A radiocarbon dated bat guano deposit from N.W. Romania: Implications for the timing of the Little Ice Age and Medieval Climate Anomaly
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There is considerable interest in the potential of bat guano as an alternative record of palaeoclimate in regions that are devoid of more commonly utilised archives. In this study, designed originally to evaluate the potential of cave hosted bat guano to preserve temporal variations in the flux of cosmogenic 36Cl, it was found that the guano depositional history is strongly linked to climatic conditions. Radiocarbon measurements on a 2.7 metre long core of bat guano from Măgurici Cave, N.W. Romania indicate a maximum depositional age of 1195 AD for the base of the core. Deposition of the lowermost portion of the accumulation occurred during the Medieval Climate Anomaly. The cave roost was subsequently devoid of bats during a regional cold phase linked to the Little Ice Age, with bats returning when local temperatures increased. The rate of guano accumulation then appears to increase in tandem with anthropogenic warming. This indicates that bat occupation at this roost site in Măgurici Cave is strongly linked to regional climate variability, with habitation during warm periods, possibly associated with the abundance of insects upon which the bats feed. Comparison of large peaks in anthropogenic 14C and 36Cl production associated with nuclear weapons testing indicates downward migration of 36Cl, probably reflecting post-depositional migration within the guano deposit. Elevated 36Cl/Cl at the top of the core in comparison with modern atmospheric values may indicate recycling of bomb 36Cl in vegetation. Therefore, we show that while bat guano contains abundant atmospherically-derived chloride it has severe limitations as a potential archive of atmospherically-derived 36Cl (a solar proxy), because of post-depositional mobility. However, separation of organically bound chloride, or the use of an alternative cosmogenic isotope 10Be, in bat guano, may offer an unexploited solar proxy that contains contemporaneous environmental signals, such as stable isotopes (e.g. δ13C) and pollen, in association with radiocarbon dating.