Revisiting the role of ocean ventilation in glacial CO2 sequestration using radiocarbon (ROGUE14)

在过去的100万年里，大气中的二氧化碳在“冰河时代”的最低值和“间冰期”的最大值之间反复振荡，与全球气温约5摄氏度的波动大致同步。几十年来，这些变化的原因一直是一个引起强烈兴趣和争论的话题。人们普遍认为，这些气候循环涉及海洋内部“多余”二氧化碳的反复封存(和释放)。然而，目前还不清楚到底是什么原因导致海洋在冰河时代的循环中重复储存(然后‘喷出’)二氧化碳。这些过去的全球变化为我们提供了一个极好的‘自然实验室’，在这个实验室里，我们可以发展我们对地球自然系统在面对区域和全球气候变化时的敏感性的理解。这对于理解海洋通过从大气中吸收碳来调节大气二氧化碳的自然能力特别相关。这对于理解海洋通过从大气吸收热量来调节全球气温的自然能力也是相关的。由于海洋碳库的巨大规模和与大气的紧密联系，海洋碳库可以对大气二氧化碳产生强烈影响。海洋的碳储量是大气的60倍，因此海洋碳含量的相对较小变化很容易推动大气二氧化碳的上升或下降。来自海洋的二氧化碳输入/输出的微妙平衡最终取决于固碳生物群和缓慢的颠覆海洋环流的协同作用，前者将碳从阳光充足的浅海输送到深海内陆，后者将碳从海洋内陆虹吸回海面，在那里它可以重新加入大气层。如果从表层海洋输出的生物固定碳就像一个将二氧化碳注入深海内部的‘碳泵’，那么海洋颠覆的环流，再加上海面上的气体交换，就代表着这个泵的‘泄漏’，使得二氧化碳再次逃逸到大气中。这个项目试图调查海洋碳泵‘泄漏性’的变化可能如何导致过去大气中二氧化碳的变化，从而促成了冰河时代的循环。一种流行的观点认为，海洋循环的减缓，加上海气交换的限制(可能与海冰有关，封顶了高纬度海洋)，是上一个冰河时代大气二氧化碳减少的主要原因。根据大约2万年前最后一次冰河时代(即最后一次冰川最大时期，简称LGM)的放射性碳测量数据，大量数据支持这一观点。然而，一些新的观察表明，类似的放射性碳测量并不支持海洋对大气二氧化碳的影响在冰川时期的早期，即40-20,000年前。全球碳循环在40年前和2万年前之间是否经历了迄今未被察觉的巨大变化；或者海洋在大气二氧化碳变化中所起的作用是否比人们以前认为的要小？为了解决这一问题，迫切需要一个涵盖从4万年到现在的整个晚期冰川时期的强大数据集。本项目着手获取这些数据。这项工作的潜在结果是令人兴奋的。如果一方面我们找到了对主流范式的支持，我们就解开了一个关于地球过去气候的长期谜团，并提供了正确理解海洋与大气的碳交换在气候剧烈多变时期如何变化所需的数据。另一方面，如果我们的新数据反驳了主流范式，那么我们的发现将完全重新定位研究海洋-大气相互作用在过去二氧化碳和气候变化中的作用。

Over the last ~1 million years, atmospheric CO2 has oscillated repeatedly between 'ice age' minima and 'interglacial' maxima, in broad synchrony with global temperature swings of ~5 degrees Celsius. The causes of these changes have been a subject of intense interest and debate for decades. It is widely believed that these climate cycles involved the repeated sequestration (and release) of 'excess' CO2 in the ocean interior. However, it remains unclear exactly what would have caused the ocean to repeatedly store (and then 'belch out') CO2 in time with the ice age cycles. These past global changes provide us with an excellent 'natural laboratory' in which to develop our understanding of the sensitivity of the Earth's natural systems, in the face of regional and global climate change. This is particularly relevant for understanding the natural capacity of the ocean to modulate atmospheric CO2, via carbon uptake from the atmosphere. It is also relevant for understanding the natural capacity of the ocean to moderate global temperatures, via heat uptake from the atmosphere.Due to its immense size and tight connection to the atmosphere, the marine carbon pool can exert a strong influence on atmospheric CO2. The ocean's carbon inventory is ~60 times larger than that of the atmosphere, so relatively small changes in the ocean's carbon content can readily push atmospheric CO2 up or down. The fine balance of CO2 inputs/outputs from the ocean is ultimately determined by the synergies of carbon-fixing biota that pump carbon from the shallow sunlit ocean to the deep ocean interior, and the slow overturning circulation of the ocean that siphons carbon back up from the ocean interior to the sea surface where it can rejoin the atmosphere. If the export of biologically-fixed carbon from the surface ocean acts as a 'carbon pump' injecting CO2 into the deep ocean interior, the ocean's overturning circulation, combined with the exchange of gases at the sea surface, represents a 'leak' in this pump, allowing CO2 to escape to the atmosphere again.This project seeks to investigate how changes in the 'leakiness' of the ocean's carbon pump may have been responsible for past changes in atmospheric CO2, and therefore contributed to the ice age cycles. One prevalent view is that a slowing down of the ocean's circulation, combined with a restriction of air-sea gas-exchange (perhaps linked to sea-ice, capping off the high-latitude oceans), was the main cause of lower atmospheric CO2 during the last ice age. A body of data, based on radiocarbon measurements from the height of the last ice age, ~20,000 years ago (i.e. the Last Glacial Maximum, or LGM), support this notion. However, some new observations suggest that similar radiocarbon measurements do not support such a strong oceanic impact on atmospheric CO2 earlier in the glacial period, from 40-20,000 years ago. Did the global carbon cycle undergo massive changes between 40- and 20,000 years ago that have hitherto gone undetected; or did the ocean play a smaller role in atmospheric CO2 change than previously has been thought? A robust dataset that spans the entire late glacial period, from 40,000 years to the present is urgently needed to resolve this question. This project sets out to obtain these data. The potential outcomes of this work are exciting. If on the one hand we find support for the prevailing paradigm, we will have resolved a long-standing puzzle regarding Earth past climate, and provided the data that are needed to properly understand how the ocean's exchange of carbon with the atmosphere has changed over a period of intense climate variability. If on the other hand our new data refute the prevailing paradigm, then our findings will completely reorient research into the role of ocean-atmosphere interactions in past CO2 and climate change.