Deposition of the 2011 Cordón Caulle Tephra (Chile, 40ºS) in Lake Sediments: Implications for Tephrochronology and Volcanology

Abstract:

Tephras preserved in lake sediments are commonly used to synchronize sedimentary archives of climate and environmental change, and to correlate them with terrestrial environments. They also provide an opportunity to reconstruct explosive volcanic activity, e.g., eruption frequency and tephra dispersal. Although it is known that sedimentary processes may affect the record of tephras in lakes, lake sediments are generally considered as one of the best archives of tephra stratigraphy. The June 2011 eruption of the Cordón Caulle (CC, Chile, 40ºS) offered an ideal opportunity to study the processes affecting tephra deposition in lakes. During the 2011 CC eruption, the prevailing westerly winds transported the erupted pyroclastic material away from Puyehue Lake. Yet, the tephra was identified within the lake with a thickness ranging from 1 cm to >10 cm. This is in contrast with smaller lakes, where tephra thickness was in agreement with ash fall distribution maps. Geomorphological observations and sedimentological analyses provide evidence that the tephra deposited in Puyehue Lake entirely consists of material reworked from the upper watershed, transported by rivers, and distributed by lake currents according to particle size and density. Proximal basins mostly received coarse tephra particles via underflows, while in distal basins the deposits consisted of low amounts of fine-grained particles transported by interflows. Pumice transported by overflows reached most lake basins. These results have important implications for tephrochronology and volcanology. First, they show that lakes do not act as passive tephra traps, and they illustrate how two nearby lakes can contain very distinct tephra records, simply due to the size and orientation of their drainage basins. Second, they show that lakes with large watersheds, such as Puyehue, record many more eruptions than smaller lakes, which only register direct ash falls, leading to very different conclusions regarding the recurrence of volcanic eruptions. Finally, our data show that using lakes with large watersheds for isopach mapping systematically leads to an overestimation of the area affected by ash deposition, and of the volume of erupted tephra. Smaller lakes with limited drainage basins are therefore better suited for volcanological studies.