Heavy metal-based bat guano chronologies: A cautionary tale

Bat guano has gained recognition as a reliable paleoenvironmental archive in recent decades, but to reconstruct paleoenvironmental conditions using bat guano, an age depth model is required. For post-bomb, modern guano deposits, radiocarbon dating is the most acceptable method for determining chronologies but is costly and not always straightforward. Thus, a reliable and inexpensive alternative is needed. Heavy metal concentrations are an untested means of dating post-bomb guano deposits, despite their widespread use in dating other geologic deposits. Here, we explore the possibility of using peaks of heavy metal concentrations of lead (Pb) and molybdenum (Mo) to identify singular dates within a bat guano core collected from Cripps Mill Cave in Tennessee, USA. Based on published environmental studies, we identified four potential chronostratigraphic markers: (1) one peak in Pb concentration potentially indicative of 1974 CE, synchronous with peak leaded gasoline combustion in the United States and (2) three peaks in Mo concentrations in the Cripps Mill Cave guano core potentially corresponding to published ice core Mo concentration peaks dating to 1975, 1980, and 1989 CE. To test the reliability of these heavy metal-based dates, we undertook radiocarbon sampling of the core. The independent radiocarbon dating shows that the metal-based age-depth model is not reliable. Our approach shows that peaks of Pb, Mo, and potentially many other heavy metal peaks are not an effective method for dating modern post-bomb guano deposits and are likely to reflect metal mobilization within the guano core. This study serves as a cautionary tale for misattributing dating significance to peaks of metal concentrations in guano.