Amzad H. Laskar

Late-Holocene climate in the Lower Narmada valley, Gujarat, western India, inferred using sedimentary carbon and oxygen isotope ratios

We report the late-Holocene climate and vegetation of the Lower Narmada valley, Gujarat, western India as inferred from the stable carbon and oxygen isotopic composition (δ13C and δ18O) of sedimentary carbonates and the associated organic carbon. The alluvial plain from the surface to a depth of ~2 m consists of late-Holocene sediments, deposited during last ~3 kyr, probably by large historic and paleofloods. δ13C of both carbonates and organic carbon in the sediments suggest that climate was subhumid throughout the late Holocene (~ 3 kyr) as it is today, and the vegetation was of mixed C3-C4 type with little change in their relative proportions. The modern vegetation mostly comprises shrubs and scattered woody plants (C3) with a little grass (C4) in some places. The recent change in vegetation is attributable to anthropogenic disturbance: the natural grasslands (C4) are replaced by shrubs and woody plants (C3). Two comparatively drier events at ~2.1 ka and ~1.3 ka are observed, consistent with widespread proxy paleoclimatic records and are attributed to a weaker southwest monsoon rain. Marine influence on the isotopic compositions is observed in a cliff section of the Narmada estuary throughout its depositional history of ~4.2 kyr. The radiocarbon ages of the sediments are of the order of decades to a century at the surface and increase almost linearly with depth, being 1000–3000 yr at ~100 cm and 3200–4200 yr at ~200 cm.

Potential of Stable Carbon and Oxygen Isotope Variations of Speleothems from Andaman Islands, India, for Paleomonsoon Reconstruction

The Indian monsoon activity, coinciding with the Inter-Tropical Convective Zone (ITCZ), progresses from the southern Indian Ocean during the boreal summer and withdraws towards the south in winter. Islands situated to the south of India receive, therefore, the first and last showers of the monsoon; speleothems in such islands have not yet been explored for their potential to reconstruct past monsoon rainfall. Here, we present the first measurements of stable carbon and oxygen isotopic compositions (δ13C and δ18O) of a stalagmite collected from the Baratang Island of Andamans, along with new data on δ18O of modern monsoon precipitation (May to July 2010). The aim was to detect (i) whether these samples are amenable to dating using 14C, (ii) whether their oxygen isotopes indicate precipitation under isotopic equilibrium, and (iii) if (i) and (ii) above are true, can we reconstruct monsoon activity during the past few millennia? Our results indicate that while δ18O of speleothem does show evidence for precipitation under isotopic equilibrium; dating by 14C shows inversions due to varying contributions from dead carbon. The present work highlights the problems and prospects of speleothem paleomonsoon research in these islands.

Chronology of major terrace forming events in the Andaman islands during the last 40 kyr

Major earthquakes that trigger tsunamis are great natural hazards. The devastations caused by the December 26, 2004 Sumatran earthquake, and the March 11, 2011 Japan earthquake, and associated tsunamis will remain in our memories for a long time. Such events reaffirm the need for studying the cause and effects of large earthquakes of the past and to prepare the world better for the future. In such an effort, to understand the pattern of earthquakes and their effects on the geomorphic evolution, we have studied deformation history in the Andaman and Nicobar Islands, located in one of the most active convergent margins of the world. Focusing on tectonically formed coastal terraces and determining the timing of their formation from the exposed dead corals, we have been able to reconstruct the history of major earthquakes in these islands for the last 40 kyr. Our results in conjunction with the existing radiocarbon age data from coastal terraces of these islands appear to suggest that the frequency of major earthquakes (M > 7) in the region has increased during the last 9 kyr. In confirmation with some earlier work, we find evidences for a major earthquake and a tsunami between 500-600 cal yr BP and possibly 4 others during 6–9 cal kyr BP. Our results also indicate that there has been a continuous subsidence of the south Andaman Islands.

Isotope signature study of the tea samples produced at four different regions in India

India ranks second in the world for tea production and is well known for Darjeeling tea, which has great demand in the world market due to its unique flavor. In the present study, the combination of isotopic composition of Sr (as 87Sr/86Sr) and C (as δ13C) was studied as geographic tracing signatures for Indian tea samples grown in different regions. Authentic tea leaves as well as soil samples were collected from different tea producing regions; namely, Assam, Darjeeling, Munnar and Kangra, which are geographically distinct from one another. Isotopic analyses were performed by Multi-Collector Inductively Coupled Plasma Mass Spectrometry and Elemental Analyzer-Isotope Ratio Mass Spectrometry for Sr and C, respectively. On the basis of Sr isotopic data, Darjeeling tea samples were found to be more radiogenic than the other tea samples, with changes in the 87Sr/86Sr ratio being an excellent geographic indicator. Variations in δ13C proved to be an excellent geographic indicator for Munnar and Kangra teas. The 87Sr/86Sr values were statistically identical in both the soil and the tea. Principal Component Analysis (PCA), using the combination of 87Sr/86Sr, δ13C and strontium concentration data, was used to effectively differentiate among different tea producing regions.

Stable carbon isotopes in dissolved inorganic carbon: extraction and implications for quantifying the contributions from silicate and carbonate weathering in the Krishna River system during peak discharge

We present a comparative study of two offline methods, a newly developed method and an existing one, for the measurement of the stable carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC; δ13CDIC) in natural waters. The measured δ13CDIC values of different water samples, prepared from laboratory Na2CO3, ground and oceanic waters, and a laboratory carbonate isotope standard, are found to be accurate and reproducible to within 0.5 ‰\ (1σ). The extraction of CO2 from water samples by these methods does not require pre-treatment or sample poisoning and can be applied to a variety of natural waters to address carbon cycling in the hydrosphere. In addition, we present a simple method (based on a two-end-member mixing model) to estimate the silicate-weathering contribution to DIC in a river system by using the concentration of DIC and its δ13C. This approach is tested with data from the Krishna River system as a case study, thereby quantifying the contribution of silicate and carbonate weathering to DIC, particularly during peak discharge.

Seasonal variation in stable isotope compositions of waters from a Himalayan river: Estimation of glacier melt contribution

Stable isotopic compositions (delta O-18 and delta D) in water samples collected from Parbati River in Himachal Pradesh, India, during 2002-2005 were measured to delineate the contributions from different sources in different seasons. A seasonal cycle with high delta O-18 and delta D values (in parts per thousand) during the spring (March to May; -9.2, -58.6), intermediate values during the winter (December to February; -10.1, -65.6), and low values during the south-west monsoon (July to September; -10.9, -71.8) is observed. The d-excess values (15.2 +/- 2.1 parts per thousand) are higher compared with the global meteoric waters indicating significant contribution (similar to 26%) of moisture carried by western disturbances. Higher delta-values during the spring are ascribed to enhanced contribution from snow melt. The lower delta-values during the monsoon are due to various Rayleigh effects (altitude, continental, and amount effects) and large-scale convection effect on rains. A three component-mixing model using the isotopic data and some plausible end member isotopic values was applied to identify contributions to the river discharge from different sources such as groundwater or subsurface base flow, glacier/surface snow melt, and the monsoon rain. Meltwater from glacier and snow combines with the base flow in spring season. South-west monsoon rain and glacier melt along with the base flow constitute the monsoon discharge. The post-monsoon season and winter are dominated by groundwater contribution. About 80% of the discharge is contributed by glacier melts in spring season. In the rainy monsoon season, glacier/snow melt contributes similar to 41% of the discharge. The mean annual glacier melt contribution to the river water is estimated to be 44 +/- 15%. The present estimate along with some previous studies suggest that glacier contribution to the river discharge increased from similar to 35% to 50% during the period 1990 to 2011. This is consistent with recent data on glacier retreats in the Himalayan region.