Triple oxygen isotope systematics has evolved as a powerful tool in understanding various earth system processes. It has proven reliable in assessing paleoenvironmental conditions from natural archives (e.g., waters, ice cores, biota, sediments, etc.) owing to recent advances in high-precision mass spectrometric analysis. Toward the standardization of triple oxygen isotope analyses in carbonates, we describe a procedure of high-precision Δ′17O analysis of carbonates by a two-step protocol: acid digestion of carbonates to evolve CO2 followed by the catalytic CO2–O2 exchange method. The Δ′17O values of a suite of carbonate reference materials and several carbonates of different origins have been determined with good precision (∼0.007‰). The accuracy of sample Δ′17O values is dependent on the accuracy of Δ′17O composition of the reference CO2 used in determining the effective fractionation (θs) in the experimental setup. The obtained Δ′17O values (λ = 0.528, versus VSMOW) for NBS18-CO2 (−0.119‰) and NBS19-CO2 (−0.169‰) show a difference of 0.050‰, similar to that obtained elsewhere via complete fluorination. The analyzed carbonates mostly conform to equilibrium mass-dependent fractionation laws, but we encountered a suite of samples from cold seeps, caves, and metasomatic environments that have Δ′17O values indicative of disequilibrium fractionation. We show that a combination of clumped isotope composition (Δ47) that provides estimates of formation temperature and triple oxygen isotope ratios in carbonates can help in reconstructing past environments, where paired carbonate data (δ13C−δ18O−Δ47–Δ′17O) and parent water data (δ17O−δ18O−Δ′17O) are particularly useful.

Variations in the isotope ratios (18O/16O and D/H) of meteoric water at continental stations serve as valuable tracers for the hydrological cycle. In the present study, we investigated the role of sea surface temperature, wind speed, relative humidity, and rainout processes on the stable isotopic composition of the 2010 monsoon rainwater of Bangalore. The wind over the ocean influences the sea surface temperature and humidity which in turn influence the vapor isotopic composition. The rainout over the Arabian Sea and the land mass (Western Ghats) during advection of the air parcel to Bangalore and its recycling further modify the vapor composition. The isotopic ratios (δ18O, δD, and d‐excess) of the precipitation at Bangalore was estimated following a Rayleigh fractionation model involving rainout and recycling processes yield values consistent with our observation. In some samples, however, the observed isotopic ratios are higher (by 2 to 5‰), and a few of them are associated with high d‐excess values. These discrepancies could be due to limitation of the model assumptions.

Stable Hydrogen and Oxygen isotopic composition of water vapor, rainwater and surface seawater show a distinct trend across the latitude over the Southern Indian Ocean. Our observations on isotopic composition of surface seawater, water vapor and rainwater across a transect covering the tropical Indian Ocean to the regions of the Southern Ocean showed a strong latitudinal dependency; characterized by the zonal process of evaporation and precipitation. The sampling points were spread across diverse zones of SST, wind speed and rainfall regimes. The observed physical parameters such as sea surface temperature, wind speed and relative humidity over the oceanic regions were used in a box model calculation across the latitudes to predict the isotopic composition of water vapor under equilibrium and kinetic conditions, and compared with results from isotope enabled global spectral model. Further, we obtained the average fraction of recycled moisture across the oceanic transect latitudes as 13.4 ± 7.7%. The values of recycled fraction were maximum at the vicinity of the Inter Tropical Convergence Zone (ITCZ), while the minimum values were recorded over the region of subsidence and evaporation, at the Northern and Southern latitudes of the ITCZ. These estimates are consistent with the earlier reported recyling values.

Isotopic ratios of rainwaters are believed to decrease with the amount of rainfall. However, analyses of the isotopic composition of rainwater and water vapor samples collected from Bangalore during the monsoon period of 2013 fail to show any simple relationship with the local meteorological parameters whereas show good correlation with the regional integrated convective activity. The correlation is particularly high when the averaging is done over the preceding 8 to 15 days, showing the influence of mixing or residence time scale of atmospheric moisture. This observation emphasizes the role of regional atmospheric circulation driving the isotopic values. A comparison between observed isotope ratios in water vapor and rainwater with Isotope‐enabled Global Spectral Model shows discrepancies between the two. The observed values are relatively enriched, indicating a systematic bias in the model values. The higher observed values suggest underestimation of the evaporation in the model, which we estimate to be about 28 ± 15% on average. Simultaneous analyses of rainwater and water vapor isotopic composition again show definitive presence of raindrop evaporation (31 ± 14%). We also documented a distinct pattern of isotopic variation in six samples collected at Bangalore due to mixing of vapor from a cyclonic system in close proximity that originated from the Bay of Bengal. It seems that large‐scale isotopic depletion occurs during cyclones caused by Rayleigh fractionation due to massive rainout. These results demonstrate the power of rainwater and water vapor isotope monitoring to elucidate the genesis and dynamics of water recycling within synoptic‐scale monsoon systems.

Rationale The doubly substituted isotopologues (e.g., 18O18O, 17O18O) in atmospheric O2 are potential tracers for ozone photochemistry and atmospheric temperatures. Their low abundances and isobaric interference are the major analytical challenges. The 253 Ultra high‐resolution stable isotope ratio mass spectrometer is suitable for resolving isobaric interferences. Methods O2 from air is purified using gas chromatography on a packed column filled with molecular sieve 5 Å and cooled to −78°C. The δ17O, δ18O, Δ17O, Δ35 and Δ36 values are measured on the extracted O2 with the 253 Ultra at medium mass resolution (M/ΔM ~10000) using Faraday detectors for the singly substituted isotopologues and ion counters for the doubly substituted isotopologues. Results Interferences from isobars, mainly 35Cl for 17O18O and H35Cl and 36Ar for 18O18O, are sufficiently resolved to enable high‐precision determination of Δ35 and Δ36. The Δ35 and Δ36 values of O2 after photochemical isotope equilibration at −63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ35 and Δ36 values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O2 differ from those expected at equilibrium. Conclusions The 253 Ultra allows interference‐free clumped isotope measurements of O2 at medium mass resolution. The Δ35 and Δ36 signatures in atmospheric O2 are mainly governed by O3 photochemistry, temperature and atmospheric transport. Tropospheric O2 is isotopically well mixed and retains a significant stratospheric signature.