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

在冈瓦纳的几个地方有证据表明，在奥陶世晚期和志留纪早期，至少有一个主要的大陆冰盖。然而，各种来源的地质证据表明，当时大气中的二氧化碳分压很高，而Berner（1990,1991）的地球化学循环模型表明，其含量可能高达或高于目前大气水平的13倍。为了解决这个明显的矛盾（高二氧化碳分压意味着无冰的地球），将采用以下方法：1)将研究两个时期：缺乏极地冰证据的中奥陶世，以及有证据表明冰延伸最大的阿什吉尔（晚奥陶世）。2)将大气环流模型GCM应用于奥陶纪气候研究，并利用其结果驱动空间解析的地球化学循环模型（考虑气候和古岩性的地理变化）。我们将首先用13xPAL的二氧化碳水平来模拟这两个时期的气候。如果地理上的差异不能解释气候差异，那么我们将逐步研究奥陶世晚期的二氧化碳水平，直到我们发现全年都有雪，地质证据表明有冰盖。通过这种方式，我们可以确定二氧化碳分压值的范围，该范围与冰川学证据在气候上是相容的。然后，我们将运行一个全球地球化学循环模型，通过观察哪种气候（即风化机制）产生的二氧化碳消耗速率与通过火山作用和变质作用推断出的二氧化碳产生速率一致，在一定程度上提供了一种估算/确认二氧化碳分压的额外方法。特别是，我们将研究中奥陶纪（陆相）火山活动增加导致晚奥陶纪（陆相）硅酸盐风化作用增加的假设。风化作用的增加意味着碳循环在比当前模式显示的二氧化碳分压水平更低的情况下处于平衡状态，从而允许发生冰川作用。这项对中奥陶世和晚奥陶世的研究将使我们看到冰川状态的演变，并将对大气中二氧化碳作为地球气候的主要统治者的重要性进行测试。

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.