GRGC: Weathering, Plants and the Long-Term Carbon Cycle

9417325 Berner 大陆上钙和镁硅酸盐矿物的风化，以及随后在海洋中以碳酸盐形式释放的钙和镁的沉淀，对地质时期大气中二氧化碳的水平产生了主要控制。 这个过程的一个重要方面是植物在风化过程中改变矿物质溶解速率所发挥的作用。 显生宙上维管束陆地植物的兴起和演化是否影响了硅酸盐风化的速度，进而影响了大气中二氧化碳的水平？ 通过使用电子显微探针的岩相学研究，我们发现，在夏威夷岛上，维管束植物在年轻玄武岩流上的生长会导致植物根部附近硅酸钙和硅酸镁的强烈蚀刻和去除。 结果发现，高等植物下的初始风化相对速率比地衣下或没有任何植物生命的情况下要大得多。 同样，在圣海伦斯山，我们发现十年前的火山灰沉积物显示出紧邻植物根部的化学变化的证据，但没有证据表明远离根部的地方。 尽管这项岩相学研究在定性到半定量方面很有用，但仍需要一种更好的方法来量化植物对硅酸盐风化的影响。 这可以通过研究排出岩石的溶液的化学性质来完成。 我们在新罕布什尔州哈伯德布鲁克实验森林站的自然户外条件下进行了一项多年持续的对照实验，研究幼苗和草对硅酸盐风化的影响，初步水化学结果表明，树木的硅酸盐风化率至少提高了 1.5-3.5 倍。 我们建议通过检查排出冰岛玄武岩的水的化学成分来检查这一结果。 冰岛是我们提出的研究的理想选择，因为岩石富含火山玻璃和硅酸钙和硅酸镁，不含碳酸钙，年龄足够小，植被覆盖程度存在差异，不存在人为酸雨，并且可以轻松采集泉水和溪水样本。 （由于那些年轻的玄武岩流的渗透性很强，植物覆盖度存在差异，因此不可能在夏威夷岛上进行水采样。）对冰岛的勘察显示，有几个地方可以从植被和无植被区域收集水，这些区域彼此非常接近，具有相同的基岩岩性、小气候和方面。这使得可以对当地植物生命对硅酸盐风化的定量作用进行实验。 此外，Sigurdur Gislason 博士在冰岛已经拥有相当多的耐候溶液化学方面的专业知识，我们将在该领域得到他的协助。 吉斯拉森博士对冰岛泉水的初步化学数据表明，植物对风化率的影响应该是巨大的。

9417325 Berner The weathering of Ca and Mg silicate minerals on the continents, followed by the precipitation of the liberated Ca and Mg in the oceans as carbonates, has exerted a major control on the level of atmospheric CO2 over geologic time. An important aspect of this process is the role played by plants as they modify the rate of mineral dissolution during weathering. Did the rise and evolution of vascular land plants over the Phanerozoic affect the rate of silicate weathering, and in turn the level of atmospheric CO2? From petrographic studies using the electron microprobe we have discovered that on the island of Hawaii the growth of vascular plants on young basalt flows results in intense etching and removal of Ca and Mg silicates in the vicinity of plant rootlets. As a result relative rates of initial weathering under higher plants were found to be much greater than under lichens or in the absence of any plant life. Similarly at Mt. St. Helens we found that a ten year old volcanic ash deposit shows evidence of chemical alteration immediately adjacent to plant rootlets but no evidence away from the rootlets. Although this petrographic study has been useful in a qualitative to semi-quantitative sense, there is a need for a better way to quantify the effects of plants on silicate weathering. This can be done through the study of the chemistry of solutions draining the rocks. Our preliminary water chemical results of a multiyear ongoing controlled experiment on the effects of young growing trees and grass on silicate weathering, conducted under natural outdoor conditions at the Hubbard Brook Experimental Forest Station in New Hampshire, indicate a minimum silicate weathering rate enhancement by the trees of a factor of 1.5-3.5. We propose to check this result by examining the chemistry of waters draining the basaltic rocks of Iceland. Iceland is ideal for our proposed study in that the rocks are rich in volcanic glass and Ca and Mg silicates , contain no CaCO3, are young enough that variations in the degree of vegetational coverage is present, anthropogenic acid rain is absent and spring and stream waters can be easily sampled. (Water sampling on the island of Hawaii was impossible due to the great permeability of those young basalt flows that exhibit variations in plant coverage.) A reconnaissance visit to Iceland shows that there are several localities where waters can be collected from vegetated and unvegetated areas that are sufficiently nearly to one another that they share the same bedrock lithology, microclimate, aspect. etc. This makes it possible to conduct an experiment on the quantitative role of the local plant life on silicate weathering. Also, there is considerable expertise already available on the chemistry of weathering solutions in Iceland, in the person of Dr. Sigurdur Gislason, and we will be assisted by him in the field. Dr. Gislason's preliminary chemical data on Icelandic spring waters suggest that the effect of plants on weathering rate should be substantial.