Paleoenvironmental changes of the Common Era in core sediments of Nakaumi Lagoon, Southwest Japan: Correlation with solar activity

In the Common Era coastal lagoon environments change through anthropogenic modification of landforms and the landscape evolution that occurs with eustatic sea level change. The recent lake water environment in Nakaumi Lagoon, Southwest Japan, has undergone large changes due to both of these factors. In this study, the paleoenvironment of the Common Era is examined using high-resolution sedimentologic and geochemical analysis of three cores collected from Nakaumi Lagoon. This lagoon is located on the Sea of Japan side of southwest Japan, and is a brackish-water lake of ~86 square kilometers. The three cores are 3.5~4m in length, and consist of mud with shell fragment at most horizons. Samples were collected at 1cm intervals from these cores, and grain size analysis and C, N, S element analyses were performed. In the N1 core, total organic carbon (TOC) content has a peak (ca 3.5%) at 380cm, and declines step-wise higher in the core. TOC content is lowest (ca 1.5%) at 30~40cm. TOC content increases above this horizon. Similar changes of TOC content occur in the other cores. Total Sulfur (TS) content shows a very similar pattern of change to TOC content. The horizon of the TOC peak at 380cm (BC200) in the N1 core indicates the most reduced environment based on the TS content. This suggests the final closure of the lagoonal area (Nakaumi Lagoon) formed by the progradation of the Yumigahama sand bar. After that, TOC content decreases as a new water circulation system became established in the lagoon and water depth shallowed through aggradation and increasing sedimentation rate. Mean grain size fluctuates in the range of 6.5~8.0 phi, and is around 7.5 phi in most samples. Mean grain size in the N2 core tends to coarsen above 70cm. This anomaly in mean grain size in the N2 core probably reflects the modification of the Iinashi River channel in AD1665. An additional excursion in mean grain size above 40cm may indicate a later anthropogenic channel diversion of the Iinashi River in AD1840. The pattern of variation in mean grain size in the N1 core is similar to variation in atmospheric radiocarbon 14C (Delta 14C). This pattern may therefore reflect climate change driven at least in part by solar activity. Coarser grain size in Nakaumi Lagoon sediments reflects flood events that bring a greater sediment load into the lake. Increased flood event frequency in the temperate zone is associated with warmer climates.