Exploring cycad foliage as an archive of the isotopic composition of atmospheric nitrogen

Molecular nitrogen (N-2) constitutes the majority of Earth's modern atmosphere, contributing similar to 0.79 bar of partial pressure (pN(2)). However, fluctuations in pN(2) may have occurred on 10(7)-10(9) 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 N-2 in deep time: the foliage of cycads. Cycads are ancient gymnosperms that host symbiotic N-2-fixing cyanobacteria in modified root structures known as coralloid roots. All extant species of cycads are known to host symbionts, suggesting that this N-2-fixing capacity is perhaps ancestral, reaching back to the early history of cycads in the late Paleozoic. Therefore, if the process of microbial N-2 fixation records the delta N-15 value of atmospheric N-2 in cycad foliage, the fossil record of cycads may provide an archive of atmospheric delta N-15 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 N-2. We find that neither biological nor environmental factors significantly influence the delta N-15 values of cycad foliage, suggesting that they provide a reasonably robust record of the delta N-15 of atmospheric N-2. 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 N-2-fixing symbiosis between cycads and cyanobacteria.