RationaleOxygen isotopic ratios of silicates are excellent tools to reconstruct paleotemperature and isotopic composition of the precipitating fluid. However, the measurement of 17O/16O is difficult due to the low abundance of 17O. The present study reports a simplified high‐precision analytical technique for measuring the two oxygen isotope ratios, 17O/16O and 18O/16O, in silicates.MethodsSilicate samples were ablated by a CO2 laser in a BrF5 environment. The released oxygen (O2) was then cryogenically trapped in a molecular sieve zeolite (MSZ). Associated contaminants such as BrF5, F2, NF3 etc. were cleaned by passing the gas through a NaCl trap followed by a cooled (−25°C) MSZ‐packed U‐tube trap. The purified O2 was analysed in a MAT 253 isotope ratio mass spectrometer for oxygen isotope ratios.ResultsThe δδ17O and δ18O values of the working gas were calibrated by NBS‐28 and crosschecked by inter‐laboratory references UWG‐2, SCO and IMAU‐O2. The average analytical precisions (using aliquots of NBS‐28, UWG‐2, SCO, and laboratory internal standards IIT‐KGP‐SQ quartz and IIT‐KGP‐NQ quartz) of the δ17O, δ18O and ∆′17O values were 0.04‰, 0.08‰ and 4 per meg, respectively.ConclusionsA new cryogenic cleaning technique was developed that does not require GC but efficiently removes NF3‐contaminants from oxygen gas produced by laser fluorination of silicates. The technique is simple, quick and cost‐effective and provides highly precise and accurate δ17O, δ18O and ∆′17O values.

Rationale The stable isotopic compositions of biogenic carbonates like fish otoliths (ear bones) are widely used for palaeoclimatic reconstruction. The conventional method using acid‐digestion of micro‐milled samples is a multi‐step time‐consuming process. Here we report a fast method based on laser heating of otolith carbonates to obtain accurate and high‐resolution stable isotopic compositions. Method Otoliths of catfish from the Gulf of Kutch were analysed to check the precision, accuracy and time‐resolution of the isotope ratios. The CO2, generated by heating otoliths with a 50 W CO2 laser, was analysed for its oxygen and carbon isotope ratio [δ18O and δ13C, with precision: 0.12 and 0.17‰ (1σ), accuracy: 0.13 and 0.25‰, respectively] using a continuous‐flow isotope ratio mass spectrometer. The effect of laser power (0.7–2 W) was assessed for reproducible data. Samples were roasted and analysed to account for the effect of the inherent organic matter on the isotopic values. Results Roasting did not alter the δ18O of the otoliths but increased the δ13C slightly. High‐resolution (125 μm) analysis of the right and left otolith of a fish yielded similar δ18O and δ13C values, suggesting the suitability of either of them for deriving the climate signal. An increase in δ18O values from ~ −2‰ to ~ −1‰, observed across the ontogeny, is consistent with the known migratory behaviour of the catfish between freshwater and the sea. Conclusions The otolith δ18O value of an adult fish records the sea surface temperature (with ~3°C uncertainty) on a monthly scale. The otolith δ13C values, with the knowledge of dietary δ13C, provide the mean annual δ13C value of dissolved inorganic carbon. The study provides a rapid method for retrieving high‐resolution seasonal climate data from otoliths found aplenty in geological/archaeological records.

Miller, Tanaka and Greenwood (MTG) 1 have critically assessed our article Ghoshmaulik, Bhattacharya, Roy and Sarkar (GBRS) 2 in a recent

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<p>The intertrappean sediments and the bole beds of the Deccan volcanic province hold clues to the climatic condition in India during the Cretaceous/Paleocene transition. Earlier isotopic studies of the bulk clays from the ‘bole beds’ showed that the rainwater composition was lighter (δ<sup>18</sup>O  -8‰) relative to the present-day (δ<sup>18</sup>O ~ -5‰). This was ascribed to an increase in the rainfall (amount effect). However, later reconstruction of the mean annual precipitation (MAP) from the intertrappean paleosol carbonates suggested that the amount was no different than the modern-day precipitation. One possible reason for this disagreement can be due to the low preservation potential of proxies used in these studies. The present study was carried out by analysing authigenic silica which is resistant to post-depositional modifications. Such silica deposits are abundant throughout the Deccan intertrappean sediments occurring as cherts, chertified limestone and silicified fossils. They form during the interaction of silica-rich water with the existing sediments or fossils, the silica being derived by leaching of the volcanic ash by surface run-off and/or from siliceous hydrothermal waters. Silicified woods were analyzed for their triple oxygen isotope ratios (expressed as δ<sup>17</sup>O and δ<sup>18</sup>O) to determine the silicification temperature and the isotopic composition of the silicifying fluid. The distribution of the obtained silicification temperature and water composition of diverse samples indicates a widely variable silicification environment. The silicification took place at temperatures from 25°C  (near surface temperature)  to 90°C (at relatively shallower levels of 50-100 m). In addition, the δ<sup>18</sup>O (VSMOW) values of silicification fluid varied from -14‰ to near 0‰. The geological, floral and faunal evidence suggest deposition of these woods in a continental fluvio-lacustrine environment. Isotope modelling of the data suggest a two-component fluid mixing between hydrothermal water and a lake water. Assuming this fluid to be derived from a mixture of meteoric water and volcanic hydrothermal water, the δ<sup>18</sup>O value of the local meteoric water is estimated to be -14‰ to -12‰. These values are lower by about 9‰ to 7‰ compared to today (mean annual δ<sup>18</sup>O over central India being ~-5‰). We ascribe this to an increase in the mean annual rainfall by about 400 mm. It is possible that the late cretaceous precipitation increased due to the warming caused by a high CO<sub>2</sub> environment.</p>