Worms v sediment: the rise of burrowing and oxygen levels in the early Paleozoic
Worms v sediment: the rise of burrowing and oxygen levels in the early Paleozoic
Paul Wignall (SOEE), Simon Poulton (SOEE), Jeff Peakall (SOEE)
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 if they are so readily destroyed by burrowing? 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 by tiny burrows (Schieber and Wilson 2020).
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 there is a surface mixed layer, of completely burrowed sediment, that is typically 10 cm thick thanks to the efficiency and intensity of modern burrowing organisms such as worms, shrimps and sea cucumbers. But, it took a long time for such impressive sediment bulldozers to evolve, early burrowers appear not to have been as effective. Studies of ancient sediments suggest that the earliest Cambrian burrowers were only capable of producing a surface mixed layer < 1cm thick with the result that thin beds had a much greater chance of survival at this time (Tarhan 2018). Consequently, laminated sediments are currently interpreted as evidence for oxygen-poor or anoxic depositional conditions unless they are of early Paleozoic age in which case the preservation of thin beds may be because burrowers were a bit rubbish at that time. But, this raises a major problem. The Paleozoic is a key time in Earth history when it is thought that ocean oxygenation gradually improved (along with atmospheric oxygen levels) but values only reached modern conditions by the Carboniferous (Sperling et al. 2021). So, is the presence of laminated sediment in the earliest Paleozoic a consequence of the prevailing environmental conditions (anoxia/dysoxia at the seabed), or the stage of evolution achieved by burrowers? Or perhaps both factors are important, but how can we tell? These are key questions of macroevolution. The lack of seabed mixing has fundamental consequences for the cycling of nutrients and functioning of the marine realm whilst the purported low oxygenation levels of the early Paleozoic may account for the high global extinction rates of the time (Sperling et al. 2022). But if the supposed anoxic, laminated sediments are in fact unborrowed oxygenated sediments then we need new hypotheses.
This project will address these questions regarding the evolution of the marine biosphere in the early Paleozoic 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, and several new approaches are available, notably in assessing the different redox states of sediment iron (Poulton 2021) and the petrography of pyrite (framboid size analysis) (Wignall and Newton 1998). The combination of independent geochemical and petrographic techniques is a powerful approach for determining seafloor oxygenation levels and importantly it does not rely on the use of trace fossils. Thus, trace fossil occurrences can be compared with seafloor redox proxies without the need to use trace fossils to determine oxygen levels.
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 multiple independent approaches. They will become a skilled practitioner at assessing ancient oxygen levels.
The chemistry of iron is strongly controlled by mineralogy and redox conditions: ferric iron oxides occur if oxygen is plentiful but as levels decline more soluble ferrous iron occurs which reacts with sulphides to form pyrite. A sequential extraction scheme has been developed to assess the different species of iron allowing their variable proportions to be quantified (Poulton 2021). These iron proxies can be compared with the values of sediment trace metals which are also strongly controlled by redox: molybdenum is strongly enriched in strongly anoxic/sulphidic conditions for example.
Petrographic analysis will allow the smallest scale burrows to be analysed which, combined with field study of outcrops, will allow the key parameters of bioturbation to be assessed: mixed layer depth, burrow diversity, density and size. Scanning electron microscopy of samples is used to assess the population size of the tens-of-micron-scale pyrite framboids where present. These are strongly controlled by water column redox conditions (Wignall and Newton 1998).
Impacts and publications
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? These issues feed into equally important questions: why were extinction rates so much higher in the early Paleozoic, does this relate to oxygenation levels? Were early Paleozoic oceans more like those of the Precambrian, with unburrowed sediment and poor oxygenation, or were they unique in being well oxygenated but lacking efficient organisms to fully exploit all habitats? It is hoped the project will help answer these questions and lead to publication of papers in the highest impact scientific journals.
Training and research environment
The student will join the large and supportive research groups of the three supervisors and receive training in both the geochemical laboratory skills and fieldwork skills required to assess early Paleozoic marine sediments. The great strength of the training will be its cross disciplinary nature: the student will be able to draw on diverse skills and approaches to interpreting ancient sediments thereby avoiding the pitfalls of narrowly focussing on single analytical techniques.
Each supervisor is an international expert in their field and they have extensive records of successful research supervision going back over many years, and have fostered many high profile research publications from their students. Their research labs have produced dozens of successful research scientists many of whom now populate university faculties around the world.
Poulton, S.W. 2021. The Iron Speciation Paleoredox Proxy. Cambridge Elements: Geochemical Tracers in Earth System Science, 25 pp.
Schieber, J. and Wilson, R.D. 2021. Burrows without a trace – How meioturbation affects rock fabrics and leaves a record of meiobenthos activity in shales and mudstones. PalZ 95, 767-791.
Sperling, E.A. et al. 2021. A long-term record of early to mid-Paleozoic marine redox change. Science Advances 7, eabf4382.
Sperling, E.A. et al. 2022. Breathless through time: Oxygen and animals across Earth history. The Biological Bulletin 243.
Tarhan, L.G. 2018. The early Paleozoic development of bioturbation – Evolutionary and geobiological consequences. Earth-Science Reviews 178, 177-207.
Wignall, P.B. and Newton, R.J. 1998. Pyrite framboid diameter as a measure of oxygen deficiency in ancient mudrocks. American Journal of Science 298, 537-552.