Exploring the controls of Early Aptian Tethyan and proto-Atlantic reefal palaeoecology turnover

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Previous work by the proponents and colleagues in field areas in Oman, Croatia, Spain, and Portugal documented that Tethyan and proto-Atlantic lower Aptian reefal ecosystems temporally collapsed and were replaced by problematic micro-encruster communities spreading over more than 100000 km2 such as in the Oman platform. The abrupt transition from coral-rudist to microencruster facies represents a profound palaeo-ecological turnover in these shallow tropical platform settings. Important Tethys-wide differences in the timing, facies, and taxonomy of the micro-encruster events are recognized. Based on high-resolution chemostratigraphy, the microencruster intervals (durations of ca. 300 kyr) have been dated as harbinger of the OAE1a, the OAE1a-equivalent interval itself (C4-C6), and the immediate post-OAE1a phase. In sections in Spain and Portugal, Lithocodium facies is intensely bored and forms meter-sized carbonate bodies encrusting coral rubble. In Oman and Croatia, rudist-coral fragments are scarce to absent and bacinelloid facies forms meter to km-sized superstructures. Motivated by the very substantial data set available, the proponents suggest that the hitherto unknown controls triggering these remarkable facies patterns merit an adequate level of attention. The starting point is the observation of a Tethys-wide (and beyond) feature that coincides in time with a major environmental perturbation event (OAE1a) in the Cretaceous. The present proposal suggests exploring early Aptian epeiric-neritic seawater properties with focus on dissolved oxygen levels (coastal hypoxia). In the Recent world, oxygen-depleted dead zone water bodies are known from not less than 375 coastal zones worldwide. The mechanisms leading to seawater oxygen depletion in the Recent (high temperatures, enhanced nutrient influx, microbial consumption of dissolved seawater oxygen etc.) have arguable been far more intense in the vast, warm and saline shallow epeiric seas of the early Aptian (kettle effect). We here hypothesize that seawater properties temporally reached above-tolerance levels for rudist-coral communities and were replaced by even more hardy microencruster facies. First data sets exploit the well-established REE and uranium isotope (238U/235U) proxies sensitive to seawater redox conditions. Both of these proxies independently point to lower coastal seawater oxygen levels during OAE1a at the sites studied. The present proposal is important for the following reason: The parameters controlling coastal ecosystem turnover during OAE1a have, when better understood, a high potential to shed light on processes governing OAE1a in general. This is particularly significant in the context of coupled continent-ocean processes during time periods of environmental perturbation.