Differentiating taphonomic from evolutionary effects in early Palaeozoic carbonate systems

The Cambrian-Ordovician was a unique interval in terms of feedback between biological processes, early diagenesis, substrate composition and biodiversity (Wright & Cherns, 2016). As the depth and extent of bioturbation increased through this interval major changes were induced in the nature of early diagenetic processes, including carbonate dissolution and cementation within the shallow taphonomically active zone (TAZ). These changes influenced sediment composition, the nature of sea floor substrates for sclerobionts (encrusters, borers) and even impacted upon the preservation of soft bodied organisms. Two related hypotheses are proposed for testing:

Firstly, did the shallow TAZ which developed in the Cambrian–mid Ordovician produce a different taphonomic filter for skeletal groups compared with that which characterised the late Ordovician and younger seas? The taphonomic filter shown to have operated from the late Ordovician selectively removed labile aragonitic skeletal remains in low energy settings, hugely skewing the fossil record (Cherns & Wright 2009). Its impact in the earliest Palaeozoic has yet to be evaluated, but there is some evidence that even the more stable calcitic skeletal material may have been removed from Cambrian low energy settings.

What role does taphonomy play in the Great Ordovician Biodiversification Event (GOBE)? The second hypothesis to be tested is that changes recorded in the grain composition of early Palaeozoic limestones may largely reflect changes in taphonomic processes and not simply indicate changes in biodiversity. After the first appearance of metazoan skeletons there was a lag of >40 m.y. before skeletal debris came to dominate carbonate sediments. It is likely that the decline in peloidal and intraclastic sediments during the mid Ordovician reflected the deepening of the carbonate cementation zone as the TAZ deepened, such that these cemented zones were less frequently reworked. Could the relative increase in skeletal remains in later Early Palaeozoic limestones also be a taphonomic effect caused by reduced dissolution of calcitic skeletal grains in a thickened TAZ?

This taphonomic and petrological study will extend earlier studies of the effects on biodiversity of early diagenetic skeletal dissolution in the TAZ (e.g. Cherns & Wright 2009). Field studies on Cambro-Ordovician successions will be complemented by comparison of seafloor heterogeneity in modern carbonate settings.