Worms v sediment: the rise of burrowing and oxygen

Nearly all marine strata are intensely reworked by the burrowing activities of animals, especially worms. These produce churned-up sediment in which fine-scale features, such as thin bedding, are destroyed in a surface mixed layer. This makes it difficult to evaluate sedimentary processes, especially in fine-grained sediments, where deposition often occurs in millimetre-thick layers. Nonetheless, great strides have been made in recent years in interpreting the depositional processes of ancient, fine-grained, finely-laminated sediments. But how come such fine laminae exist? Traditionally it has been argued that laminated seafloor sediments must have accumulated beneath waters lacking oxygen, thereby stopping animals from living there. Fine lamination is therefore used as evidence for anoxic deposition. Even under low oxygen (dysoxic) conditions large burrowing organisms struggle to survive although tiny worms, such as nematodes, can still thrive. Dysoxic sediments can therefore also be laminated but on close inspection the thin layers are visibly disrupted. A second factor that may be responsible for preservation of laminae in some ancient rocks concerns their age. Burrowing organisms appeared at the start of the Cambrian and sediments have been churned over ever since. Today the surface mixed layer is 10 cm thick, but early burrowers may not have been as good as modern ones. This layer may have been < 1cm thick in the Cambrian with the result that thin beds had a much greater chance of survival at this time. Consequently, finely laminated sediments are currently interpreted as evidence for oxygen-poor conditions unless they are of Early Palaeozoic age in which case the preservation of thin beds may be because burrowers were a bit rubbish at that time. But, this raises a problem. The Palaeozoic is thought to have been a time during which ocean oxygenation greatly improved (Sperling et al. 2021, Sci. Adv.). So, is lamination in the earliest Palaeozoic a consequence of the prevailing environmental conditions (anoxia/dysoxia at the seabed), or the stage of evolution achieved by burrowers as assumed presently? Or perhaps both factors are important, but how can we tell?

This project will address these questions regarding the evolution of the marine biosphere in the Early Palaeozoic by combining studies of trace fossils, sedimentology and geochemistry of Cambrian - Silurian fine-grained strata from a diversity of marine settings. There are diverse geochemical and sedimentological tools available for assessing oxygenation conditions of such rocks which will allow the role of oxygenation to be independently assessed from the burrowing activity. Work at Leeds has pioneered many of these techniques, notably in assessing the different redox states of sediment iron and the petrography of pyrite (framboid size analysis). The student will be trained in a broad range of cross disciplinary techniques from geochemistry, sedimentology and palaeoecology and conduct a series of integrative case studies in which the seafloor oxygen levels are assessed using several independent criteria. The burrowing activity will be documented both at a macroscale at outcrop and at the microscale.

The project will answer the large-scale question: was the early Palaeozoic evolution of the biosphere in lockstep with global oxygenation levels or were intrinsic, long-term evolutionary factors more important in controlling the "efficiency" of marine benthic life?