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

支持者和同事在阿曼、克罗地亚、西班牙和葡萄牙的实地研究表明，特提斯和原大西洋下阿普提亚珊瑚礁生态系统暂时崩溃，取而代之的是分布在超过10万平方公里的有问题的微壳群群落，比如在阿曼平台。从珊瑚滩相到微壳状相的突然转变代表了这些热带浅海台地环境中深刻的古生态转换。重要的特提斯范围内的差异在时间，相，和分类的微壳层事件被认识。基于高分辨率化学地层学，微壳层段（持续时间约300 kyr）被确定为OAE1a、OAE1a等效层段本身（C4-C6）和OAE1a后阶段的先兆。在西班牙和葡萄牙的剖面上，岩芯岩相被强烈钻孔，形成了米大小的碳酸盐体，包裹在珊瑚碎石上。在阿曼和克罗地亚，原始珊瑚碎片很少或不存在，杆状相形成一米至公里大小的上层建筑。在大量可用数据的推动下，支持者认为，迄今为止未知的控制因素引发了这些显著的相模式，值得给予足够的关注。研究的出发点是在白垩纪一次主要的环境扰动事件（OAE1a）发生的同时，观察到一个特提斯范围（及更远）的特征。目前的建议建议探索早期阿普提亚浅海-浅海海水的性质，重点是溶解氧水平（海岸缺氧）。在最近的世界里，在全世界不少于375个沿海地区发现了缺氧死区水体。导致近期海水缺氧的机制（高温、营养物质流入增强、微生物对溶解海水氧的消耗等）在阿普tian早期广阔、温暖和含盐的浅海中更为强烈（水壶效应）。我们在这里假设海水的性质暂时超过了珊瑚群落的容忍度，并被更耐寒的微壳相所取代。第一批数据集利用了对海水氧化还原条件敏感的稀土和铀同位素（238U/235U）指标。这两个指标都独立地指出，在OAE1a期间，研究地点的沿海海水氧含量较低。目前的建议很重要，原因如下：在OAE1a期间控制沿海生态系统更替的参数，如果得到更好的理解，就有很大的潜力揭示OAE1a的一般控制过程。这在环境扰动期间大陆-海洋耦合过程的背景下尤为重要。

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.