Manoj K. Jaiswal

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.

Timing of recent out-of-sequence active deformation in the frontal Himalayan wedge: Insights from the Darjiling sub-Himalaya, India

Recent studies of India-Eurasia convergence suggest that the entire convergence in the Himalayan wedge is almost exclusively accommodated along its basal detachment fault (Main Himalayan thrust, MHT) and its near-surface equivalent (Main Frontal thrust, MFT). Using direct dating of fault-zone gouge and strath terrace deposits, we conclude the following. (1) The present mountain front in the Darjiling sub-Himalaya was emplaced by ca. 40 ka. (2) Out-of-sequence deformation on surface-breaking faults north of the MFT in the Darjiling sub-Himalaya began ca. 20 ka and has probably continued since. (3) The Tista River responded to the ca. 20 ka deformation by migrating 150 m eastward (average rate ∼13 mm yr −1 ) and by incising 48 m vertically (average rate ∼4.4 mm yr −1 ), creating unpaired, disjointed strath terraces between 11.3 ± 1.3 ka and 1.4 ± 0.3 ka. Out-of-sequence, surface-breaking faults in the Himalaya indicate partial accommodation of active convergence within the Himalayan wedge. Using the results from the Bhuj earthquake of 2001, we suggest that active deformation along the out-of-sequence faults is a potential seismic hazard in the Himalaya, and Himalayan seismic hazard models must account for this. We also propose a conceptual model for active deformation in the Himalaya.

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 frontalmost 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 2 ranging from ~ 19 ka to ~ 340 ka using Optical 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 (N118E) is estimated to be 1010 +/- 160 m. An analytical fold model derived from a sandbox experiment [Bernard, et al., in press] 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 tectonics, 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 on blind thrust faults.

Changes in natural OSL sensitivity during single aliquot regeneration procedure and their implications for equivalent dose determination

Measurement of low temperature (90°C–120°C) Thermoluminescence (TL) sensitivity of natural quartz samples subjected to pre-heating and optical stimulation indicate that significant sensitivity changes can occur during measurement of the natural Optically Stimulated Luminescence (OSL). During the measurement of natural signal, the luminescence sensitivity of samples can change by 40%. The sensitivity changes both during the initial preheat and the measurement of natural OSL. The currently used version of Single Aliquot Regeneration (SAR) protocol measures and corrects for the sensitivity changes after preheat and readout of natural OSL. However, it does not take into account the changes in sensitivity during the readout of the natural signal.We therefore developed a correction procedure so that both the natural and the regenerated OSL intensities can be measured and plotted with the same sensitivity and suggest that in the absence of such a correction, a considerable fraction of the SAR based ages could have systematic errors. The correction for the sensitivity is based on the use of sensitivity of 110°C TL quartz peak, which is correlated to OSL signal (Murray and Roberts, 1998). The use of 110°C peak provides a reasonable measure of the changes in OSL sensitivity of quartz. A modified Natural Sensitivity Corrected-SAR (NSC-SAR) procedure, that comprises the measurement of, 1) the TL intensity of 110°C peak for a test dose on sample as received (i.e. natural sample) and, 2) the sensitivity of the 110°C peak of the same sample after the preheat and read out of the natural OSL, is proposed. This ratio, termed as Natural Correction Factor (NCF), then provides a way to correct for sensitivity changes. Results on samples from diverse depositional environments indicated that the NSC-SAR consistently (without exception) provided improved distribution in paleodoses i.e. a lower scatter compared to the standard SAR protocol. In addition, the use of this protocol also resolved anomalous cases where the intensity of natural OSL was significantly above the saturation intensity of the regenerated OSL. Implicitly, this study implies a caution on the use of palaeodoses obtained from single grains as such a correction is not possible in the currently used automated single grain OSL measurement systems. The only way now on will be to analyze aliquots with only a grain on them.

Deciphering cumulative fault slip vectors from fold scarps: Relationships between long‐term and coseismic deformations in central Western Taiwan

We document the 30 ka cumulative slip history and long-term slip vector azimuth on the northern Chelungpu fault based on a series of fault-bend folded alluvial terraces and draw quantitative relationships between geological structure, deformation observed from the geomorphology, and coseismic displacements during the 1999 Mw = 7.6 Chi-Chi earthquake. In our study area, three main terrace levels show progressive folding by kink band migration in relation to the underlying fault geometry, forming a main N-S fold scarp up to ~193 m high and secondary E-W scarps. Detailed analysis using 5 m resolution digital elevation model allows us to characterize the scarp morphology and quantify the deformation parameters, namely, terrace heights, fold scarp relief, and fold limb width and slope angle. The 3-D deformation of the highest terrace, dated by optically stimulated luminescence at 30.2 ± 4.0 ka, enables to simultaneously determine amplitude and azimuth of the long-term slip vector based on scarp relief. The long-term slip vector, oriented N338° ± 6°, is found to parallel the Chi-Chi coseismic displacements in this area. Cumulative slip and dating results yield a constant slip rate of 17.7 ± 2.2 mm/a in the direction N338° ± 6°. Late Quaternary shortening rates observed at four sites vary along strike in a similar way to Chi-Chi coseismic displacements. Together with the collinearity of long-term and coseismic slip vectors at our study site, this suggests that Chi-Chi earthquake is a characteristic earthquake for the Chelungpu thrust with recurrence interval ~470 years. We also discuss implications for the regional and long-term distribution of shortening in the central Western Foothills.

PALAEOFLOOD RECORDS FROM UPPER KAVERI RIVER, SOUTHERN INDIA: EVIDENCE FOR DISCRETE FLOODS DURING HOLOCENE

Palaeoflood Records from Upper Kaveri River, Southern India: Evidence for Discrete Floods During Holocene A record of six discrete middle Holocene floods has been established based on sedimentological and stratigraphical studies in the upper Kaveri catchment at Siddapur. The flood events are represented by six discrete, sharp-bounded, sand-silt couplets. Texturally and geochemically the suite of couplets is quite distinct from the overlying and underlying structureless fluvial deposits. Based on OSL ages the suite of couplets cover the Holocene from ~8 to ~2 ka. Such evidence is not present or reported from any other river originating in the Western Ghat in the Indian Peninsula. We argue that the six couplets represent short-term, high discharge events or flash floods. The initiation of this phase of flash floods broadly corresponds with the southward migration of ITCZ and a gradual decline in Indian summer monsoon precipitation starting at ~7.8 ka. Comparison of the elevation of the highest couplet with the high flood level (HFL) of the 1961 extraordinary flood on Kaveri demonstrates that the 20th century flood was higher than the mid-Holocene palaeofloods.

Feasibility of the Sar Technique on Quartz Sand of Terraces of NW Himalaya: A Case Study from Devprayag

Feasibility of the Sar Technique on Quartz Sand of Terraces of NW Himalaya: A Case Study from Devprayag Optically Stimulated Luminescence (OSL) dating technique based on the Single Aliquot Regenerative dose (SAR) protocol is being used increasingly as a means of establishing sediment burial age in the late Quaternary studies. Thermal transfer, low and changing luminescence sensitivity of quartz grains of young sedimentary belts of the New Zealand Alps and the north-east Himalaya poses problems in using SAR protocol. Records of active tectonics and signatures of palaeo-climate are preserved in the Quaternary - Holocene terrace sediments. Therefore, to unfold the history of successive tectonic and palaeo-climate events, robust chronological technique is needed. Palaeoflood deposits in NW Lesser Himalayan region receive quartz from the weathering of various rock types such as quartzite and phyllite in the Alaknanda Basin. A series of tests e.g. dose recovery, preheat plateau, thermal recuperation and change in sensitivity, were performed to check the suitability of quartz grains collected from the terrace sediment of Devprayag of the NW Himalaya, for OSL studies. Inferences were drawn regarding the source of the quartz grains on the basis of the geochemistry and luminescence intensity of the terrace sediment. The study shows that though quartz from the North West Himalaya are low in luminescence intensity but the reproducibility of De value makes the quartz sand suitable for SAR dating technique. Relation between luminescence intensity with CIA values help to predict the provenance of quartz sand. Tests show that the quartz from NW Himalaya is suitable for SAR protocol in OSL.

Residual Luminescence in Quartz from Slack Water Deposits in Kaveri Basin, South India: A Single Aliquot Approach

Residual Luminescence in Quartz from Slack Water Deposits in Kaveri Basin, South India: A Single Aliquot Approach Optical bleaching of quartz may be incomplete due to decreased daylight intensity and narrower wavelength spectrum in water column, leading to age overestimation in young fluvial sediments. This hypothesis was tested on a low energy slack water deposits from Kaveri Basin in south India using the SAR protocol. Various tests suggest successful application of SAR on quartz from the study area despite low sensitivity and few numbers of bright grains. Partial bleaching can be ruled out if using SAR on small aliquots. The results suggest a uniform residual luminescence and a common sedimentary condition in this area. Due to partial bleaching, the lowest values of palaeodoses should be considered in preference to the average values.

Luminescence chronology of alluvial fan in North Bengal, India: Implications to tectonics and climate

Abstract Alluvial fans are important geomorphic archives because of its strategic location at mountain front which can provide clues to the past climate and tectonics. In general, they provide information about optimum climate condition suitable for fan formation. We provide a detailed luminescence chronology of alluvial fan in north West Bengal, India. The regional fan surface (T44) has been dissected by E-W trending Himalayan thrusts known as Matiali and Chalsa thrusts and have various terraces named as T3, T2 and youngest one as T1. Luminescence ages suggest that the formation of the alluvial fan (regional surface) started before 171 ka and continued till 72 ka covering a time span of nearly 100 ky; suggesting of weaker monsoon at 72 ka. Matiali fault activated after 171 ky. Chalsa fault is suggested to be active during 48–41 ka. The last aggradational phase was around 6 ka which led to the formation of T1a terrace. The study suggests that climatic fluctuation during the period were shaping the morphology of the alluvial fan, along with tectonic activities on the two faults.

Metabarcoding of PalEnDNA as An Efficient Tool to Recover Ancient Bacterial Diversity

Abstract Past bacterial diversity of a paleosol was reconstructed using metabarcoding of paleo environmental DNA (PalEnDNA). The paleosol was subsampled from a sediment core which was excavated from a palaeo beach-ridge located 2.6 km away from present sea shore and identified that it was deposited under marine influence ∼6000 years ago, using geological proxies. The bacterial community contained 37 bacterial phyla and dominated by Proteobacteria, followed by Bacteroidetes, Firmicutes, Actinobacteria, Verrucomicrobia, and Chloroflexi. The bacterial community was a mix-up of marine and terrestrial population, and thereby diversity was higher than marine populations. The result shows metabarcoding of PalEnDNA can effectively reconstruct past bacterial community structure.