Seasonal freshwater flux estimation using mollusc from the tropical Mandovi Zuari estuary, Goa, India

Abstract

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.

Freshwater plume-like condition near the north-eastern coastal Arabian Sea during early Miocene: Evidence from the stable isotope record in the growth bands of gastropods (Turritella sp.)

“…The same analytical method was followed in other studies as well (Ghosh et al, 2021, Banerjee et al, 2023. In this method, a floating boat made from Pyrex glass loaded with 50ug of carbonate powder was introduced into the glass vial prefilled with 1 ml 105% phosphoric acid.…”

“…3 and Supplementary section 6) from the Kachchh basin (eastern Arabian Sea) with the coeval  18 O records from the Quilon formation (south-eastern Arabian Sea from Prasanna and Kapur 2022) and exceptionally well-preserved planktonic foraminifera from coastal Tanzania (western Arabian sea from . We have further compared the early Miocene  18 O records with the modern-day mollusc shell-based  18 O records from the western Indian coastal locations -Mandvi beach of Bhuj (Present study), MZ estuary (Ghosh et al, 2021), Cochin estuary of Southern India (Ghosh et al, 2018) and Hooghly estuary (Banerjee et al, 2018). This comparison shows the Kachchh basin, situated in the north-eastern part of the Arabian Sea was receiving more freshwater influx than the coeval basins at the south eastern (Quilon formation) coastal Arabian Sea and Tanzania representing the eastern coastal Indian Ocean.…”