Avetoichnus Uchman & Rattazzi, 2011

Uchman, Alfred & Wetzel, Andreas, 2024, Sequestrichnia - an ethological category of marine trace fossils recording the collection and stowage of nutritional material within burrows, Comptes Rendus Palevol 23 (22), pp. 325-338 : 330-331

Ichnogenus Avetoichnus Uchman & Rattazzi, 2011

( Fig. 4)

Avetoichnus (Paleocene-Oligocene) represents a mid-tier burrow consisting of a mostly horizontal to subhorizontal helix enveloping a central tube that was probably an open tunnel when the tracemaker lived. The tube contains grey mud, whereas the spiral is filled with black, probably organic-rich mud that likely originated from the overlying pelagic sediment ( Uchman & Rattazzi 2011). The tube resembles a pinched open burrow that truncates the inner edges of the helical turns, which were already filled with dark mud when the tunnel was still inhabited. Chemichnia like Chondrites co-occur with Avetoichnus ( Fig. 4).

The colour of the helical fill suggests that it was displaced downward. Furthermore, the partial truncation of the inner turns of the spiral points to later reworking by the tracemaker. Very likely, the organic material in the spiral fostered the development of reducing conditions therein whereas the central tunnel was open and oxygenated water could be circulated through it by the inhabitant. Thus, a steep geochemical gradient developed across the tunnel margin between anoxic host sediment and oxic water in the lumen, a situation known to enhance microbial activity (e.g. Meysman et al. 2010). Thus, the trace-fossil producer sequestering organic-rich material on the seafloor appears to have constructed an organic-rich subsurface domain that rapidly became anoxic. Therefore, this trace is classified as a sequestrichnion.

OTHER POSSIBLE SEQUESTRICHNIA

The burial of labile organic matter in anoxic sediments deep below the surface mixed layer strongly suggests priming induced by the trace producers, probably wormlike animals such as echiurans, sipunculids, and polychaetes together with crustaceans. Besides utilizing the sequestered burrow fill directly as indicated by subsequent reworking, some trace-fossil producers burrowed down and construct a cache underneath a domain, which is already enriched in organic matter like seagrass or wood (e.g. Griffis & Suchanek 1991; Bojanowski & Wetzel 2024). From this perspective, some other trace fossils could also be sequestrichnia.

At least some specimens of the spreite burrow Teichichnus zigzag Frey & Bromley, 1985 occurring in Jurassic deposits record sequestrichnial behaviour (e.g. Wetzel et al. 2023). Teichichnus zigzag exhibits: 1) a spreite enriched in organic-rich material relative to the host sediment; 2) no reworking halo around the spreite (thus indicating that the organic matter was introduced from above); 3) lamellae produced later that systematically crosscut previously produced ones, and hence indicate reworking by the producer. It can thus be excluded that reworking represents a response to erosion of the sediment surface, because reworking of the spreite is locally restricted. Furthermore, in calm lagoonal bayfill deposits, T. zigzag is common ( Knaust 2018). In these settings, hyperpycnal flows may provide organic matter. Therefore, a sequestrichnial behavior of the T. zigzag tracemakers in calm lagoonal settings is highly likely though it has not yet been demonstrated.

Other spreite burrows, for instance Rhizocorallium commune Schmid, 1876 , are extensively filled with pellets. They were produced in dysoxic, low-energy mud ( Knaust 2013), and could represent sequestrichnia. Unfortunately, TOC data of the spreite fill and host sediment are not available and, therefore, clear evidence of sequestrichnial behaviour is lacking although it is not unlikely.

Halimedides Lorenz von Liburnau, 1902 , Cambrian (Series 2) to Recent in age, is a system of horizontal tunnels with chambers strung along it, not rarely in the context of poorly oxygenated deposits ( Fernández-Martínez et al. 2021). When preserved in full relief, the chambers are filled with darker material than in the surrounding rock (e.g. Uchman 1999). The chambers, which have been interpreted as food caches ( Gaillard & Olivero 2009; Lukeneder et al. 2012), could be good places for microbial processing of organic matter. Lepidenteron mantelli ( Geinitz, 1850) is a burrow in Upper Cretaceous marls of the deeper continental shelf. It is filled with sediment rich in plant material, which is strongly pyritized, foremost with pyrite framboids ( Jurkowska et al. 2018). The plant material was actively collected, was a substrate for microbial activity and probably promoted priming. Therefore, this trace fossil could be partly sequestrichnial and partly chemichnial. Gyrophyllites Glocker, 1841 , Cambrian(?) and Early Ordovician to Cenozoic in age, is composed of a vertical to subvertical shaft with lateral, radial, petal-like lobes that may be repeatedly distributed at a few levels. The lobes may be filled with organic-rich material from the underlying or overlying bed, and their position at different levels was modulated by the migration of the redox boundary ( Strzeboński & Uchman 2015; Muñoz et al. 2019). The fill of the lateral lobes could promote microbial processing including priming. However, in the case of Gyrophyllites , the mud appears to have been taken not only from the seafloor but also from of a bed beneath it, in cases where the proper mud was not present on the seafloor, in particular if a decelerating, low-erosive gravity flow rapidly deposited sand that covered and preserved the previous seafloor including the topmost organic-rich interval. Some Planolites Nicholson, 1873 from Cretaceous marls was evidently filled actively with darker sediment from above ( Locklair & Savrda 1998), possibly to induce priming. As Planolites can be produced by diverse organisms in a wide range of environments, only certain representatives of this ichnogenus might be sequestrichnia. Also, Alcyonidiopsis Massalongo, 1856 , a simple, cylindrical burrow filled with pellets, commonly darker than the surrounding deposits, and known at least since the Ordovician (see Uchman 1999) can belong to sequestrichnia.