Preservational modes in the Ediacaran Gaojiashan Lagerstätte: Pyritization, aluminosilicification, and carbonaceous compression

Abstract

Burgess Shale-type (BST) fossils often are preserved as two-dimensional carbonaceous compressions, sometimes aided by two mineralization processes: pyritization and aluminosilicification, defined by a thin and sometimes localized coating of authigenic pyrite or aluminosilicate minerals on the carbonaceous materials. Here we report similar mineralization modes within the late Ediacaran Gaojiashan Lagerstätte of southern Shaanxi Province, South China. Examination of two common Gaojiashan fossils, Conotubus hemiannulatus and Gaojiashania cyclus (hereafter referred by generic names only), indicates that pervasive pyritization is the primary taphonomic pathway in this deposit, responsible for the preservation of ~80% of the collected fossils. Pervasive pyritization of Gaojiashan fossils results in their three-dimensional preservation, as opposed to two-dimensional carbonaceous compression with thin pyrite coatings seen in some BST Lagerstätte. However, microscale investigation using environmental scanning electron microscopy (ESEM) and energy dispersive X-ray spectroscopy (EDS) shows that some flattened specimens of Conotubus and Gaojiashania are preserved as carbonaceous compressions, which are in some cases associated with aluminosilicate minerals. Rare compressed specimens of Conotubus are replicated by greenish clay minerals, which are interpreted as diagenetic products of aluminosilicate precursors. Our taphonomic analysis shows that pervasive authigenic pyritization tends to replicate more labile tissues than aluminosilicate-aided carbonaceous compression, but both pyritization and aluminosilicification processes can facilitate the replication and preservation of soft-bodied organisms in siliciclastic sediments. Building upon this conclusion, we propose that multiple taphonomic processes, including kerogenization (or polymerization of organic molecules), pyritization, and aluminosilicification, can contribute to the mode of BST preservation. Viewed in this light, various siliciclastic-hosted Lagerstätten that are facilitated by kerogenization, pyritization, and aluminosilicification can be plotted on a ternary diagram based on the relative importance of these taphonomic processes.