Interplanetary dust laced with helium-3 that has settled on the sea floor has provided climate scientists with an urgently needed historical record of sea ice. That urgency stems from climatologists battling with understanding how the Arctic will respond to the worsening climate crisis.
The amount of ice on the Arctic Ocean has depleted by more than 42% in response to rising temperatures since regular satellite monitoring began in 1979 — and the Arctic continues to warm faster than anywhere else on Earth, particularly due to human-driven global warming caused by things like burning coal for cheap power. In a few decades time we could see the Arctic Ocean free of ice all summer long. Besides the resultant rising sea levels as the ice melts, scientists want to learn more about how this change in sea ice affects the habitability of the Arctic and the wider world.
Until now, it has been difficult to make accurate predictions about the Arctic sea ice in part because there have been no historical records to base predictions on. If we don't know how the sea ice responded to changes in climate in the past, we can't say for certain how it will respond in the future.
Which is where the cosmic dust comes in.
We are being gently doused in dust from space every day. If you place a bowl outside for a week, some of the dirt that gathers in it will be from space.
When the Arctic Ocean is covered in ice, the dust is prevented from reaching the sea floor. So when the ocean is largely absent of ice, more of the cosmic dust is able to settle as sediment.
Pavia led a team who went searching for this dust in sedimentary cores taken from three locations in the Arctic Ocean: one near the North Pole where there is ice present all year, one near the edge of the ice in September when ice coverage is at its annual lowest, and another at a site that was covered in ice in 1980, but no longer is.
In particular, Pavia's team was looking for sedimentary layers of the isotopes helium-3 and thorium-230. Each has a different origin. Helium-3 is present in cosmic dust, having been captured by dust grains from the sun's solar wind, whereas thorium is a decay product of naturally occurring uranium that has become dissolved in the ocean. At times of high ice abundance on the ocean, the ratio of thorium-230 to helium-3 should be higher than at times when there is less ice and more cosmic dust can reach the seabed.
"It's like looking for a needle in a haystack," said Pavia. "You've got this small amount of cosmic dust raining down everywhere, but you've also got Earth sediments accumulating pretty fast."
The cores provided a historical record chronicling periods when greater and smaller amounts of cosmic dust have reached the bottom of the ocean, corresponding to differing amounts of sea ice. The ice has waxed and waned over millennia, and the cores indicate that the dawn of the most recent ice age, beginning about 20,000 years ago, saw a decrease in the amount of cosmic dust on the seabed as ice covered the entirety of the Arctic all year round.
"During the last ice age there was almost no cosmic dust in the Arctic sediments," said Pavia.
When the ice began to melt and retreat as the ice age started to come to an end 15,000 years ago, the cores show that the amount of cosmic dust in the sediment on the sea floor began to increase.
What's most intriguing is what the cores tell us about what governs the amount of sea ice and how its presence, or lack thereof, can influence the balance of nutrients and hence the biosphere of the ocean.
The assumption had been that the loss of ice from the Arctic Ocean was governed by the temperature of the ocean, but the results from Pavia's group indicate that it has more to do with atmospheric temperatures instead. This is a crucial piece of information because the ocean takes longer to respond to climate change than the atmosphere. If true, then we may lose sea ice in the Arctic Ocean more quickly than we expected.
They also found that sea-ice coverage is correlated with how quickly nutrients in the ocean are consumed by biological processes. Tiny shells that were once worn by microbes called foraminifera were present in the cores, and a chemical analysis revealed how much of the total available nutrients they consumed when the microbes were alive at different points in the historical record. Pavia’s team found a correlation between increased consumption of nutrients and a lack of sea ice.
"As ice decreases in the future, we expect to see increased consumption of nutrients by phytoplankton in the Arctic, which has consequences for the food web," said Pavia. Long term, such productivity might not be maintained, causing delicate ecosystems both in the ocean and on the coast to collapse.
The results still leave some questions unanswered for now, such as why nutrient availability changes with the amount of sea ice present. One possible explanation is that with less ice, there is more room on the surface of the ocean for photosynthesizing algae that produce more nutrients. However, a competing effect would be the dilution of the nutrients by the melting sea ice, meaning there must be a delicate balance between the two processes.
The results were published on Nov. 6 in the journal Science.
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