Possible climatic controls on the accumulation of Peru's most prominent alluvial fan: The Lima Conglomerate

Litty, Camille; Schlunegger, Fritz; Akçar, Naki; Lanari, Pierre; Christl, Marcus; Vockenhuber, Christof (2019). Possible climatic controls on the accumulation of Peru's most prominent alluvial fan: The Lima Conglomerate. Earth surface processes and landforms, 44(5), pp. 991-1003. Wiley 10.1002/esp.4548

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Sediment accumulation and the formation of terrace levels can occur in response to a change in either a tectonic or climatic driving force. This is particular the case for mountain ranges adjacent to plate boundaries such as the Peruvian Andes, where tectonics and an active seismicity have been shown to drive surface mass fluxes but where modern erosion has also been related to precipitation rates and patterns thereof. Here, we explore these controls on the deposition of the Lima Conglomerate, situated in Lima, Peru, on the western Andean margin adjacent to the Pacific coast, and we relate these results to the broader context of controls on erosion in the Andes. We use a combination of quantitative methods to explore the age of sediment accumulation, the provenance of the material and the paleo-erosion rates recorded by these deposits. Isochron burial dating with cosmogenic 10Be and 26Al yield an age of c. 500 ka for the base (490 ± 70 ka) and the uppermost sample situated c. 30 m higher upsection (490 ± 80 ka). Results of paleo-erosion rate estimates with concentrations of in-situ 10Be, which were measured on sand embedded within the conglomerates, show a c. 60% increase from 105 ± 10 mm ka-1 for a sample of the lowermost level, to 169 ± 14 mm ka-1 for a sample of the highermost level. Finally, provenance tracing with in-situ U/Pb ages on detrital zircon imply that the material has been derived from the entire drainage basin including the low-relief region in the headwaters. The combination of these results suggests that sediment accumulation occurred in response to an erosional pulse, which affected the entire basin within a short time interval. Because 10Be data represents a large spatial record of erosion, we exclude the possibility where a breakout of a lake, dammed by e.g. a large landslide, was the cause for the material pulse. For the same reasons, we exclude the possibility of a focused release of material in response to earthquakes to be responsible for the large material fluxes. Instead, the inferred erosional pulse was likely to have occurred at the scale of nearly the entire basin, supporting the idea of a larger-scale, most likely climate driven control. In this context, the accumulation age of c. 500 ka falls into an orbital cycle (eccentricity), thus fostering the emerging picture in the literature that erosion and sediment routing in the Andes has most likely been driven by climate and cyclic changes thereof. We thus suggest that the Andean mountain range offers an ideal laboratory to explore the erosional history in relation to climate patterns, at least in Peru.

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