A radiocarbon dated bat guano deposit from N.W. Romania: Implications for the timing of the Little Ice Age and Medieval Climate Anomaly Vanessa E. Johnston a, ⁎, Frank McDermott a , Tudor Tămaş b,c a UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland b Faculty of Biology and Geology, Department of Geology, “Babeş-Bolyai” University, Cluj-Napoca, Romania c “Emil Racoviţă” Institute of Speleology, Cluj-Napoca, Romania abstract article info Article history: Received 15 October 2009 Received in revised form 20 February 2010 Accepted 24 February 2010 Available online 2 March 2010 Keywords: Bat guano Cosmogenic isotope Radiocarbon Little Ice Age Cl-36 Bomb pulse 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 36 Cl, 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 14 C and 36 Cl production associated with nuclear weapons testing indicates downward migration of 36 Cl, probably reflecting post-depositional migration within the guano deposit. Elevated 36 Cl/Cl at the top of the core in comparison with modern atmospheric values may indicate recycling of bomb 36 Cl 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 36 Cl (a solar proxy), because of post-depositional mobility. However, separation of organically bound chloride, or the use of an alternative cosmogenic isotope 10 Be, in bat guano, may offer an unexploited solar proxy that contains contemporaneous environmental signals, such as stable isotopes (e.g. δ 13 C) and pollen, in association with radiocarbon dating. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The under-exploited archive of bat guano has recently been shown to provide high resolution palaeoclimate data (e.g. Leroy and Simms, 2006; Bird et al., 2007; Wurster et al., 2008). In an attempt to understand solar forcing of the climate, we test, here for the first time, the suitability of bat guano as an archive of atmospheric 36 Cl. In principle, this solar irradiance proxy (Bard et al., 2000; Beer et al., 2002; Muscheler et al., 2007) could be used in conjunction with stable isotope data from the guano to circumvent correlation uncertainties associated with deriving solar and climate proxy records from dif- ferent archives. 36 Cl is strongly hydrophilic and is therefore removed rapidly from the atmosphere by wet precipitation. This behaviour contrasts with 14 C which becomes incorporated in biogeochemical cycles. 36 Cl is thought to enter bat guano through nutrient and water intake by bats, similar to the mechanism proposed previously for packrat middens (Plummer et al., 1997). Insectivorous bat guano is predominantly composed from insect remains, bat hair and mucus (Maher, 2006). Its 36 Cl/Cl ratio is therefore expected to reflect the local contemporaneous atmospheric composition. Here, a stratigraphic sequence of bat guano, extracted from Măgurici Cave, N.W. Romania, is measured for 14 C and 36 Cl/Cl isotopes to assess the possibility of utilising this under-exploited archive as a palaeocli- mate and solar irradiance proxy. In Adam Cave, S. Carpathian Mountains, Romania, a 250 cm thick bat guano deposit had been dated previously using radiocarbon to 7600±80 14 C yrs BP (Carbonnel et al., 1999); however the 271 cm thick deposit in Măgurici Cave was undated prior to this study. This study presents the first known measurements of 36 Cl/Cl on bat guano, and hence the chemical extraction processes and sampling protocols are also evaluated. The chronology of guano deposits may be used to infer the presence of bats in a cave which in turn can have implications for habitat and environmental changes Palaeogeography, Palaeoclimatology, Palaeoecology 291 (2010) 217–227 ⁎ Corresponding author. Tel.: +353 1 7162138; fax: +353 1 2837733. E-mail address: vanessa.johnston@ucd.ie (V.E. Johnston). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.02.031 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo