RECONSTRUCTING LATITUDINAL TERRESTRIAL TEMPERATURE GRADIENTS AT THE CRETACEOUS-PALAEOGENE BOUNDARY: TESTING THE "EQUABLE EARTH" HYPOTHESIS

重建白垩纪-古近纪边界的纬度陆地温度梯度：检验“平等地球”假说

• 批准号: NE/S002324/1
• 负责人: Rhodri Jerrett
• 金额: $ 74.16万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2019
• 资助国家: 英国
• 起止时间: 2019 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FS002324%2F1
• 关键词: RECONSTRUCTING; LATITUDINAL; TERRESTRIAL; TEMPERATURE; GRADIENTS

As anthropogenic atmospheric warming is forecasted to exceed 2C above preindustrial temperatures by 2100, a key uncertainty in predicting the impact of this change is the quantitative understanding of how this warming will be distributed in the oceans and atmosphere. One means of assessing this is to look to the geological past, especially the late Cretaceous to Eocene (100-34 Ma ago), when atmospheric pCO2 levels were last as high as the 700 ppmv forecasted for 2100, and global mean annual temperatures (MAT) were up to 8C warmer than today (the so-called "Greenhouse World"). Fossil data suggest that temperature-sensitive organisms, such as reptilians, were living in the Arctic-circle during this period, and led to the emergence of the "Equable Earth" hypothesis - a scenario that invokes near total collapse of the meridional, equator-to-pole temperature gradient at this time. This indicates a climate system that operated in a fundamentally different way to the modern "Icehouse World", with a different/enhanced means of transporting heat from the tropics to the poles. A fundamental problem for scientists aiming to predict future climate change, is that state-of-the-art models are not able to reproduce the degree of collapse of the global meridional temperature gradient suggested by fossil data, reflecting a problem with either the "Equable Earth" hypothesis, or with climate modelling. Either way, this uncertainty impedes our ability to confidently predict the impact of future climate change with far-reaching implications.This research will be the first robust test of the "Equable Earth" hypothesis. We will reconstruct meridional variation in land surface MAT in a transect along the North American Continent, spanning mid- to high-palaeolatitude for several discrete time-equivalent instantaneous time-slices spanning the Cretaceous-Palaeogene (K-Pg) boundary - an interval in the middle of the "Greenhouse World". The MATs will be reconstructed using the brGDGT palaeotemperature proxy from collected coal samples. brGDGTs are lipids produced by bacteria thriving in terrestrial environments, whose distribution is a function of land surface MAT and can be used to reconstruct land surface MATs. We have identified ten separate sites, spanning 47-75N of palaeolatitude, where coals (fossil peats) were demonstrably accumulating coevally, by the occurrence within each of the coals of the globally synchronous Iridium (Ir)-enriched layer that settled from the atmosphere after the impact of a meteorite at the K-Pg boundary. In addition to the Ir-enriched layer, the coals contain datable tephra horizons, which will constrain vertical rates of change of MAT from time-slice to time-slice. They also contain distinctive carbon isotopic events before, during and after the Ir- enriched layer, which provide additional correlatable time lines between all locations. Combined, this provides an unique opportunity to generate serial time-slice reconstructions of meridional land surface MAT gradients, spaced at sub-orbital durations, at this critical period in Earth history. This will provide us with the opportunity to critically test the "Equable Earth" hypothesis, by placing numerical bounds on meridional MAT gradients for a series of time slices in continental interiors at this time. By generating meridional MAT gradients for multiple intervals, and by generating a tephrochronologically-based time-series through the succession, it will be possible to place bounds on the rates of change of MAT in time, from mid- to high- latitude. This will also reveal, for the first time, the dynamics in space and time of the "Greenhouse Earth" climate system, and will also allow us to assess MAT in the aftermath of meteorite impact at the K-Pg boundary, giving insight into the response of the climate system to catastrophic change, and allowing us to test competing hypotheses of climate change as the driver for the mass extinction at the K-Pg boundary.

据预测，到2100年，人为造成的大气变暖将比工业化前的温度高出2摄氏度，预测这种变化的影响的一个关键不确定性是对这种变暖在海洋和大气中的分布的定量理解。评估这一点的一种方法是查看地质历史，特别是白垩纪晚期到始新世（100-34 Ma前），当时大气pCO 2水平最后高达2100年预测的700 ppmv，全球平均年气温（MAT）比今天高出8摄氏度（所谓的“温室世界”）。化石数据表明，在这一时期，对温度敏感的生物，如爬行动物，生活在北极圈，并导致了“地球均衡”假说的出现--这一假设认为，此时赤道到两极的温度梯度几乎完全崩溃。这表明，气候系统的运作方式与现代的“冰屋世界”有着根本的不同，它具有不同的/增强的将热量从热带输送到两极的方式。科学家在预测未来气候变化时面临的一个根本问题是，最先进的模型无法重现化石数据所显示的全球纬向温度梯度的崩溃程度，这反映了“地球均衡”假说或气候建模的问题。无论如何，这种不确定性阻碍了我们自信地预测未来气候变化影响的能力，这将产生深远的影响。这项研究将是对“地球均衡”假设的首次有力检验。我们将重建沿沿着北美大陆的一个样带中的陆地表面MAT的纬向变化，跨越中高古纬度的几个离散的时间等效的瞬时时间切片跨越古近纪（K-Pg）边界-中间的“温室世界”的间隔。将使用采集的煤样的brGDGT古温度代用指标重建MAT。brGDGT是由在陆地环境中生长的细菌产生的脂质，其分布是陆地表面MAT的函数，并且可以用于重建陆地表面MAT。我们已经确定了10个独立的网站，跨越47- 75 N的古纬度，在那里煤（化石泥炭）显然是积累coevally，发生在全球同步的铱（Ir）富集层，从大气中的陨石在K-Pg边界的影响后，从每个煤。除了富铱层，煤还含有可定年火山灰层位，这将限制MAT从时间片到时间片的垂直变化率。它们还包含在Ir富集层之前、期间和之后的独特的碳同位素事件，这提供了所有地点之间的额外可验证的时间线。结合起来，这提供了一个独特的机会，以产生连续的时间片重建的纬向陆地表面MAT梯度，间隔在亚轨道的持续时间，在地球历史的这一关键时期。这将为我们提供一个机会，严格测试“均衡地球”的假设，通过放置在一系列的时间片在大陆内部的时间轴MAT梯度的数值界限在这个时候。通过生成多个间隔的经向MAT梯度，并通过生成整个序列的基于灰岩年代学的时间序列，可以对从中纬度到高纬度的MAT随时间的变化率进行限制。这也将首次揭示“温室地球”气候系统的空间和时间动态，也将使我们能够评估陨石撞击K-Pg边界后的MAT，深入了解气候系统对灾难性变化的反应，并使我们能够测试气候变化作为K-Pg边界大规模灭绝驱动力的相互竞争的假设。

项目成果

Steady decline in mean annual air temperatures in the first 30 k.y. after the Cretaceous-Paleogene boundary

前30年年平均气温稳步下降

• DOI: 10.1130/g50588.1
• 发表时间: 2023
• 期刊: Geology
• 影响因子: 5.8
• 作者: O'Connor L