以同位素組成探討台灣南部集水區化學風化通量與沿海地下水輸出之控制因素

Abstract

Detailed hydrographic surveys are presented by using environmental isotope and trace element compositions (δD, δ18O, δ13CDIC, Tritium, 14C, Δ87Sr and Ba) to investigate submarine groundwater discharge (SGD) off the Pingtung Plain coast, SW Taiwan. Surface seawater δ18O distributions clearly reflecting seasonal variations and different freshwater contributions. Waters from the submarine groundwater discharge (SGD) tongue along the Kaoping Canyon are distinct in terms of δ18O, d13CDIC and Δ14C. Isotopic compositions of Canyon seawaters are characterized by significant variability and heavy isotope enrichment. The groundwater end-member can be represented by data of groundwater wells monitored in the Pingtung Plain. Stable isotopes data show a good separation of freshwater with seawater groups. Important reactions have occurred in the mixing zones which would affect the local distribution of chemicals. 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 400-700 m and 1200 m along deep parts of Kaoping Canyon. Assuming two end-member mixing model, fresh submarine groundwater was estimated 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 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 ~1200 m). Areas with notable SGD are located a long distance from the shoreline (~25 km), 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.