Shell size variation of Neogloboquadrina pachyderma sin. in the Norwegian-Greenland Sea during the last 1.3 Myr

We present measurements of the maximum diameter of the planktonic foraminifer Neogloboquadrina pachyderma sin. from six sediment cores (Ocean Drilling Program sites 643, 644, 907, 909, 985 and 987) from the Norwegian-Greenland Sea. Our data show a distinct net increase in mean shell size of N. pachyderma sin. at all sites during the last 1.3 Ma, with largest shell sizes reached after 0.4 Ma. External factors such as glacial-interglacial variability and carbonate dissolution alone cannot account for the observed variation in mean shell size of N. pachyderma sin. We consider the observed shell size increase to mirror an evolutionary trend towards better adaptation of N. pachyderma sin. to the cold water environment after 1.1-1.0 Ma. Probably, the Mid Pleistocene climate shift and the associated change of amplitude and frequency of glacial-interglacial fluctuations have triggered the evolution of this planktonic foraminifer. Oxygen and carbon stable isotope analyses of different shell size classes indicate that the observed shell size increase could not be explained by the functional concept that larger shells promote increasing sinking velocities during gametogenesis. For paleoceanographic reconstructions, the evolutionary adaptation of Neogloboquadrina pachyderma sin. to the cold water habitat has significant implications. Carbonate sedimentation in highest latitudes is highly dependent on the presence of this species. In the Norwegian-Greenland Sea, carbonate-poor intervals before 1.1 Ma are, therefore, not necessarily related to severe glacial conditions. They are probably attributed to the absence of this not yet polar-adapted species. Further, transfer function and modern analog techniques used for the reconstruction of surface water conditions in high latitudes could, therefore, contain a large range of errors if they were applied to samples older than 1.1-1.0 Myrs.

我们报道了取自挪威-格陵兰海6个沉积岩芯（大洋钻探计划（Ocean Drilling Program）站位643、644、907、909、985及987）的浮游有孔虫（planktonic foraminifer）厚壳新球虫（Neogloboquadrina pachyderma sin.）最大直径的测量数据。我们的研究数据显示，在过去1.3 Ma间，所有站位的N. pachyderma sin.壳体平均尺寸均呈现显著的净增长，且最大壳体尺寸出现于0.4 Ma之后。仅冰期-间冰期旋回变率、碳酸盐溶解作用等外部因素，不足以解释N. pachyderma sin.壳体平均尺寸的观测变化。我们认为，此次观测到的壳体尺寸增大，反映了N. pachyderma sin.在1.1~1.0 Ma后对冷水环境适应性提升的演化趋势。中更新世气候转型及其伴随的冰期-间冰期波动振幅与频率变化，或已触发这一浮游有孔虫的演化进程。对不同壳体尺寸级别的氧碳稳定同位素分析表明，观测到的壳体尺寸增大无法通过“更大壳体可提升配子发生过程中的沉降速率”这一功能假说得到解释。对于古海洋学重建工作而言，厚壳新球虫对冷水栖息环境的演化适应具有重要指示意义。高纬度海域的碳酸盐沉积作用高度依赖该物种的存续。因此，挪威-格陵兰海在1.1 Ma之前的碳酸盐贫化区间，未必与强冰期环境相关，其成因可能是当时该物种尚未适应极地环境，尚未形成有效种群。此外，若将用于重建高纬度表层水体环境的传递函数（transfer function）与现代类比技术（modern analog techniques）应用于年龄老于1.1~1.0 Ma的沉积物样品，可能会引入较大的误差范围。