Ice Cores: Noble Gases as Past Ocean Temperature Proxies

This essay is based on my master thesis and published at Geophysical Research Letters. Please click ResearchGate link here for full text of both.

Ice Cores: Time Capsules for Paleoclimate

Antarctica ice cores are a powerful tool for retrieving past climate data: they are like time capsules containing dust and air bubbles trapped in there from millennia ago. The use of ice cores in paleoclimate studies was first proposed in the 1950s by Willi Dansgaard [1]. In polar regions, snow keeps accumulating every year on the continental ice sheet, forming a permeable layer called firn (Fig 1).The firn layer is about 50-100m deep(Schwander, 1996), and it may take up to 3 kyr (1 kyr = 1000 years) before the airbubble enters the close-up depth [1]. Most ice core samples have layers in chronological order, which are known as continuous cores. Currently the oldest continuous ice core traces back to 800,000 years ago [2]. Scientists have been seeking increasingly older ice to extend our understanding of the planet’s climate history. However, finding older ice is challenging due to the dynamic nature of Antarctica's ice, which flows toward the continent's edges and eventually melts into the ocean. Scientists (DEEPICE project led by EU and COLDEX led by Oregon State University) are currently aiming to retrieve continuous ice cores that extend up to 1.5 million years back.

Fig.1 [2].

Mean Ocean Temperature Reconstruction

The global mean ocean temperature (MOT) is an important measure for the state of Earth climate as it reflects the global energy balance. The potential of using noble gases as MOT proxies was seen due to their unique properties: they are chemically inert and their solubility is temperature-dependent. The solubility of noble gases in seawater decreases with increasing ocean temperature, and thus a lower MOT should result in a lower atmospheric inventory. This change can be measured in the noble gas mixing ratios from air bubbles trapped in ice cores.

Many proxies have been proposed for the paleoclimatic reconstruction of the oceantemperature, for instance, the ratio of magnesium to calcium in benthic foraminiferal species (Bryan and Marchitto, 2008), heavy oxygen isotope content of fossils or carbonaterocks, and Sr to Ca ratio in the corals (Lea, 2014). However, these proxies carry stronglocal temperature signals instead of representing the global mean and also involve chemicaland biological processes that are not fully understood yet (Ritz et al., 2011). It is thereforenecessary to have a reliable proxy with only physical processes involved representing theglobal mean temperature.

Noble Gas Proxies

Noble gases refer to elements of group 18 in the periodic table. With the outermost electronic shells being completely filled, noble gases exhibit chemical inertness (Pan et al.,2013). Apart from Helium, which can escape from the Earth’s gravity, and Radon, whichis radioactive, the other four noble gases - Neon, Argon, Krypton and Xenon have no essential sinks or sources and therefore have conserved total inventories in the Earthsystem (Bereiter et al., 2018b). This constraint of the mass conservation of any specific noble gas can be expressed asItot = Iatm + Iocn, (1)where Iatm and Iocn are their oceanic and atmospheric inventories. This equation also holds for nitrogen, as the change of N2 inventories (nitrification and denitrification) in nature is negligible for the total inventories (Gruber, 2004; Schlesinger, 1997). Given the inert nature of noble gases, Headly and Severinghaus (2007) first suggested the potential of using δKr/N2 measured from air bubbles trapped in the ice cores as theproxy to reconstruct the MOT during the Last Glacial Maximum (LGM). Owing to the similarities in their chemical properties, δAr/N2 and δXe/N2 have also been used as MOT proxies in later research.

Fig.2 .

Simulation results show that two factors, the sea ice and the state of the At- lantic Meridional Overturning Circulation (AMOC), have influence on noble gases responses to MOT change via air-sea gas exchange.

While proxy studies suggest that a collapsed AMOC existed in the past, the Bern3D 2.0 model was not able to reproduce it in LGM conditions, neither has there been strong disagreement between the current noble gas reconstructed MOT records and other temperature proxy reconstructions. Overall, noble gases can be used as reliable proxies for the past 800 kyr MOT reconstructions, although sea ice may cause small deficit by partially blocking the air-sea gas exchange and generating undersaturation. Rare situations are discussed when proxy reliability is compromised by shutdown of AMOC.

References

[1] Winckler, G. and Severinghaus, J. (2013). Noble Gases in Ice Cores: Indicators of theEarth’s Climate History, pages 33–53.

[2]