Abrupt Ocean Acidification Events

全球变暖并不是大气中二氧化碳水平上升的唯一后果。由于二氧化碳是一种酸性气体，而人类排放的很高比例的气体最终会混合到海洋中，海洋正在变得更加酸性。由于人类旨在通过设定目标来限制排放，因此预测这种酸化将对海洋生物和海洋中的化学过程产生什么影响是至关重要的。实验研究表明，由碳酸钙(钙化)制成外壳的浮游生物可能会受到特别的影响。预测未来的一种方法是研究地质历史中的类似事件。数百万年前，大气中的二氧化碳水平高于现在，导致海洋酸化。地质记录包含一些突如其来的事件，其中酸化作用因自然排放而迅速增加。这些事件的大小各不相同，因此可以被视为全球范围内海洋酸化的一系列“自然实验”。到目前为止，这些事件期间的酸化程度还没有得到充分测量，海洋生物受到影响的细节也没有得到充分调查。这项提案旨在纠正这一点。目前，我们面临着一个明显的悖论。地质证据表明，过去的一些酸化事件可能相当严重，特别是最大的酸化事件，它界定了古新世和始新世之间的界线。虽然有一些灭绝、迁徙和其他影响，但总体上浮游生物的反应并不是很大。这包括像球藻和有孔虫这样的钙化浮游生物。因此，这些群体对pH变化的适应能力可能比实验室实验所显示的更强。变化的速度可能是最重要的因素，浮游生物可以在一定范围内适应。或者，我们可能对这些生物生活的地表水中发生的酸化程度有误。这些不确定性必须作为紧急事项加以解决。过去海洋的酸度变化在一组钙化有机体-有孔虫的贝壳上留下了化学指纹，其中一些作为浮游生物生活在水柱的不同深度，另一些生活在海底。这个指纹是进入贝壳中的硼同位素的比例。通过测量这一点，我们可以计算出有孔虫生活的水的酸度。一个潜在的问题是，海水中硼同位素的总体比例随着时间的推移而变化，尽管我们可以通过几种方法来限制这一点，例如通过检查生活在不同水深的有孔虫的酸度剖面。该提案将调查深海和大洋边缘的各种地质记录。其中独一无二的是坦桑尼亚的一个穿过海洋沉积物的钻孔。初步工作揭示了古新世/始新世事件开始的高度扩展(厚)记录的存在。关键的是，有孔虫保存得很好。2008年，该工地使用了先进的钻井技术进行了重新钻探，目前正在等待详细的采样和研究。这一记录将使我们能够确定pH值变化的速度和幅度，以及研究对该地点保存完好的浮游生物群落的影响。其他网站提供了一系列不同规模的事件。通过评估酸化程度和生态反应，我们可以校准影响。然而，这项任务并不容易；除了酸度之外，还有其他因素参与其中，而且这些事件前后的酸度水平与现代海洋不同。只有将这项研究的结果与其他小组正在进行的实验工作结合起来，我们才能对未来不同排放情景下可能发生的情况提供更准确的预测。

Global warming is not the only consequence of rising levels of carbon dioxide in the atmosphere. Because CO2 is an acidic gas, and a high proportion of the human emissions eventually get mixed into the ocean, the oceans are becoming more acidic. As humanity aims to limit emissions by setting targets it is vital to predict what effect such acidification will have on marine life and chemical processes in the ocean. Experimental work suggests that plankton that make their shells out of calcium carbonate ('calcify') may be particularly affected. One way to predict the future is to study similar events in the geological past. Millions of years ago atmospheric carbon dioxide levels were higher than they are at present, causing ocean acidification. The geological record contains a number of abrupt events where acidification increased rapidly in response to natural emissions. These events are of different sizes so can be viewed as a set of 'natural experiments' in ocean acidification on a global scale. So far the amount of acidification during these events has not been measured adequately and details of the way marine life was affected have not been fully investigated. This proposal aims to rectify this. At present we are faced with an apparent paradox. Geological evidence indicates that some of the past acidification events were likely quite severe, especially the biggest of all which defines the boundary between the Paleocene and Eocene epochs. Although there were some extinctions, migrations and other effects, overall there was not a very great response in the plankton. This includes calcifying plankton like coccolithophorids and foraminifera. Hence it may be that these groups are more resilient to pH changes than laboratory experiments suggest. Possibly the rate of change is the most important factor, and the plankton can adapt within limits. Alternatively we may be wrong about the degree of acidification that occurred in the surface waters where these organisms live. These uncertainties must be resolved as a matter of urgency. Acidity changes in the past ocean leave a chemical fingerprint in the shells of one group of calcifying organism, the foraminifera, some of which live as plankton at different depths in the water column and some on the sea floor. This fingerprint is the ratio of isotopes of boron incorporated into the shell. By measuring this we can calculate the acidity of the water in which the foraminifera lived. One potential problem is that the overall ratio of boron isotopes in seawater has changed through time, although there are several ways in which we can constrain this, for instance by examining acidity profiles based on foraminifera that live at different water depth. The proposal will investigate a variety of geological records from the deep sea and the margins of the oceans. Unique among these is a borehole through marine sediments in Tanzania. Preliminary work revealed the presence of a highly expanded (thick) record of the onset of the Paleocene / Eocene event. Crucially the foraminifera are excellently preserved. In 2008 this site was redrilled using superior drilling technology and the excellent cores currently await detailed sampling and study. This record will enable us to define the rate and magnitude of pH change as well as study the effects on the extraordinarily diverse communities of well-preserved plankton at that site. Other sites provide a set of events of various magnitudes. By assessing the degree of acidification and the ecological responses, we can calibrate the effects. The task is not easy however; factors other than acidity were involved, and the 'background' level of acidity before and after these events was different from the modern ocean. Only by integrating the results of this study with ongoing experimental work by other groups can we aim to provide more accurate predictions of what might happen under different emissions scenarios in the future.