(Table 1, 2) Foraminifera biometric data, and boron isotopic composition of Globigerinoides sacculifer of sediments from the Ontong-Java Plateau

Sediment samples from the Ontong-Java Plateau in the Pacific and the 90° east ridge in the Indian Ocean were used to investigate whether shell size and early diagenesis affect d11B of the symbiont-bearing planktonic foraminifer Globigerinoides sacculifer. In pristine shells from both study locations we found a systematic increase of d11B and Mg/Ca with shell size. Shells in the sieve size class 515–865 µm revealed d11B values +2.1 to +2.3 per mil higher than shells in the 250–380 µm class. This pattern is most likely due to differences in symbiont photosynthetic activity and its integrated effect on the pH of the foraminiferal microenvironment. We therefore suggest smaller individuals must live at approximately 50–100 m water depth where ambient light levels are lower. Using the empirical calibration curve for d11B in G. sacculifer, only shells larger than 425 µm reflect surface seawater pH. Partial dissolution of shells derived from deeper sediment cores was determined by shell weight analyses and investigation of the shell surface microstructure by scanning electron microscopy. The d11B in partially dissolved shells is up to 2 per mil lower relative to pristine shells of the same size class. In agreement with a relatively higher weight loss in smaller shells, samples from the Ontong-Java Plateau show a more pronounced dissolution effect than larger shells. On the basis of the primary size effect and potential postdepositional dissolution effects, we recommend the use of shells that are visually pristine and, in the case of G. sacculifer, larger than 500 μm for paleoreconstructions.本研究采集了取自太平洋翁通爪哇海台（Ontong-Java Plateau）与印度洋东经90°海岭的沉积物样品，旨在探究壳体尺寸与早期成岩作用（early diagenesis）是否会对带共生体浮游有孔虫（symbiont-bearing planktonic foraminifer）泡抱球虫（Globigerinoides sacculifer）的d11B比值产生影响。在两处研究区域的完好壳体样本中，我们观测到d11B比值与Mg/Ca比值随壳体尺寸增大呈现系统性升高的趋势。筛分级别为515–865 μm的壳体，其d11B比值较250–380 μm级别的壳体高出2.1‰至2.3‰。该规律极可能源于共生体光合活性的差异，及其对有孔虫微环境pH的综合调控效应。据此我们推测，个体较小的泡抱球虫大致栖息于水深50–100 m的水层，此处的环境光照水平相对较低。借助泡抱球虫d11B比值的经验校准曲线，仅尺寸大于425 μm的壳体能够准确反映表层海水pH。通过壳体重量分析与扫描电子显微镜（scanning electron microscopy）观测壳体表面微观结构，我们确定了取自较深沉积物岩芯的壳体发生了部分溶解现象。与同尺寸的完好壳体相比，部分溶解壳体的d11B比值最高可降低2‰。与小型壳体更高的重量损失程度相吻合，翁通爪哇海台的样品中，小型壳体表现出比大型壳体更为显著的溶解效应。综合上述原生尺寸效应与潜在的沉积后溶解影响，我们建议在古环境重建（paleoreconstructions）工作中，优先选取外观完好、且泡抱球虫壳体尺寸大于500 μm的样品开展分析。