EXPLORING PATHWAYS OF SOFT-TISSUE PRESERVATION USING GEOCHEMICAL MODELING

Exceptional preservation of soft-bodied organisms is a prevalent feature of the Neoproterozoic–lower Paleozoic fossil record and has played a critical role in shaping study of early multicellular and animal life. Fossils in strata of this age are exceptionally preserved in a variety of modes linked to authigenic minerals formed during early diagenesis, including pyrite, silica, phosphates, and clays. Recent results from geochemical analyses of fossils as well as taphonomy experiments have prompted new questions regarding these modes of fossilization and their paleoenvironmental and paleobiological implications, including the processes responsible for preservation of individual taxa in disparate preservational modes. Moreover, how seafloor diagenetic factors intersect at the molecular level to shape distinct modes of fossilization remains, in many cases, poorly quantified. Answering these questions will allow us to better understand the impact of particular geochemical conditions on soft-tissue preservation and more robustly recognize associated biases in the fossil record. A more quantitative framework for the processes regulating authigenic mineralization pathways will also facilitate access to fossil-hosted paleoenvironmental data, and refine our search image for future fossil discoveries.

To address these questions, we have developed a new one-dimensional reactive-transport model simulating the early diagenetic evolution of chemical species in response to carcass burial in marine sediments. Using this model, we explore the biogeochemical conditions under which different authigenic minerals form. Our model includes reaction-based, empirically grounded representation of carbon, iron, sulfur, phosphorus, and silica cycling. Importantly, our diagenetic model also includes precipitation of iron- and magnesium-rich clays that may have been particularly common phases in marine sediments underlying Neoproterozoic–early Paleozoic ferruginous and silica-rich bottom-waters. Clay minerals are rarely included in diagenetic models; our model allows for new insights into how clay authigenesis is coupled to major marine biogeochemical cycles and how the interplay between these cycles facilitated the fossilization of some of Earth’s earliest seafloor animal communities.