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.

During the end-Permian mass extinction, a global decline in seafloor sediment mixing and burrowing (bioturbation) provides critical evidence for the collapse of marine ecosystems, likely triggered by rapid ocean warming and deoxygenation. However, the decline and subsequent recovery of bioturbation after the extinction event may not only have been a symptom of environmental change but also a driver, influencing nutrient exchange and reductant burial across the sediment-water interface and thus water column oxygen availability and seafloor habitability more broadly. Here we test this hypothesis through combined analyses of bioturbation and sedimentary geochemistry, focusing on marine siliciclastic records of the Permian-Triassic transition from Svalbard. We find that total organic carbon, total sulfur, and organic phosphorus decrease with increasing bioturbation intensity, whereas inorganic reactive phosphorus phases (authigenic and iron oxide-bound phosphorus) increase. These differences are most strongly associated with biodiffusion (particle mixing) rather than bioirrigation (solute exchange). Our findings suggest that bioturbation primarily influenced sediment chemistry by enhancing organic matter oxidation, in contrast to some modern settings where downward mixing may promote organic matter preservation within the anoxic portion of seafloor sediments. The early return of shallow-tier bioturbators in this region < 200 kyr after the extinction event likely promoted a rapid restoration of efficient carbon and sulfur cycling within benthic ecosystems. In contrast, efficient phosphorus burial via sink-switching may not have resumed until deeper-tier bioturbators achieved pre-extinction levels of sediment mixing > 1 Myr after the mass extinction.

Elevated temperatures persisted for an anomalously protracted interval following pulsed volcanic carbon release associated with the end-Permian mass extinction, deviating from the expected timescale of climate recovery following a carbon injection event. Here, we present evidence for enhanced reverse weathering—a CO 2 source—following the end-Permian mass extinction based on the lithium isotopic composition of marine shales and cherts. We find that the average lithium isotopic composition of Lower Triassic marine shales is significantly elevated relative to that of all other previously measured Phanerozoic marine shales. Notably, the record generated here conflicts with carbonate-based interpretations of the lithium isotopic composition of Early Triassic seawater, forcing a re-evaluation of the existing framework used to interpret lithium isotopes in sedimentary archives. Using a stochastic forward lithium cycle model, we demonstrate that elevated reverse weathering is required to reproduce the lithium isotopic values and trends observed in Lower Triassic marine shales and cherts. Collectively, this work provides direct geochemical evidence for enhanced reverse weathering in the aftermath of Earth’s most severe mass extinction.

The activities of marine sediment-dwelling invertebrates play a fundamental role in mediating major biogeochemical cycles and have profoundly shaped the evolution of marine systems. Yet there remains a paucity of global marine data describing bioturbation intensities and mixed layer depths and interrogating how these vary with multiple environmental and ecological factors at a system scale. We applied an ensemble of tree-based machine learning techniques to resolve a global map and determine the environmental and ecological correlates most closely associated with bioturbation. We find that bioturbation intensity and the depth of the sediment mixed layer each reflect different associations with a consortium of environmental and ecological parameters, and that bioturbation intensities are much more readily predicted than sediment mixed layer depths from these correlates. Furthermore, we find that the bioturbation intensity, the depth of the sediment mixed layer, and their environmental and ecological correlates differ between shallow marine and open-ocean settings. Our findings provide new insights into the importance of potential drivers of ancient sediment mixing recorded by geologic archives. These results also highlight that climate change may, in the near future, drive shifts in bioturbation and reciprocal fundamental changes in benthic functioning.

The advent of bioturbation in the early Paleozoic and the subsequent intensification of sediment mixing by benthic animals dramatically altered marine ecosystems and biogeochemical cycling. However, the timing and pace of the transition to ubiquitous well-bioturbated sediments in the uppermost seafloor remain under debate. Skolithos 'piperock', a classic Cambrian ichnofabric composed of densely packed vertical burrows, has often been invoked as evidence for an early ramping up of infaunalization and intense sediment disruption, but its biogeochemical impact has not been quantitatively explored in this context. Here, we relate high-resolution sedimentological and bioturbation data from 182 m of the Zabriskie Quartzite of the Death Valley region of California-a locality known for hosting piperock-to a new numerical three-dimensional diagenetic model to investigate the effect of Skolithos on sediment oxygen penetration and organic carbon remineralization. Our modeling indicates that increasing Skolithos burrow density substantially expands the local oxic zone and, in contrast with other modes of bioturbation, can also result in decreased sulfur oxidation. However, our field data indicate true 'piperock' is relatively scarce in the Zabriskie Quartzite, even among horizons that are of identical lithology to those containing piperock and that house individual Skolithos burrows. This suggests that, in the Zabriskie and potentially other Cambrian piperock-bearing units, the broader impact of piperock on seafloor ecology, chemistry, and sediment properties may have been restricted in scope.

The boron isotopic composition (δ 11 B) of bulk carbonates may provide an archive to reconstruct changes in ocean pH. Reconstructions from ancient carbonates typically assume that no significant resetting of δ 11 B occurred during marine burial diagenesis. However, our understanding of B isotopic behavior associated with this process remains limited. Here we provide measurements of B/Ca and B isotopic composition (δ 11 B) from a modern peri-platform carbonate sequence near the Great Bahama Bank that has undergone marine burial diagenesis. Our results reveal significant decreases in both δ 11 B (~13 ‰) and B/Ca (~80 %) of bulk carbonates with depth. We attribute this pattern to the release of isotopically light B (δ 11 B ~20 ‰) to porewater during aragonite dissolution, with uptake of substantially isotopically lighter borate ions (δ 11 B ~-1 ‰) from porewater by newly forming low-Mg calcite. A quantitative model adds further support for this interpretation and provides an estimate of average neomorphism rate ( k0) in the range of 1×10 -6 to 5×10 -6 yr -1 , which is comparable to previous rate estimates for neomorphism and/or recrystallization during meteoric diagenesis. Our results demonstrate the strong potential for resetting δ 11 B signatures in bulk carbonates during recrystallization, which must be considered in future attempts to reconstruct pH and p CO 2 from these records and may require reinterpretation of existing records. Our results also suggest the potential of B isotopes as a proxy for carbonate recrystallization/neomorphism and original carbonate mineralogy.