Past hydroclimate and vegetation variation in Romania inferred from isotopic geochemistry and pollen of cave bat guano

While an abundance of paleo-records related to hydroclimate and vegetation exist in East-Central Europe, currently there is a scarcity of reconstructions that have the resolution to effectively capture the past 2000 years. A more complete understanding of this interval is important as it includes significant climatic events such as the Medieval Warm Period, Little Ice Age, and the post-industrial revolution human induced climate change. A solution to increasing our understanding of these events is the use of cave bat guano, a relatively underutilized source of climatic information. Cave bat guano piles commonly have near annual deposition and in Europe can often be found to accumulate for over 1000 years. Additionally, as bats are primarily ingesting herbivorous insects, geochemical and palynological records sourced from guano can be linked to the local environment that lies within proximity to the cave. Variation of nitrogen and carbon stable isotopes can potentially occur in response to fluctuations in certain climatic parameters, such as temperature and water availability. These signals are then transferred from plant foliage to insect to bat and ultimately preserved in guano. Multiple factors also allow for the deposition of pollen grains in guano piles. Insectivorous bats can ingest pollen grains attached to insects, wind dispersed pollen can latch on to foraging bat’s fur later dislodging onto the guano pile, or cave ventilation. Therefore, through isotopic and palynological analysis of bat guano high- resolution late-Holocene records of climate and vegetation can be produced. Guano cores were obtained from three caves along a north to southwest transect in Romania for analysis of carbon and nitrogen isotopes and pollen. In the Măgurici record (northwestern Romania) a 2‰ increase in δ13C, 1.5‰ decrease in δ15N, and the presence of thermophilous species between AD 881 and 1240 indicate a warm and dry Medieval Warm Period occurred in the northwestern region of the country. This event is followed by the Little Ice Age (~AD 1240 – 1850), wherein a decrease from 13.9 to 8.5‰ in δ15N values and a 2.3‰ increase in δ13C values of Topolnița guano (southern Romania) suggest a wetter summer and winter climate. It was also demonstrated that the North Atlantic Oscillation (NAO) is the primary control of winter precipitation in at each study location. In AD 1970 a 4‰ decrease in δ15N values was found to correspond with a notable transition to a positive phase of the NAO (drier conditions). Additional positive phases of the NAO were found to be recorded in the Măgurici δ15N series between AD 1820 and AD 1855. In contrast, a 2‰ rise in δ15N values from AD 1940 until AD 1970 occurs contemporaneously with an increased frequency of negative NAO phases (wetter conditions). Pollen extracted from aliquots of guano were investigated to reconstruct past environments within proximity of the caves. In doing so it was possible to discern whether fluctuations in primary forest taxa occurred in response to climatic and/or human impact. It was found that the development of patchy forests consisting of Tilia, Quercus, and Carpinus betulus following the Little Ice Age around Topolnița and Gura Ponicovei caves is likely associated with the lower water availability at this time. Additionally, the significant contribution of ruderal pollen indicators since AD 1985 is suggesting an increase of agricultural practices around Topolnița Cave