CYTOPLASMIC MASSES PRESERVED IN EARLY HOLOCENE DIATOMS: A POSSIBLE TAPHONOMIC PROCESS AND ITS PALEO-ECOLOGICAL IMPLICATIONS

In Lake Suigetsu, central Japan, greenish/light-brown granules identified as cytoplasmic masses had been preserved in siliceous cell walls of freshwater diatoms in annual layers of lacustrine muds since the early Holocene. The lacustrine muds consisted of alternating dark-colored (rich in diatom valves, clay, and organic matter) and light-colored (mainly diatom valves) laminae. The greenish/light-brown granules were predominately preserved in frustules of the genus Aulacoseira preserved in the dark-colored laminae. The dark-colored laminae were inferred to have formed annually under stratified water caused by surface water warming in summer that caused the formation of an organic-rich anoxic layer on the lake bottom that favored granule preservation. The good preservation of cytoplasmic masses in dark-colored laminae suggested a cause for diatom assemblage periodicity, a phenomenon that was commonly noted in temperate lakes: the cells containing these masses could be potential seed stocks for subsequent spring blooms. Frustules of the most abundant granule-containing species, Aulacoseira nipponica (Skvortzow) Tuji, in the dark-colored laminae of the Early Holocene muds were abundant in the overlying light-colored laminae, suggesting that these species reproduced abundantly in springtime yielding a massive diatom bloom.