Investigation of the seasonality of N. pachyderma in the North Atlantic Ocean during Heinrich Stadial 1 using an ecosystem modeling approach

Fossil shells of planktonic foraminifera serve as the prime source of information on past changes in surface ocean conditions. Because the population size of planktonic foraminifera species changes throughout the year, the signal preserved in fossil shells is biased towards the conditions when species production was at its maximum. The amplitude of the potential seasonal bias is a function of the magnitude of the seasonal cycle in production. Here we use a planktonic foraminifera model coupled to an ecosystem model to investigate to what degree seasonal variations in production of the species Neogloboquadrina pachyderma may affect paleoceanographic reconstructions during Heinrich Stadial 1 (~18-15 cal. ka B.P.) in the North Atlantic Ocean. The model implies that during Heinrich Stadial 1 the maximum seasonal production occurred later in the year compared to the Last Glacial Maximum (~21-19 cal. ka B.P.) and the pre-industrial era north of 30 ºN. A diagnosis of the model output indicates that this change reflects the sensitivity of the species to the seasonal cycle of sea-ice cover and food supply, which collectively lead to shifts in the modeled maximum production from the Last Glacial Maximum to Heinrich Stadial 1 by up to six months. Assuming equilibrium oxygen isotopic incorporation in the shells of N. pachyderma, the modeled changes in seasonality would result in an underestimation of the actual magnitude of the meltwater isotopic signal recorded by fossil assemblages of N. pachyderma wherever calcification is likely to take place.

浮游有孔虫（planktonic foraminifera）的化石壳体是重建表层海洋古环境变化的核心信息载体。由于浮游有孔虫各物种种群规模随年度周期性变动，其化石壳体中保存的环境信号会偏向于该物种繁殖量达到峰值时期的海洋环境条件，潜在季节性偏差的幅度则取决于该物种种群繁殖量季节循环的强度。本研究采用耦合生态系统模型的浮游有孔虫模型，旨在探讨北大西洋海因里希冰阶1（Heinrich Stadial 1，约18~15校准千年BP（cal. ka B.P.））时期，厚壳新球虫（Neogloboquadrina pachyderma）的繁殖季节变化对古海洋学重建的影响程度。模型结果显示，相较于末次盛冰期（Last Glacial Maximum，约21~19校准千年BP）与北纬30°以北的前工业时代，海因里希冰阶1的物种季节繁殖峰值出现时间明显延后。对模型输出的诊断分析表明，这一变化源于该物种对海冰覆盖与食物供给季节循环的敏感性，二者共同导致模型模拟的繁殖峰值时间从末次盛冰期到海因里希冰阶1发生了最长达6个月的偏移。假设厚壳新球虫壳体的氧同位素沉积处于平衡状态，那么这种季节模式的变化，会使得所有可能发生钙化作用的区域中，厚壳新球虫化石组合所记录的融水同位素信号的实际幅值被低估。