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

在白垩纪地层中可以找到一些关于环境突变对地球生物圈和地球化学循环影响的最佳证据。这些岩石包含了大气成分和海洋环流剧烈变化的明确证据，这些变化导致了广泛的、偶发的和地质上短暂的（~0.5到1米）海洋缺氧事件（oae）。最近在一些中白垩世OAE矿床基底上的负C同位素偏移记录，以及早期阿普tian陆生植物群的d13C值所描绘的可能更大的同时期同位素异常，表明中白垩世的环境扰动可能是温室气体驱动的变暖的结果，其速度与古新世-始新世热极大期推断的速度相当，并且是地球目前在上个世纪经历的。因此，对这些中白垩纪事件的详细、综合研究，如本研究将进行的，有可能改善我们对温室变暖期间气候突变的理解。本研究旨在确定中白垩世oae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。研究了两个代表最极端和最广泛环境变化的海洋缺氧事件：(1)阿普梯早期OAE1a （119.5 ~ 120.5 Ma）和(2)阿普梯晚期~阿普梯早期OAE1b （~108.5 ~ 113 Ma）。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，我们为此开发了详细的年代地层格架，这是主要的研究领域。这些台地沉积的成藏速率比典型的深海层序的成藏速率高3 ~ 15倍，这应该能在不同的代理记录之间提供明确的超前-滞后关系。我们还将研究选择的深海oae记录，以定义更全球的信号和更广泛的古深度视角。这项研究的一个基本和以前未被探索的方面是，在可能的情况下，在103到105年的时间尺度上，揭示代理记录与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。该研究将整合一系列地层学、古生物学、地球化学和碳和营养循环、生产力、底水氧合和海平面的同位素代用指标。正在开展详细的纳米化石、浮游和底栖有孔虫组合研究，以确定地表和深水环境的变化，包括营养收支、生产力、水柱结构、温度梯度和深水氧合的变化。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、生产力和缺氧的代表。为了更全面地了解地球化学和同位素代用记录对环境的影响，我们将把它们的速率/相位/振幅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。通过深海岩心（398站点）的巴雷米亚至阿尔比安段陆相维管植物物质的同位素分析，将为正常碳循环的中断以及二氧化碳通量和碳库的变化提供一个独立和高度敏感的指标。最后，我们将使用海洋碳同位素系统的盒子模型和相关的生物地球化学-三维全球海洋模型进行数值实验，以评估代理记录及其推断的环境条件如何约束与oae起源的假设机制相关的一系列地质“强迫”。由此产生的一组高分辨率代理记录是由统计扩展的剖面同时开发的，再加上数值模拟和数据模型比较，将为了解海洋缺氧事件背后的海洋学和大气条件提供重要的见解。此外，这项与白垩纪经典oae相关的环境突变研究将为其他地质时期（包括当前地球系统）温室变暖期间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.