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

由于预计到2100年，人为造成的大气变暖将比工业化前的温度高出2摄氏度以上，因此预测这一变化影响的一个关键不确定性是对这种变暖将如何在海洋和大气中分布的定量理解。评估这一点的一种方法是回顾地质历史，特别是白垩纪晚期到始新世（100-34 Ma前），当时大气中二氧化碳分压水平最后一次高达2100年预测的700 ppmv，全球年平均气温（MAT）比今天（所谓的“温室世界”）高出8摄氏度。化石数据表明，在这一时期，对温度敏感的生物，如爬行动物，生活在北极圈内，并导致了“地球均衡假说”的出现——这一假说认为，在这一时期，经向、赤道到极点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“冰窖世界”根本不同，将热量从热带输送到两极的方式不同/增强。旨在预测未来气候变化的科学家面临的一个根本问题是，最先进的模型无法再现化石数据所显示的全球经向温度梯度崩溃的程度，这反映了“地球平等”假说或气候模型的问题。不管怎样，这种不确定性阻碍了我们自信地预测未来气候变化的深远影响的能力。这项研究将是对“地球平等”假说的首次有力检验。我们将重建沿北美大陆样带的陆地表面MAT的经向变化，该样带跨越中至高古纬度，为几个离散的时间等效瞬时时间片，跨越白垩纪-古近纪（K-Pg）边界-“温室世界”中间的间隔。利用采集的煤样的brGDGT古地温代用物对垫层进行重建。brGDGTs是在陆地环境中繁殖的细菌产生的脂质，其分布是陆地表面垫的函数，可用于重建陆地表面垫。我们已经确定了10个独立的地点，跨越47-75N的古纬度，煤（化石泥炭）明显聚集在一起，通过在K-Pg边界陨石撞击后从大气中沉降的全球同步富集铱（Ir）层的每个煤中出现。除了富ir层外，煤中还含有可测定的热层，这将限制MAT在不同时间片上的垂直变化率。它们还包含富Ir层之前、期间和之后的独特碳同位素事件，这为所有地点之间提供了额外的相关时间线。结合起来，这提供了一个独特的机会，在地球历史的这个关键时期，以亚轨道持续时间为间隔，生成经向陆地表面MAT梯度的连续时间片重建。这将为我们提供一个机会，通过对大陆内部的一系列时间片的经向MAT梯度设置数值界限，来严格检验“地球均衡”假说。通过生成多个间隔的经向MAT梯度，并通过演替生成基于温度年代学的时间序列，将有可能确定从中纬度到高纬度的MAT随时间变化的速率界限。这也将首次揭示“温室地球”气候系统在空间和时间上的动态，并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响应，并允许我们测试气候变化作为K-Pg边界大规模灭绝驱动因素的相互竞争的假设。

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.