Tracing coastal waster masses with Sr/Ca ratio and salinity in Nanwan Bay, Taiwan

We propose a method of using two tracers, water Sr/Ca ratio (with a precision of better one per mil), and salinity, to differentiate the freshwater and marine sources of oceanic water for coastal areas with strong upwelling. In general, water column Sr/Ca ratios increase from a surface value of 8.50 mmol/mol to 8.64 mmol/mol, found below 1000 m in the Equatorial Pacific. The fresh water Sr and Ca concentrations are over ten times lower than those in seawater and its input to the ocean will not significantly change the seawater Sr/Ca ratio. The regional water Sr/Ca value, therefore, is essentially controlled by the mixing of surface and deeper seawaters. However, the variation of seawater salinity is dominated by fresh water input. We used a Sr/Ca-salinity diagram to determine the end-member composition of the surface seawater in a semi-enclosed bay (Nanwan Bay located in the southernmost Taiwan) with well-known tidally induced upwelling. We divided the sources of the Nanwan water into two components during the dry season: 69% upwelled water from a deep ocean source, and 31% offshore surface water. During the 1994 wet season an additional 2-2.5 % fresh water, mainly from rainfall, was added to the bay. A residence time of 30-40 days is estimated for the Nanwan water. These results are supported by the relation between seasonal variations of seawater oxygen isotope value, salinity and precipitation and demonstrate the practicality of a Sr/Ca-salinity diagram in distinguishing the freshwater and oceanic water contributions to the geochemistry of the surface seawater in an enclosed bay that exhibits strong upwelling. This comparison could also be applied to other regions of strong upwelling.