Seasonal-to-diurnal scale isotopic signatures of tidally-influenced submarine groundwater discharge to the Bay of Bengal: Control of hydrological cycle on tropical oceans

Abstract

Submarine groundwater discharge (SGD) acts as major pathway to transport solute-laden terrestrial-sourced fresh groundwater, as well as re-circulated marine water to the global oceans. The study area, Bay of Bengal (BoB), a part of the Indian Ocean, receives one of world’s highest terrestrial riverine fresh water discharge, sediment and solute flux from the adjacent Himalayan and cratonic South Asia. Thus, together with the monsoon- dominated tropical climate, it forms one of the most complicated, productive and interactive global hydrological systems. However, understanding such topical phenomena needs intricate mechanistic understanding, based on high resolution data, which are barely available from the BoB. Delineation of stable isotopic and chemical signature of hydrologic-sourced components in the SGD to the BoB would help to identify the intraannual to diurnal-scale impact of seasonality and tidal cycles, as well as interactions with other surface water bodies. This study provides one of the first documentation of such high-resolution, temporally-variable, stable isotope patterns of SGD in coastal systems of the BoB, and possibly of any tropical ocean. During post-monsoon season, the discharging groundwater was observed to have depleted δ18O (ranges −2.12‰ to −4.19‰) and low Cl− concentrations (745 to>11,500 ppm) (seepage water), which is closely associated with the groundwater δ18O composition (−3.18‰ to −4.05‰) and Cl− content (775 to>5900 ppm) range. In pre-monsoon season depleted δ18O values suggests that regional groundwater contributes up to 45m from high tide line (HTL) (up to 88%), and re-circulated seawater-sourced SGD dominates 45m to 110m (extent of study transect) offshore. In post-monsoon season, terrestrial-sourced groundwater predominates the SGD composition (up to 99%) till 110 m. Changes in δ18O and Cl− content, in pre-monsoon season indicates enhanced infiltration of seawater in the seepage face, due to lower terrestrial-sourced freshwater discharge, whereas, in post-monsoon terrestrialsourced, resident freshwater dominates in the seepage face. The study suggests that SGD are sourced to interactions between local-regional hydrological systems, and do reflect their compositional variability. It also provides insight of influencing physico-chemical mechanisms, ranging from seasonal to daily-tidal time-scales. The outcome of this study thus may provide intricate insights in delineating the coastal hydrologic and biogeochemical processes, as well as detecting, carbon sinks, nutrient sources and primary productivity in a tropical ocean.