Global Change: Atmospheric pCO2 and Glaciation- A Coupled Paleoclimatic and Geochemical Cycling Study of the Late Ordovician/Early Silurian

在晚奥陶世和志留纪早期，冈瓦那的几个地区至少有一个主要的大陆冰盖的证据。然而，各种地质证据表明，此时大气中二氧化碳含量很高，伯纳(1990,1991)的地球化学循环模型表明，二氧化碳含量可能是目前大气水平的13倍或更高。为了解决这一明显的悖论(预计高二氧化碳将意味着地球没有冰)，将采用以下方法：1)将研究两个时期：中奥陶世，缺乏极地冰的证据，以及阿什吉尔(晚奥陶世)，有证据表明最大的冰延伸。2)大气环流模式GCM将被应用于奥陶系气候的研究，其结果将被用来驱动空间分辨的地球化学循环模式(该模式考虑了气候和古岩石学的地理变化)。我们将首先模拟这两个时期的气候，二氧化碳水平为13xPAL。如果地理上的差异不能解释气候的差异，那么我们将逐渐改变奥陶纪晚期的二氧化碳水平，直到我们发现，在地质证据表明有冰盖的地方，全年都有雪。通过这种方法，我们可以确定与冰川学证据相一致的PCO2值范围。然后，我们将运行全球地球化学循环模型，部分原因是通过观察哪种气候(即风化制度)产生的二氧化碳消耗率与通过火山作用和变质作用推断的生产率一致，从而提供另一种估计/确认二氧化碳的方法。特别是，我们将检验这一假说，即中奥陶世(陆地)火山活动增加导致晚奥陶世(陆地)硅酸盐风化增加。更多的风化将意味着碳循环在比当前模型所显示的更低的二氧化碳水平上处于平衡，从而允许冰川作用。这项对中、晚奥陶世的研究将使我们能够观察到冰川状态的演化，并将检验大气二氧化碳作为地球气候主要监控者的重要性。

Evidence exits from several parts of Gondwana for at least one major, continental ice sheet during the Late Ordovician and Early Silurian. Yet various sources of geological evidence suggest high atmospheric pCO2 at this time, and Berner's (1990,1991) Geochemical cycling model indicates that the amount may have been as high or higher than 13 times the present atmospheric level. To resolve this apparent paradox (high pCO2 would be expected to imply an ice- free Earth) the following approach will be followed: 1) Two periods will be studied: the Middle Ordovician, which lacks evidence for polar ice, and the Ashgill (Late Ordovician) which has evidence for the largest ice extend. 2) An atmospheric general circulation model GCM will be applied to the study of Ordovician climates, and the results of this will then be used to drive a spatially resolved geochemical cycling model (which considers geographical variations in climate and paleolithology). We will first model the climates of both periods with a CO2 level of 13xPAL. If differences in geography cannot account for the climate difference then we will progressively lover the Late Ordovician CO2 levels until we find that snow remains throughout the year where the geological evidence indicates an ice cap. In this way we can determine the range of pCO2 values which is climatically compatible with the glaciological evidence . We will then run a global geochemical cycling model, in part to provide an additional means of estimating/confirming pCO2 by seeing which climate, i.e., weathering regime, generates a CO2 consumption rate that is consistent with the inferred rate of production via volcanism and metamorphism. In particular we will examine the hypothesis that increased (terrestrial) volcanism in the Middle Ordovician led to increased (terrestrial) silicate weathering in the Late Ordovician. Increased weathering would mean that the carbon cycle was in balance at a lower pCO2 level than current model indicate, and thus allow glaciation. This study of the Middle and Late Ordovician will allow us to look at the evolution to a glaciated state, and will be a test of the importance of atmospheric CO2 as a prime governor of climate on Earth.