Far-Drifting Antarctic Icebergs Are Trigger of Ice Ages, Scientists Say

by |January 13, 2021

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.

icebergs breaking off of antarctica

Icebergs broken off and drifting far from the coast of Antarctica are key to the initiation of ice ages, says a new study. (Pierre Dutrieux/Lamont-Doherty Earth Observatory)

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 .

Researchers examine a sediment core

Researchers examine a sediment core taken off southern Africa. (Courtesy Sidney Hemming/Lamont-Doherty Earth Observatory)

“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.


Subscribe
Notify of
guest
3 Comments
Oldest
Newest
Inline Feedbacks
View all comments
Desmond
Desmond
1 year ago

So in short we may be preventing another ice age. Bad for earth as it can’t kill off humans. Probably good for humans until overpopulation leads to implosion.

ralph ellis
ralph ellis
1 year ago

This paper proposes that icebergs from Antarctica getting rafted into the tropics, bring fresh water to the North Atlantic and change oceanic cycles, to cause cooling and invigorate an already existing ice age. But in no way do their arguments demonstrate or explain causation.   As you can see below, their observations are merely a consequence of the ice age cycle, not a cause.
(Note:  IRD = Ice Rafted Debris found at the bottom of the ocean, in mud cores. It shows how many icebergs reached and melted at that location and latitude.) 
.
a.  They are proposing that (quote): 
  i. Glacial conditions in Antarctica favour the transport of icebergs away from Antarctica (and towards the tropics).  
  ii. Increasing sea ice extent allows more northerly iceberg melt, which precedes (full) ice age conditions.
So a cooling world with ever more sea ice allows icebergs to drift closer to the tropics – and you can see that in their graph.  The three big IRD peaks 250 – 450 ky ago coincide with the depths of an already-established ice age. 
So they have not demonstrated anything.  If cooling is a pre-requisite for tropical IRD deposition, then they have not explained the ice-age’s cooling mechanism at all.  All they have demonstrated is that when Antarctic sea-ice spreads further north, it can carry icebergs (and their rock debris) further north.  
In fact, the most interesting aspect of their graph is that it demonstrates that tropical IRD (blue plot 2nd down) precedes decreasing d18O (ie: warming) (black plot 3rd down).   And while that may be true, this observation in no way demonstrates causality.  Ice-age conditions allow tropical IRD because sea-ice spreads towards the tropics, and then interglacial warming stops and reverses that process.  That is logical – but the temperature is controlling IRD and not vice versa.
.
b. Their theory does NOT explain the full ice age cycle in any shape, form or fashion. There are numerous precessional and obliquity cycles within each full ice age cycle, which do not produce any warming or cooling.  So why the selective nature of the ice age cycle, which can accept or reject orbital cycles at will?  This IRD theory cannot explain this feature.
In reality, it is albedo that controls the ice age cycle, as a feedback agent.  The cold portion of the Milankovitch cycle promotes cooling, and the amount of initial cooling in the first 5 ky of an ice age is significant.  Subsequent warm portions of the Milankovitch cycle cannot reverse these nascent ice age conditions, because the albedo of the expanding northern ice sheets is too high.  It is only after these ice sheets have expanded, and sea-ice has likewise expanded, that IRD deposition can spread towards the tropics.
So what triggers interglacial warming?  Answer – dust.  Dust always peaks just before an interglacial, and it is that dust, which lowers the albedo of the northern ice sheets and allows much greater insolation absorption, that can precipitate interglacial warming (in concert with a northern Milankovitch warm period – a northern Great Summer). 
Please see my latest science paper on ice ages.
Modulation of Ice Ages via Dust and Albedo:
https://www.sciencedirect.com/science/article/pii/S1674987116300305

.

c.  Oh, do note that CO2 is not mentioned until the last line of the paper – so they can get a grant.  Do they know how absurd this looks?  

Ralph

Tracy Sullens
Tracy Sullens
1 year ago

What about the proposed slowing of oceanic circulation due to dilution of saline water versus fresh water from glacial and iceberg melts? Is there a study that hypothesizes this may trigger the end of the current interglacial period?