Effect of diet, anthropogenic activity, and climate on δ15N values of cave bat guano


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Few studies have attributed δ15N values of guano to a factor other than diet. A δ15N record obtained from a 1.5-m core of bat guano deposit from Zidită Cave (western Romania) provides a record of anthropogenic and climatic influence on the regional nitrogen pool. Nitrogen content is nearly constant (%N > 9) for over 1 m of the core, indicating limited diagenesis. The δ15N and δ13C time series are compared and the δ15N is also interpreted in light of previously published pollen assemblage from the same core. Using these comparisons the influence of anthropogenic activity and water availability is interpreted. Although some δ15N variation can be attributed to major changes in anthropogenic activities, additional variation implies an alternative control. The correlation between δ15N and δ13C values is significant (p < 0.01), but not strong, suggesting that both variables are influenced by water availability, known to be a primary control of δ13C values within C3 ecosystems. Drier periods indicated by higher δ13C values correspond with lower δ15N values and vice-versa for wetter conditions. The instrumental climate record (precipitation amount) for the past 50 years nearby the cave supports this relationship. From 1000 to 1800 CE, the δ15N values fluctuated between 11.5 and 14‰, then decreased in two abrupt steps, at 1800 and 1947 CE. The most significant decrease occurred after 1947 CE when values fell from 12.5 to below 7‰, suggesting the N-cycle transitioned towards a more conservative state. The correlations between δ15N and δ13C, and the instrumental precipitation record, along with the apparent negligible effect of diet on long term δ15N variation indicate that the δ15N values of guano can be used as an integrator of the local N-cycle.


Climatic changes, Romania, Guano, Caves

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Volume 461