COLLABORATIVE RESEARCH: Understanding Abrupt Climatic Disturbance in the Aptian-Albian (Mid-Cretaceous)

关于环境突变对地球生物圈和地球化学循环的影响的一些最好的证据可以在白垩纪地层中找到。这些岩石包含了明显的证据，表明大气成分和海洋环流发生了巨大的变化，导致了大范围的、间歇性的和地质上的短暂(约0.5至1英里)。海洋缺氧事件(OAES)。最近记录的一些白垩纪中期OAE矿床底部的负C同位素漂移，以及由早期阿普提亚陆地植物群的d13C值所圈定的可能更大规模的同时代同位素异常，表明白垩纪中期的环境扰动可能是温室气体驱动的变暖的结果，其速度与推测的古新世-始新世最高温度相当，目前地球在上个世纪经历了这一变化。因此，对这些白垩纪中期事件的详细、综合的研究，如本研究将进行的，将有助于完善我们对温室气体变暖期间气候突变的理解。本研究旨在建立中白垩世OAEs期间环境边界条件变化的本质，并在潜在强迫机制的背景下解释这些变化的时间和机制关系。目前正在研究的两个海洋缺氧事件代表了最极端和最广泛的环境变化：(1)早阿普提亚OAE1a(119.5~120.5 Ma)和(2)晚阿普提斯至早阿尔卑斯OAE1b(~108.5~113Ma)。墨西哥东部马德雷山脉的陆架和斜坡剖面代表了现有最广泛的沉积记录，我们为其制定了详细的年代地层格架，是主要的研究领域。这些台地沉积的累积速率比深海层序的典型沉积速率高3到15倍，这应该会在不同的替代记录之间提供明显的超前-滞后关系。我们还将调查精选的海洋环境事件深海记录，以确定更全球化的信号和更广泛的古深度视角。这项研究的一个基本方面和以前未曾探索过的方面是，在可能的情况下，在103到105年的时间尺度上解开代理记录与它们所推断的环境变化的相互作用、响应和反馈之间的精确相位关系。研究将整合一系列关于碳和营养循环、生产力、海底氧化作用和海平面的地层学、古生物学、地球化学和同位素代理。正在进行详细的超微化石以及浮游和底栖有孔虫组合研究，以确定表层和深水环境的变化，包括营养平衡、生产力、水柱结构、温度梯度和深水氧气的变化。正在开发地球化学和同位素记录(d13Ccarb、d13Corg、TOC、%CaCO3和d34S)，作为碳循环、有机碳埋藏、生产力和缺氧的替代。为了更全面地了解我们的地球化学和同位素替代记录的环境影响，我们将把它们的速率/相位/幅度关系与从微化石组合数据推断的古生态趋势以及相对海平面历史进行比较。对深海岩芯(398站点)巴雷米期至阿尔比亚期的陆地维管植物物质进行同位素分析，将提供一个独立且高度灵敏的指标，指示正常的碳循环，以及二氧化碳通量和碳库的变化。最后，我们将使用海洋碳同位素系统学的盒子模型和相关联的生物地球化学-3D全球海洋模型进行数值实验，以评估替代记录及其推断的环境条件如何约束与OAE起源假说机制相关的一系列地质‘作用力’。由此产生的一组高分辨率替代记录是从统计上扩大的截面同时开发的，再加上数字建模和数据模型比较，应能提供对海洋缺氧事件背后的海洋和大气条件的重要洞察。此外，这一与经典白垩纪OAEs相关的环境突变研究将为包括现代地球系统在内的其他地质时期温室变暖过程中的生物地球化学循环提供一个端元模型。

PROJECT SUMMARYSome of the best evidence for the effects of abrupt environmental change on the Earth's biosphere and geochemical cycles can be found in Cretaceous strata. These rocks contain clear evidence for drastically altered atmospheric composition and oceanic circulation that resulted in widespread, episodic, and geologically ephemeral (~0.5 to 1 m.y.) oceanic anoxic events (OAEs). The recent documentation of negative C isotope excursions at the base of some mid-Cretaceous OAE deposits, and a likely contemporaneous isotope anomaly of greater magnitude delineated by the d13C values of early Aptian terrestrial flora, suggest that environmental perturbation during the mid-Cretaceous may have been the result of greenhouse-gas-driven warming at rates comparable to those inferred for the Paleocene-Eocene Thermal Maximum, and currently experienced by Earth over the last century. Thus detailed, integrative study of these mid-Cretaceous events, such as will be carried out by this study, holds potential for refining our understanding of abrupt climatic disturbance during greenhouse warming.This investigation is designed to establish the nature of changes in environmental boundary conditions during mid-Cretaceous OAEs, and to interpret the temporal and mechanistic relationship of these changes within the context of potential forcing mechanisms. Two oceanic anoxic events that represent the most extreme and widespread environmental change are being studied: (1) the early Aptian OAE1a (119.5 to 120.5 Ma), and (2) the late Aptian to early Albian OAE1b (~108.5 to 113 Ma). Shelf and slope sections in the Sierra Madre Oriental of Mexico that represent the most expanded sedimentary records available, and for which we have developed a detailed chronostratigraphic framework are the primary field area of study. These platform deposits have accumulation rates 3 to 15 times higher than those typical of deep-sea sequences that should provide clear lead-lag relationships between the different proxy records. We will also investigate select deep-sea records of the OAEs to define a more global signal and a wider paleo-depth perspective. One of the fundamental and previously unexplored aspects of this investigation is to unravel, where possible, on a 103 to105 yr time scale the precise phase relationship between proxy records and the interactions, responses, and feedbacks of their inferred environmental changes.The research will integrate a series of stratigraphic, paleontologic, geochemical and isotopic proxies of carbon and nutrient cycling, productivity, bottom-water oxygenation, and sea level. Detailed nannofossil, and planktic and benthic foraminiferal assemblage studies are being carried out to determine variation in surface- and deep-water environments including changes in nutrient budgets, productivity, water column structure, temperature gradients, and deep-water oxygenation. Geochemical and isotopic records (d13Ccarb, d13Corg, TOC, %CaCO3, and d34S) are being developed as proxies of carbon cycling, organic carbon burial, productivity and anoxia. In order to develop a more complete understanding of the environmental implications of our geochemical and isotopic proxy records, we will compare their rate/phasing/amplitude relationships to paleoecological trends inferred from microfossil assemblage data, and to the relative sea-level history. Isotopic analysis of terrestrial vascular plant matter in the Barremian through Albian interval of a deep-sea core (Site 398) will provide an independent and highly sensitive indicator of disruption to normal carbon cycling, as well as shifts in CO2 fluxes and carbon reservoirs.Finally, we will carry out numerical experiments using a box model for the carbon isotopic systematics of the ocean and a linked biogeochemistry- 3D global ocean model in order to assess how the proxy records and their inferred environmental conditions constrain a series of geological 'forcings' relevant to the hypothesized mechanisms for the origin of the OAEs. The resulting set of high-resolution proxy records developed simultaneously from statigraphically expanded sections, coupled with numeric modeling and data-model comparisons should provide significant insight into the oceanographic and atmospheric conditions underlying oceanic anoxic events. Moreover, this study of abrupt environmental change associated with the classic Cretaceous OAEs should provide an end-member model for biogeochemical cycling during greenhouse warming of other geologic periods, including the current earth system.