Ichnoliths as results of authigenesis associated with aquatic animal traces

Maciej Jakub Bojanowski, Andreas Wetzel

Animal-made bioturbational structures modify physio-chemical conditions and biota on the sediment surface and below. The behavior of the trace makers chiefly causes such changes by sediment irrigation, mucus lining, organic matter storage, microbial gardening etc. Such traces are preferential loci for accumulation of organic material that may foster syn- to post-bioturbational mineral authigenesis. Similar processes associated with plant roots have already been described and termed. In this study, accordingly, an analogous terminology is proposed for authigenic mineralization of and around animal traces in aquatic environments for which the general, subsuming term “ichnolith” (equivalent to “rhizolith”) is suggested. Two types of ichnoliths are recognizable so far. (1) “Ichnopetrifications” are mineralized animal traces preserving their original morphology. (2) “Ichnocretions” are concretions, which preferentially developed around animal traces. These two categories supposedly do not cover all phenomena and thus, additional ones could be introduced in the future. Moreover, new terms “ichnostrome” and “ichnosax” are introduced for beds and irregular rock masses, respectively, which formed through early-diagenetic cementation along dense animal traces.
Ichnoliths form by cementation mostly close to the sediment surface. They are compaction-resistant and become not deformed or destructed by mechanical and chemical processes. Consequently, they may preserve primary sedimentary and bioturbational structures in detail. Thus, ichnoliths potentially store environmental data enabling more reliable and detailed reconstruction of depositional settings and ecological conditions than the surrounding sediment. In particular, carbonate cements are important archives of biogeochemical reactions and composition of fluids circulating shallow in sediment during or shortly after bioturbation.