal AP, las cuencas de la parte oriental mantuvieron lagos salinos e hipersalinos y la parte occidental tuvo un lago intermitente. El cambio de una playa salina-hipersalina a un lago perenne con condiciones profundas ocurrió en la margen oriental alrededor de los 15 ka cal AP a medida que la insolación de verano aumentó tanto la temperatura superficial del mar (TSM) del Océano Índico como la fuerza del monzón del suroeste. Durante la transición del Pleistoceno al Holoceno, la mayor insolación de verano y la alta TSM del Océano Índico aumentaron la cantidad de lluvia de verano y facilitaron la expansión del monzón a todo el desierto. Sin embargo, el predominio de las precipitaciones de invierno sobre las lluvias de verano durante el Holoceno temprano y medio mantuvieron lagos perennes a lo largo del desierto. Durante el Holoceno medio y tardío, las condiciones más secas en la región fueron contemporáneas a los intervalos de reducción en la insolación de verano, la ubicación de la Zona de Convergencia Intertropical en una latitud sureña y el aumento en la actividad de El Niño-Oscilación del Sur

Luminescence chronology of incision and channel pattern changes in the River Ganga, India

The River Ganga in the central Gangetic plain shows the incision of ~20 m of Late Quaternary sediments that form a vast upland terrace (T<SUB>2</SUB>). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T<SUB>1</SUB>) and the present-day flood plain (terrace-T<SUB>0</SUB>). Terrace-T<SUB>1</SUB> shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T<SUB>0</SUB> and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided. Optical dating of samples from three different terraces has bracketed the phase of incision to be &lt; 6 and 4 ka. Different ages of the top of terrace-T<SUB>2</SUB> show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka

Pleistocene climatic changes and landscape evolution in the Kashmir Basin, India: paleopedologic and chronostratigraphic studies

The Kashmir Basin is filled with Pliocene-Pleistocene fluvio-lacustrine sediments. On the flank of the Pir Panjal these sediments, called "Lower Karewas", are covered with loess deposits up to 25 m thick that contain numerous middle and late Pleistocene paleosols, mostly polygenetic pedocomplexes, which were classified by micromorphological studies. Thermoluminescence dates provide a chronostratigraphy of the late Pleistocene loess-paleosol sequences. On the Himalayan flank, the "Upper Karewas" are covered only with late Pleistocene loess-paleosol sequences. Their base is the last interglacial soil, developed ca. 110,000±10,000 yr B.P. This soil is genetically comparable to the modern soil and, therefore, is thought to have developed under a deciduous forest in a "xeric" soil moisture regime. The loesses of last glacial age on both flanks of the basin contain three humus-rich Ah, mostly Aht, horizons, indicating three warm and mostly humid climatic episodes between ca. 80,000 and 50,000 yr B.P. The middle Pleistocene loesses contain at least four Bwt, or thick Bt, horizons developed during four interglacial periods, having climates similar to the present. Large parts of the Karewa Lake must have lasted until the end of the penultimate glacial age