Rationale: Aerodyne Tunable Infrared Laser Differential Absorption Spectrometer (TILDAS) allows for the measurement of carbonate-clumped (∆ 638) and stable oxygen isotope ratios (δ 628) without the mass interference of 17O, and understanding acid fractionation factors during phosphoric acid digestion is essential for inter-laboratory data comparison. Here, we present δ 628 and ∆ 638 ratios in CO 2 generated during phosphoric acid digestion of a reference carbonate at different temperatures. Methods: Carrara Marble (MAR-J1) calcite is digested with ⁓104% phosphoric acid using a) Break Seal method is a modified version of a McCrea-type reaction vessel where complete equilibration is achieved between product CO 2 and H 2O generated from the digestion and b) Individual Acid Bath (IAB) where the samples are digested with fresh aliquot of acid each time and the product CO 2 and H 2O simultaneously frozen in a U-trap connected to the reaction chamber by liquid N 2. Results: The regression equations for acid fractionation of δ 628 are: a) Break Seal Method: 1000 ln α (dig. temp- 25℃) = (0.532 ± 0.030) × 10 6/T 2 + (-6.185 ± 0.308) ; R 2 = 0.98. b) IAB: 1000 ln α (dig. temp- 25℃) = (0.359 ± 0.027) × 10 6/T 2 + (-4.114 ± 0.247) ; R 2 = 0.97. The regression equations for acid fractionation of ∆ 638 are: a) Break Seal Method: 1000 ln α (dig. temp - 25℃) = (0.0189 ± 0.0017) × 10 6/T 2 + (-0.2151 ± 0.0174) ; R 2 = 0.95. b) IAB: 1000 ln α (dig. temp - 25℃) = (0.0529 ± 0.0041) × 10 6/T 2 + (-0.6072 ± 0.0356) ; R 2=0.97. Conclusion: The shallow slope of the IAB method for δ 628 and the steeper slope for ∆ 638 indicate minimal resetting of the isotope composition of product CO 2 due to re-equilibration or interaction with free H 2O molecules. This observation enables inter-laboratory comparisons of δ 628 and ∆ 638 in calcite.

Himalayan rivers are prone for drying subject to continuous drop in the glacial meltwater contribution and groundwater level. The present study is conducted in the region of West Kameng District of Arunachal Pradesh, India, covering catchment of 2 tributaries of Kameng river, viz., Tenga & Dirang-Bichom. We used stable isotopic method to trace the origin of water feeding the river as it flows from the headwater to the region of flood plain. Our observation allowed defining the Local Water line (LWL) in this region for the dry period, with the relation δD = (8.1 ± 0.3)×δ 18O + (11.6 ± 2.5‰). This equation is derived from analysis of river water sample collected during March 2021. This LWL is identical to that reconstructed using the monthly (from India, April – October 2007 ) precipitation isotope data from a station at Mawlong, Meghalaya, [δD = (8.1 ± 0.1) ×δ 18O + (11.8 ± 0.9) ‰]. The d-excess values from the two set of data are similar at 11.2± 1.8 ‰ and 11.3± 2.7‰, respectively, implying that river water is mainly derived from rainwater. Such coincidence of observation is interpreted as a common source of water for river and groundwater. Further, we compared present observation with other studies on the surface water composition in other Himalayan River systems and documented a consistent elevation pattern for stable isotopes. Our observation on spatial variability showed maximum altitude effect in the North-Western Himalaya and drop in isotope ratios with height with pronounced participation of recycled moisture in the Eastern Himalaya with presence of terrestrial biosphere.