Ice ages are not that easy to define. It may seem intuitive that an Ice Age represents a frozen planet, but the truth is often more nuanced than that.
An Ice Age has constant glaciations and deglaciations, with ice caps that vibrate in line with climate change. These giants have constantly increased and decreased, exerting and lifting pressure from the ocean floor.
Several studies also show that the most recent deglaciation, the Holocene (about 21 ka-15 ka ago) of the Barents Sea had a huge impact on the release of methane to water. A most recent study in Geology looks even deeper into the past, some 125,000 years ago, and contributes to the conclusion: The melting of the Arctic ice caps releases the powerful greenhouse gas methane from the ocean floor.
“In our study, we extend the geological history of methane release in the Arctic to the penultimate interglacial, the so-called Eemian period. We found that the similarities between the Holocene and Eemian deglaciation events point to a common driver for the episodic release of geologic methane – the retreat of the ice caps. “says researcher Pierre-Antoine Dessandier, who conducted this study as a postdoctoral fellow at the CAGE Center for Arctic Gas Hydrate Environment and Climate at UiT The Arctic University of Norway.
See thousands of years of methane release in tiny shells
The study is based on measurements of different isotopes found in sediment cores taken from the Arctic Ocean. Isotopes are variations of chemical elements, such as carbon and oxygen, in this case. Different isotopes of the same element have different weight and interact with other chemical elements in the environment in specific ways. This means that the composition of certain isotopes is correlated with environmental changes – such as temperature or the amount of methane in the water column or in sediment. Isotopes are absorbed and stored in the shells of tiny organisms called foraminifera and thus are archived in the sediments for thousands of years as the tiny creatures die. In addition, if the methane has been released for longer periods of time, the archived shells get carbonate growth which in itself can also be tested for isotopes.
“Isotopic recording showed that as the ice sheet melted and the pressure on the seabed decreased during the Eemian, methane was released by violent pushes, slow seeps, or a combination of the two. completely disappeared, a few thousand years later, methane emissions had stabilized, “says Dessandier.
Where does methane come from?
Arctic methane reservoirs are made up of gas hydrates and free gas. Gas hydrates are solids, usually methane, frozen in a cage with water and extremely sensitive to changes in pressure and temperature in the ocean. These reservoirs are potentially large enough to increase atmospheric concentrations of methane if they are released during the melting of glacial ice and permafrost. the Geology The study reinforces the hypothesis that the release of this greenhouse gas is strongly correlated with the melting of the ice caps. It is also an example of the past showing what the future may hold for us.
“The current acceleration of Greenland’s ice melt is an analogue of our model. We believe that the future release of methane from below and near these ice caps is likely.” Said Dessandier
The increase in methane emissions is a major factor in the increase in the concentration of greenhouse gases in the earth’s atmosphere and is responsible for up to a third of global warming in the short term. In 2019, about 60% (360 million tonnes) of the methane released worldwide came from human activities, while natural sources contributed about 40% (230 million tonnes).
The amount of methane that eventually reached the atmosphere during the Eemian and Holocene deglaciations remains uncertain. Part of the problem with quantification is the microbial communities that live on the seabed and in the water and use methane to survive.
But these two past deglaciations occurred over thousands of years, while the current retreat of the ice caps is unprecedented according to geological data.
“Projections of future climate change should certainly include the release of methane from shrinking ice caps. The past can be used to better inform the future.”