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Biogeochemistry

Dust detectives: tracing the origins of Antarctic ice core debris

The paper:

Gili, S., D. M. Gaiero, S. L. Goldstein, F. Chemale Jr, E. Koester, J. Jweda, P. Vallelonga, and M. R. Kaplan (2016), Provenance of dust to Antarctica: A lead isotopic perspective, Geophysical Research Letters, 43, 2291–2298, doi:10.1002/ 2016GL068244

Dust and ocean productivity

Microscope image of dust particles (Creative Commons)

Microscope image of dust particles (Creative Commons)

The tiniest of particles can have a huge impact on the ocean ecosystem. Microscopic particles of soil or rock can be

picked up by wind and trapped in the atmosphere, then transported halfway around the world in wind currents. This process can be critical for life in the ocean because in much of the ocean (especially the southern ocean surrounding Antarctica) the productivity of the ecosystem is limited by iron availability. For example, see this post! Dust is very rich in iron, and dust deposition represents the only source of iron to some remote ocean ecosystems. Dust plumes blowing off the Saharan desert (shown below) have been shown to fertilize algae growth in the eastern Atlantic Ocean, and can even provide nutrients to the Amazon rain forest.

Dust blowing off the Sahara over the Atlantic Ocean (NASA)

Dust blowing off the Sahara over the Atlantic Ocean (NASA)

Dust can provide interesting insights into the patterns of wind currents and atmospheric circulation. By determining the origin of deposited dust, we can reconstruct the wind patterns required to get it from its origin to its destination. Antarctica is a particularly interesting place to study dust deposition because it produces little dust of its own. The continent is covered in snow and ice, so exposed mountain faces are the only significant local dust sources. Most dust therefore comes from atmospheric transport across the ocean, providing an opportunity to examine wind patterns.

In addition, Antarctic ice cores provide a window into dust deposition in the distant past. Without any way to directly measure how the world worked hundreds of thousands of years ago, we rely on oblique clues like ice core records to gradually piece together the puzzle of the prehistoric earth system.  The glaciers have been accumulating for the last 800,000 years, so examining the origins of dust particles trapped in ice cores can provide a massive historical record of dust transport and deposition. This is important for understanding past changes in ocean iron availability which, as you can imagine, is not easy to figure out!

Tracing dust origins with lead isotopes

The chemical composition of dust particles is used to track their source, because their composition is preserved during transport and reflects their origins. This study uses lead isotopes to identify the “fingerprint” of dust sources in different regions. Dust sources on different continents have subtly different fingerprints, so examining the lead isotopes of dust particles trapped in ice cores can be used to determine the relative contributions of the different sources. Few studies have used this sort of fingerprint because it’s hard to make a measurement without contamination. If even a tiny bit of dust from the lab gets in the ice core sample, the contamination ruins the measurement.

Previous studies using lead isotopes were inconclusive because the differences between the sources were small and subtle. Gili and her colleagues analyzed the lead isotopes of potential dust sources in much greater detail. To do this, they collected samples of dust and loess (a rock formed by dust deposits) across a north-south transect of South America (Figure 3). The lead isotopes of these samples were then compared with previous results from dust trapped in Antarctic ice cores and dust sources collected from Australia.

The origins of dust in Antarctica

Figure 3: Map of South America showing the locations of dust and loess sampled for lead isotopes as possible sources of dust deposited in Antarctic ice cores. From Gili et al. 2016

Figure 3: Map of South America showing the locations of dust and loess sampled for lead isotopes as possible sources of dust deposited in Antarctic ice cores. From Gili et al. 2016

Previous attempts to trace dust sources had trouble distinguishing dust from Australia and South America, since their chemical signatures are similar. The new intensive data from South America provided by this study indicate Australian sources are unlikely. Dust from the ice cores appears distinct from Australian dust, and much closer to that of southern South America with possible contributions from local Antarctic sources as well (Figure 4). More precisely, most of the dust appears to be from the Patagonia region of Argentina, with smaller inputs from other dust sources in the Andes Mountains of Chile and northwest Argentina.

There was less change in dust emission and transport between cold and warm periods in history than previously thought. If the source region is covered by a glacier, less dust is generated, so sources can change dramatically as glaciers expand and contract with climate change. However, it appears Patagonia remained the dominant source in both warm and cool climate regimes. This consistency adds to our understanding of ancient patterns of atmospheric circulation and iron transport to the ocean. The lack of change in dust source suggests dust production processes and wind patterns were broadly similar over the last 800,000 years. This is one more puzzle piece for understanding how the earth worked in the distant past when we weren’t around to observe it!

Figure 4: The lead isotope composition of dust samples collected from the source regions (top) and the ice cores (bottom). The blue line and blue dots in the top panel show the composition of dust collected from Australia, while the colored shapes show the range of lead isotope values for the various South American sources. The isotopic composition of the samples from the ice cores (red points in the bottom panel) are closer to the South American than Australian sources and can be explained by mixing between these and local Antarctic dust. From Gili et al. 2016

Figure 4: The lead isotope composition of dust samples collected from the source regions (top) and the ice cores (bottom). The blue line and blue dots in the top panel show the composition of dust collected from Australia, while the colored shapes show the range of lead isotope values for the various South American sources. The isotopic composition of the samples from the ice cores (red points in the bottom panel) are closer to the South American than Australian sources and can be explained by mixing between these and local Antarctic dust. From Gili et al. 2016

Michael Philben
I recently completed a PhD in Marine Science at the University of South Carolina and am now a postdoc at Memorial University of Newfoundland. I research the effects of climate change on soil organic matter in boreal forests and peatlands. I spend my free time picking berries and exploring “The Rock” (Newfoundland).

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