Earth’s Climate History, Written in Dust
What happens in the Southern Ocean can affect the carbon budget of the entire planet. The details of how exactly carbon flows into and out of the ocean, though, aren’t fully understood yet. These new sediment cores from the South Pacific allowed my colleagues from the Alfred Wegener Institute in Germany and I to look at a million years of climate history from this key area of the planet. In our results, published this week in Science, we figured out how the amount of terrestrial dust that falls onto the ocean surface (and ends up at the bottom of the ocean, in our sediment cores) changed in sync with other climate parameters in these cores and from other parts of the southern hemisphere.
Why are we interested in dust, anyway? There are two main reasons. First, dust contains iron, an essential and often limiting nutrient for tiny phytoplankton (algae) that live in the surface ocean. The algae need iron to grow, and as they grow, they also grab CO2 and use it to build their shells. When they die and sink to the bottom of the ocean, they take the carbon from that CO2 with them—storing it at the ocean floor, far away from the atmosphere, so the total amount of CO2 in the air gets smaller.
Secondly, dust in the atmosphere influences the radiative balance of the planet. Dust is made of little ground-up particles of rocks, some of which are shiny and reflective, and bounce incoming sunlight back away from the planet.
Previous studies have shown that during ice ages, there’s more dust getting to the Southern Ocean, and that the phytoplankton living at the surface of the ocean may use nutrients more efficiently than they do in warmer times—effectively lowering the load of atmospheric CO2 in those glacial periods. However, all of the work done thus far looked at either a small part of the Atlantic sector of the Southern Ocean, or ice cores on the Antarctic continent. A whole half of the region—the Pacific sector—was unstudied.
The Pacific sector of the Southern Ocean has remained something of a terra incognita for researchers. It is one of the most remote parts of the world oceans, and has been explored only patchily. There’s a good reason, explains Rainier Gersonde, a scientist at the Alfred Wegener Institute and chief scientist of this leg of the Polarstern cruise, who points out that “the region is influenced by extreme storms and swells in which wave heights of 10 meters or more are not uncommon.” The Polarstern made a voyage of 10,000 nautical miles (18,500 km) through this particularly inhospitable part of the Antarctic Ocean in order to obtain high quality and sufficiently long sediment cores.
Was the effort worthwhile? The answer is a definite yes. The new records from the South Pacific now close the gaping hole in our understanding of this dynamic region of the hemisphere. The exciting—and somewhat surprising—news is that we found the same pattern in the South Pacific as in the cores from the South Atlantic and in Antarctica. This tells us, says Frank Lamy from Alfred Wegener Institute, that the increased dust input was a phenomenon affecting the entire southern hemisphere during cold periods.
And this is a somewhat surprising result. Many scientists expected dust input to the South Pacific to be lower than what we found in this study. They were convinced that dust supply to the Pacific area could not have been higher during the ice ages than during warmer periods of the Earth’s climate history. So where could larger dust quantities in this area of the Earth’s oceans come from?
Dust must come from the continents, and there’s not many continents in the Southern Hemisphere it could come from. In the eastern (Atlantic) half of the Southern Ocean and in the East Antarctic ice cores, the dust comes from Patagonia, and many researchers think that the big glaciers that grow during cold, glacial periods grind up the rock they slide over, making huge amounts of rock powder, which means huge amounts of dust. Less is known where the dust in the Pacific sector comes from, but a logical option would be large, dry, dusty Australia (or perhaps New Zealand).
If Australia is the source of the dust, and the temporal pattern of dust flux is tightly coupled to the pattern of dust flux we see in the ice cores, then the correlation suggests that the commonly assumed process of glaciers grinding down bedrock and supplying most of the dust during ice ages is not the dominant process driving dust generation in the mid latitudes of the Southern Hemisphere.
Rather, the tight correlation between all Southern Hemisphere records seems to point to another potential driver: shifts in the wind picking up the dust particles from the ground. Or, the winds get stronger or gustier, and can pick up and carry the particles more easily out into the ocean and beyond.
One of the possible explanations of the causes of the increased dust flux we saw is that the very strong wind belts prevalent in this region—the “Roaring Forties” and the “Furious Fifties,” in sailors’ parlance—move northward during the cold glacial periods. Moving the raging winds northward would put them squarely over dry, dusty parts of Australia, allowing that continent to be a major dust supplier to the Pacific part of the Southern Ocean at those times.
However, there’s more work to be done in order to pin this hypothesis down. In fact, the very question of where the dust found in the south Pacific is coming from is the next question we are tackling. In a collaboration with our colleagues at the Alfred Wegener Institute and ex-Lamonter Katharina Pahnke (now in Oldenburg), we will chemically fingerprint the dust we find in the South Pacific and trace it back to its sources. This technique will tell from where the dust was blown out to the South Pacific during the ice ages: Australia, South America or perhaps the mountain glacial outwash plains in New Zealand. These sediment cores have opened doors to many new, exciting lines of questioning, which may help us much better understand how the climate system has changed so dramatically over the past million years.
Gisela Winckler is a geochemist at Lamont-Doherty.