| 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(pCO2-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, 732 m 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 pCO2-atm. Values of the partial pressure of carbon dioxide of sea surface(pCO2-sw) in interglacial are relatively higher than those in glacial, while the values of sea surface p H obey the opposite trends. The correlation between the carbonate system parameters of sea surface and pCO2-atm indicate that the change of sea surface pH and pCO2 in the tropical western Pacific is generally controlled by the pCO2-atm during the last 150 kyr.However, the analysis of difference between surface pCO2-sw and pCO2-atm(?pCO2(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 pCO2-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 pCO2-sw value; in interglacial period, low primary productivity leads to seawater pH with minimum value and pCO2-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 Ni?o, the area is a relatively strong carbon source with relatively low seawater p H value and relatively high pCO2-sw value; In La Ni?a, the area has weak carbon source or sink with relatively high seawater value and relatively low pCO2-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 pCO2-sw increase in this area during the transitions. |
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