4 - Bioturbation

Summary

Truth is the daughter of time.

Aulus Gellius (ad 130–175)

Introduction

As discussed in Chapter 3, bioturbation – the mixing of sediment by organisms – has a tremendous impact on porewater chemistry and preservation. Moreover, any signal that is incorporated into the stratigraphic record, whether it be seasonal shell inputs or instantaneous volcanic ash layers, must pass through the low-pass filter of bioturbation, in which high-frequency events are damped or removed and lower-frequency events preserved. Only in rare cases do organisms impede bioturbation, such as the extensive root systems of marsh plants (e.g., Nydick et al., 1995), but even in this instance, plant roots no doubt pump oxygen into the subsurface and alter porewater chemistry.

On one hand, bioturbation may be viewed favorably because it erases high-frequency “noise” and leaves behind evidence of longer-term patterns and the processes that generated them. On the other, bioturbation is a major impediment to bridging the gap between ecological (short-term) and geological or evolutionary (long-term) processes. Although methods have recently been developed for the classification and semi-quantitative estimation of the extent of bioturbation in both cross-section and on bedding planes (e.g., Droser and Bottjer, 1986; Miller and Smail, 1997; see also Bertness and Miller, 1984) and in estimating the population sizes of bioturbators of ancient sediments (Kowalewki and Demko, 1997), most earth scientists have avoided study of this process because unraveling its effects is quite complex mathematically and because it involves the integration of diverse disciplines.