Methane and Nitrous Oxide distributions in natural waters around Taiwan Hsiao-Chun Tseng, Chen-Tung Arthur Chen, Ting-Yu Chen ＊, Hung-Ling Chen,Meng-Chia Chen, Wen- Yen Yang Institute of Marine Geology and Chemistry, National Sun Yat-Sen University. 804 Kaohsiung, Taiwan ＊ Abstract It is now apparent that tropical seas such as the South China Sea (SCS) and the West Philippines Sea (WPS) are either close to neutral or are a small source of CO 2 to the atmosphere. Methane (CH 4 ) and nitrous oxide (N 2 O), however, are clearly supersaturated in the SCS. In the case of the SCS, the CH 4 and N 2 O released from the surface waters contribute more than two times as much to the greenhouse effect as CO 2 does. The SCS also seems to export CH 4 and N 2 O to the WPS, which is also a source of CH 4 but seems to be a weak sink for N 2 O. The sampling stations are shown in Fig.1. And Fig.2. shows the distribution of nitrate and nitrite in the SCS and WPS. Because there are many NO 3 - +NO 2 - -rich rivers flowing into the SCS. Further, the deep water of WPS flows across the deepest sill (2200 m) into the SCS, becomes the deep and the bottom waters of the SCS. The vertical mixing brings NO 3 - +NO 2 - to the upper water, making the concentration of NO 3 - +NO 2 - higher than the WPS above 600m but lower below. CH 4 is formed during zooplankton grazing or in the anoxic microenvironment within particles which exist mainly in the euphotic zone. Higher nutrients lead to higher primary production, making the average surface CH 4 concentration in the SCS (4.32±2.46 nM, N=55) higher than in the WPS (3.18±1.57 nM, N=60) (Fig. 3(a)). In the SCS, the average surface N 2 O concentration measured in summer is 8.23±1.5 nM (n=55), and in the WPS in summer is only 4.64±0.39 nM (n=23), which is slightly lower than in winter (5.73±0.64 nM, n=6), (Fig. 3(b)). The relation between N 2 O and AOU shows a positive correlation before AOU Max (Fig. 4), which indicates that the majority of N 2 O in seawater is produced by nitrification. Fig. 3 (b). Vertical distribution of N 2 O Fig. 3 (a). Vertical distribution of CH 4 Some high CH 4 values, mostly in the SCS, exist at mid-depths. These signals may be related to the sediments on the upper continental slope which are richer in organic carbon, and the anoxic generation of CH 4 may have occurred. Since gas hydrates have been detected in the general area, it is also reasonable to link these high sub-surface values with a release of CH 4 from the gas hydrates in the sediments (Chen and Tseng, 2006). Same here, we assume some rather high N 2 O concentrations (N 2 O>30nM) in the SCS are released from sediments on the continental slope. In terms of air-sea exchange, the sea-to-air fluxes (F) of CH 4 and N 2 O in the WPS are 0.23±0.33μmol/m 2 /hr and -0.23±0.28μmol/m 2 /hr, respectively. In the SCS, CH 4 and N 2 O fluxes are, respectively, 0.33±0.35μmol/m 2 /hr and 0.20±0.24μmol/m 2 /hr in summer on average. These values are lower than the sea-to-air flux of CO 2 (26.3±6.9μmol/m 2 /hr; Chou et al., 2005 ； 30.4μmol/m 2 /hr, Chen et al., 2006), but nevertheless, the releases of CH 4 and N 2 O contribute more than two times to the greenhouse effect than CO 2 does. Study Area a. OR-I 725 (2004/8) OR-I 695(2003/9) b. OR-III 896(2003/8) C.d. OR-III 983(2004/7) 圖 1 、 (a) 西菲律賓海採樣站位圖 (b) 南海北部及珠江口採樣站位圖 (c) 、 (d) 南海東北部及台灣西南海域 OR-III 1126 (2006/1) N 2 O shows a maximum at about 1000m in the WPS where there is also an DO minimum. However, as the seawater under 1000m depth in the WPS mixes with NPDW (North Pacific Deep Water), the concentration of N 2 O decreases because the N 2 O concentration in the m depth range of the North Pacific is only about nM (Usui et al., 1998). A shallower N 2 O maximum occurs around 700m in the SCS, presumably because of the intense upwelling and vertical mixing in the SCS basin. Fig. 2. Vertical distribution of NO 3 - +NO 2 - Fig. 4. Correlation between N 2 O and AOU Fig.1. Study area and station locations (a) West Philippine Sea. (b) 、 (C) 、 (d) South China Sea Reference: Chen, C. T. A. and Tseng, H. C., Abnormally high CH 4 concentrations at mid-depths on the continental slopes of the northern South China Sea, TAO, 17 (4), Chen, C. T. A., Wang, S. L., Chou, W. C. and Sheu, D. D., Carbonate chemistry and projected future changes in pH and CaCO 3 saturation state of the South China Sea, Marine Chemistry, 101, 277–305. Chou W. C., Sheu, D. D. D. and Chen, C. T. A. et al., Seasonal variability of carbon chemistry at the SEATS time-series site, northern South China Sea between 2002 and 2003, TAO, 16 (2): Usui, T.,. Koike, I. and Ogura, N., Vertical profiles of nitrous oxide and dissolved oxygen in marine sediment. Marine Chemistry, 59, Mixes with NPDW Released from sediments on the continental slope Released from sediments on the continental slope or gas hydrates