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”的最小值和“间冰期”的最大值之间反复振荡，与全球温度波动约5摄氏度大致同步。几十年来，这些变化的原因一直是人们强烈关注和辩论的主题。人们普遍认为，这些气候循环涉及海洋内部“过量”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周期中反复储存（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很好的“自然实验室”，以发展我们对地球自然系统在区域和全球气候变化面前的敏感性的理解。这对于了解海洋通过从大气中吸收碳来调节大气CO2的自然能力特别重要。这也有助于了解海洋通过从大气中吸收热量来调节全球温度的自然能力，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中的CO2产生很大影响。海洋的碳存量是大气的60倍，因此海洋碳含量的相对较小的变化可以很容易地推动大气中的二氧化碳上升或下降。海洋二氧化碳输入/输出的精细平衡最终取决于碳固定生物群的协同作用，这些生物群将碳从阳光照射的浅海泵到深海内部，以及海洋的缓慢翻转环流，将碳从海洋内部虹吸到海面，在那里它可以重新加入大气。如果从海洋表面输出的生物固定碳作为一个“碳泵”将二氧化碳注入深海内部，海洋的翻转环流，结合海面的气体交换，代表了这个泵的“泄漏”，让二氧化碳再次逃逸到大气中。这个项目旨在调查海洋碳泵的“泄漏”变化可能是如何影响海洋的碳排放的。负责过去大气中二氧化碳的变化，因此有助于冰河时代的循环。一种流行的观点是，海洋环流的减缓，加上海气交换的限制（可能与海冰有关，覆盖了高纬度海洋），是上一个冰河时代大气中二氧化碳含量较低的主要原因。一系列基于放射性碳测量的数据，来自大约20，000年前的最后一个冰河时代（即末次冰期最大期或LGM）的高度，支持这一观点。然而，一些新的观测结果表明，类似的放射性碳测量结果并不支持在冰河时期早期（40- 20，000年前）海洋对大气中二氧化碳的强烈影响。全球碳循环是否在40至20，000年前经历了迄今未被发现的巨大变化;或者海洋在大气CO2变化中所起的作用比以前认为的要小？迫切需要一个跨越整个晚冰期（从4万年到现在）的强大数据集来解决这个问题。本项目旨在获取这些数据。这项工作的潜在成果令人兴奋。如果一方面我们找到了流行范式的支持，我们将解决一个长期存在的关于地球过去气候的难题，并提供所需的数据，以正确理解海洋与大气的碳交换如何在强烈的气候变化期间发生变化。另一方面，如果我们的新数据反驳了流行的范式，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.