Ina Neugebauer, Camille Thomas , Nicolas Waldmann, Cristina Recasens, Daniel Ariztegui 

Changing redox conditions in lakes are captured in their sediments, and are often influenced by climate. Their study therefore allows tracing past climate change on (sub-) annual to longer time-scales. In Lago Fagnano (54°S Argentina/Chile), an oligotrophic and deep-ventilated soft-water lake, cyclic alternations of light grey clay and dark green to black laminae are preserved throughout the Holocene sedimentary record. This study aims at clarifying the mechanism, frequency, and climatic forcing of laminae formation and preservation in Lago Fagnano, and their relation to changing redox conditions in the lake. Using high-resolution XRF scanning and mapping, thin section, XRD and SEM analyses of sediment cores, Fe- and Mn-oxides were identified as generating the lamination on (sub-) decadal time-scales. Black and greenish laminae are interpreted as buried palaeo-oxidation fronts that underwent early diagenetic processes. The burial of redox fronts in Lago Fagnano is most likely promoted by cyclic rapid increases of sedimentation due to higher runoff and mass-wasting events. Increased runoff is related to the strength of the Southern Hemisphere Westerlies that, in turn, is modified by climate oscillations. Therefore, it is suggested that the cyclic repetition of the buried palaeo-redox fronts, showing periodicities of ~52 and ~4.5 years in the western and eastern sub-basins of Lago Fagnano, respectively, is forced by climate modes. The most likely candidates are the Antarctic Oscillation (AO) and the El Niño Southern Oscillation (ENSO), both impacting southernmost South America and showing similar sub-decadal modes and multi-decadal variations. Although the burial of redox fronts in lake or marine sediments is well-known, this study is the first to present a record of cyclic, high-frequency recurrence of these fronts most likely triggered by changing climate.