Molecular nitrogen (N₂) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN₂). However, fluctuations in pN₂ may have occurred on 10⁷–10⁹ year timescales in Earth's past, perhaps altering the isotopic composition of atmospheric nitrogen. Here, we explore an archive that may record the isotopic composition of atmospheric N₂ in deep time: the foliage of cycads. Cycads are ancient gymnosperms that host symbiotic N₂‐fixing cyanobacteria in modified root structures known as coralloid roots. All extant species of cycads are known to host symbionts, suggesting that this N₂‐fixing capacity is perhaps ancestral, reaching back to the early history of cycads in the late Paleozoic. Therefore, if the process of microbial N₂ fixation records the δ¹⁵N value of atmospheric N₂ in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ¹⁵N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N₂. We find that neither biological nor environmental factors significantly influence the δ¹⁵N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ¹⁵N of atmospheric N₂. Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N₂‐fixing symbiosis between cycads and cyanobacteria.

中文翻译：

---

探索苏铁科植物的叶子，作为大气氮同位素组成的档案。

分子氮（N ₂）构成了地球现代大气的大部分，贡献了约0.79 bar的分压（p N ₂）。但是，p N _2的波动可能发生 在地球过去的10 ⁷ –10 ⁹年时间尺度上，可能改变了大气氮的同位素组成。在这里，我们探索了一个档案，该档案可以记录深层大气N ₂的同位素组成：苏铁的叶子。苏铁是拥有共生N _2的古老裸子植物。在被称为珊瑚根的改良根结构中固定蓝藻。已知所有现存的苏铁种类都可以容纳共生体，这表明这种固定的N ₂可能是祖先的，可以追溯到古生代晚期苏铁的早期历史。因此，如果微生物氮过程_2个定影记录的δ ¹⁵的大气N N值₂在苏铁叶子，铁树的化石记录可提供大气δ的存档¹⁵的N值。为了探索这种潜在的代理，我们进行了在一系列现代环境中生长的野生苏铁的调查，以确定苏铁叶是否可靠地记录了大气N ₂的同位素组成。我们发现，无论是生物还是环境因素显著影响δ ^15个苏铁叶的N个值，这表明他们提供δ的合理稳健的记录¹⁵的大气N N ₂。该代理人在碳素苏铁化石记录中的应用不仅可以帮助抑制自古生代以来大气氮同位素比的变化，而且还可以揭示苏铁与蓝细菌之间固定N _2的共生的古代。