沿岸域堆積物の過去数百～数千年間を対象としたパレォ研究 一豊後水道•別府湾を例として

摘要

沿岸域の堆積物を用いた過去数百～数千年という比較的新しい時代を対象としたパレォ研究 は，環境や生態系の長期動態とその現状を理解するという点で，今後も第四紀学の発展に重要な 役割を果たすことが期待される.ここでは，これまで筆者と共同研究者が行ってきた豊後水道に おける海洋の温暖化や.豊後水道•別府湾における沿岸域海洋生態系の十年規模変動．イワシ類 個体数の長期変動の3つのトピックを紹介し，環境や海洋生態系の長期変動に対する我々の理解 が沿岸域のパレォ研究によってどこまで進んだか，どのようなパレ才研究が今後求められるかに ついて述べる.%To help further understand the long-term dynamics of environments and ecosystems and those modern situations which have been identified by observations, paleo studies on the last centuries to millennia using coastal sediments play important roles. Here I introduce three coastal paleo studies in the Seto Inland Sea and the Bungo Channel, Japan, which I and collaborators have performed so far ： ocean warming trend in the Bungo Channel; decadal-scale marine ecosystem variability in the Bungo Channel and the Seto Inland Sea; and long-term dynamics of Japanese sardine and anchovy populations. From the studies on benthic foraminiferal Mg/ Ca-based bottom water temperature reconstruction in the eastern Bungo Channel over the last 200 years, we found an onset of the ocean warming trend in the early 20th century and the highest temperature stage seen in the late 20th century over the last 200 years in the bottom water. Fossil diatom and pigment records from the Bungo Channel and Beppu Bay showed decadal-scale variations in primary productivity, which are likely due to their responses to variations in nutrient supply from shelf slope regions off the Bungo Channel associated with. Pacific decadal climate variability. This phenomenon is potentially linked to the recent oligotrophication in the Seto Inland Sea. On the basis of fish scale records reconstructed from Beppu Bay sediments, we found that multidecadal alternations between Japanese sardine and anchovy as seen in the catch records during the 20th century were unstable for the last 2800 years. Paleo records rather show centennial and millennial scale variations in fish populations, indicating that there may be a wide variety of sardine and anchovy population dynamics in response to centennial to millenial climate modes before the observational era. Therefore, the paradigm for Pacific climate/ marine ecosystem decadal variability, such as regime shifts based on modern observations, may be transient These paleo studies on the past centuries and millennia using Japanese coastal sediments have improved our understanding of long-term dynamics of climate and marine ecosystems and these modern situations. For projections and management of marine environments and ecosystems, there still remain many things for coastal paleo researchers on the past centuries and millennia to do.