Astonishingly long-range "fertilizer transport": At the beginning of the last two glacial periods, mineral particles from South America provided a plentiful source of iron for the algae in distant waters of the Deep South, researchers report. Apparently, the jet stream transported the dust at high altitude almost all around Antarctica, according to the analysis of a drill core from the sea floor. The fertilization likely significantly increased carbon sequestration in the region and may have played a role in climate cooling, the scientists say.
Atmospheric dust is already known to be an important factor in the Earth's climate system. There is a direct and an indirect effect: Since the fine particles reflect the sunlight, they influence the earth's radiation balance - they have a cooling effect. Ultimately, this is also the effect of the second aspect: when the mineral particles finally trickle down, they can bring fertilizing substances into nutrient-poor sea areas. An important deficiency nutrient that limits the growth of algae there is known to be iron: if the availability increases, the phytoplankton can build up significantly more biomass, which binds atmospheric carbon dioxide.
When the algae finally die and sink into the deep sea, the greenhouse gas can remain bound there for a long time, which is known to have a cooling effect on the earth's climate. These mechanisms can be particularly effective in the iron-limited subpolar Southern Ocean. It is therefore already suspected that changes in the dust cycle in the southern hemisphere played an important role in the natural alternation between cold and warm periods in the past.
Well-travelled dust particles in a drill core
In order to gain insights into the history of dust transport in the south, the researchers led by Torben Struve from the University of Oldenburg have now analyzed a sediment core that comes from the soil of the subpolar South Pacific west of South America. The chronologically definable deposits in the drill core date back 260,000 years and thus span two ice age cycles. The researchers explain that the dust particles contained can be assigned to their regions of origin based on their geochemical fingerprints. In this way, they were able to determine how high the proportion of particles from the adjacent landmasses was in the various phases of the two ice ages.
The researchers were surprised to find that up to two-thirds of the particles came from South America. Interestingly, this maximum was apparent at the beginning of each cold age cycle. Land masses such as Australia and New Zealand contributed more than half of the deposited dust only in comparatively short periods of time. Their contribution increased especially towards the end of the ice ages when the climate warmed again. "Amazingly, dust from South America dominated, even though it had to travel a very long way from the source to our sampling site," says Struve. Because, as the researchers explain, eastward-directed air currents prevail in South America – so the particles cannot have reached the sampling point over the relatively short distance from the west.
The researchers conclude from the data that the South American dust apparently entered the jet stream and was then able to circle Antarctica in the higher reaches of the atmosphere. In contrast, the dust particles from the low-lying source regions of Australia and New Zealand could probably be washed out of the air more quickly with the rain and therefore rarely reached the heights necessary for long-distance transport, the scientists explain.
Dust from Andean heights surrounds Antarctica
They were also able to locate the origin of the South American dust even more precisely based on its characteristics: The largest proportion came from regions up to 5000 meters high in the Andes, which stretch across the north-west of today's Argentina and the south of Bolivia. As the researchers found, the dust from there contained a form of iron that is biologically useful. The bottom line was that dust production from all terrestrial sources increased significantly during ice ages compared to interglacials, probably due to wind changes and drought. The researchers say that the amount of iron could have increased by a factor of three to six.
According to them, the results can now help to better understand the alternation between ice ages and interglacial periods in the southern hemisphere: "Exactly how natural iron fertilization in the Southern Ocean intensified these climate changes has not yet been fully clarified," says Struve. However, the new data offer valuable information that can now be incorporated into Earth system models in order to better understand the complex processes in the climate system.
Finally, Struve comments on another thought that may arise from the research topic: Whether it makes sense to fertilize nutrient-poor marine areas with iron in a targeted manner in order to slow down the current climate change cannot be assessed on the basis of the study, says the scientist: "I would be very careful with that. In order to achieve a noteworthy effect, one would have to supply large areas of the sea with biologically usable iron over a long period of time. That hardly seems practicable,” says the geochemist.
Source: University of Oldenburg, specialist article: PNAS, doi: 10.1073/pnas.2206085119