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

沉积岩旋回的起源一直存在争议。至少在过去的几百万年里，有一个令人信服的例子可以证明，这些周期通常是由地球轨道和自转的周期性变化以及与附近行星的相互作用控制的，这些行星每隔20、40、100和400天就会产生一次气候变化。然而，在更古老的岩石中，这个问题要困难得多，因为缺乏独立的年龄控制，这导致在测试轨道强迫假说之前对沉积物积累速率的明显变化进行先验修正。周期性记录不完整的反复出现的证据加剧了这个问题，因为任何在轨道时间尺度内“缺失”的地层都无法被现有的前新近纪年表检测到。例如，如果有人能记录下这些周期发生在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周期相匹配，以前所未有的精度来判断过去5000万年的岩石时间。我们提出了一个高分辨率的U-Pb锆石年代学方案，以解决旋回沉积物的一个经典例子——早始新世绿河组的轨道强迫争论。这些保存完好的岩石大约在5000万年前沉积在一个大湖里。提出的工作目标是定量地、高精度地检验绿河Fm的循环性假设。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。我们建议利用来自丰富层间灰层的U-Pb锆石年代学，每~100 ky区间至少提供一个年龄，期望精度小于正负50 ky。此外，与沉积物旋回起源的争论部分相关，本研究还将测试气候在湖相地层结构中的作用。早始新世气候最佳期是在长时间变冷之前最温暖的时期，而长时间变冷在当前的冰川气候中达到顶峰。因此，鉴于人类活动导致的二氧化碳分压增加预测了未来更温暖的条件，寻找可靠的、高度分辨率的气候记录尤为重要。高度确定的始新世早期气候历史将对理解气候对哺乳动物进化的影响具有深远的意义，并允许对太阳系动力学模型的鲁棒性进行详细评估，直至大约50 Ma。这项提议的研究是EARTHTIME （www.earth-time.org）的成果，该倡议汇集了广泛的地质年代学家、古生物学家和地层学家，目的是在分辨率大大提高的水平上对地球历史进行排序。该研究将提供一种新的方法来校准早始新世部分的时间尺度，并将允许在40Ar/39Ar和U-Pb计时器之间进行详细的交叉校准。

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.