Characteristic of Biogenic Traces in the Modern Sediments of Intertidal Flat in Andong Area, Hangzhou Bay

Key words: 
• Hangzhou Bay /
• intertidal flat /
• sedimentary environment /
• modern biogenic traces

Abstract: Various trace-makers have been found in the soft ground of intertidal zone in Andong area, Hangzhou Bay. The trace-makers discovered in the study area include Bullacta exarata(Philippi), Cerithidea cingulata (Gmelin), Periophthalmus cantonensis(Osbeck), Ilyoplax ningpoensis, Nereis succinea, Solen strictus, Moerella iridescens, Sea anemone, as well as birds and plants. They can be classed as 8 genus, belonging to 5 categories including mollusca, arthropod, annelida, chordate, and Coelenterate. The traces made by these animals include repichnia(crawling, creeping, trails, trcaceways), domichnia, fodinichnia, augerisignia, cubichnia, digestisignia, fugichnia, avipedia and root traces. Based on the analysis of these traces, it is showed that their main types, spatial distribution, abundance and diversity and makers are different in various subenvironments of the intertidal zone. 1) The types and the distribution characteristics of trace-makers and their traces are obviously different in various subenvironments of the intertidal zone in the study area. In the upper tidal zone, the sediments mainly are sandy mud containing abundant organic material, the traces discovered in the area include trails, cubichnia and augerisignia made by Cerithidea cingulata (Gmelin), trails and cubichnia made by Bullacta exaratas(Philippi), fodinichnia, domichnia and trackways made by Ilyoplax ningpoensis, trails, fodinichnia, domichnia and digestisignia made by Nereis succinea,trails and domichnia made by Periophthalmus cantonensis(Osbeck),as well as avipidia made by birds and root traces. Among these traces, the trails and fodinichnia made by Cerithidea cingulata and Nereis succinea are the most abundant on the bedding plane. However, in the substrate, the most abundant traces are dwelling burrows made by Nereis succinea, and their distribution is also quite extensive, showing intensive disturbed structures. Meanwhile, the root traces (up to 50~60 cm in depth) and dwelling burrows made by Ilyoplax ningpoensis concentrate locally. These traces always coexist with parrallel ripple marks and interference ripple marks. In the middle tidal zone, the sediments are made up of gray silt or silt mixed with sand, with high water content and sparser vegetation. The higher abundance traces in the flat zone mainly consist the dwelling burrows made by Moerella iridescens and dwelling burrows and fodinichnia made by Solen strictus, containing a small amount trails made by Bullacta exarata (Philippi) as well as root traces and birds' traces. Along both sides of the tidal channels, the sediment is finer, trails made by Cerithidea cingulatas, domichnia and traceways made by Ilyoplax ningpoensis, and repichnia made by Periophthalmus cantonensis are commonly discovered. On the bedding plane of the middle tidal zone, linguoid ripple marks are very common, and biogenic traces always coexist with linguoid ripple marks. In the lower tidal zone, the sediments are made up of gray silt or silty sand, with high water content, most of them are soup ground and no vegetation in the area, the traces mainly are domichnia made by Moerella iridescens and Solen strictus, along with a few dwelling burrows made by sea anemone's. 2)The clearness of the traces was influenced by the grain size and the water content in the substrate sediments. 3)Because of the scouring of the tidal current and the wave, the epichnial on the bedding plane may be destroyed easily, the preservation rate of traces is extremely low, however, the domichnia made by the endogeny animals in the soft ground can be preserved easily. This study provides not only a lot of new information of modern biogenic traces in order to the sedimentary environment research in the intertidal area of Hanzhou Bay, but also use for reference to paleoichnology study in ancient similar intertidal flats.