Antarctic ice core records show that atmospheric CO₂ concentration during the Last Glacial Maximum (~20,000 years ago) was about 100 ppm lower than the current warm interglacial called the Holocene (~10,000 years ago to A.D. 1850). Understanding the processes that led to past atmospheric CO₂ changes is crucial and may help to constrain future carbon cycle and climate changes in face of the on-going atmospheric CO₂ increase.

Many studies have attempted to investigate past carbon cycle changes and their impacts on atmospheric CO₂. However, data interpretation is complicated by carbon redistributions within the ocean, which means that for example, ocean carbon content changes at one location cannot be simply linked to atmospheric CO₂ changes. The prevailing view is that suppressed outgassing of CO₂ in the Southern Ocean is key to lowering atmospheric CO₂ during ice ages. Like all other systems, however, atmospheric CO₂ concentration is controlled by both CO₂ gains (e.g., via Southern Ocean outgassing) and losses (e.g., via North Atlantic absorption). Crucially, past changes in oceanic CO₂ uptake remain unconstrained to date, preventing our understanding of the entire system.

In this new study, we present a novel approach to estimate the air-sea exchange component CO₂, whose changes can be more directly linked to atmospheric CO₂ variations. This new method suggests that CO₂ absorption in the North Atlantic was about twice as efficient during the Last Glacial Maximum compared to the Holocene. This new estimate, based on geochemical data from multiple sediment cores, indicates an additional ~100 Gigatonne sequestration of carbon by the North Atlantic during the Last Glacial Maximum, equivalent to a ~50 ppm atmospheric CO₂ decrease. This highlights an important role of the North Atlantic in carbon sequestration in the glacial deep ocean.

A greater high-latitude cooling and enhanced nutrient utilization could be responsible for more efficient glacial CO₂ absorption in the North Atlantic, but further studies are required to figure out the exact mechanisms. As the Southern Ocean has long been suggested to be an important region in driving changes in atmospheric CO₂, it would be interesting to apply the new approach to estimate past air-sea CO₂ exchange in the Southern Ocean.

This work is led by Dr. Yu from the Research School of Earth Sciences at the Australian National University through collaborations with 14 scientists from Australia, China, UK and USA, including Professor JIN Zhangdong and Dr. ZHANG Fei from Institute of Earth Environment, Chinese Academy of Sciences.

Schematics to illustrate more efficient atmospheric CO₂ uptake during the Last Glacial Maximum (LGM) than during the Holocene. Stronger solubility and biological pumps, indicated by meridional temperature (T; cyan arrows) and nutrient (PO₄; green arrows) gradients, are thought to enhance glacial CO₂ absorption (red arrows). DIC_as represents air-sea exchange component CO₂ derived from multiple carbonate chemistry reconstructions including phosphate and carbonate ion concentrations. Circles indicate sediment cores used for palaeoceanographic reconstructions. Black arrows denote movements of major ocean water masses including Gulf Streams, North Atlantic Deep Water (NADW), and Glacial North Atlantic Intermediate Water (GNAIW). (Image by YU, et al)

Contact: Bai Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: baijie@ieecas.cn