THE INFLUENCE OF BIOEROSION ON TAPHONOMY - A CASE STUDY USING NAUTILUS MACROMPHALUS

Bioerosion, i.e., chemical and mechanical degradation of largely carbonate skeletal material, was studied in detail in shells of Nautilus for the first time. Nautilus typically lives along slopes of coral reefs in water depths between 300-400 m, with a range of 100-700 m and occurs in the Indo-Pacific Ocean exclusively. After death of the animal its conch will either float (up to hundreds of km until it sinks or is washed onto the beach) or it will rapidly sink to the seafloor close to where Nautilus has lived and died.

Two sets of N. macromphalus shells from New Caledonia are included in this study: 1) Backshore collected (from two beaches on Lifou (Loyalty Islands)) and 2) Deep-sea dredged (395 m: halfway from Grand Terre and the Isle of Pines; 589 m: western edge of the Lansdowne Bank west of Grand Terre). After preparing synthetic resin casts these were analyzed with the SEM and ichnotaxa were categorized semi-quantitatively in the plan view.

In total, 30 ichnotaxa were identified, some of them are new. Seven ichnotaxa were produced by cyanobacteria and five traces by chlorophytes. Further traces were produced by sponges, fungi, foraminifers, and by rhodophytes. Six traces were produced by some unknown heterotroph and two have an unknown affinity so far. The 23 ichnotaxa occurring in the backshore ichnocoenosis contain eleven traces that are produced by phototrophic and ten by heterotrophic organisms; two forms are of unknown origin. The 15 traces from the 395 m sample contain both, traces of photoautotroph producers and those of heterotrophs. All of the eight taxa recorded from 589 m water depth are of heterotrophic origin. Interpretation of the results is largely based on the ichnobathymetrical scheme combining the key ichnospecies and the mode of penetration: 1) Backshore: Shallow euphotic zone III to the deep euphotic zone; key ichnotaxa of the intertidal are present as well reflecting transport from nearshore to backshore. 2) 398 m: Transport must have taken place from the deep euphotic zone down into the aphotic zone. 3) 589 m: The shell had rapidly sunk to the seafloor after death of the specimen into aphotic conditions and had likely stayed close to the living site of the animal.