Comparison of 14C and 210Pb - 137Cs - 241Am dating methods of a recent bat guano deposit (Lot, SW France)

Abstract

Bat guano deposits are increasingly used as records of past environmental changes, an approach that requires a precise chronology of the guano layers. This paper presents a comparison between the well-established 14 C dating method and methods based on natural 210 Pb excesses, ( 210 Pb) ex , and artificial radionuclides 137 Cs and 241 Am. The studied example is a bat guano deposit from a cave in SW France (the Brantites III cave), which is currently investigated for paleo-environmental reconstructions using stable isotopes. 14 C data show that the upper part of the guano deposit accumulated during the last 150 years with a marked increase in accumulation rates after around 1960 AD. While the incorporation of atmospheric 14 C in guano is a well-understood process, the origin of 210 Pb excesses is more complex. Based on consideration of 137 Cs and ( 210 Pb) ex inventories recorded in undisturbed soils in France, and the measured inventories in the guano deposit, we suggest that most of the 210 Pb excess is produced by 222 Rn decay in the cave air and then adsorbed onto the guano. As Radon concentrations in caves can vary significantly on both short and long-term timescales, one needs to be cautious before applying the often-used CRS (constant rate of supply of 210 Pb excess) model to guano dating. Our ( 210 Pb) ex data are best interpreted by two successive periods of roughly constant, but widely different accumulation rates (0.3 cm/y and 2.6 cm/y before and after 1960, respectively) and ( 210 Pb) ex fluxes. We suggest that these relatively abrupt variations result from a change in cave ventilation leading to a more favourable shelter for bats after 1960. The upper 40 cm of the deposit shows evidence of 210 Pb mobility, adding a further complexity to the interpretation of ( 210 Pb) ex profiles in guano deposits. However, the existence of well-defined 137 Cs and 241 Am peaks allows a precise identification of the year of maximum atmospheric fallouts (∼1963–1964). When the ages provided by artificial radionuclides are combined with the 210 Pb-derived accumulation rates, an age model can be built, which is in good agreement with the 14 C age model. This example shows that the ( 210 Pb) ex method, when associated with 137 Cs ( 241 Am) data, can be used to date recent guano deposit, although its application is not as straightforward as the 14 C method.