Morphology and size of Neogloboquadrina pachyderma in the North Atlantic

The variability in size and shape of shells of the polar planktonic foraminifer Neogloboquadrina pachyderma have been quantified in 33 recent surface sediment samples throughout the northern Atlantic Ocean and correlated with the properties of the ambient surface waters. The aim of the study was to determine whether any of the morphological features could be used to reconstruct sea surface properties in the polar realm of the North Atlantic, where most paleotemperature proxies appear to fail. The analyses revealed that shell morphology is only weakly controlled by habitat properties, whereas shell size showed a strong correlation with sea surface temperature. The regression of mean shell size on sea surface temperature revealed the presence of two trends among the sinistrally coiled shells: a continuous increase in shell size with decreasing SST in sediments deposited under polar water masses and a continuous increase in shell size with increasing SST in samples from transitional waters. The second trend mirrors the trend observed for dextrally coiled shells, which are frequent in the same samples and signal the presence of N. incompta. The identical mean shell size trends among the sinistral and dextral specimens in the temperate samples confirms the results of earlier genetic studies which indicated the existence of a small but distinct proportion of opposite coiling in N. incompta, to which the sinistral shells in the temperate samples could be attributed. The linear correlation between mean shell size and sea surface temperature in the polar domain (summer SST < 9 °C) has been used to develop an empirical formula for the reconstruction of past sea surface temperatures from shell sizes in fossil samples. The standard error of the residuals of the linear regression is 2.36 °C (1 sigma), which implies a much larger error than for most paleothermometers, but enough precision to allow resolution between results by individual paleothermometers in the polar domain. The resulting regression model has been applied on two sediment cores spanning the interval from the Last Glacial Maximum (LGM) to the present day. The results from core PS1906-1 are consistent with ice-free conditions during the LGM in the Norwegian Sea. The SST estimates for the LGM inferred from N. pachyderma shell size are similar or slightly higher than those for the latest Holocene. The results do not indicate anomalously high SST during the glacial and the LGM reconstructions thus appear more consistent with the results from foraminiferal transfer functions and geochemical proxies. Both sediment cores show the highest reconstructed SST during the early Holocene insolation optimum.

本研究对采自北大西洋北部的33份现代表层沉积物样品中的极地浮游有孔虫（polar planktonic foraminifer）厚壳新球等形虫（Neogloboquadrina pachyderma）的壳体大小与形态变异进行了定量分析，并将其与周围表层海水的理化性质进行关联。本研究旨在探究，在多数古温度代用指标难以发挥作用的北大西洋极地海域，是否可借助该物种的任一形态特征重建表层海水性质。分析结果显示，壳体形态仅受栖息环境性质的微弱调控，而壳体大小则与海水表层温度（sea surface temperature，后文简称SST）呈现显著相关性。针对壳体平均大小与SST的回归分析显示，左旋壳（sinistrally coiled shells）存在两种变化趋势：在极地水团沉积的沉积物中，壳体大小随SST降低而持续增大；而在过渡水域的样品中，壳体大小则随SST升高而持续增大。第二种趋势与右旋壳（dextrally coiled shells）的变化趋势一致——右旋壳在同组样品中较为常见，其指示了不完整新球等形虫（Neogloboquadrina incompta）的存在。温带样品中左旋与右旋壳体的平均大小变化趋势一致，这验证了此前遗传学研究的结论：该研究指出，不完整新球等形虫中存在一小部分但比例明确的反向旋向个体，而温带样品中的左旋壳正属于此类。基于极地海域（夏季SST<9℃）内壳体平均大小与SST的线性相关性，本研究构建了一套经验公式，可通过化石样品的壳体大小重建古海水表层温度。该线性回归的残差标准误为2.36℃（1 sigma），其误差幅度大于多数古温度计，但仍具备足够精度，可区分极地海域内不同古温度计的计算结果。本研究将所得回归模型应用于两根覆盖末次冰盛期（Last Glacial Maximum，后文简称LGM）至现代的沉积物岩芯。岩芯PS1906-1的分析结果与挪威海在末次冰盛期无冰覆盖的结论一致。基于厚壳新球等形虫壳体大小重建的末次冰盛期SST估算值，与全新世晚期的估算值相近或略高。本次研究结果未显示冰期存在异常偏高的SST，因此末次冰盛期的温度重建结果与有孔虫转换函数及地球化学代用指标的结果更为吻合。两根沉积物岩芯均显示，在早全新世日照最佳期，重建得到的SST达到峰值。