Phytoplankton assemblages in shallow marine environments are being impacted by anthropogenic climate change, but long-term outcomes of these changes are uncertain. Investigation of past neritic calcareous nannoplankton can help us understand the fate these ecosystems face. In this study, a Canonical Correspondence Analysis (CCA) of calcareous nannofossils and X-ray fluorescence geochemistry was used to determine how past planktonic ecosystems were influenced by paleoenvironmental parameters on the eastern side of the Late Cretaceous Western Interior Seaway in the Cenomanian (ca. 95–93 Ma). Samples were collected every 10 cm from the Graneros Shale Greenhorn Formation at an outcrop in northwestern Iowa to determine high resolution changes in assemblages and paleoenvironments. Nannofossil diversity outside of a few small intervals ranges is high (generally 30–60 species) with abundant small Biscutum constans, confirming other publications that show elevated diversity in Cretaceous nearshore settings. The CCA results imply assemblages were most influenced by terrigenous influence, wet vs. dry climate, and changes in water mass source. Cretaceous nannofossil paleoecology was also revised based on the CCA results. Size differentiation of nannofossil taxa may highlight more complexity in environmental preferences that have been largely overlooked. After the initial transgression of the Greenhorn Sea into the region, the climate became wetter and terrigenous influence in the area was high. The peak of terrigenous influence corresponds with elevated nannofossil diversity but a lack of microfauna, which may indicate a similar oceanographic setting to the modern Gulf of Mexico nearshore dead zone. As sea level continued to rise above this point, nannoplankton assemblages indicate a potential shift to a higher productivity, stratified water column. As the muddy Graneros Shale transitioned to a further offshore chalky Greenhorn Formation, a normal marine, cosmopolitan nannofloral assemblage became established. Nannofossil and geochemical evidence indicates high productivity from upwelling might be related to the change of deposition to chalk in the Greenhorn Formation. While only a single outcrop was investigated, the novel use of an integrated micropaleontological and geochemical analysis has shed light on the dynamics of how phytoplankton ecosystems were established and modified in shallow marine environments of the Cretaceous and could have important implications on modern shallow marine settings.

浅海环境中的浮游植物群落正在受到人为气候变化的影响，但这些变化的长期结果尚不确定。对过去浅海钙质超微浮游生物的调查可以帮助我们了解这些生态系统面临的命运。在这项研究中，利用钙质超微化石和 X 射线荧光地球化学的典型对应分析 (CCA) 来确定塞诺曼阶 (约100 年前) 晚白垩世西部内海航道东侧过去的浮游生态系统如何受到古环境参数的影响。 95–93 Ma）。从爱荷华州西北部露头的 Graneros 页岩 Greenhorn 地层中每隔 10 厘米采集一次样本，以确定组合的高分辨率变化和古环境。少数小区间范围之外的超微化石多样性很高（通常为 30-60 种），具有丰富的小型Biscutum constans ，这证实了其他出版物显示白垩纪近岸环境中多样性较高。CCA 结果表明组合受陆地影响、潮湿与干燥气候以及水源变化的影响最大。白垩纪超微化石古生态学并根据 CCA 结果进行了修订。超微化石类群的大小差异可能突显出在很大程度上被忽视的环境偏好的复杂性。新手海最初侵入该地区后，气候变得更加湿润，该地区的陆源影响也很大。陆源影响的峰值对应于超微化石多样性的增加，但微型动物群的缺乏，这可能表明与现代墨西哥湾近岸死区类似的海洋学环境。随着海平面继续上升到这一点以上，超小型浮游生物组合表明可能会转向生产力更高的分层水柱。随着泥泞的 Graneros 页岩过渡到更远的近海白垩质 Greenhorn 地层，一个正常的海洋、世界性的纳米花卉组合开始建立。超微化石和地球化学证据表明，上升流产生的高生产力可能与 Greenhorn 地层中沉积物向白垩的变化有关。虽然只研究了一个露头，但综合微古生物学和地球化学分析的新用途揭示了白垩纪浅海环境中浮游植物生态系统如何建立和改变的动态，并可能对现代浅海环境产生重要影响。