海洋地质与环境重点实验室知识库

    浮游有孔虫B/Ca主要受控于海水pH，从而可作为海水pH值的替代性指标，其在探索海洋对大气二氧化碳分压（pCO_2-atm）冰期旋回贡献作用的研究中广受关注。西太平洋暖池区作为全球海气相互作用最活跃、全球大洋年平均气温最高的地区，了解该海区海水pH和二氧化碳分压（pCO₂）地史时期的演化特征及其影响因素，对探究大洋酸化和全球碳循环具有重要意义。
    本次研究选取西太平洋暖池北部边缘柱状岩心MD06-3052孔（14°48.6042′ N、123°29.3983′ E，水深732米，孔长19.48 m），通过部分典型层位浮游有孔虫G. ruber和N. dutertrei的B/Ca分析，探讨了生命效应和溶解作用与有孔虫B/Ca指标的关系，利用全柱连续层位浮游有孔虫G. ruber的B/Ca重建了海水的pH和pCO₂，进而探究了15万年来热带西太平洋表层海水pH和pCO₂的演化特征和影响因素。
    G. ruber的B/Ca随壳体粒径的增大，总体呈增高趋势，主要由钙化速率加快所致；N. dutertrei的B/Ca随壳体粒径的增大，总体呈降低趋势，呼吸作用可能是导致该趋势的主要原因。总之，生命效应对浮游有孔虫B/Ca有重要影响。同一层位、相同粒径条件下，G. ruber和N. dutertrei不同厚度壳体的B/Ca各自相近，表明溶解作用对浮游有孔虫B/Ca影响不大。浮游有孔虫B/Ca作为海水pH的替代性指标是有效的，但挑选有孔虫时，应选择优势粒径范围的样品分析B/Ca，以减少生命效应对该指标的影响。
    热带西太平洋浮游有孔虫G. ruber B/Ca重建的表层海水pH和pCO₂与氧同位素、pCO_2-atm一样，都呈现出明显的冰期-间冰期的变化特征。表层海水二氧化碳分压（pCO_2-sw）在间冰期值相对冰期要高，海水pH的变化趋势则与pCO_2-sw相反，在间冰期值相对冰期要低。表层海水碳酸盐系统参数与pCO_2-atm的协变特征表明15万年来热带西太平洋表层海水pH和pCO₂变化总体上受pCO_2-atm控制。
    然而，表层pCO_2-sw与pCO_2-atm差值（DpCO_{2（sw-atm）}）分析表明15万年来热带西太平洋表层并不处于CO₂的海-气平衡状态，具体表现为大气CO₂的源。这表明除主控因素pCO_2-atm外，一些因素或过程打破了这种平衡，其中营养物质和生产力、类ENSO过程以及南极中层水/亚南极模态水（AAIW/SAMW）扮演重要角色。冰期时，初级生产力高，浮游植物的生长对表层海水溶解无机碳的利用增强，导致海水pH为相对高值，pCO_2-sw为相对低值；间冰期时，初级生产力较低，海水pH为极小值，pCO_2-sw为极大值。此外，MD06-3052孔所在海区类ENSO变化与海水pH和pCO₂有较好对应：在El Niño盛行时，海水pH为相对低值，pCO_2-sw为相对高值，海区为相对强碳源；在La Niña盛行时，海水pH为相对高值，pCO_2-sw为相对低值，海区为弱碳源或碳汇。环南极深层水的上涌，通过增强的AAIW/SAMW将碳同位素低值信号和溶解的CO₂输送到MD06-3052孔所在海域，造成冷期向暖期过渡阶段的四次碳同位素低值事件、海水pH的降低和pCO_2-sw的升高。

  The planktonic foraminiferal B/Ca is mainly controlled by seawater pH, thus a geochemical proxy for seawater pH. In order to explore the role of ocean in the atmospheric partial pressure of carbon dioxide (pCO_2-atm) glacial cycle, the proxy has been widely concerned. The western Pacific warm pool which has the warmest water in the global ocean plays an important role in climate change as a major source of heat and moisture. Understanding the variations and controls of local seawater pH and pCO₂ in the geological history has great significance to explore ocean acidification and global carbon cycle.
    Core MD06-3052 (14°48.6042′ N; 123°29.3983′ E, 732m water-depth, 19.48 m length) in the north margin of western Pacific Warm Pool was selected in this study. The relationships between vital effects, dissolution and B/Ca were discussed by analyzing the B/Ca of planktonic foraminifera G. ruber and N. dutertrei in some typical layers. The seawater pH and pCO₂ were reconstructed with B/Ca of G. ruber in continuous layers of core, then the variations and controls of sea surface pH and pCO₂ in the tropical western Pacific during last 150 kyr were explored.
    The B/Ca of G. ruber generally increases with increasing shell size, which is controlled by increasing calcification rate; the B/Ca of N. dutertrei generally decreases with increasing shell size which probably controlled by respiration. In conclusion, vital effects has important influence on foraminifera B/Ca. The B/Ca of G. ruber and N. dutertrei with the different shell thinkness do not show obvious discrepancy between same sample layer but same shell size, respectively, suggesting that dissolution has little effect on B/Ca. The planktonic foraminiferal B/Ca is a valid proxy for seawater pH, as long as we choose planktonic foraminifer of dominant shell size to reduce the influence of vital effects on its B/Ca.
    The sea surface pH and pCO₂ reconstructed by the B/Ca of planktonic foraminifera G. ruber in the tropical western Pacific are characterized by glacial-interglacial cycles, which is similar to oxygen isotopes and pCO_2-atm. Values of the partial pressure of carbon dioxide of sea surface (pCO_2-sw) in interglacial are relatively higher than those in glacial, while the values of sea surface pH obey the opposite trends. The correlation between the carbonate system parameters of sea surface and pCO_2-atm indicate that the change of sea surface pH and pCO₂ in the tropical western Pacific is generally controlled by the pCO_2-atm during the last 150 kyr.
    However, the analysis of difference between surface pCO_2-sw and pCO_2-atm（DpCO_{2（sw-atm）}）suggests that sea surface in the tropical western Pacific during the last 150 kyr is not in balance of ocean-atmosphere, but a source of atmospheric CO₂. It suggests that other factors or process broke the balance except the main control factor of pCO_2-atm. Nutrient and productivity, ENSO-like process and Antarctic Intermediate Water/ Subantarctic Mode Water(AAIW/SAMW) play an important role in it. In glacial period, high primary productivity show that the exploit enhanced of the growth of phytoplankton to the dissolved inorganic carbon of sea surface, which leads to relatively high seawater pH value and relatively low pCO_2-sw value; in interglacial period, low primary productivity leads to seawater pH with minimum value and pCO_2-sw with maximum value. In addition, ENSO-like pattern in the sea area of core MD06-3052 coincides with seawater pH and pCO₂: In El Niño, the area is a relatively strong carbon source with relatively low seawater pH value and relatively high pCO_2-sw value; In La Niña, the area has weak carbon source or sink with relatively high seawater value and relatively low pCO_2-sw value. The upwelling of Cicumpolar Deep Water was enhanced in the transitions from cold to warm period, bringing the minimum signals of carbon isotope and dissolved CO₂ to the area of core MD06-3052 through AAIW/SAMW. As a result, carbon isotope and seawater pH decrease and pCO_2-sw increase in this area during the transitions.