Carbonate properties and oxygen concentrations at time series station DYFAMED

Monthly measurements of pH, alkalinity and oxygen over two years (February 1998-February 2000) at the Dyfamed site in the central zone of the Ligurian–Provençal Basin of the Mediterranean made it possible to assess the vertical distributions (5-2000 m) and the seasonal variations of these properties. Alkalinity varies linearly with salinity between surface water and the Levantine Intermediate Water (marked by a maximum of temperature and salinity). In deep water, total alkalinity is also correlated linearly to salinity, but the slope of the regression line is 15% less. In surface water, the pH at 25°C varies between 7.91 and 8.06 on the total proton scale depending upon the season. The lowest values are observed in winter, the highest in spring and in summer. These variations are primarily due to biological production. The pH goes through a minimum around 150-200 m and a small maximum below the intermediate water. The total dissolved inorganic carbon content (deduced from pH and alkalinity) is variable in surface water (2205-2310 ?mol/kg) and has a maximum in intermediate water, which is related to the salinity maximum. Normalized total inorganic carbon at a constant salinity is strongly negatively correlated with pH at 25°C. The fugacity of CO2, (fCO2) varies between 320 and 430 ?atm in surface water, according to the season. Below the seasonal thermocline, the maximum fCO2 (about 410 ?atm) is located around 150-200 m. The presence of a minimum of oxygen in the intermediate water of this area has been observed for several years, but our measurements made it possible to specify the relationship between oxygen and salinity in deep water. Data from the intense vertical mixing during the winters of 1999 and 2000 were used to calculate the oxygen quantity exchanged with the atmosphere during these periods. The estimated quantity of oxygen entering the Mediterranean Sea exceeds that deduced from exchange coefficients calculated with the formula of Wanninkhof and McGillis. During the vertical mixing in the 1999 winter, fCO2 in surface water was on average below equilibrium with atmospheric fCO2, thus implying that CO2 was entering the sea. However, on this time scale, even with high exchange coefficients, the estimated CO2 uptake had no significant influence on the inorganic carbon content in the water column.

1998年2月至2000年2月的两年间，研究人员对地中海利古里亚-普罗旺斯盆地中部的Dyfamed站位开展了pH、碱度（alkalinity）、溶解氧的逐月观测，借此得以评估上述参数在5~2000米深度的垂直分布特征及其季节变化。表层水与黎凡特中层水（Levantine Intermediate Water，该水团以温度与盐度（salinity）的峰值为标识）之间，碱度随盐度呈线性变化。深水中的总碱度同样与盐度呈线性相关，但回归直线的斜率低15%。表层水中，25℃下的pH值以总质子尺度（total proton scale）为基准，随季节在7.91至8.06之间波动：冬季测得最低值，春夏季则为最高值，这类变化主要由生物生产过程驱动。pH值在约150~200米深度处出现极小值，在中层水下方存在一个小幅极大值。由pH与碱度推算得到的总溶解无机碳（total dissolved inorganic carbon）含量在表层水中存在波动，范围为2205~2310 μmol/kg，且在中层水中达到峰值，该峰值与盐度峰值相关联。恒定盐度下的归一化总无机碳与25℃时的pH值呈显著负相关。二氧化碳分压（fugacity of CO₂, fCO₂）在表层水中随季节在320~430 μatm之间变化。季节温跃层（seasonal thermocline）下方，fCO₂的最大值（约410 μatm）出现在150~200米深度处。多年来研究者已观测到该区域中层水存在溶解氧极小值，但本研究的测量结果明确了深水中溶解氧与盐度之间的关联关系。研究利用1999年与2000年冬季强垂直混合过程中的观测数据，计算了这两个时期内海水与大气间的氧气交换量。经估算进入地中海的氧气量，高于采用Wanninkhof与McGillis公式计算的交换系数所推导出的数值。1999年冬季垂直混合期间，表层海水的fCO₂平均值低于大气fCO₂的平衡值，这意味着二氧化碳正从大气进入海洋。然而在该时间尺度下，即便采用较高的交换系数，估算得到的二氧化碳吸收量对水柱中的无机碳含量并无显著影响。