Fluvial fans as stratigraphic recorders of suborbital climate cyclicity: an example from the Lima fluvial fan in Peru

Fluvial fans, as opposed to alluvial fans, have only been recognised over the past two decades as important end members in the classification of alluvial-fluvial landforms. There is still a lack of knowledge regarding the factors driving their formation, their geomorphology, stratigraphic build-up and significance as quantitative recorders of terrestrial (sub)orbital climate change. Here, we present a stratigraphical and grain-size analysis of ~5,000 gravel clasts of the Lima fluvial fan in Peru. An age-depth, and derived sedimentation rate, model was constructed by means of Monte Carlo Markov Chain iterations of thirteen, new luminescence ages of the fluvial fan. Our data showed near-continuous sedimentation from 121.7 ± 4 ka at the base of the exposed stratigraphic section at 10 m above sea level (asl) until 6.3 ± 1.6 ka at the top (62.5 m asl). Stratigraphical unconformities, both erosional and non-erosional, systematically coincided with the initiation of coarsening upward sequences as shown by increases in the D₅₀ grain size. The unconformities and coarsening-upward sequences, in turn, coincided with both orbital and suborbital pluvial periods, as known from Peruvian lake and speleothem records. The precession cycle was the primary driver for increased precipitation, runoff and modelled sedimentation rates during the lower half of the last glacial period, coinciding with the Ouki and Salinas pluvial periods. Imprinted on the precession cycle, one-to-five-ka long suborbital pluvial periods constituted a secondary driver, coinciding with the stratigraphical unconformities and increases in D₅₀ grain size. Throughout the Salinas wet phase, the amplitude of the precession cycle diminished and, at the end of it, the precession cycle ceased altogether to be a driver for fluvial sedimentation. From 50 ka onwards, stratigraphical unconformities, coarsening upward cycles and an increase in sedimentation rate systematically coincided with the onset of the Atlantic Heinrich events, which have been recognised as three-to-five-ka long pluvial events in the aforementioned lake and speleothem records. Most fluvial sedimentation events have been recognised in fragmentary records of other fluvial systems in Peru, but the Lima fan constitutes the most complete fluvial record to date. As such, the Lima stratigraphical record shows the value of fluvial fans as quantitative recorders of fluvial landscape change due to last glacial climate cyclicity.