Flood and earthquake records over the past 2000 years revealed by varved lake sediments from Lake Xinluhai, southeastern Tibetan Plateau

Abstract

The southeastern margin of the Tibetan Plateau experiences frequent natural disasters, yet long-term records are scarce for risk assessment. We established a 2000-year varve chronology from Lake Xinluhai using a 1150 cm sediment core, combining varve counting and AMS ¹⁴C dating. A multi-proxy analysis, including grain-size, total organic carbon (TOC), charcoal and pollen concentrations, and μ-XRF surface scanning of elemental composition in laminae and turbidite layers, revealed two distinct types of turbidites: (1) Flood-induced turbidites, which exhibit graded bedding with coarse sand at the base transitioning upward to fine silt, capped by a clay-rich layer. At the turbidite base, these layers show a sharp increase in D₅₀, D₉₀, TOC, charcoal, and pollen concentrations, along with the poorest sorting. Upward, these parameters decrease, while sorting improves, reflecting a systematic hydrodynamic sorting process. (2) Earthquake-triggered turbidite contains mixed bedrock fragments and clay, with erratic grain-sizes, poor sorting, and slightly elevated TOC, but without notable changes in charcoal or pollen assemblages relative to background sediments. Our findings indicate that ten turbidites were deposited by extreme rainfall-induced floods (recurrence interval: 73–279 years; last event: 1796–1806 CE), while one turbidite was triggered by a slope collapse associated with a Ms ≥ 6.0 earthquake on the Manigange segment of the Ganzi-Yushu Fault during 842–867 CE. Given that over 200 years have passed since the last flood, the probability of future flooding is high. This study provides a scientific basis for long-term natural hazard assessment in the southeastern Tibetan Plateau.