Researchers have known for some time that fine-grained dust from space blankets the surface of Earth, falling from the cosmos at a constant rate and settling into ocean sediments. A study published Nov. 6 in Science shows that tracking where cosmic dust has fallen - and where it hasn't - can reveal how sea ice coverage has changed over millennia.
"If we can project the timing and spatial patterns of ice coverage decline in the future, it will help us understand warming, predict changes to food webs and fishing, and prepare for geopolitical shifts," said Frankie Pavia, a UW assistant professor of oceanography, who led the study.
Ice floating on the surface of the sea blocks cosmic dust from accumulating in the sediment, which is evident in the ratio of compounds present. When the surface is clear, cosmic dust reaches the seafloor, leaving traces of helium-3.
Cosmic dust swirls through space after stars explode and comets collide. Passing the sun, cosmic dust is implanted with a rare form of helium - helium-3. Scientists measure helium-3 to distinguish cosmic dust from earthly debris.
"It's like looking for a needle in a haystack," Pavia said. "You've got this small amount of cosmic dust raining down everywhere, but you've also got Earth sediments accumulating pretty fast."
In this study, Pavia was more interested in the absence of cosmic dust.
"During the last ice age, there was almost no cosmic dust in the Arctic sediments," he said.
The researchers hypothesized that cosmic dust could stand as a proxy for ice before there were satellites to monitor changes in coverage. Ice at the sea surface blocks cosmic dust from reaching the seafloor, while open water allows cosmic dust to settle into sediment. By analyzing the amount of cosmic dust in sediment cores from three sites, researchers reconstructed the history of sea ice for the past 30,000 years.
In the study, UW researchers measured the amount of cosmic dust in sediment cores from the three sites marked in black. The colored lines show historic margins and the shaded circles show ice coverage thousands of years ago, as indicated by cosmic dust accumulation or absence.
The three sites featured in the study "span a gradient of modern ice coverage," Pavia said. The first, located near the North Pole, is covered year-round. The second borders the edge of the ice during its annual low in September, and the third was ice-bound in 1980 but is now seasonally ice-free.
The researchers found that year-round ice coverage corresponded with less cosmic dust in the sediment. This was also observed during the last ice age, around 20,000 years ago. As Earth began to thaw, cosmic dust once again appeared in samples.
The researchers then matched ice coverage to nutrient availability, showing that nutrient consumption peaked when sea ice was low and decreased as ice built up.
The data on nutrient cycling comes from tiny shells once occupied by nitrogen digesters called foraminifera. Chemical analysis of these organisms' shells shows what percentage of the total available nutrients were consumed when they were alive.
"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," Pavia said.
Additional research is needed to show what is driving changes in nutrient availability. One hypothesis suggests that sea ice decline increases the amount of nutrients used by surface organisms because there is more photosynthesis, but another argues that nutrients are diluted by ice melting.
Both scenarios present as more consumption, but only the first indicates an increase in marine productivity.
Additional co-authors include Jesse R. Farmer at the University of Massachusetts Boston; Laura Gemery and Thomas M. Cronin at the United States Geological Survey; and Jonathan Treffkorn and Kenneth A. Farley at Caltech.
This study was funded by the National Science Foundation and a Foster and Coco Stanback Postdoctoral Fellowship.
Research Report:Cosmic dust reveals dynamic shifts in central Arctic sea-ice coverage over the last 30,000 years
Related Links
University of Washington
Beyond the Ice Age
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