Worms v sediment: the rise of burrowing and oxygen

几乎所有的海洋地层都受到动物，尤其是蠕虫的挖洞活动的强烈改造。这些产生了搅碎的沉积物，其中细尺度的特征，如薄层，在表面混合层中被破坏。这使得很难评估沉积过程，特别是在细粒沉积物中，沉积通常发生在毫米厚的层中。尽管如此，近年来在解释古代、细颗粒、细层状沉积物的沉积过程方面取得了很大进展。但这么细的纹层是怎么存在的呢？传统上，人们一直认为叠层海底沉积物一定是在缺氧的水下积累起来的，从而阻止了动物在那里生活。因此，细层积被用作缺氧沉积的证据。即使在低氧（缺氧）条件下，大型穴居生物也难以生存，而微小的蠕虫，如线虫，仍然可以茁壮成长。因此，厌氧沉积物也可以层压，但仔细观察，薄层明显被破坏。在一些古老的岩石中，纹层得以保存的第二个因素可能与它们的年龄有关。穴居生物出现在寒武纪初期，从那时起沉积物就被搅动了。今天，地表的混合层有10厘米厚，但早期的穴居者可能没有现代穴居者那么好。这一层在寒武纪的厚度可能小于1厘米，因此薄层在这个时候存活的机会要大得多。因此，细层状沉积物目前被解释为缺氧条件的证据，除非它们是早古生代的，在这种情况下，薄层的保存可能是因为当时的穴居者有点垃圾。但是，这就产生了一个问题。古生代被认为是海洋氧合大大改善的时期(Sperling et al. 2021, Sci。放置)。那么，最早古生代的层压是当时环境条件（海底缺氧/缺氧）的结果，还是目前假设的穴居者所达到的进化阶段？或者这两个因素都很重要，但我们怎么知道呢？本项目将结合不同海洋环境下寒武—志留纪细粒地层的微量化石、沉积学和地球化学研究，解决这些关于早古生代海洋生物圈演化的问题。有各种各样的地球化学和沉积学工具可用于评估这类岩石的氧化条件，这将允许从穴居活动中独立评估氧化作用。利兹的工作开创了许多这样的技术，特别是在评估沉积物铁的不同氧化还原状态和黄铁矿的岩石学（树状体尺寸分析）方面。该学生将接受广泛的跨学科技术培训，包括地球化学、沉积学和古生态学，并进行一系列综合案例研究，其中使用几个独立的标准评估海底氧气水平。将在露头的宏观尺度和微观尺度上记录挖洞活动。该项目将回答一个大规模的问题：早期古生代生物圈的进化与全球氧合水平是同步的，还是内在的、长期的进化因素在控制海洋底栖生物的“效率”方面更重要？

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?