Large numbers of icebergs drifting unusually far from Antarctica before melting into ocean waters have been key to initiating ice ages of the past, says a new study. According to the study, the icebergs introduced freshwater into certain areas of the ocean that triggered a series of chain reactions, culminating with Earth plunging into prolonged periods of cold. The findings were published this week in the journal Nature.
Scientists have long agreed that repeated ice ages over millions of years have been paced by cyclic changes in Earth’s orbit around the sun. These cycles increase or decrease the amount of solar radiation that reaches the planet’s surface. However, the changes are relatively small, so it has been a mystery how they bring such dramatic shifts in climate.
In the new study, scientists from more than a dozen institutions propose that when Earth’s orbit is just right, icebergs broken off from the coasts of Antarctica drift further and further away from the continent before ultimately melting. This, in turn, shifts huge volumes of freshwater away from the Southern Ocean and into the Atlantic. Then, as the Southern Ocean gets saltier and the North Atlantic gets fresher, global ocean circulation patterns change dramatically. This causes the ocean to pull increased amounts of carbon dioxide from the atmosphere, reducing the so-called greenhouse effect, and pushing Earth into ice-age conditions.
The scientists used multiple techniques to reconstruct past conditions. These included identifying tiny fragments of rock that Antarctic glaciers picked up as they scraped seaward over the continent’s surface. When the ice reached the coast, icebergs calved off, and the rocky debris was rafted along before getting dropped to the bottom as the icebergs melted.
The scientists based their conclusions on an ocean-bottom sediment core drilled in deep water 800 kilometers off the southern tip of Africa. The core was brought up by the International Ocean Discovery Program Expedition 361 in 2016. Analysis showed it contained layers of ice-rafted debris from Antarctica. “This is way far north for Antarctic icebergs to travel,” said study coauthor Sidney Hemming, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory. “It’s also where several key factors in ocean currents intersect. The study makes a powerful case that there is a series of factors all connected to the ice ages.” Hemming was co-chief scientist on the cruise.
The recovered sediments, encompassing more than 1.6 million years of history, contained one of the longest detailed records of Antarctic ice-rafted debris ever collected by researchers. Using climate-model simulations, the team determined that huge volumes of freshwater must have been transported by the icebergs. They further found that each layer of ice-rafted debris consistently presaged subsequent changes in deep ocean circulation. They reconstructed the changes from the chemistry of tiny shell-building creatures called foraminifera, also found in the sediments .
“We were astonished to find that this lead-lag relationship was present during the onset of every ice age for the last 1.6 million years,” said the study’s lead author, Aidan Starr of the United Kingdom’s Cardiff University. “Such a leading role for the Southern Ocean and Antarctica in global climate has been speculated, but seeing it so clearly in geological evidence was very exciting.”
“Our results provide the missing link into how Antarctica and the Southern Ocean responded to the natural rhythms of the climate system associated with our orbit around the sun,” said coauthor Ian Hall of Cardiff University. Hall served along with Hemming as the cruise’s co-chief scientist..
Over the past 1.5 million years, Earth has plunged into at least 25 ice ages documented by scientists. The planet is currently experiencing an interglacial period, in which temperatures are warmer. In the natural course of things, it would eventually dip into another ice age, starting thousands of years from now. But the study’s authors suggest that due to rising temperatures resulting from human emissions of greenhouse gases, the cycle may be disrupted; the Southern Ocean will likely become too warm for icebergs to travel far enough to trigger the necessary changes in ocean circulation, they say.
Hall says the study may help scientists understand how the climate may respond to human interference in the future. “As we observe an increase in the mass loss from [Antarctica] and iceberg activity in the Southern Ocean, [our] study emphasizes the importance of understanding iceberg trajectories and melt patterns in developing the most robust predictions of their future impact,” he said.
Adapted from a press release by Cardiff University.