Dating an Amazonic ice core:disentangling a complex chemical record

Ice cores are one of the best palaeoarchives for the most recent geological record. Their resolution is unmatched as it is possible to retrieve seasonal information of thousands of years of climatic archive. Therefore, dating is fundamental to interpreting these archives. Polar and temperate ice cores are well studied and have provided valuable records for paleoclimate interpretation. However, tropical ice cores remain under- studied because of many technical difficulties inherent to it, even though they have precious information on tropical climate dynamics. One of the biggest challenges is dating tropical ice cores. The relationship between ice depth and age is rarely straightforward and typically requires a multi-proxy approach - specially in tropical records, as they are not submitted to the typical polar and high-latitude climatic dynamic, due to its complex ice flow patterns, post-depositional processes like melting, and high background noise for chemical markers. Here we present results of a 128.3 m long ice core, collected from the Quelccaya Ice Cap, Peru (at 13°55’46,099”S, 70°49’21,557”W, 5.674 m above the sea level) during the austral winter of 2022. . In this study, we used refractory black carbon (rBC), ion concentration depth profiles and a series of frequency analysis to perform annual layer counting (manual and automated) based on seasonal variations. We try to assign to the dating reference horizons using volcanic signatures from historically known events and the El Niño Southern Oscillation (ENSO) index as tie points. The very low mobility of black carbon in ice and snowpack causes it to remain effectively locked in place after deposition, thereby creating a clear and consistent seasonal archive in the ice core data, with pronounced seasonality marked by peaks during the dry season (June – August). Ionic signal is less seasonal and presents intense remobilization indicating that the ice pack is rapidly losing part of its climatic signal that is so important for the understanding of tropical paleoclimate dynamics.
Keywords: ice core, Amazon, black carbon, paleoclimate