Data from: Evolutionary ecology of Early Paleocene planktonic foraminifera: size, depth habitat and symbiosis

The carbon stable isotope (δ13C) composition of the calcitic tests of planktonic foraminifera has an important role as a geochemical tracer of ocean carbon system changes associated with the Cretaceous/Paleogene (K/Pg) mass extinction event and its aftermath. Questions remain, however, about the extent of δ13C isotopic disequilibrium effects and the impact of depth habitat evolution on test calcite δ13C among rapidly evolving Paleocene species, and the influence this has on reconstructed surface-to-deep ocean dissolved inorganic carbon (DIC) gradients. A synthesis of new and existing multispecies data, on the relationship between δ13C and δ18O and test size, sheds light on these issues. Results suggest that early Paleocene species quickly radiated into a range of depths habitats in a thermally stratified water column. Negative δ18O gradients with increasing test size in some species of Praemurica suggest either ontogenetic or ecotypic dependence on calcification temperature that may reflect depth/light controlled variability in symbiont photosynthetic activity. The pattern of positive δ13C test-size correlations allows us to (1) identify metabolic disequilibrium δ13C effects in small foraminifera tests, as occur in the immediate aftermath of the K/Pg event, (2) constrain the timing of evolution of foraminiferal photosymbiosis to 63.5 Ma, ∼0.9 Myr earlier than previously suggested, and (3) identify the apparent loss of symbiosis in a late-ranging morphotype of Praemurica. These findings have implications for interpreting δ13C DIC gradients at a resolution appropriate for incoming highly resolved K/Pg core records.

浮游有孔虫（planktonic foraminifera）钙质壳体的碳稳定同位素（δ¹³C）组成，作为与白垩纪/古近纪（K/Pg）大灭绝事件及其后续过程相关的海洋碳系统变化的地球化学示踪剂（geochemical tracer），具有重要研究价值。然而，针对古新世快速演化类群，目前仍存在若干待解问题：一是δ¹³C同位素不平衡（isotopic disequilibrium）效应的影响程度，二是深度栖息环境（depth habitat）演化对壳体方解石（test calcite）δ¹³C的作用机制，三是该效应对重建的海洋表层-深层溶解无机碳（DIC, dissolved inorganic carbon）梯度的影响。本研究通过整合全新数据与现存多物种数据（multispecies data），分析δ¹³C、δ¹⁸O与壳体大小之间的关联，为解答上述问题提供了新的视角。研究结果显示，古新世早期类群在热分层水体中快速辐射至多个深度栖息环境。部分普氏虫属（Praemurica）物种中，壳体δ¹⁸O值随壳体增大呈现负向梯度变化，这表明其钙化温度存在个体发育的（ontogenetic）或生态型的（ecotypic）依赖性，该依赖性可能反映了受深度与光照调控的共生体（symbiont）光合活性（photosynthetic activity）差异。壳体δ¹³C与大小呈正相关的模式，使我们得以实现三方面研究突破：（1）识别出K/Pg事件发生后即刻出现的小型有孔虫壳体中的代谢性同位素不平衡δ¹³C效应；（2）将有孔虫光合共生（photosymbiosis）作用的演化时间限定在63.5 Ma，比此前推测的时间早约0.9 Myr；（3）识别出普氏虫属一个晚期分布的形态类群中，共生作用似乎出现了丢失。上述研究结果对于以适配于未来高分辨率K/Pg岩芯记录的分辨率来解读DIC梯度具有重要指导意义。