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

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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.