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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >RECONCILING LATE ORDOVICIAN (440 MA) GLACIATION WITH VERY HIGH (14X) CO2 LEVELS
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RECONCILING LATE ORDOVICIAN (440 MA) GLACIATION WITH VERY HIGH (14X) CO2 LEVELS

机译：以极高（14倍）的CO2水平调和晚奥陶纪（440 MA）冰川

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摘要

Geochemical data and models suggest a positive correlation between carbon dioxide changes and climate during the last 540 m.y. The most dramatic exception to this correlation involves the Late Ordovician (440 Ma) glaciation, which occurred at a time when CO2 levels may have been much greater than present (14-16X?). Since decreased solar luminosity at that time only partially offset increased radiative forcing from CO2, some other factor needs to be considered to explain the glaciation. Prior work with energy balance models (EBMs) suggested that the unique geographic configuration of Gondwanaland at that time may have resulted in a small area of parameter space permitting permanent snow cover and higher CO2 levels. However, the crude snow and sea ice parameterizations in the EBM left these conclusions open to further scrutiny. Herein we present results from four experiments with the GENESIS general circulation model with CO2 levels 14X greater than present, solar luminosity reduced 4.5%, and an orbital configuration set for minimum summer insolation receipt. We examined the effects of different combinations of ocean heat transport and topography on high-latitude snow cover on Gondwanaland. For the no-elevation simulations we failed to simulate permanent summer snow covet. However, for the slightly elevated topography cases (300-500 m), permanent summer snow cover occurs where geological data indicate the Ordovician ice sheet was present. These results support the hypothesis based on EBM studies. Further results indicate that although average runoff per grid point increases substantially for the Ordovician runs, the decreased land area results in global runoff 10-30% less than present, with largest runoff reductions for flat topography This response has implications for CO2-runoff/weathering parameterizations in geochemical models. Finally, simulated tropical sea surface temperatures (SSTs) are the same or only marginally warmer than present. This result is consistent with evidence from other warm time intervals indicating small changes in tropical SSTs during time of high CO2. [References: 52]
机译：地球化学数据和模型表明，最近540 y期间二氧化碳变化与气候之间呈正相关。这种相关性最戏剧性的例外是奥陶纪晚期（440 Ma）冰期，这发生在二氧化碳水平可能远大于目前（14-16X？）的时候。由于当时太阳光度的降低仅部分抵消了来自CO2的辐射强迫的增加，因此需要考虑其他一些因素来解释这种冰川作用。先前使用能量平衡模型（EBM）的工作表明，冈瓦纳群岛当时的独特地理构造可能导致较小的参数空间区域，允许永久性积雪和较高的CO2水平。但是，EBM中的原始雪和海冰参数设置使这些结论有待进一步研究。在这里，我们介绍了GENESIS普通循环模型的四项实验的结果，该模型的CO2水平比当前高14倍，太阳光度降低4.5％，并设定了最低夏季日照量的轨道配置。我们研究了海洋热传输和地形的不同组合对冈瓦纳大陆上高纬度积雪的影响。对于无高程模拟，我们未能模拟永久性的夏季积雪。但是，对于地形略高的案例（300-500 m），在地质数据表明存在奥陶纪冰盖的情况下会发生永久性夏季积雪。这些结果支持基于EBM研究的假设。进一步的结果表明，尽管奥陶系的每个栅格点的平均径流量大大增加，但减少的土地面积使全球径流量比目前减少了10-30％，平坦地形的径流量减少量最大。此响应对CO2径流量/风化有影响地球化学模型中的参数化。最后，模拟的热带海面温度（SST）与现在相同或仅略高。该结果与其他温暖时间间隔的证据一致，表明高CO2期间热带SST的变化很小。 [参考：52]

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