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

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences

Abstract

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.

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