Institute of Marine Geology and Chemistry, National Sun
Yat-Sen University, Kaohsiung, Taiwan 804, Tel: +886-7-525-5148, Fax:
+886-7-7-525-5348, E-mail: ddsheu@mail.nsysu.edu.tw
Seasonal variability of seawater d13C
at the South China Sea time-series station
In response to the increasing needs of a greater geographic coverage of oceanic CO2 time-series study, a comprehensive time-series program in the South China Sea (Southeast Asia Time Series Study; SEATS) was initiated in 1998 by groups of scientists in Taiwan. The SEATS site (18o15’N; 115o35’E) is thought to be important for a better understanding of the global ocean carbon flux not only because it is the only time-series station in the largest marginal sea (the South China Sea) on the west side of the tropical Pacific, a region believed to have a great impact on global climate, but also because it is located inside an oceanic regime covered by the east Asia monsoon, thus providing a unique opportunity to study the influence of climate-induced physical forcing on the seasonal and interannual variability of upper ocean biogeochemistry. Together with other designated core measurements of seawater chemistry (major nutrients, AOU, pH, NTA, and NTCO2) at this station, seawater d13C analysis on a seasonal basis was commenced from year 2002. Among our isotopic results, one of the most striking features we found is that there is a distinct decrease of d 13C (~0.2 ‰) in the mixed layer (0-50 m) during winter as compared to the other seasons. Since the overall procedural error of our isotopic analysis is better than ±0.05 ‰ and has intercalibrated recently against seawater samples dispatched by Dr. A. G. Dickson for carbon isotope analysis, the ~0.2 ‰ depletion observed in winter cannot be fortuitous, instead it has to be resulted chiefly from the enhanced mixing of water from the underneath thermocline during strong winter monsoon.