Neoichnology of black backed sea gulls trails in the intertidal flat at the Mont Saint Michael hypertidal system, France

Abstract

Neoichnology is a powerful tool in linking the geological record of bioturbation with modern examples produced by invertebrates and vertebrates. Nevertheless, not always the use of actualism is possible. It is clear the absence of modern analogues of certain extinct groups or behavioral convergence difficult the connection of producer in the past with neoichnological examples. The value and limitations of the neoichnology were analyzed in detail by Zonneveld (2016). Notably, other reasons preclude the study of the ichnofossils from present day records of bioturbation. For instance, some of them have extremely low preservation potential, with an ephemeral existence, mainly as a consequence of controls related to the environment where they are produced. In general, sedimentary environments with high frequency changes of physical parameters tend to preserve low ichnological record. However, it is important to distinguish if the bioturbation is produced on surfaces (epichnia) exposed to high environmental changes or into the bed (endichnia). In the first case, the preservation potential is scarce, while in the second one it is high. In this sense, tidal flats are an example where both situations occur, particularly in the case of hypertidal ones with ranges higher than 4m (Archer, 2013). Interestingly, sediment grain-size, climatic-zone location and fluvial discharges are also influential factors. The UNESCO heritage site of bay of Mount Saint Michael situated in Normandy, France, is one of the places studied by Archer (2013). The walk-out of a part of the bay in the summer of 2018 during the ebb tide, close to the spring part of the tidal cycle, allowed to observe the famous and incorrect named “quick sands”. In this site, abundant sedimentary structures and bioturbation in areas between tidal channels, have also been documented. The attention is here particularly focused in trails with a W-shape ending part and with a series of transversal arcuate ridges that present alternation in the grain-size noted by differences in the sediment color. Similar trails were reported from the Wadden Sea, one of the largest tidal-flats in the world, and others from China and New Zeland. The latter, was investigated by Gregory (1999) that assigned the trails to black backed sea gulls foot-paddling for feeding, as a peculiar case of “aviturbation”. This type of gulls is abundant in the Mount Saint Michael and is the most suitable producer of the trails. Nevertheless, some features observed in the trails differ from the studied previous case. For instance, they are recorded mostly in a rippled bed rather than a flat one, and evidence some records with almost not ridges, suggesting a dominant resting behavior. Regarding the orientation, they are parallel to the flood-ebb currents and not necessarily are related with the wind direction as was suggested. Moreover, the bioturbation was not observed at the moment of the generation but occurred just before exposure. The zigzagging or curved courses observed in a previous study are in favor of other control than wind and looks like related to the feeding activity.