16:30 〜 16:45
[SEM13-10] Environmental magnetic study of a Holocene sediment core from the Yeongsan Estuary, Southwest Korea
キーワード：完新世堆積物、磁気特性、エスチュアリー、古環境、続成作用
The sensitivity of magnetic properties to environmental processes may provide useful information on paleoenvironmental changes in estuarine environments. Magnetic property studies of estuaries are less common than other environments. Owing to the west coast of South Korea having an abundance of estuaries, it can provide a good place to study these processes in estuarine environments. We analyzed a variety of magnetic properties based on magnetic susceptibility, hysteresis parameters, progressive acquisition of isothermal remanent magnetization (IRM), and first-order reversal curve data, in addition to electron microscopic observations of magnetic minerals, from a Holocene muddy sediment core recovered from the Yeongsan Estuary on the west coast of South Korea. The age of the studied sediment core was well constrained based on radiocarbon dates. We examined diagenetic effects on magnetic properties and tested their availability as proxies of paleoenvironmental change. The presence of generally low magnetic susceptibilities, ubiquitous authigenic magnetic components such as pyrite and greigite suggested that the analyzed sediments had undergone considerable early diagenetic alteration. Our results confirm that the use of initial bulk susceptibility as a stand-alone environmental change proxy is not recommended unless it is supported by additional magnetic analyses. We recognized the existence of ferromagnetic-based variabilities related to something besides the adverse diagenetic effects, and examined possible relationships with sea-level and some of major climate changes during the Holocene. The most remarkable finding of this study is the two distinct intervals with high values in magnetic coercivity (Bc), coercivity of remanence (Bcr), and ratio of remanent saturation moment to saturation moment (Mrs/Ms) that were well coincident with the respective abrupt decelerations in the rate of sea-level rise occurred at around 8.2 and 7 thousand years ago. Furthermore, it is inferred that such condition with abrupt drop in sea-level rise rate would be favorable for the abrupt modification of grain size distribution of ferromagnetic minerals toward more single-domain-like content.