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.
The majority of Japanese depositional coasts are dominated by wave and wave-induced sedimentary processes. Long-term sedimentation in the wave-dominated coasts associated with sandy and gravelly beaches results in development of beach ridges and coastal dunes. While beach ridges and dunes are expected as valuable sedimentary archives of palaeoenvironment, they have not attracted much attention in Japanese coasts for decades. Recent development of the ground-penetrating radar (GPR) and optically-stimulated luminescence (OSL) dating enabled the continuous survey of subsurface structures and quantitative dating of sandy and gravelly deposits ; and if these techniques are applied, Japanese coasts can be a tremendous repository of palaeoenvironmental records. This paper reviews geographic features of beach ridges and dunes in Japanese coasts, and their possibilities as records of past shorelines, sea level, winter monsoon, and coastal hazards.
The abundances of 58 elements in volcanic glass shards from tephra samples of the Shobudani Group along the Kinokawa River, Japan, were analyzed using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The age of the Bodaitouge tephra near the Negoro Fault was estimated by fission-track (FT) dating, combined with U-Pb dating by LA-ICP-MS. The element patterns of the Gojo 4 and Shobudani 1 tephras are generally consistent with those of the Shishimuta-Pink and Ebisutouge-Fukuda tephras, respectively. Those of the Bodaitouge and Shin’ike tephras are similar to that of the Aira-Tn (AT) tephra. However, the age of the Bodaitouge tephra sample estimated by FT and U-Pb dating is 1.6-1.3Ma, which is much older than the age of AT. The stratigraphic sequence of the Shin’ike tephra also suggests that it should be older than AT. However, in addition to the element abundances, incompletely hydrated volcanic glasses and the refractive index of coexisting ferrosilite suggest that the studied Shin’ike tephra sample is AT itself. The Shin’ike tephra may have been derived from a younger layer unconformably covering the Shobudani Group. The element pattern of the Nagayagawa tephra is similar to that of the Kikai-Akahoya rather than AT.
The emergence of grinding stones with grooves is one of the hallmarks indicating the beginning of bow-and-arrow hunting. In the Japanese islands, grinding stones with grooves appeared from the Incipient Jomon period, which coincides with the timing when arrowheads drastically increased. Although the grinding stone with grooves has been considered to be used as an arrowshaft smoother, its real function is little known so far. The present study conducted a morphometric analysis of a grinding stone with grooves from the Torihama shell mound in Fukui Prefecture, central Japan, in order to better understand the function. To confirm the chronological position of the grinding stone with grooves, we undertook radiocarbon dating of two walnuts (Juglans ailantifolia) recovered from the layer from which the grinding stone with grooves was unearthed. The radiocarbon determinations show 10,160±35BP and 10,070±30BP that are calibrated to 12,030-11,650calBP and 11,770-11,400calBP respectively using IntCal13 calibration curve and OxCal platform. The age range suggests that the grinding stone with grooves from the Torihama shell mound dates from the Pleistocene-Holocene transition. The morphometric analysis of the 3D-data on a groove that most clearly formed on this grinding stone indicates that the groove runs straight and its surface shape shows a small deviation in morphometric values. The high straightness and significantly high morphometric standardization of the groove suggest that the groove was formed due to smoothing of a rod-like material, such as an arrowshaft.