Sediment Mixing Depths and Rates in Continental Environments and the Creation of Macrochannels and Macropores: Lessons Learned and Implications for Alerting Porosity and Permeability by Bioturbation
An abundance of research has been conducted on sediment mixing rates and patterns, mostly with marine macrobenthic invertebrate biota (less so with marine vertebrates). These research results have been applied to bioturbation patterns of aquatic and terrestrial organisms in continental environments without adequate testing. Field and laboratory research on continental organisms, however, demonstrates that sediment mixing rates and patterns of these organisms exhibit a wider range of outcomes over vastly different physicochemical conditions that are controlled by the environment factors unique to the continental realm. Freshwater aquatic and terrestrial biota exhibit local and non-local sediment mixing patterns, with those exhibited by terrestrial organisms occurring over several of orders of magnitude greater than freshwater and marine organisms. For example, organisms (annelids, mollusks, and crustaceans) living in rivers and lakes can bioturbate commonly to depths of 10-20 cm; marine organisms (annelids, vermiform animals, mollusks, and crustaceans) to depths of 1-4 m; and terrestrial organisms (plant roots, crustaceans, insects) to depths of 9-100 m. Consequently, the volume of sediment modified by these organisms also increases by several orders of magnitude in the respective environments. Spatial and temporal patterns of the physicochemical conditions that govern bioturbation patterns in continental and marine aquatic environments are not parallel. Such conditions appear to be more variable in lakes based on terrigenous input, seasonal variations in temperature, and water column mixing patterns that control bottom water oxygenation. The vast majority of sediment mixing in terrestrial environments occurs as part of soil formation within the vadose zone (pore space dominated by air) above the phreatic zone (100% water-filled pore space), both of which are part of the groundwater profile. Under these conditions, biota move sediment by local and non-local patterns, with many organisms using specific behaviors and bioturbation patterns at all stages of their life cycles. Macrochannels and macropores that result from the collection of bioturbation patterns are produced in gravitational dewatered and subaerially compacted sediments that also experience abiotic pedoturbation (e.g., wetting-drying of soils). Thus, marine sediment mixing models depicting sediment movement, irrigation, and biogeochemical cycling are inappropriate to for terrestrial bioturbation.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California