Tracking bioturbation through time: The evolution of the marine sedimentary mixed and transition layers

The physical, biogeochemical, and ecological properties of the modern seafloor are extensively shaped by the activities of burrowing and sediment-mixing animals, processes collectively known as bioturbation. Bioturbation is primarily recorded by homogenized sediments of the seafloor mixed layer and the underlying transition layer of discrete burrows. Although these two zones can be readily measured today, there has been limited understanding of how the mixed and transition layers evolved over the Phanerozoic since animals first began to extensively colonize the seafloor. Here, we provide a record for the depths of the sedimentary mixed and transition layers through the Phanerozoic. We find that although deepening of the sediment mixed layer spanned hundreds of millions of years, a deep transition layer was established as early as the Cambrian and did not further deepen until the Mesozoic—trajectories reflecting evolutionary radiations, changes in nutrient cycling, and alleviation of oxygen stress.