Worms v sediment: the rise of burrowing and oxygen

几乎所有的海洋地层都被动物，特别是蠕虫的穴居活动强烈改造。这些产生了搅动的沉积物，其中细尺度特征，如薄层理，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，沉积往往发生在毫米厚的地层中。尽管如此，近年来在解释古老的、细粒的、细层状沉积物的沉积过程方面取得了很大的进展。但是为什么会有这么好的纹层呢？传统上，人们认为层状海底沉积物一定是在缺氧的沃茨下积累起来的，因此阻止了动物在那里生活。因此，精细的叠层被用作缺氧沉积的证据。即使在低氧条件下，大型穴居生物也难以生存，尽管线虫等微小蠕虫仍然可以茁壮成长。因此，Dysoclonal沉积物也可以分层，但仔细观察，薄层明显被破坏。第二个因素可能是负责保存纹层在一些古老的岩石有关他们的年龄。穴居生物出现在寒武纪之初，从那以后，沉积物就一直被搅动着。今天，地表的混合层有10厘米厚，但早期的穴居人可能没有现代的穴居人那么好。在寒武纪，这一层的厚度可能小于1厘米，因此薄层在此时有更大的生存机会。因此，精细的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人有点垃圾。但是，这提出了一个问题。古生代被认为是海洋氧合大大改善的时期（Sperling等人，2021，Sci. Adv.）。那么，最早古生代的层积是当时环境条件（海底缺氧/缺氧）的结果，还是目前假设的穴居动物所达到的进化阶段？也许这两个因素都很重要，但我们怎么知道呢？该项目将通过对来自多种海洋环境的寒武纪-志留纪细粒地层的遗迹化石、沉积学和地球化学的研究，解决有关早古生代海洋生物圈演化的这些问题。有各种各样的地球化学和沉积学工具可用于评估这些岩石的氧化条件，这将使氧化作用独立于挖掘活动进行评估。在利兹的工作开创了许多这些技术，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相学（framboid大小分析）。学生将接受来自地球化学，沉积学和古生态学的广泛的跨学科技术培训，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。将在露头处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：早古生代生物圈的演变是否与全球氧合水平同步，或者内在的长期演变因素在控制海洋底栖生物的“效率”方面是否更为重要？

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?