Deep submarine groundwater discharge indicated by tracers of oxygen, strontium isotopes and barium content in the Pingtung coastal zone, southern Taiwan

We have conducted detailed hydrographic surveys off the Pingtung Plain coast, SW Taiwan, using environmental isotope and trace element compositions (δ¹⁸O, Δ⁸⁷Sr, and Ba) as the main tracers to investigate submarine groundwater discharge (SGD) in the region. The δ¹⁸O values of seawater in this area show significant variability with heavy isotope enrichment relatively to groundwater, and two δ¹⁸O-depleted layers are found at depths of 400-700m and 1200m along deep parts of Kaoping Canyon. Assuming two end-member mixing model, we estimate that fresh submarine groundwater may constitute a few percent of the total SGD in the canyon. This is reflected in lower salinity, higher Δ⁸⁷Sr values and Ba concentrations in deep canyon water than in adjacent seawater. Pore water samples collected from the canyon floor have light δ¹⁸O values and low chloride concentrations, consistent with the overlying δ¹⁸O-depleted water. Elsewhere, seabed pore waters have normal seawater compositions. The results of this study indicate that the deep SGD in the Pingtung Plain coastal zone is mainly fed by recirculating seawater with some admixture of fresh groundwater in canyon area (down to ~1200m). Areas with notable SGD are located a long distance from the shoreline (~25km), and the 'deep' SGD may be widespread off southern Taiwan. This discharge must be considered as a significant source of trace elements and other chemical constituents to the coastal ocean.