Studies of long‐term ecological changes are crucial for understanding the factors that govern the distribution and abundance of taxa in time and space. The present study analyses the long‐term patterns of palaeoecological changes during the Late Cretaceous, using the marine bivalve fossil record of the Ariyalur Sub‐basin, south India. Multivariate quantitative analyses reveal a conspicuous shift in the bivalve palaeocommunity structure from the middle Turonian to Santonian Garudamangalam Formation to the early Maastrichtian Kallankurichchi Formation. A concomitant decline in diversity and evenness from the Garudamangalam to the Kallankurichchi is also associated with a near disappearance of infaunal groups in the latter, where a few stationary epifaunal taxa exhibit overwhelming dominance. Both formations appear to have suffered taphonomic alteration and lithification bias that may have contributed to the reduction of species diversity. The dominance of shallow burrower bivalves in the siliciclastic‐dominated Garudamangalam Formation reflects a scenario comparable to the post‐Palaeozoic marine assemblages. In contrast, the development of a firmer substrate (firmground), aided by a possible reduction in the siliciclastic supply during an inferred sea‐level rise during the deposition of the Kallankurichchi Formation, was more favourable for epifaunal bivalves. Reduced sedimentation rate may have resulted in a low nutrient environment and the development of firmgrounds that offer the required substrate stability for epifaunal bivalves but decrease infaunal burrowing efficiency in the Kallankurichchi Formation. The worldwide prevalence of recliner bivalves in carbonate and mixed siliciclastic‐carbonate systems during the Late Cretaceous underscores the importance of substrate conditions in determining bivalve palaeocommunity structure.

Chemical composition heterogeneity in carbonate rocks ranging from the microscale to several meters preclude their direct age determination. Therefore, stepwise dissolution of several bulk rock samples or subsamples of an individual specimen can provide information on the primary composition of the carbonate material and refine the resulting Pb–Pb age. A millimetre-thick carbonate-rich (white) and shale-rich (black) layer from the Rohtas Formation, Lower Vindhyan Group, was divided into eight subsamples each and subjected to three leaching steps. The first leachate (L1) was discarded, and subsequent leachates, L2 and L3, were obtained after 24 hours of leaching in 0.6M HBr. A Pb-Pb isochron age of 1666±25 Ma (n=23, MSWD=13) is obtained by regressing L2 and L3 leachates from both layers. Since initial leachates (e.g., L2) contain epigenetically altered carbonate material, data points in the 207Pb/204Pb vs 206Pb/204Pb plot are well correlated but show a large scatter. In contrast, regressing only the L3 leachates of both layers yielded a Pb-Pb isochron age of 1644±49 Ma (n=12, MSWD=7), with a reduced scatter and comparable with earlier published work. Increasing the number of leaching steps minimizes the degree of scatter and significantly improves the precision of the obtained Pb-Pb age. Two subsamples from phosphoritic stromatolite-bearing carbonate rocks of the Tirohan Dolomite from the Jankikund river section, Lower Vindhyan Group, were subjected to seven leaching steps, of one hour each, in 0.5M HBr. The Pb isotopic composition of the carbonate material dissolved in five steps (L3 to L7) was regressed to yield an isochron age of 1579±16 Ma (n=10, MSWD=1.3). The Pb-Pb age of the Tirohan Dolomite Member obtained by multi-step leaching in this study is indistinguishable within error from an earlier reported age [1] of 1650±89 Ma (n=5, MSWD=89), moreover shows a better correlation of the 207Pb/204Pb vs 206Pb/204Pb data. All the analyses were performed in static mode on a Thermo-Fisher Neptune Plus MC-ICPMS. The instrumental mass fractionation was corrected using both thallium-spiking and sample-standard bracketing. Furthermore, a generalized power law correction was applied to the 204Pb-corrected ratios of unknown samples using the 205Tl/203Tl ratios of the bracketing NBS-981 standards. The two-stage mass bias corrected ratios were then normalized by the ratios of NBS-981 during each analytical session. The long-term isotopic ratios of NBS-981 standard (n=71) are 206Pb/204Pb= 16.9371±0.0026 (2σ), 207Pb/204Pb= 15.4906±0.0046 (2σ), 208Pb/204Pb= 36.7042±0.0113 (2σ). [1] Bengtson, S., Belivanova, V., Rasmussen, B., Whitehouse, M. (2009). The controversial “Cambrian” fossils of the Vindhyan are real but more than a billion years older. Proceedings of the National Academy of Sciences, 106(19), 7729-7734.

The carbon cycle perturbations in geological history are preserved in the form of changes in stable carbon isotope ratios (δ13C values) in different carbon-bearing sedimentary archives. The carbon cycle perturbation that occurred across the Paleocene-Eocene boundary (~56 Ma) is known as the Paleocene Eocene thermal Maximum (PETM). After more than three decades of research, the exact magnitude of the negative carbon isotope excursion (CIE) is still fuzzy. The shallow marine sedimentary archive, deposited far above the lysocline, is considered to be the best archive to quantify the carbon cycle perturbation because the deep marine (carbonate) was likely to be affected by carbonate dissolution and terrestrial sedimentary records influenced by different climatic parameters. However, different biotic and abiotic processes could influence the magnitude of the CIE during the perturbed carbon cycle-climate state in a shallow marine environment. For this reason, the present study investigated the early Paleogene marine carbonate rocks deposited in the eastern Tethyan Sea (Ladakh, NW India) to check the possible presence of the PETM CIE and test whether shallow marine carbonate is a good archive for measuring the CIE magnitude. The presence of age-diagnostic larger benthic foraminifera and detailed micro-facies analysis indicates the investigated shallow marine carbonate rocks were deposited during the ~56 to 54 Ma (Shallow Benthic Zone - 4 to 7) and are likely to hold the PETM CIE. The secular variation in the δ13C values of unaltered bulk carbonate, screened through the cathodoluminescence microscopic study, reveals a PETM CIE magnitude of -3.6 ‰. The observed CIE magnitude is similar to the globally accepted CIE magnitude (-4 ± 0.4 ‰) for PETM and suggests that shallow marine carbonate can be used to assess the magnitude of PETM and other carbon cycle perturbations.

Under rising anthropogenic CO2, the future of the tropical climate states and the response of the biosphere, specifically the fate of the tropical rainforest (TRF), is uncertain. Therefore, deep-time climate proxy records and model simulations are being extensively utilized to understand the possible response of the TRF community during extreme climate states. However, comprehensive climate-TRF proxy data from the tropical/equatorial region for the paleo-global warming episodes, e.g., Late Paleocene - Early Eocene interval (-56 to 51 Ma, encompassing transient hyperthermal events like Paleocene-Eocene Thermal Maximum [PETM], Eocene Thermal Maximum2 [ETM2]/H1/ Eocene Layer of Mysterious Origin [ELMO], H2, I1, and I2), are very limited and create difficulties in the validation of simulated results. Here we present long-term land surface temperature and precipitation (delta 2H and delta 18O of pedogenic clay mineral-derived) and TRF diversity (palynology) data from a paleo-equatorial region, spanning the -56 to 51 Ma interval. Present data suggest that the hydrological response to global warming was not temporally uniform in the paleo-equatorial land. While a significantly increased rainfall buffered the terrestrial temperature during the PETM, an insignificant increase in precipitation and negligible temperature lowering can be observed during the ETM2 hyperthermal event. However, the climate system's response during the other Early Eocene hyperthermals, i.e., H2, I1, and I2, was very similar to the PETM. Despite these small aberrations, the long-term average equatorial land surface temperature (27 +/- 4 degrees C) during the Early Eocene greenhouse episode remained very similar to the modern equatorial temperature (28-30 degrees C). Rainfall proxy and plant diversity data suggest that the precipitation aided TRFs' resilience and proliferation, possibly through temperature buffering, during this paleo-greenhouse episode.

Since diagnostic primary depositional sedimentary structures and depth-dependent grain-size trends are rarely preserved, building a comprehensive sequence stratigraphic framework for the vast majority of the Phanerozoic carbonate platform sequences is pending. Among the two most important sequence stratigraphic surfaces, while the subaerial unconformity can be reliably identified by either karst development or the appearance of siliciclastic materials, the demarcation of the maximum flooding surface remains difficult in lithologically uniform shallow marine carbonate sequences. The present study attempts to identify the globally documented maximum flooding surface within the body of the negative carbon isotope excursion of the Palaeocene-Eocene Thermal Maximum recorded in the shallow marine carbonate platform sequences. The results show that, along with the carbonate microfacies, the yttrium to holmium ratio (Y/Ho ratio) of the carbonate fraction reliably records the sea-level changes. A Y/Ho ratio between 70 and 80 demarcates the stratigraphic position of the maximum sea-level state (the most open marine condition in the studied interval) and maximum flooding surface in the studied sections. Since the Y/Ho ratios remain relatively stable throughout diagenesis, they can be used for maximum flooding surface identification in shallow marine carbonate platform sequences. The possibility exists that the same method can also be applied to the mixed siliciclastic-carbonate systems.

The extraction of stable carbon and oxygen isotope ratios (delta C-13 and delta O-18) of primarily precipitated calcite from partially dolomitized limestone is of prime interest because the delta C-13 and delta O-18 values of primary calcite are widely used in paleoclimate research. The differential acid extraction method (DAE) has been widely used to separate CO2 evolved during the calcite-orthophosphoric acid reaction from CO2 evolved from the dolomite- orthophosphoric acid reaction. However, the various laborious offline methods with inevitable uncertainties make DAE very challenging. We proposed a fully automatic-online DAE method in the present study, using an autosampler-GasBench II-Isotope Ratio Mass Spectrometer (IRMS). The results show that 6-values of calcite can be obtained from the mixture (grain size between 180 and 250 mu m) having >50% calcite after similar to 15-100 min of reaction with orthophosphoric acid at 30 degrees C. Further, both the precision and the accuracy of the measurement using the autosampler-GasBench II-IRMS can be improved by attaching micro-vibrators to the sample tray.

ABSTRACTIncreasing stable carbon isotopic ratio (δ13C) of sedimentary organic matter (SOM) has traditionally been interpreted to reflect an increase in C4 vegetation abundance, though microbial degradation or increasing δ13C values of C3 plants in response to precipitation change can also cause a similar effect. Therefore, δ13CSOM values alone cannot reveal the true origin of the observed 13C enrichment in SOM. Here, employing a wet‐oxidation method on modern sediment, we have demonstrated that this treatment removes partly degraded and degradation‐prone components of the C3 plant‐derived organic matter (OM). Therefore, it helps to understand the real contribution of C4‐derived organic carbon (OC) in the modern sediment and identify the C3 plant‐derived OC in disguise. As a test case, we extend our inference to Middle to Late Holocene lower Gangetic floodplain records, which were supposed to register a complete switchover from a C3‐dominated to a C4‐dominated system (~10‰ positive shifts in δ13CSOM values). However, the present study showed that ~60% (~6‰) of the observed positive shift in δ13CSOM values actually register a temporal change in the C3 plant end‐member δ13C value in response to reduction in Indian summer monsoon precipitation.

The hydrogen isotopic composition (δ 2 H) of authigenic clay minerals has been used extensively in paleoclimate studies. The separation of clay minerals from sediments/soils, using various chemicals, is a prerequisite for isotope ratio measurements, where carbonate, Fe-(oxyhydr)oxides, and organic matter are removed successively from the sediments for a greater clay yield. The commonly adopted organic matter-removal method using hydrogen peroxide (H 2 O 2 ) is thought to either alter directly the pristine δ 2 H values of the smectite clay minerals or to introduce organic hydrogen-bearing impurities through the ineffective removal of organic matter. The objective of the present study was to test whether H 2 O 2 treatment can alter the δ 2 H values of kaolinite (Kln) by comparing two organic matter-removal methods, namely, H 2 O 2 and disodium peroxodisulfate (Na 2 S 2 O 8 ) combined with a neutral buffer. In doing so, kaolinite-rich, old (~56 Ma) sediment samples and pure kaolinite internal laboratory reference materials were used to understand the effectiveness and suitability of the above-mentioned methods in clay-sample preparation for δ 2 H measurements. The δ 2 H values of the H 2 O 2 -treated aliquots show smaller δ 2 H values than those for the Na 2 S 2 O 8 -treated aliquots. Estimated ambient water δ 18 O values (−4‰) from the Na 2 S 2 O 8 -treated aliquots agreed well with the bio-phosphate (fish vertebrae) based environmental water δ 18 O estimation (−3.3‰). The present study indicated, therefore, that δ 2 H values obtained after Na 2 S 2 O 8 treatment are likely to be more realistic for paleoclimate reconstruction.

Sedimentary lipid biomarker proxies are extensively used for paleoclimatic and paleoenvironmental research. Among all the lipid compounds, the non-polar n-alkanes (chain lengths n-C-14 to n-C-37) are one of the most widely used lipid fractions for such studies. However, the lack of optimized automated extraction and post-extraction sample workup procedure, using conventional polar solvent, from environmental samples, has been challenging. Conversely, extraction of the n-alkanes using non-polar solvents restricts the co-extraction of other polar lipid compounds like ketones, alcohols, acids, etc. which may be of interest to many researchers. Here we report an optimized total lipid extraction method using the conventional polar dichloromethane/methanol solvents with standardized Automated Solvent Extractor (ASE) parameters and post-extraction sample work-up protocols. The robustness of the method has been tested by extracting eight samples containing algae, leaf, and sediment/ rock of varying age (spanning from recent to similar to 252 Ma). The results of the extraction efficiency (yield) of the method were compared with commonly used methods, those using either non-polar n-hexane or polar dichloromethane/methanol solvents. For n-alkane extraction from algal biomass, the performance of the proposed method and the conventional polar solvent method are statistically comparable, but the new method performs better than the non-polar solvent method. Likewise, for the leaf and sediment samples, the performance of the proposed method is equivalent to the non-polar solvent method but outperforms the commonly used polar solvent method. The Terrestrial Aquatic Ratio (TAR) and long-chain hydrocarbon/short-chain hydrocarbon (LHC/SHC) ratios calculated from the algae and sediment samples also show that our recommended extraction approach can be confidently used in calculating n-alkane ratio proxies, which critically depend on the extraction efficiency of the short-chain n-alkanes. Taken together, the new method will be highly useful for studying the n-alkanes from a wide spectrum of natural sample matrices and provide reliable data for paleoclimatic inference.

The coastal upwelling zones, occupying only ~0.5% of the global ocean, account for ~10% of the global primary productivity. The CO2 fixation by primary producers amplifies in the upwelling zones during global warming due to the higher nutrient supply. Based on the presumption that the nutrient-deficient coastal ocean is less productive, the state of the oligotrophic coastal ocean is often neglected in the productivity-climate change studies. The present study investigated the changes in the primary productivity, redox condition, and nutrient content, using algal abundance, total organic carbon, and various major, trace, and rare earth elements and yttrium (REY) proxies, of the oligotrophic equatorial eastern Tethyan coastal ocean across the Paleocene-Eocene Thermal Maximum (PETM), a prominent paleo-global warming event. Despite the lower nutrient (lower NiEF, CuEF, and ZnEF) contents, and invariable salinity, pH, and light conditions, the PETM interval shows extensive growth of coralline red algae in the hypoxic-oxic water column. Based on these observations, and inferences drawn from the previous laboratory experiments, conducted on the algal growth in varying pCO2 by others, we postulate that the increased atmospheric CO2 concentrations during the PETM probably enhanced the primary productivity of the oligotrophic Tethyan coastal ocean. If so, then the oligotrophic coastal ocean may be considered as an effective CO2 sink and likely to play a pivotal role in carbon cycle-climate connection studies.

The Himalayan foreland basin and other parts of the world had witnessed a dramatic change in the ecological structure during the Late Miocene, as indicated by the increase in abundance of C 4 plants in a C 3 dominated ecosystem. However, the asynchronous expansion of C 4 plants across the latitudes, as well as within the different sub‐basins of the Himalayan foreland basin, tentatively suggests that regional climatic factors and sedimentary architecture vis‐à‐vis geomorphological settings might have an important influence in controlling the abundance of C 4 plants. This study has carried out sedimentological and palaeohydrological analyses of the Siwalik channels at Naladkhad and Ranital regions of the Kangra sub‐basin, north‐west Himalaya, and estimated slope and sinuosity of the modern Himalayan fan rivers ( n > 100), which provides a hydrological analogue for the Siwalik channels. The results suggest that the Siwalik Group in the Kangra sub‐basin was deposited by braided rivers, situated at the proximal part of palaeo‐alluvial fan. In addition to climate‐driven changes, the comparison between sedimentary architecture and published carbon isotope data from the Kangra sub‐basin suggests that channel‐fill dominated fan‐proximal Naladkhad and Ranital regions favoured the growth of C 3 plants until ca 6 Ma, a time when Pakistan Siwaliks were dominated by C 4 plants. In comparison to the Kangra sub‐basin, Pakistan Siwaliks are characterized by a higher abundance of floodplain sediments and possibly represent a distal fan deposit. Data from other parts of the world similarly suggest that, in addition to climate forcing, the dominance of overbank fines favoured the growth of C 4 plants. Therefore, detailed knowledge of the depositional environment and palaeo‐geomorphic setting of the sedimentary archives is essential to understand the influence of sedimentary architecture on the spatio‐temporal variation in the abundance of C 4 vegetation, especially for foreland basin settings, where lateral facies transition rate is high.

A number of Precambrian sedimentary basins of the Indian subcontinent offer scope for tracking early oxygenation history of atmosphere and hydrosphere. Available studies, though certainly not exhaustive, record signatures of pre-great oxygenation event (GOE) whiffs of atmospheric oxygenation between 3.29 and 3.02 Ga. Besides, available geochemical signatures . from Precambrian sedimental.) , rocks (BIE sulfides, sulfates, argillaceous sediments and phosphorites) suggest a generally sub-oxic shallow marine and bipartite oxicsulfidic condition in late Paleoproterozoic and Mesoproterozoic time, respectively. In this backdrop, occurrence of phosphorites in several late Paleoproterozoic basins possibly indicates formation of local oxygen oasis in presence of cyanobacterial community. From heavy to very heavy delta S-34 values in sulfides (pyrite) present in a number of Mesoproterozoic basins and Mo, Mo/TOC values from argillaceous intervals of the Vindhyan Supergroup, it is inferred that the deep hydrosphere was, in general, anoxic and, at times, euxinic.

ABSTRACTRadiocarbon dating of archaeological carbonates from seven cultural stages of Dholavira, Great Rann of Kachchh (GRK), the largest excavated Harappan settlement in India, suggests the beginning of occupation at ~5500 years BP (pre‐Harappan), and continuation until ~3800 years BP (early part of the Late Harappan period). The settlement rapidly expanded under favourable monsoonal climate conditions when architectural elements such as the Citadel, Bailey, Lower and Middle Town were added between the Early and mid‐Mature Harappan periods. Abundant local mangroves grew around the GRK sustaining prolific populations of the edible gastropod Terebralia palustris. Oxygen isotope (δ18O) sclerochronology of Early Harappan gastropod shell suggests seasonal mixing of some depleted (δ18O ~ −12‰) river water in summer/monsoon months (through ancient Saraswati and/or Indus distributary channels) with seawater that periodically inundated the GRK. Evaporation from this semi‐enclosed water body during the non‐monsoon months enriched the δ18O of water/shell carbonates. The humid fluvial landscape possibly changed due to a catastrophic drought driving the final collapse of the settlement of Dholavira exactly at the onset of the Meghalayan (Late Holocene) stage (~4300–4100 years BP). Indeed, Dholavira presents a classic case for understanding how climate change can increase future drought risk as predicted by the IPCC working group. Copyright © 2019 John Wiley & Sons, Ltd.

Large river systems have lately gained much attention in past as well as modern climate change studies because of their ability to transfer and sequester massive amounts of terrestrial organic matter (OM) in their delta-fan archives. Because of their long- uninterrupted- sedimentation history, the delta-fan repositories of the large rivers are also thought to be excellent paleo-climate archives. Being one of the largest, the delta-fan of the Ganges-Brahmaputra (G-B) River system has been extensively studied. OM-based paleo-vegetation proxy records, recovered from the floodplain archive, however, showed poor concurrence with the G-B delta-fan records. Using carbon isotopic composition (δ13C) of modern C3 and C4 plant produced OM, soil, and bedload sediments from the lower Gangetic plain, we have shown that riverine sediments and therefore delta-fan sedimentary archives fail to capture the floodplain vegetation composition in terms of C3–C4 plant abundance. The bias arises because of the faster removal of complete C4, and partial degradation of C3 plant derived carbon during OM transfer from plant to the soil, before its final storage in the delta-fan archives. Using a global compilation of riverine OM ages and their δ13C values, we propose that multiple deposition-erosion cycles of sediments, during its transportation through large River systems, possibly cause sedimentary OM ageing and play a key role in determining the quality of OM based proxy records. We, therefore, suggest that bulk OM ages are necessary to assess the quality of OM based proxy records when retrieving paleo-climate information from the large delta-fan archives.

Two hitherto unexplored settlements at Karim Shahi and Vigakot are reported from the uninhabited hyper-arid region of the western Great Rann of Kachchh (GRK), located near southern fringe of Thar Desert, Gujarat, NW India. The archaeological evidence, supported by radiocarbon and optical chronology indicate presence of settlement from the Early Iron Age to Early Historic (similar to 3100-2300 years B.P.) and Historic to Medieval (similar to 1500-900 years B.P.) periods. This would imply that following the Harappan decline, the GRK was still a hospitable terrain for the sustenance of human settlements' during the Early Iron Age. Isotopic and microbotanical evidence indicate that relatively higher rainfall than today sustained the hydrological system until around the Medieval period. We propose that the early withdrawal of the late Holocene Inter Tropical Convergence Zone (ITCZ) and resultant monsoon decline accounts for the abandonment of settlement in the western (Sindh-Baluchistan) domain. As a consequence, the rivers in the western domain dried early, while the fluvial system in the southern/eastern domain (Sorath/Cholistan in Gujarat/Ghaggar-Hakra-Nara interfluve) was sustained due to monsoonal rain at least up to 1600 years B.P. The settlements were subsequently disrupted due to a combination of hyper-arid climate and infrequent tectonic activity sometime between the late Medieval and recent time. The settlement migration in the southern/eastern domain, as far into the Rann and desert fringe during this period was possibly a nonpareil ancient analogue of modern "climate refugee and refugia" (Warner, 2011; Morelli et al., 2016) induced by anthropogenic climate change.

Kyanite-eclogite xenoliths from Wajrakarur are considered as remnants of subducted ocean-floor crust [1]. Here trace element concentration and isotopic data are presented in garnet (Grt) and kyanite (Ky) from xenoliths KL-2 E1-E4, characterized by [2]). We use the precise Sr/Sr host kimberlite groundmass perovskite ratio (0.70312-0.70333, [3]) as a proxy for the extent of kimberlitic magma infiltration at 1.1 Ga. The xenolithic Grt and Cr-rich (upto 1506 ppm) Ky have more radiogenic Sr/Sr values than kimberlite, at 1.1 Ga, of 0.703829-0.705203 and 0.703811-0.704502, respectively. Furthermore, the Grt and Ky Nd/Nd ratios, at 1.1 Ga, are 0.509321-0.511372 and 0.510951-0.511156, respectively, and are distinctly lower than those of the host kimberlite (0.511870-0.512290, [4]). This indicates that the infiltration of kimberlitic fluid has not altered the Sr/Sr and Nd/Nd ratios in the Grt and Ky, and therefore their isotope compositions must be inherited and predate the kimberlite magma generation event at 1.1 Ga. Trace elements in Grt and Ky indicate extreme metasomatism (Sr in Grt 104-296 ppm, in Ky 672-8713 ppm [limit Sr<2ppm] and Nb in Grt 0.64-1.78 ppm, in Ky 1.7-4.54 ppm [limit Nb<0.5ppm]). The xenoliths underwent at least one major melting event inferred from extreme depletions in Re, Os and Os/Os ratios [5]. Their mantle-like δ18O values (Grt 5.3-5.4‰, Ky 5.3-5.9‰), positive Eu anomalies in both Grt and Ky (similar to Group 1 HREE-depleted garnets of [1]) suggests that the protolith likely was a chromite-bearing leucogabbro, emplaced as a high-pressure cumulate at the crust-mantle boundary, which was later eclogitized due to deep-seated subduction and underwent episodes of extreme melting and metasomatism before 1.1 Ga and at least before 1.7 Ga, as inferred from their youngest Re depletion dates [5].

The exact magnitude of the carbon isotopic excursion (CIE) for the Palaeocene–Eocene Thermal Maximum (PETM) is essential for our understanding of the carbon cycle perturbation. Global compilation of the PETM CIE magnitudes indicates that the shallow-marine inorganic carbonate could be a potential candidate to decipher the actual CIE magnitude. The present study, therefore, made an attempt to explore the thick Palaeogene shallow-marine carbonate sequence of the Sylhet Limestone exposed in the Jaintia Hills of northeast (NE) India, in terms of the preservation and magnitude of the PETM CIE. Exploratory sampling carried out across the Sylhet Limestone suggests that this sequence was deposited during the Late Palaeocene and Early Eocene, as evident from the age-diagnostic foraminifera. The observed \({\sim }3.4\permille \) CIE at the top of the Lakadong Limestone, resting above the Miscellanea miscella and Ranikothalia nuttalli foraminifera-bearing horizon, can, therefore, be correlated with the PETM CIE. Although the magnitude of the CIE from our limited data set agrees well with the global compilation, the absence of a stepped profile questions the preservation of the CIE reported elsewhere from the Tethyan sequence. Further work is needed for a better understanding of the PETM interval in NE India.

Rationale The elaborate sampling and analytical protocol associated with conventional dual-inlet isotope ratio mass spectrometry has long hindered high-resolution climate studies from biogenic accretionary carbonates. Laser-based on-line systems, in comparison, produce rapid data, but suffer from unresolvable matrix effects. It is, therefore, necessary to resolve these matrix effects to take advantage of the automated laser-based method. Methods Two marine bivalve shells (one aragonite and one calcite) and one fish otolith (aragonite) were first analysed using a CO2 laser ablation system attached to a continuous flow isotope ratio mass spectrometer under different experimental conditions (different laser power, sample untreated vs vacuum roasted). The shells and the otolith were then micro-drilled and the isotopic compositions of the powders were measured in a dual-inlet isotope ratio mass spectrometer following the conventional acid digestion method. Results The vacuum-roasted samples (both aragonite and calcite) produced mean isotopic ratios (with a reproducibility of ±0.2 ‰ for both δ18O and δ13C values) almost identical to the values obtained using the conventional acid digestion method. As the isotopic ratio of the acid digested samples fall within the analytical precision (±0.2 ‰) of the laser ablation system, this suggests the usefulness of the method for studying the biogenic accretionary carbonate matrix. Conclusions When using laser-based continuous flow isotope ratio mass spectrometry for the high-resolution isotopic measurements of biogenic carbonates, the employment of a vacuum-roasting step will reduce the matrix effect. This method will be of immense help to geologists and sclerochronologists in exploring short-term changes in climatic parameters (e.g. seasonality) in geological times.

The antiquity and decline of the Bronze Age Harappan civilization in the Indus-Ghaggar-Hakra river valleys is an enigma in archaeology. Weakening of the monsoon after ~5 ka BP (and droughts throughout the Asia) is a strong contender for the Harappan collapse, although controversy exists about the synchroneity of climate change and collapse of civilization. One reason for this controversy is lack of a continuous record of cultural levels and palaeomonsoon change in close proximity. We report a high resolution oxygen isotope (δ(18)O) record of animal teeth-bone phosphates from an archaeological trench itself at Bhirrana, NW India, preserving all cultural levels of this civilization. Bhirrana was part of a high concentration of settlements along the dried up mythical Vedic river valley 'Saraswati', an extension of Ghaggar river in the Thar desert. Isotope and archaeological data suggest that the pre-Harappans started inhabiting this area along the mighty Ghaggar-Hakra rivers fed by intensified monsoon from 9 to 7 ka BP. The monsoon monotonically declined after 7 ka yet the settlements continued to survive from early to mature Harappan time. Our study suggests that other cause like change in subsistence strategy by shifting crop patterns rather than climate change was responsible for Harappan collapse.