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

点击翻译按钮获取中文摘要

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.