ENGINEERING BENTHIC BIOGEOCHEMISTRY: REACTIVE-TRANSPORT MODELING OF BIOMIXING AND BIOIRRIGATION BEHAVIORS ACROSS THE EDIACARAN-CAMBRIAN BOUNDARY

The evolution of bioturbation and the subsequent Cambrian Substrate Revolution marks the shift from microbial mat-dominated, reduced seafloor sediments during the Neoproterozoic to bioturbated, oxidized sediments during the Paleozoic. It is hypothesized that increased biomixing (solid-phase sediment mixing) deepened the sedimentary mixed layer and homogenized sediments, while increased bioirrigation (flushing burrows with overlying water) resulted in the oxidation and removal of reduced species in the sediments. Increased bioturbation during the Cambrian Substrate Revolution has been suggested as the mechanism for major changes in biogeochemical cycles, such as significant increases in oceanic sulfate concentrations. However, these effects have not yet been attributed to specifically biomixing or bioirrigation behaviors. Moreover, changes in the relative proportion of biomixing and bioirrigation behaviors during the Ediacaran-Cambrian transition and their impacts on sediment biogeochemistry are unresolved. Here, we integrate data from the trace fossil record with reactive-transport modeling to investigate the effects of early bioturbation on sediment biogeochemistry around the Ediacaran-Cambrian boundary. Trace fossil slabs were collected from the Nama Group, Namibia, and the White-Inyo Mountains, California and were point-counted for all trace fossils as a proxy for biomixing intensity. Individual ichnotaxa were characterized in terms of bioirrigation potential. These data were incorporated into a reactive-transport model to quantify the sedimentary biogeochemical effects of various trace fossil assemblages. Our results show that trace fossil assemblages with predominantly biomixing behaviors, such as those found in the latest Ediacaran, may have reduced the oxygen penetration depth in the sediment by supplying more organic matter and other reactive compounds used in microbial respiration, resulting in increased oxygen consumption. In contrast, bio-irrigators supply more oxygen to deeper sediment layers, leading to more oxidizing conditions. These results are critical for future work on how the different early bioturbation behaviors may have altered sediment biogeochemistry to influence benthic ecosystem habitability during the Ediacaran-Cambrian transition.