BIOGENIC SEDIMENT MIXING: BRIDGING THE GAP BETWEEN THE MODERN AND THE ANCIENT

Abstract Biogenic sediment mixing is a key process in modern environments, which has played a major role in triggering macroevolutionary breakthroughs, including those that took place during the Ediacaran–Cambrian transition. In the modern, several procedures are used to quantify key metrics such as the thickness of the mixed layer, the maximum depth of sediment mixing, and the intensity of biogenic sediment reworking. Although assessing the extent and role of biogenic sediment mixing in the fossil record has been informed by our knowledge of bioturbation in modern oceans, extrapolating concepts and quantifying proxies is problematic. Complications arise from a series of conceptual barriers, which are sociological, epistemological, and ontological in nature. Sociological barriers reside in the fact that separate scientific communities deal with bioturbation in the modern and fossil record. These obstacles can be effectively removed through increased collaboration among marine benthic ecologists and ichnologists, which will result in enhanced cross-fertilization between fields. Epistemological barriers involve inconsistencies in terminology and conceptual frameworks, such as divergent interpretations of the term “mixed layer”. These can be mitigated through the standardization of definitions, clarification of misconceptions, and adoption of unified methodological protocols. Ontological barriers, however, are the most difficult to overcome stemming from the fundamental differences between the nature of the modern and the fossil record, which impact on their corresponding datasets. This is illustrated by the need to adjust functional modes employed for modern bioturbators for the analysis of the fossil record and integration with paleobiology, and by the difficulties in quantifying biogenic sediment reworking in the fossil record. There are also notable differences in the availability of environmental and ecological correlations most closely associated with bioturbation, which lead to differences in the ability to determine and compare the relative importance of influential parameters reflected in the fossil record versus the modern. Hence, ancient bioturbated intervals do not fully represent a snapshot of the modern, precluding quantification of some parameters, most notably the thickness of the mixed layer. These limitations underscore the importance of contextualizing bioturbation within its sedimentological framework to better understand the unique nature/characteristics of trace fossil assemblages. Ultimately, a nuanced evaluation of the interplay of bioturbation and sedimentation is essential for advancing interpretations of paleoenvironmental and evolutionary dynamics and increase awareness of the idiosyncratic aspects of the trace-fossil record.