Radiocarbon constraints on lake succession and sedimentation processes in a small mountainous lake in NE Taiwan

    Monsoon rainfall in East Asia is a subsystem of the global monsoon system and affects nearly one-quarter of the world's population. Taiwan, an island in the subtropical East Asian monsoon system, provides the unique opportunity to study monsoon and typhoon variability over the western subtropical Pacific on both historical and geological time scales. However, the Holocene paleoclimate records retrieved from various lakes in Taiwan have shown discrepancies in their recorded climate variability. This could partly be caused by the lack of modern sedimentological observations, limiting our understanding of the mechanisms controlling lake sedimentation and thus introducing uncertainty in the interpretation of lake records. To understand the paleoclimate records archived in lacustrine sediments, a robust age model is essential. The aim of this study is to understand the complex processes that led to the formation of a small wetland (24°30' N, 121°35' E, 2095 meters above sea level), which is located in the Taiping Mountains, Yilan County, NE Taiwan.

    In this study, we use several cores from different parts of the wetland, including wooded swamps, fens, and the open pond, to construct a robust model for lake formation and succession. However, cores from different parts of the wetland display puzzling differences in both maximum ages and sedimentation rates. Moreover, one core displays a major age reversal. Multiple radiocarbon dates corroborate that this reversal is not a sampling or analytical artifact but actually reflects an inversion of the sediment sequence. Possible hypotheses explaining this age reversal include typhoons, landslides, and overturned floating fens. Examples from other mountain lakes in Taiwan demonstrate that floating islands consisting of a mixture of aquatic plants, bryophytes, helophytes, and sediment can move hundreds of meters during typhoons. If a floating island is blown by typhoon winds, it may become turned upside down, resulting in an inverse age sequence.