Transport of Dust in the Upper Atmosphere

Transport of Dust in the Upper Atmosphere

During the last two ice ages, dust particles from central South America were the most important source of iron in the South Pacific. The findings were published in the journal PNAS by a team led by Oldenburg geochemist Torben Struve.

Dust from Argentina’s dry Puna Plateau was an important source of iron for the nutrient-deficient South Pacific during the last two glacial cycles, particularly at the beginning of these cycles. This was the main finding of a study published in the scientific journal PNAS by a team of researchers led by geochemist Dr. Torben Struve of the University of Oldenburg.

According to the team’s theory, the jet stream circulation – powerful air currents flowing from west to east at several kilometers in altitude – picked up the fine mineral particles on the east side of the Andes and carried them almost all the way around the Antarctic continent to the Southeast Pacific. The researchers were able to reconstruct the contributions from various dust sources located on the surrounding continents using a sediment core from the seafloor as a climate archive.

Dust in the atmosphere is an important component of the climate system. On the one hand, fine dust particles have an impact on the Earth’s energy budget because they reflect incoming sunlight at high altitude, causing it to cool. On the other hand, mineral particles can carry nutrients such as iron and manganese to remote ocean areas where they stimulate the growth of algae.

We were surprised to find that dust from South America predominated throughout the study period, despite having to travel a long distance from the source to our sampling site.

Dr. Torben Struve

When the algae die and sink to the deep ocean, they remove carbon dioxide from the atmosphere, which also has a cooling effect. These mechanisms can be particularly effective in the remote and iron-deficient subpolar Southern Ocean so that changes in the Southern Hemisphere dust cycle are ascribed a significant role in the natural alternation between cold glacial and warm interglacial periods in the past. Therefore, the sources and transport pathways of dust have been the subject of intensive research for some time now.

Dust from South America dominated throughout the study period

Struve’s team examined a sediment core from the subpolar South Pacific seafloor with deposits dating back 260,000 years, spanning two glacial cycles. The researchers were able to determine the proportion of particles from South America, South Africa, Australia, and New Zealand in the different phases of the two glacial cycles using the geochemical fingerprint of the dust fraction in the core.

“We were surprised to find that dust from South America predominated throughout the study period, despite having to travel a long distance from the source to our sampling site,” says Struve, the paper’s lead author.

Dust transport in the upper levels of the atmosphere

According to the analysis, up to two-thirds of the particles originated there, and this proportion was particularly high at the beginning of the glacial cycles. Land masses located closer to the sampling site, such as Australia and New Zealand, contributed only just over half of the deposited dust, and over relatively short periods of time. Their contributions increased particularly towards the end of the glacial periods, when global temperatures started to increase again.

The researchers conclude from this data that the South American dust was emitted from the high-elevation source regions of the eastern Andes into the jet stream and traveled around Antarctica in the upper levels of the atmosphere. In contrast, dust particles from the low-elevation source regions in Australia and New Zealand were washed out of the atmosphere more quickly with the rain so they rarely reached such heights for long-distance transport.

According to the study, the majority of the South American dust originated in the Andes, which stretch from northwest Argentina to southern Bolivia and reach altitudes of up to 5,000 meters. Parts of the Puna-Altiplano Plateau and the dry high valleys of the Central Andes are included in this area. However, it has received little attention from researchers as a potential source of iron for the Southern Ocean until now. The team reports that during the ice ages, dust from this region contained higher proportions of bioavailable iron, most likely due to increased glacial activity in the source regions.

Dust production from all sources increased in the glacial periods

The study concludes that dust production from all sources increased in glacial periods compared to warmer interglacial periods, resulting in an increase in iron input from the dust of a factor of three to six – a finding that confirms previous studies that it was drier and presumably windier in cooler climates than in warmer climates. The team also discovered evidence in the data that the westerly winds that blow around Antarctica shifted southwards or slowed down at the end of the ice ages and during warm interglacial periods.

These findings could help scientists better understand the transition between glacial and interglacial periods in the Southern Hemisphere, according to Struve: “How exactly natural iron fertilization in the Southern Ocean amplified these climatic changes is not yet fully understood,” the geochemist adds, but he emphasizes that the new data provides valuable insights and can be incorporated into current Earth system models, providing a more detailed picture of the processes involved.

However, Struve emphasizes that this study cannot answer the question of whether it makes sense to artificially fertilize nutrient-deficient ocean areas with iron in order to slow the current rate of climate change. “I would be very cautious about that; to have a significant impact, you would need to supply bioavailable iron to remote ocean areas over long periods of time and on a large scale. That doesn’t seem likely.”