Lake morphometry controls the remobilization and long-term geochemical imprint of distal tephra deposition

摘要

Deposition of distal tephra-fall deposits in lake systems with low watershed sediment fluxes can represent large geochemical anomalies in sediment elemental composition. We examine the persistence of compositional effects of fallout from the 7.63 kyr BP Mt. Mazama caldera-forming event and other tephra deposits in the micro-laminated abyssal sediments of Pettit Lake, central Idaho, which received 50 kg m(-2) of Mazama tephra-fall material. Persistent post-depositional dominance of the Mazama material is reflected in elemental composition and grain size distribution and is related to lake hypsometry and the presence of a large in-lake sediment reservoir subject to remobilization. Decoupling of recycled clastic mineral material with Mazama compositional affinity from both biogenic matter and from clastic material derived from within the watershed is shown by coherence between dated primary tephras and a radiocarbon chronology.