Ancient ice tells story of carbon storage

Scientists better understand the history of carbon dioxide storage in the Southern Ocean after analysing sea ice from the past 20,000 years.

Writing in the journal Nature Geoscience, they say extreme seasonal variation during a period known as the Antarctic Cold Reversal caused a proliferation of diverse marine plankton populations. The plankton sequester large amounts of carbon dioxide (CO₂) from the atmosphere, which gets stored in the deep ocean.

The last Ice Age, around 18,000 years ago, saw a drastic change in CO₂ levels as the Earth transitioned from a cold, icy world to the warm interglacial planet that we enjoy now.  

Over a 7000-year period, Earth was warmed by a rapid increase in CO₂ levels, from around 190 parts per million (ppm) to 280 ppm. But this wasn’t a steady rise; it was punctuated by intermittent plateaus.

One of these stands out: a 1900-year plateau that started around 14,600 years ago, when CO₂ levels levelled out at around 240 ppm. Why this happened is a mystery, says lead author Christopher Fogwill from Keele University, UK.

To resolve it, he and an international team from Europe and Australia travelled to the Patriot Hills Blue Ice Area in the Ellsworth Mountains of Antarctica. [The trip was cold but productive; Cosmos recently reported on a separate paper describing how rising ocean temperatures drove the melting of Antarctic ice sheets.]

“Blue ice areas are the perfect laboratory for scientists due to their unique topography,” says Fogwill; “they are created by fierce, high-density katabatic winds.”

With unrestricted amounts of ice to sample, the team developed four new measures of marine biomarkers, including fluoresce, imaging flow cytometry liquid chromatography and ancient DNA analysis.

“Organic biomarkers and DNA from the Southern Ocean are blown onto Antarctica and preserved in the ice, providing a unique record in a region where we have few scientific observations,” says co-author Chris Turney from Australia’s University of NSW.

The team’s discovery of the striking increase in marine plankton is “unprecedented” in ice core science, according to Fogwill.

Subsequent climate modelling revealed it was driven by large seasonal variation in sea ice expansion. During winter, ice grew extensively across the Southern Ocean, but would break down during summer as a result of rapid warming.

When fully formed, sea ice prevents exchange with the air, but when it breaks up, the ice can increase CO₂ drawdown, providing critical nutrients to the ocean that would have driven the marine plankton blooms seen during the plateau.

Interestingly, other research has found it is happening today as melting sea ice is triggering significant amounts of ocean carbon storage, helping to mitigate increasing CO₂ levels with a warming planet.

This needs to be explored further, the international team says, as escalated Antarctic ice melting “may impact the efficiency of the Southern Ocean as a carbon sink in the future”.