UCL Earth Sciences

Oxygen burrowed away

6 August 2014

Multicellular animals probably evolved at the seafloor after a rise in oceanic oxygen levels. In 2013, Graham Shields-Zhou and his Chinese colleague Maoyan Zhu proposed that when these animals began to rework (bioturbate) the seafloor for the first time close to the Precambrian-Cambrian boundary, they triggered a negative feedback that reduced and stabilised global atmospheric and ocean oxygen levels. 

Now, in a new paper in Nature Geoscience entitled "Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation", Shields-Zhou and colleagues outline biogeochemical model simulations that support the existence of this negative feedback which continues to stabilize atmospheric composition and possibly even climate till today.



Animal burrowing and sediment-mixing (bioturbation) began during the run up to the Ediacaran/Cambrian boundary, ini-tiating a transition between the stratified Precambrian and more well-mixed Phanerozoic sedimentary records, against the backdrop of a variable global oxygen reservoir probably smaller in size than present. Phosphorus is the long-term limiting nutrient for oxygen production via burial of organic carbon, and its retention (relative to carbon) within organic matter in marine sediments is enhanced by bioturbation. Here we explore the biogeochemical implications of a bioturbation-induced organic phosphorus sink in a simple model. We show that increased bioturbation robustly triggers a net decrease in the size of the global oxygen reservoir—the magnitude of which is contingent upon the prescribed difference in carbon to phosphorus ratios between bioturbated and laminated sediments. Bioturbation also reduces steady-state marine phosphate levels, but this effect is offset by the decline in iron-adsorbed phosphate burial that results from a decrease in oxygen concentrations. The introduction of oxygen-sensitive bioturbation to dynamical model runs is sufficient to trigger a negative feedback loop:the intensity of bioturbation is limited by the oxygen decrease it initially causes. The onset of this feedback is consistent with redox variations observed during the early Cambrian rise of bioturbation, leading us to suggest that bioturbation helped to regulate early oxygen and phosphorus cycles.

Full Reference: Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation. R. A. Boyle, T. W. Dahl, A. W. Dale, G. A. Shields-Zhou, M. Zhu, M. D. Brasier, D. E. Canfield and T. M. Lenton. Nature Geoscience (2014) doi:10.1038/ngeo2213

Related Link: Biogeochemistry: Oxygen burrowed away by Filip J. R. Meysman.  Nature Geoscience (2014) doi:10.1038/ngeo2218