A 121-Ka Record of Western Andean Fluvial Response Tosuborbital Climate Cycles Recorded by Rhythmic Grain Sizevariations of the Lima Fluvial Fan

Abstract

[Abstract] A complete, fluvial stratigraphic record for the last glacial period of the WesternAndes in Peru is not available due to preservation issues and spatial variability in sed-imentation. Deposits are typically restricted to incomplete records of fluvial terracesor localised occurrences of alluvial fans and landslides. These landforms are thoughtto have formed under a regime of climate cyclicity controlling increases in precipita-tion. Because of the fragmented preservation of these deposits, as well as datinguncertainties, it remains unclear if orbital climate cycles, such as the precession cycle,or suborbital cycles, such as the wet Heinrich events, are driving Andean sedimenta-tion. In this paper, we try to answer this question through a sedimentological–stratigraphical analysis of a much more complete sedimentary sequence than usuallyfound in the region. We present the results of a grain size analysis of 5000 clasts and13 new luminescence ages of a 52-m-long, stratigraphic section of the Lima fluvialfan in Peru. Bayesian age–depth modelling resulted in a robust chronostratigraphicframework and derived sedimentation rates. The stratigraphic record registered sedi-mentation from 121.7 ± 4 to 6:3þ1:5 1:6 ka. Three major sedimentation periods occurredbetween 121.7 to 110þ4 5 , 87 ± 1 to 67þ2 3 , and 31þ4 3 to 6:3þ1:5 1:6 ka. These periods regis-tered various unconformities and coarsening–fining upward sequences which chro-nologically correlate to suborbital pluvial periods, recognised from speleothems andlake records, that drove fluvial deposition. They also correlate with the timing ofother recognised sedimentation events throughout the Western Andes. Marineregression resulted in fan progradation and not in incision. The Lima fan stratigraphyrepresents therefore the most complete, last glacial fluvial record for the PeruvianWestern Andes to date and it highlights the potential of fluvial fans as recorders ofsuborbital climate variability.