Understanding controls on stanols in lake sediments as proxies for palaeopopulations in Mesoamerica

Faecal stanols in lake sediments have been used as a proxy for human populations in the past in a variety of contexts, with the assumption that variability in faecal stanol concentration or ratios is a reliable proxy for relative catchment-scale human populations. Despite that, the specific controls on faecal stanol concentrations and ratios in lake sediments remain poorly understood. In this study we analyse faecal stanol concentrations in lake surface sediments across Guatemala and the Yucatán Peninsula of Mexico in order to constrain geographical and biogeochemical variables controlling stanol concentrations and ratios in lake sediments in this region. We propose and test the hypothesis that the stanol ratios coprostanol:(coprostanol + stigmastanol) and coprostanol:(coprostanol + cholestanol) scale according to the proximity to and size of nearby population centres. The key controls on stanol concentrations that we identify are the proximity to human population centres and the human population within 5 km of the sampling point. Based on a transect across Lake Petén Itzá, the ratio coprostanol:(coprostanol + cholestanol) does not appear to be an accurate proxy for proximity to human population centres, nor does it correlate with catchment human population. We suggest that normalising stanol concentrations to TOC is an appropriate way to take into account the effects of mineral dilution as well as the potential effects of organic matter deposition and preservation, and that the ratio coprostanol:(coprostanol + stigmastanol) does not scale with human population but may be an effective approach to determine the relative contribution of coprostanol-producing mammals and herbivores. Further, we discuss the current limitations of the proxy as well as its future directions, including the implications of our results for sediment core siting, the use of stanol ratios in palaeolimnology, as well as the storage, transport, and diagenesis of stanols.