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

在白垩纪地层中可以找到环境突变对地球生物圈和地球化学循环影响的一些最佳证据。 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，导致了广泛的，幕式的和地质短暂的（~0.5至1百万年）。海洋缺氧事件（OAE）。最近的文件显示，在白垩纪中期的一些OAE沉积物的底部，碳同位素出现了负偏移，早期阿普第阶陆地植物群的d13 C值显示了一种可能的更大幅度的同时期同位素异常，这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，其速率与古新世-始新世热最大期的推断相当，地球在上个世纪所经历的。因此，详细的，综合研究这些白垩纪中期的事件，如将进行这项研究，有可能完善我们的理解突然气候扰动在温室warming.This调查的目的是建立在白垩纪中期的OAE环境边界条件的变化的性质，并解释这些变化的时间和机制的背景下，潜在的强迫机制。两个代表最极端和最广泛的环境变化的海洋缺氧事件正在研究中：（1）早期Aptian OAE 1a（119.5至120.5 Ma），（2）晚期Aptian到早期Albian OAE 1b（~108.5至113 Ma）。 墨西哥马德雷山脉东方的陆架和斜坡剖面代表了最广泛的沉积记录，我们已经制定了详细的年代地层框架，是主要的研究领域。这些台地沉积物的积累速率比深海层序的典型积累速率高3至15倍，这应能在不同的代用记录之间提供明确的超前-滞后关系。我们还将调查OAE的深海记录，以确定更全球的信号和更广泛的古深度视角。该研究的一个基本的和以前未探索的方面是在可能的情况下，在103至105年的时间尺度上揭示代用记录与其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、古生物、地球化学和同位素代用指标，包括碳和营养循环、生产力、底层水的氧合和海平面。正在进行详细的超微化石、浮游有孔虫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（d13 Ccarb，d13 Corg，TOC，%CaCO3和d34 S）正在被开发作为碳循环，有机碳埋藏，生产力和缺氧的替代品。 为了更全面地了解我们的地球化学和同位素代用记录的环境影响，我们将比较它们的速率/相位/振幅关系，从微体化石组合数据推断的古生态趋势，以及相对海平面的历史。深海岩心巴列姆统至阿尔布统层段陆生维管植物物质的同位素分析（398号站点）将提供一个独立的、高度敏感的指标，以显示正常碳循环的中断，以及二氧化碳通量和碳库的变化。我们将使用海洋碳同位素系统学的箱模型和相关的海洋地球化学进行数值实验，三维全球海洋模型，以评估代理记录及其推断的环境条件如何约束一系列与OAE起源的假设机制相关的地质“强迫”。 由此产生的一组高分辨率的代理记录同时开发从统计扩展部分，再加上数值模拟和数据模型比较，应该提供显着的洞察海洋和大气条件下的海洋缺氧事件。 此外，与经典的白垩纪OAE的环境突变的研究应该提供一个端元模型在温室效应的其他地质时期，包括目前的地球系统中的地球化学循环。

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.