Seawater incursion and freshwater discharges into the tropical Mandovi Zuari (MZ) estuary is investigated here using stable oxygen isotope data on monthly water samples from locations spatially separated within the estuary. Surface water samples were analysed for d 18 O and salinity relationship, d 18 O ¼ 0:0843 Â S À 2:1; where freshwater end member is designated as À2.1%; similar to the composition recorded for the rain water. We have estimated percentage of monthly freshwater Cuxes into the estuary using mass balance equation adopting an isotopic value of seawater end member. Our estimates showed freshwater contribution to the estuary water during monsoon time was 63%, while in the postmonsoon season the fraction drops to 34.7%. The contribution of the freshwater registered a minimum value of 6.8% during the pre-monsoon season. Further, we analysed the seasonal growth band secreted by the mollusc from the same estuary to understand the potential of mollusc as a recorder of seasonal water composition. The d 18 O of mollusc shell growth layers varied over a range between À4.3% and À2.1%. We interpreted this as seasonal signal assuming the growth rate from the culture experiment. The d 18 O of estuary water and observed temperature are used to simulate the isotopic composition of seasonal growth bands. The lighter d 18 O value of À4.3% precipitated during the month of July 2010, which coincides with the time of low productivity (d 13 C = À3.5%). While the heavier d 18 O (À2.1%) is recorded in the growth layer generated during November 2010 deBning the period of post-monsoon growth.

Efficient high-precision stable isotope ratio determination using the GasBench II peripheral for carbonates involves loading of a reaction vial with carbonate powder and injection of phosphoric acid of high density for carbonate digestion. Herein, we present an alternative method, which bypasses the need for acid dosing with an automated pump. The advantages of the new method include minimization of clogging within capillaries caused by the acid, acid spillage, and diffusive fractionation due to repeated piercing of the septa. Methods:The alternative method involves the use of low-cost boats preloaded with carbonate powder introduced into an Exetainer vial preinjected with phosphoric acid and placed within the compartment of a heated block maintained at a constant temperature in the GasBench II. Results:The new method yielded an improvement in precision for δ 13 C VPDB and δ 18 O VPDB values during replicate analyses of NBS 19, with an overall precision of ±0.04‰ and ±0.06‰, respectively. The accuracy and precision of analysis using the conventional method and the floating boat method were statistically re-evaluated using a bootstrap error analysis and Monte Carlo simulation methods. Conclusions:The proposed floating boat method of acid digestion showed significant improvement in analytical procedure and overall precision. This method is easily adoptable in other laboratories and is free from frequent issues of needle clogging and irregular fractionation due to diffusion facilitated by repeated puncturing of septa, and can serve as an alternative method for high-precision carbonate stable isotope analysis.

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