A PRELIMINARY EVALUATION ON B/CA OF PLANKTIC FORAMINIFERA AS A PROXY FOR SEAWATER PH: VITAL EFFECTS AND DISSOLUTION
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摘要: 浮游有孔虫B/Ca主要受控于海水pH,可作为海水pH值的替代性指标,其在探索海洋对大气pCO2冰期旋回贡献作用的研究中广受关注。然而,利用浮游有孔虫B/Ca重建海水pH,其结果的可靠性还受到温度、[CO32-]、溶解作用、生命效应(共生光合作用、呼吸作用和钙化作用)以及属种差异的制约。为评估生命效应和溶解作用对B/Ca指标有效性的影响,测定了西太平洋暖池MD06-3052岩心特征样品的浮游有孔虫表层种Globigerinoides ruber和次表层种Neogloboquadrina dutertrei在不同壳体粒径、不同壳体厚度下的B/Ca。结果表明,除两个层位外,G.ruber的B/Ca随壳体粒径的增大,总体上呈增高趋势,主要由钙化作用速率逐渐加快所致;而两个特例层位分别表现为B/Ca随粒径变化不大和随粒径先不变后增加,这可能是由钙化作用、呼吸作用与共生光合作用对G.ruber B/Ca影响的相反效应导致的竞争关系所致。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。另外,同一层位相同粒径G.ruber和N.dutertrei的B/Ca也明显不同。这些相同种属不同粒径之间以及相同粒径不同种属之间B/Ca的差异综合表明生命效应通过表征的具体过程对浮游有孔虫B/Ca有重要影响。然而,同一层位、相同粒径、不同厚度的G.ruber和N.dutertrei B/Ca各自相差不大,表明溶解作用对浮游有孔虫B/Ca影响甚微。总之,从生命效应和溶解作用角度看,只要挑选优势粒径浮游有孔虫,将生命效应对其B/Ca的影响降到最低,浮游有孔虫B/Ca是有效的海水pH的替代性指标。Abstract: B/Ca in the planktic foraminifera is mainly controlled by seawater pH,and thus could be used as ageochemical proxy for seawater pH.In order to explore the significance of ocean on the pCO2 in the atmosphere in glacial cycles,the proxy has received wide concerns among researchers.However,its reliability may be influenced by various factors,such as temperature,[CO32-],dissolution,vital effects(such as symbiont photosynthesis,respiration and calcification)as well as interspecies effects.To evaluate the influence of vital effects and dissolution on B/Ca of planktic foraminifera,we measured the B/Ca for Globigerinoides ruber and Neogloboquadrina dutertrei with different size and shell thickness from the core MD06-3052 in the western Pacific Warm Pool.Except for two cases,the B/Ca of G.ruber generally increases with increasing shell size,which is supposed to be controlled by increasing calcification rate.For the two special kinds of cases,the B/Ca of G.ruberfrom one sampling layer shows little change with the increase inshell size,and the figure from the other layer does not vary in the beginning and then increases with shell size.It is believed that these characteristics are caused by the opposite effect of calcification,respiration and symbiosis photosynthesis on the B/Ca of G.ruber.The B/Ca of N.dutertrei,however,generally decreases with increasing shell size which is probably controlled by respiration.In addition,the B/Ca of G.ruber and N.dutertrei are also different for same size shells in same sample layer.The differences in B/Ca insame species with different shell size and in different species with same shell size suggest that vital effects have important influence on foraminifera B/Ca through specific processes.However,that the B/Ca of G.ruber and N.dutertrei in the same sample layer do not show obvious discrepancy between the species with different shell thickness and same shell size suggests that dissolution has little effect on B/Ca.In summary,from the perspective of vital effects and dissolution,the B/Ca of planktic foraminifera is a valid proxy for seawater pH,as long as the dominant shell size is chosen to reduce the influence of vital effects on its B/Ca.
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Key words:
- planktic foraminifera /
- B/Ca /
- vital effects /
- dissolution
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