FESD Type I Proposal: Continent-island arc fluctuations: Linking deep Earth dynamics to long-term climate

FESD I 类提案：大陆岛弧波动：将地球深层动力学与长期气候联系起来

• 批准号: 1338842
• 负责人: Cin-Ty Lee
• 金额: $ 421万
• 依托单位: William Marsh Rice University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2020-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1338842&HistoricalAwards=false
• 关键词: FESD; Type; Proposal; Continent; island

The over-arching theme of this study is to better understand what drives long-term climate change, specifically oscillations between greenhouse and icehouse states over timescales of 100 My. These climatic oscillations, integrated over Earth?s history, profoundly influenced the evolution of life and the surface of the Earth. To first order, variations in the CO2 budget of the ocean-atmosphere-biosphere system drive climatic variation over timescales greater than 10 My: because of the greenhouse effect of CO2 in the atmosphere, Earth?s surface temperature warms when atmospheric CO2 is high and cools when CO2 levels are low, all other variables (like albedo) being equal. The C content of the Earth?s exogenic system, over long timescales, is controlled by volcanic inputs from the Earth?s interior and outputs from the exogenic system via sediment burial and subduction. Long-term climate variability is thus intimately linked to whole Earth carbon cycling, that is, the cycling of C between the endogenic and exogenic systems. Exactly how and why these inputs and outputs have changed through time is the question. This study will focus on the most recent greenhouse-icehouse transition. This begins with the Cretaceous to early Cenozoic (150-60 My) greenhouse interval when dinosaurs roamed the Earth, atmospheric CO2 pressure was possibly 4-8 times higher than today, polar ice caps were absent, and much of Earth?s economically viable hydrocarbon source rocks were generated. In contrast, the mid-Cenozoic (~55 My to present) was characterized by cooler surface temperatures, polar ice sheets, lower atmospheric CO2, and the proliferation of mammals. We will evaluate a number of hypotheses for elevated CO2 during the Cretaceous. These include enhanced carbonate subduction and subsequent output of CO2 through arc volcanoes, enhanced oceanic crust production, and an increase in the frequency of episodic flood basalts. In particular, we will also explore a new hypothesis that CO2 inputs into the exogenic system are strongly influenced by secular changes in the nature of subduction zone volcanoes. During periods of enhanced continental arc activity, carbonate sediments stored on the continents over Earth?s history, are magmatically liberated, whereas during periods dominated by island arc activity the CO2 inputs return to baseline levels because of the smaller volumes of carbonates in the oceanic upper plate. The transition from Cretaceous greenhouse to mid-Cenozoic icehouse conditions may have coincided with a decline in the number of continental arc volcanoes, suggesting that there may also be a mechanism driving long term oscillations between the nature of subduction zones. The goal of this study is to evaluate the relative importance of all these potential sources of CO2 so that a more complete model of the whole Earth carbon cycle can be developed. We are specifically interested in how deep Earth dynamics modulates these sources of CO2. To test these hypotheses and place bounds on each of these processes, we have assembled an interdisciplinary team to quantify the stability of carbonates in the shallow crust and in the deep parts of subduction zones, map out how the distribution of arc volcanoes and the extent of magmatic decarbonation has changed through time, quantify global volcanic inputs of CO2, and develop a model for long-term climate evolution coupled to the cycling of C between the deep Earth and the exogenic system.

这项研究的首要主题是更好地了解长期气候变化的驱动因素，特别是 100 My 时间尺度内温室状态和冰室状态之间的振荡。 这些气候波动贯穿了地球的历史，深刻地影响了生命和地球表面的进化。 首先，海洋-大气-生物圈系统的二氧化碳收支变化会导致气候变化超过 10 My：由于大气中二氧化碳的温室效应，当大气中二氧化碳含量较高时，地球表面温度会升高，并且当二氧化碳含量较低且所有其他变量（如反照率）相同时，温度会变冷。 地球外生系统的碳含量在很长一段时间内受到地球内部火山输入和外生系统通过沉积物埋藏和俯冲输出的控制。 因此，长期气候变化与整个地球碳循环密切相关，即碳在内源系统和外源系统之间的循环。 问题在于这些输入和输出究竟如何以及为何随着时间发生变化。这项研究将重点关注最近的温室-冰库转变。 这始于白垩纪到新生代早期（150-60 My）温室期，当时恐龙在地球上漫游，大气中的二氧化碳压力可能比今天高 4-8 倍，极地冰盖不存在，地球上大部分经济上可行的碳氢化合物生成了烃源岩。相比之下，新生代中期（〜55 My至今）的特点是地表温度较低、极地冰盖、大气二氧化碳含量较低以及哺乳动物的繁殖。 我们将评估白垩纪期间二氧化碳浓度升高的一些假设。这些包括增强的碳酸盐俯冲和随后通过弧火山输出二氧化碳、增强洋壳产量以及间歇性溢流玄武岩频率的增加。特别是，我们还将探索一个新的假设，即输入外生系统的二氧化碳受到俯冲带火山性质的长期变化的强烈影响。在大陆弧活动增强的时期，地球历史上储存在大陆上的碳酸盐沉积物通过岩浆释放出来，而在岛弧活动占主导地位的时期，由于大洋上层碳酸盐的体积较小，二氧化碳输入返回到基线水平盘子。 从白垩纪温室条件向新生代中期冰室条件的转变可能与大陆弧火山数量的减少同时发生，这表明可能存在一种驱动俯冲带性质之间长期振荡的机制。 这项研究的目的是评估所有这些潜在二氧化碳来源的相对重要性，以便开发出更完整的整个地球碳循环模型。 我们特别感兴趣的是地球动力学如何调节这些二氧化碳来源。 为了检验这些假设并为每个过程设定界限，我们组建了一个跨学科团队来量化浅层地壳和俯冲带深层碳酸盐的稳定性，绘制出弧火山的分布和范围。岩浆脱碳随着时间的推移而发生变化，量化全球火山输入的二氧化碳，并开发与地球深处和外生系统之间的碳循环相结合的长期气候演化模型。