--> The Neoichnological Basis for a Brackish-Water Ichnofacies

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The Neoichnological Basis for a Brackish-Water Ichnofacies

Abstract

By the 1950’s, Dolf Seilacher had clearly demonstrated that the proposed ichnofacies represented behavioral responses of animals to physico-chemically stable sedimentary environments. Since then, several ichnofacies have been proposed for marine settings and freshwater settings. A brackish-water (BW) ichnofacies has not been formally proposed.

Animals living in BW settings must contend with several environmental stresses, including fluctuating and lowered salinities, heterolithic substrates, inconstant current velocities, variable wave influence, variable sedimentation rates and generally abundant availability of food resources. Burrow assemblages in BW settings are expressed as low-diversity suites of small trace fossils, ichnogenera that are common to marine settings, the presence of horizontal and vertical structures of deposit-feeding generalists, and irregularly distributed bioturbation, locally with high levels of bioturbation.

Neoichnological studies in BW settings show that diminution and lowered animal and trace diversity are the result of metabolic duress and reduced survivorship, both related to low or fluctuating salinity. The composition of the burrow assemblage is dictated by the presence of abundant food resources at the sediment-water interface and in the sediment. This abundance of resources results in an “anything goes” strategy that has been incorrectly called the “mixed Cruziana-Skolithos ichnofacies”. The selection of specific behaviors into BW settings favors strategies that ensure rapid access to resources. Simple deposit-feeding strategies (e.g., Planolites, Teichichnus) exemplify the importance of rapidly ingesting food as opposed to meticulously mining it. In BW settings, vertical burrows (e.g. Skolithos, Cylindrichnus) are more often the result of infauna deposit-feeding at the sediment-water interface and not filter feeding. The resulting competition for rapid-resource exploitation leads to a range of burrows produced by ethological generalists. Although the influence of currents and variable sedimentation rates may influence the appearance of BW trace fossil assemblages, the limitations imposed by salinity and resource abundance dictate that the resulting burrows will be small, the suite diversities low, and comprising burrows that represent the efforts of ethological generalists. Clearly an ethological basis for a BW ichnofacies exists and the frontier of brackish-water ichnology lies in establishing a BW ichnofacies.