德国马克斯普朗克化学研究所Rao, Zixuan C.团队近日在研究晚白垩世鱼耳石结合有机质的氮同位素变化中取得新进展。这一研究成果发表在2024年7月29日出版的国际学术期刊《美国科学院院刊》上。

本文分析了来自美国东海岸的三个矿床中晚白垩世鱼耳石化石(如Eutawichthys maastrichtiensis，Eutawichthys zideki和Pterothrissthem sp.)耳石结合有机质的¹⁵N/¹⁴N比值(δ¹⁵N)，其中两个时代为Campanian期(83.6-77.9 Ma)，一个时代为Maastrichtian期(72.1-66 Ma)。δ¹⁵N和N含量对清洗程序和耳石形态特征的保存状态不敏感，各沉积类群间N含量差异一致，表明有机质来源于化石原生来源。从Campanian期到Maastrichtian期，3个物种耳石结合有机质δ¹⁵N均增加约4‰。就其原因而言，不同属的类似变化反驳了营养水平的变化，现代野外资料反驳了沉积位置的不同。

相反，Campanian期中较低δ¹⁵N被解释为区域或更大尺度的环境信号，可能是全球气候变暖的结果。在新生代温暖时期，有孔虫结合的δ¹5N也出现了类似的下降，这反映了在温暖条件下水柱反硝化作用的减少和海洋缺氧区(ODZs)的收缩。白垩纪耳石中相同的δ¹⁵N-气候相关性提出了，在过去约80万年中一致的ODZ-气候关系的前景，适用于白垩纪末大灭绝前后，并跨越了大陆构造的变化。

据介绍，保存在鱼耳石化石(耳石)中的有机物氮同位素是重建过去环境变化的一种很有前途的工具。

附：英文原文

Title: A nitrogen isotopic shift in fish otolith–bound organic matter during the Late Cretaceous

Author: Rao, Zixuan C., Lueders-Dumont, Jessica A., Stringer, Gary L., Ryu, Yeongjun, Zhao, Kewei, Myneni, Satish C., Oleynik, Sergey, Haug, Gerald H., Martinez-Garcia, Alfredo, Sigman, Daniel M.

Issue&Volume: 2024-7-29

Abstract: The nitrogen isotopes of the organic matter preserved in fossil fish otoliths (ear stones) are a promising tool for reconstructing past environmental changes. We analyzed the ¹⁵N/¹⁴N ratio (δ¹⁵N) of fossil otolith-bound organic matter in Late Cretaceous fish otoliths (of Eutawichthys maastrichtiensis, Eutawichthys zideki and Pterothrissus sp.) from three deposits along the US east coast, with two of Campanian (83.6 to 77.9 Ma) and one Maastrichtian (72.1 to 66 Ma) age. δ¹⁵N and N content were insensitive to cleaning protocol and the preservation state of otolith morphological features, and N content differences among taxa were consistent across deposits, pointing to a fossil-native origin for the organic matter. All three species showed an increase in otolith-bound organic matter δ¹⁵N of ~4‰ from Campanian to Maastrichtian. As to its cause, the similar change in distinct genera argues against changing trophic level, and modern field data argue against the different locations of the sedimentary deposits. Rather, the lower δ¹⁵N in the Campanian is best interpreted as an environmental signal at the regional scale or greater, and it may be a consequence of the warmer global climate. A similar decrease has been observed in foraminifera-bound δ¹⁵N during warm periods of the Cenozoic, reflecting decreased water column denitrification and thus contraction of the ocean’s oxygen deficient zones (ODZs) under warm conditions. The same δ¹⁵N-climate correlation in Cretaceous otoliths raises the prospect of an ODZ-to-climate relationship that has been consistent over the last ~80 My, applying before and after the end-Cretaceous mass extinction and spanning changes in continental configuration.

DOI: 10.1073/pnas.2322863121

Source: https://www.pnas.org/doi/abs/10.1073/pnas.2322863121