Neoichnology of siliciclastic shallow-marine environments: Invertebrates, traces, and environmental conditions

Biogenic sedimentary structures, or traces, on the modern seafloor (neoichnology) record organism behaviors in response to physico-chemical conditions; consequently, they, act as analogues to similar structures preserved in the sedimentary record. Neoichnological insights are fundamental to the broad fields of paleobiology, paleontology, and sedimentology. This paper summarizes the current state of knowledge regarding animal-sediment interactions in siliciclastic shallow-marine environments with the aim of helping workers link their observations of biogenic structures to trace-producing organisms and to predict the environmental conditions under which those traces were produced. This knowledge base is useful for reconstructing paleo-ecosystems and evolutionary trends through Earth history, as well as for constructing integrated facies models that explain the nature and distribution of sedimentary strata. Annelids, bivalves, crustaceans, echinoderms, sponges, and sea anemones produce most marine bioturbation. These organisms respond in predictable ways to major physical and chemical stressors in their environment. Such stressors include sediment texture, substrate consistency, sedimentation rate, subaerial exposure, temperature, turbidity, oxygenation, and water salinity. Invertebrates prefer to burrow in sand and firm mud, and generally avoid water saturated mud and coarse-grained sediment. Loosegrounds and softgrounds display the most diverse trace assemblages, compared to stiff, firm, and hard substrates. Low sedimentation rates result in high bioturbation intensity and diverse trace assemblages. Increasing sedimentation rate decreases the intensity of bioturbation. Subaerial exposure shifts faunal populations towards trophic generalists who produce simple structures. Highly turbid water causes deposit-feeding behaviors to predominate. Water salinity controls endobenthos and their burrowing assemblages. Normal marine conditions have diverse ichnofauna, whereas brackish water settings display low diversity but sometimes high-density bioturbation. Hypersaline waters contain low diversity ichnofaunas. Low dissolved oxygen manifests in low abundances, diversities, and densities of trace assemblages. With increasing latitude, burrowing organisms shift to deeper water, burrowing by crustaceans decreases, polychaete-generated structures are more abundant, and the size of burrows increases. Despite the current knowledge base of invertebrate neoichnology in siliciclastic shallow marine environments, the link between physico-chemical environmental stressors and burrowing remain mostly qualitative.