Bioturbation and directionality in Earth's carbon isotope record across the Neoproterozoic–Cambrian transition

Abstract

Mixing of sediments by moving animals becomes apparent in the trace fossil record from about 550 million years ago (Ma), loosely overlapping with the tail end of the extreme carbonate carbon isotope δ¹³ C_carbonate fluctuations that qualitatively distinguish the Proterozoic geochemical record from that of the Phanerozoic. These Precambrian-scale fluctuations in δ¹³ C_carbonate (PSF-δ¹³ C_carbonate ) remain enigmatic, due to their high amplitude and inclusion of global-scale negative δ¹³ C_carbonate values, below anything attributable to mantle input. Here, we note that different biogeochemical-model scenarios plausibly explaining globally synchronous PSF-δ¹³ C_carbonate converge: via mechanistic requirements for extensive anoxia in marine sediments to support sedimentary build-up of ¹³ C-depleted carbon. We hypothesize that bioturbation qualitatively reduced marine sediment anoxia by exposing sediments to oxygenated overlying waters, which ultimately contributed to decreasing the carbon cycle's subsequent susceptibility to PSF- δ¹³ C_carbonate . Bioturbation may also have reduced the quantity of (isotopically light) organic-derived carbon available to contribute to PSF- δ¹³ C_carbonate via ocean crust carbonatization at depth. We conduct a comparative modelling exercise in which we introduce bioturbation to existing model scenarios for PSF- δ¹³ C_carbonate : expressing both the anoxic proportion of marine sediments, and the global organic carbon burial efficiency, as a decreasing function of bioturbation. We find that bioturbation's oxygenating impact on sediments has the capacity to prevent PSF- δ¹³ C_carbonate caused by authigenic carbonate precipitation or methanogenesis. Bioturbation's impact on the f-ratio via remineralization is partially offset by liberation of organic phosphate, some of which feeds back into new production. We emphasize that this study is semiquantitative, exploratory and intended merely to provide a qualitative theoretical framework within which bioturbation's impact on long-term, first-order δ¹³ C_carbonate can be assessed (and it is hoped quantified in more detail by future work). With this proviso, we conclude that it is entirely plausible that bioturbation made a decisive contribution to the enigmatic directionality in the δ¹³ C_carbonate record, from the Neoproterozoic-Cambrian boundary onwards.