Analysis of stable isotopes 13C and 15N as a tool for environmental interpretation in Mesozoic floras

The study examined the occurrence of climatic and ecological stress during the time interval from the Late Triassic (Norian) to the Late Jurassic (Kimmeridgian) at selected localities in Poland. To determine changes in those circumstances, δ13C and δ15N were analyzed in fossil cuticles of leaves or leafy shoots of gymnosperms, mainly conifers. The material originated from eight localities: Upper Triassic (Norian) Patoka (Patokaea silesiaca Pacyna, Barbacka et Zdebska, 3 samples), Poręba 1 (conifer leaves of Brachyphyllum/Pagiophyllum type, 4 samples), Poręba 2 (conifer leafy shoots, 3 samples) and Marciszów-Zawiercie (conifer leaves of Brachyphyllum/Pagiophyllum type, 3 samples); Lower Jurassic (Hettangian) Sołtyków/Odrowąż (Hirmeriella muensteri (Schenk) Jung, 2 samples); Middle Jurassic (Bathonian) Zabierzów, Orlej (both Bilsdalea dura Harris, 3 and 4 samples respectively) and Grojec (Pachypteris major Raciborski, 4 samples); and Upper Jurassic (Kimmeridgian) Wólka Bałtowska (Pagiophyllum araucarinum (Pomel) Saporta, 1 sample). The number of samples used for analyses depended on availability of cuticles from the same species. Each sample weighed 4.7 mg.
The cuticles were untreated except for the sample from Wólka Bałtowska, treated with HCl to remove carbonate impurities from the sediment. Samples were placed in 1.5 mL Eppendorf tubes with tungsten carbide beads (3 mm diameter) and homogenized 2 × 1 min at 30 Hz in a Mixer Mill MM200 (Retsch, Germany); the tubes were shaken and inverted between runs. The analyses were done at the Stable Isotope Facility of the Department of Plant Sciences, University of California, Davis, California.
Changes in 13C values are usually interpreted as stress indicators. 13C reacts less with the Calvin cycle enzyme RuBisCO than 12C does, and modifies the pathway of photosynthetic CO2 fixation, engaging another enzyme, PEPC, to a greater extent. The stress may be caused by drought, lack or excess of light, salinity or air pollution (Farquhar et al., 1989). Higher values of 13C discrimination (more negative values) indicate a less stressful environment, and lower values (less negative) are connected with more stressful conditions.
The obtained δ13CVPDB values (Figure 1) differ between localities and between the Triassic and Jurassic, being generally less negative in the Norian. Within the Norian they are quite similar in material from two localities: Marciszów-Zawiercie and Poręba samples 1 and 2 (average −24.13 ‰, −24.64 ‰, −24.31 ‰, respectively). Those values differ significantly from Patoka (average −29.29 ‰), which is closer to values shown in the Hettangian (Sołtyków, average −30.76 ‰). Both of these higher discrimination values indicate the most stable conditions of all the studied localities; the lower values of Marciszów-Zawiercie and Poręba indicate the lowest environmental stability (stress).
The Bathonian values fluctuate around −26 ‰ (Zabierzów −26.16 ‰, Orlej −26.51 ‰, Grojec −26.71 ‰) and decrease slightly (more negative, higher discrimination) in the Kimmeridgian (−26.96 ‰), placing the stability of these localities between the aforementioned ones.
Content of δ15N in plant tissue is connected with nitrogen management and indicates access to nitrogen in the environment. Fluctuations in nitrogen access can be due to soil fertility and also to the level of competition among plants growing in a given habitat. Lower δ15N values indicate higher discrimination, meaning easier access to nitrogen, while higher values indicate lower discrimination and less accessible nitrogen in soil.
In our study the level of δ15NAir in samples from the Norian is quite uniform, around 5 ‰ (5.24 ‰ Poręba 1, 5.53 ‰ Poręba 2, 5.67 ‰ Patoka, 5.77 ‰ Marciszów-Zawiercie). In the Hettangian (Sołtyków) and in one Bathonian locality (Grojec) the values are low: 2.16 ‰ and 2.25 ‰, respectively. Interestingly, the other two Bathonian localities located close to each other, Zabierzów and Orlej show very different 15N values: 6.69 ‰ in Zabierzów and −5.79 ‰ in Orlej. A similar negative value came from the Kimmeridgian sample from Wólka Bałtowska (−5.09 ‰). The surprisingly high access to nitrogen in the two last localities may mean weak competition between plants at Wólka Bałtowska, having a rather poor flora characteristic for an island with dominance of a coniferalean of Brachyphyllum/Pagiophyllum type, a low number of species and a low number of fossils (Barbacka et al., 2025). At Orlej, the subsoil layer, which is weathered volcanic rock, could have had an impact on the available elements. The substrate type differed from the neighboring sites at Grojec and Zabierzów; hence the expected differences in nitrogen availability.
Acknowledgements: The study was financed by funds from the National Science Centre, Poland (2022/45/B/NZ8/02000) and by statutory funds of the W. Szafer Institute of Botany, Polish Academy of Sciences.