Collaborative Research: Testing Orbital Forcing of Terrestrial Greenhouse Climate? U/Pb Zircon Geochronology from the Eocene Green River Formation of Wyoming

沉积岩中旋回的起源一直争论不休。至少在过去的几百万年里，可以令人信服地证明，这些周期通常受到地球轨道和旋转的周期性变化以及与附近行星的相互作用的控制，这些行星每隔20、40、100和400千公里就会产生气候变化。然而，在更古老的岩石中，由于缺乏独立的年龄控制，导致在测试轨道强迫假设之前对沉积物累积速率的明显变化进行先验校正，因此问题要困难得多。循环记录不完整的反复出现的证据加剧了这个问题，因为任何层“失踪”的轨道时间尺度内不能检测到现有的前新第三纪年表。例如，如果有人能证明这些周期发生在5000万年前的岩石中，那么地球科学家就可以通过将太阳系动力学模型与岩石中的周期相匹配，以前所未有的精度告诉岩石中的时间。 我们提出了一个程序，高分辨率的U-Pb锆石年代学，以解决循环沉积物的经典例子之一，早始新世绿色河形成的轨道强迫的争论。这些保存完好的岩石沉积在一个大湖中。五千万年前拟议的工作的目标是定量测试，在高精度，在绿色河组的周期性的假设。是轨道强迫的，这种强迫是通过地球气候的变化来实现的，而地球气候的变化是由地球轨道的周期性变化来决定的。我们建议使用来自丰富的层间灰层的锆石U-Pb年代学，以提供至少一个年龄/~100 ky间隔，预期精度小于± 50 ky。除了与沉积旋回性起源的争论部分相关之外，这项研究还将测试气候在湖相地层的地层结构中的作用。早始新世气候适宜期是在长期冷却之前最温暖的时期，最终导致目前的冰川气候，因此，鉴于未来随着人为增加pCO 2而预测的更温暖的条件，找到可靠的，高分辨率的气候记录特别重要。一个高分辨率的始新世早期气候历史将有深远的影响，了解气候对哺乳动物进化的影响，并允许详细评估太阳系的动力学模型的鲁棒性，以ca。50马。这项拟议中的研究是EARTHTIME（www.earth-time.org）的成果，EARTHTIME是一项旨在汇集地质年代学家、古生物学家和地层学家的倡议，其目标是以更高的分辨率对地球历史进行测序。拟议的研究将提供一种新的方法来校准早始新世的时间尺度的一部分，并将允许详细的交叉校准之间的40 Ar/39 Ar和U-Pb计时器。

The origin of cycles in sedimentary rocks has long been debated. For at least the past few million years a convincing case can be made that the cycles are often controlled by the periodic variations in Earth's orbit and rotation as well as interactions with nearby planets that produce climate variations every 20, 40, 100, and 400 ky. However in more ancient rocks the problem is much more difficult because of a lack of independent age control which leads to a-priori corrections for apparent changes in sediment accumulation rates prior to testing the orbital forcing hypothesis. Recurrent evidence for incompleteness of cyclical records exacerbates the problem, because any strata "missing" within orbital timescales cannot be detected by existing pre-Neogene chronologies. If one could document that these cycles occur in rocks that are 50 million years old, for example, it would allow geoscientists to tell time in rocks for the past 50 million years at unprecedented levels of precision by matching models for Solar System dynamics with cycles in rocks. We propose a program, of high-resolution U-Pb zircon geochronology to resolve the orbital forcing debate for one of the classic examples of cyclic sediments, the Early Eocene Green River Formation. These spectacularly preserved rocks were deposited in a large lake ca. 50 million years ago. The goal of the proposed work is to quantitatively test, at high precision, the hypothesis that the cyclicity in the Green River Fm. is orbitally forced, where the forcing is achieved through changes in Earth's climate that are paced by the periodic changes in the Earth's orbit. We propose to use U-Pb zircon geochronology from abundant interlayered ash beds to provide at least one age per ~100 ky interval with expected precision of less than plus or minus 50 ky. In addition and partially related to the debate about the origin of sediment cyclicity, this research will also test climate's role in the stratigraphic architecture of lacustrine strata. The Early Eocene Climate Optimum is the warmest interval preceding the prolonged cooling that culminated in the current glaciated climate, and thus finding reliable, highly resolved climate records is especially relevant in view of future warmer conditions predicted with anthropogenic increases in pCO2. A highly resolved Early Eocene climate history will have far reaching implications for understanding the effect of climate on mammalian evolution and allow detailed evaluation of the robustness of dynamical models for the Solar System to ca. 50 Ma. The proposed research is an outcome of EARTHTIME (www.earth-time.org), an initiative to bring together a broad cross-section of geochronologists, paleontologists, and stratigraphers with the goal of sequencing Earth history at a much-improved level of resolution. The proposed research will provide a new approach to calibrating the Early Eocene part of the time-scale and will allow for detailed cross-calibration between 40Ar/39Ar and U-Pb chronometers.