Blooms of aberrant planktic foraminifera across the K/Pg boundary in the Western Tethys: causes and evolutionary implications

Here we report a detailed study of the different categories and types of abnormal morphologies in planktic foraminifera recognizable in the lowermost Danian mainly from the El Kef and Aïn Settara sections, Tunisia. Various types of abnormalities in the test morphology were identified, including protuberances near the proloculus, abnormal chambers, double or twinned ultimate chambers, multiple ultimate chambers, abnormal apertures, distortion in test coiling, morphologically abnormal tests, attached twins or double tests, and general monstrosities. Detailed biostratigraphic and quantitative studies of the Tunisian sections documented a major proliferation of aberrant planktic foraminifera (between approximately 5 and 18% in relative abundance) during the first 200Kyr of the Danian, starting immediately after the Cretaceous/Paleogene (K/Pg) boundary mass extinction (spanning from the Guembelitria cretacea Zone to the lower part of the P. pseudobulloides Zone). This contrasts with the proportionately low frequency of aberrant tests (generally < 2%) identified within the uppermost Maastrichtian, suggesting more stable environmental conditions during the last ~50-100Kyr of the Cretaceous. Two main pulses with abundant aberrant tests were recognized in the earliest Danian, the one recorded in the well-known K/Pg boundary clay being the more intense of those (maxima of >18%). These main pulses of aberrants coincide approximately with relevant quantitative and evolutionary turnovers in the planktic foraminiferal assemblages. In this paper we explore the relation of these high values of the foraminiferal abnormality index with the environmental changes induced by the meteorite impact of Chicxulub in Yucatan, Mexico, and the massive eruptions of the Deccan Traps, India.

本研究针对突尼斯El Kef与Aïn Settara剖面丹尼阶最底部地层中可识别的浮游有孔虫（planktic foraminifera）异常形态的不同类别与类型展开详尽探究。本次研究识别出壳体（test）形态的多种异常类型，包括初房附近突起、畸形房室、双终室/孪生终室、多终室、畸形壳口、壳体旋向扭曲、形态异常壳体、附着孪生壳体或双壳体，以及一般性畸形个体。对突尼斯剖面开展的详细生物地层学与定量研究表明，在白垩纪-古近纪（K/Pg）界线集群灭绝事件之后即刻起始的丹尼阶最初20万年期间，异常浮游有孔虫的相对丰度显著升高（占比约5%至18%），其分布层位覆盖Guembelitria cretacea带至P. pseudobulloides带下部。这一现象与马斯特里赫特阶最顶部地层中异常壳体极低的占比（通常<2%）形成显著反差，暗示白垩纪最后约5至10万年的环境条件更为稳定。丹尼阶最早期可识别出两期异常壳体富集的主要脉冲事件，其中在知名K/Pg界线黏土层中记录的峰值强度更高（相对丰度最大值超过18%）。这些异常个体的主要富集峰值大致与浮游有孔虫组合的定量与演化更替事件相契合。本文探讨了有孔虫异常指数的高值与墨西哥尤卡坦半岛希克苏鲁伯（Chicxulub）陨石撞击以及印度德干暗色岩（Deccan Traps）大规模火山活动所引发的环境变化之间的关联。