Sediments and sedimentary processes in Mutsu Bay, Japan: Pelletization as the most important mode in depositing argillaceous sediments

Mutsu Bay is a large transgressive inland basin that originated in a set of structural depressions brought about by tectonic processes in early Quaternary time. The offshore beds of the bay consist of muds rich in organic material, while sands and gravels occur as a coastal sediment facies. Active sediment palletization of muds owing to feeding by polychaetes is recognized throughout the bay, and the bed surface is mostly covered with fecal pellets. Laboratory experiments in a seawater flume suggest that isolated fecal pellets are removed more easily than ambient muds. Continued pelletization dramatically reduces the critical entrainment velocity of the bed surface. Sedimentary processes in Mutsu Bay have been influenced not only by sea-level fluctuations but also by the oceanographic conditions of the Sea of Japan. Offshore mud deposition in the bay can be attributed to warm-water circulation which came into existence about 10,000 years ago. A wet and mild climate in consequence of temperatere seawater of post-glacial warming increased biochemical weathering of the land surface, leading to a voluminous discharge of fine-grained materials into the bay. Sediment pelletization by polychaetes facilitated easy transport of argillaceous grains in response to bottom currents, resulting in the development of offshore mud layers.