Formation and diagenetic evolution of actualistic, aragonite-cemented seafloor carbonates

不连续面及其相关的间断层是浅海沉积环境中复杂而普遍的特征。该方案侧重于阿布扎比泻湖和潮间带sabkha的文石胶结硬地面。在层序地层学的术语中，这些区域被称为海侵面或准层序边界。发起人和合作者的初步工作证明，这些特征在地层和硬地面水泥岩石学方面都是高度复杂和空间可变的。通常应用的硬地层模型无法描述观测到的广泛特征。我们的数据记录了迄今为止未知但非常有特征的硬地胶结物形态趋势，从外泻湖的针状文石到内泻湖的板状和伪六边形胶结物。除了常规的海洋硬地层（即早期成岩岩化作用和随后的海底结壳和生物作用）之外，这里还提出了所谓的“非常规硬地层”的证据。这些特征是在海底下几十厘米的孔隙水氧化斜坡下的沉积物柱中的固结层形成的。在那里，复杂文石胶结物的沉淀与硫酸盐还原菌的活性、抑制剂的去除和碱度的增加有关。这一发现是相关的，因为化石凝结层可能被误认为是海洋遗漏表面，从而导致错误的古环境解释。一套新近至更新世硬地层特征的存在，使得对早期海相文石胶结层的成岩历史进行了时间分辨评估。更新世硬岩以腐蚀的前驱体胶结物和新形成的菱形方解石为代表，这种成岩演替值得详细研究。现场沉积学、岩石学和地球化学研究考虑了与早期成岩碳酸盐相有关的海水和孔隙水性质。为了阐明控制不常见水泥形态的参数，我们与格拉茨技术大学的同事合作进行非生物源沉淀实验。我们进一步依靠与纽卡斯尔大学的同事合作，在一个独立的双胞胎项目的背景下提供微生物地质学方面的专业知识。目的是解开流体性质和水泥沉淀动力学中复杂的物理、化学和微生物参数阵列。这里提出的研究具有更广泛的相关性，因为大多数硬地化石研究都集中在保存完好的方解石胶结不连续层上。相比之下，有关文石胶结硬地的实际研究和化石研究，特别是其形成和成岩演化的物理化学参数的研究还不够发达。

Discontinuity surfaces and their related hiatus intervals represent complex and very common features in shallow marine depositional environments. This proposal focusses on actualistic, aragonite-cemented hardgrounds in the lagoon and intertidal sabkha of Abu Dhabi. In the terminology of sequence stratigraphy, these qualify as transgressive surfaces or parasequence boundaries. Preliminary work by the proponent and collaborators has documented that these features are highly complex and spatially-variable both with respect to their formation and hardground cement petrography. Commonly-applied models of hardground formation fail to describe the wide range of features observed. Our data document a hitherto unknown but very characteristic trend in hardground cement morphology, ranging from needle aragonite in the outer lagoon to platy and pseudo-hexagonal cements in the inner lagoon. Next to conventional marine hardgrounds (i.e. early diagenetic lithification and subsequent encrustation and bioperforation of the seafloor), evidence for what here are referred to as “non-conventional hardgrounds” is presented. These features form as concretionary layers within the sediment column beneath the porewater redoxcline some tens of centimetres beneath the seafloor. There, precipitation of complex aragonite cements is related to the activity of sulfate-reducing bacteria, the removal of inhibitors, and an increase in alkalinity. This finding is relevant as fossil concretionary layers might be mistaken as marine omission surfaces leading to erroneous palaeoenvironmental interpretations. The presence of a suite of Recent to Pleistocene hardground features allows for a time-resolved assessment of the diagenetic history of early marine, aragonite cemented intervals. Pleistocene hardgrounds are typified by corroded precursor cements and neoformed, rhombohedral calcites, a diagenetic succession that merits detailed investigation. The field sedimentological, petrographic, and geochemical study considers seawater and porewater properties in relation to early diagenetic carbonate phases. In order to shed light on parameters that control the uncommon cement morphologies, we collaborate with colleagues from the Technical University of Graz performing abiogenic precipitation experiments. We further rely on collaboration with colleagues at Newcastle University to provide expertise in microbiogeology in the context of an independent twin project. The aim is to disentangle the complex array of physical, chemical, and microbiological parameters on fluid properties and cement precipitation kinetics. Research proposed here has wider relevance as most studies of fossil hardground have focussed on well-preserved, calcite-cemented discontinuities. In contrast, research dealing with actualistic and fossil aragonite-cemented hardgrounds, and specifically the physico-chemical parameters of their formation and diagenetic evolution, is as yet insufficiently developed.