Modern agriculture a major control of increased rates of dust flux from continent to ocean

Dust blowing from west Africa into the Atlantic Ocean (credit: NASA Earth Observatory)

Strong winds can pick up dust particles* from continents and carry them thousands of kilometers where they are deposited on the ocean floor. Deserts are especially important contributors of dust with the Sahara Desert of northern Africa being the single largest source of mineral dust in the world.  The occurrence of this process has been observed and deposits of dust have been documented in marine sediment cores for a long time, but what has been more difficult to determine is how this process changes over time.

Very recent changes in dust flux (since the 1970s) are well known and relatively well understood to be the result of recent drought conditions in the Sahel region. However, longer-term trends, at the scale of centuries and millennia, are not as well understood. A new paper by Mulitza et al., published in Nature last week, provides a robust record of dust deposition for the past 3,200 years. The sedimentary archive is from a marine site offshore northwest Africa, located in a prime position under one of the most active dust plumes.

The top three curves on the diagram at left are the dust fraction, terrigenous (i.e., continent-derived) fraction of a certain grain size, and dust flux, respectively (click on it for a bigger version). The curve at the bottom of the diagram is a δ18O record, which the authors use as a proxy for precipitation. Also note the arrows near the top of diagram denoting when various forms of agriculture became dominant in the region.

Essentially, the authors of this study are arguing that the dust deposition generally corresponds to the paleo-precipitation record until very recently. When the climate was drier, more dust was picked up from the continent and deposited in the ocean. The curves don’t match exactly, but I wouldn’t expect them to — there are numerous other factors at play here. But the general correspondence is compelling. The departure of the dust fraction and flux curves from the paleo-precipitation curve starts a few hundred years ago.

From the abstract, Mulitza et al. explain this departure:

With the help of our dust record and a proxy record for West African precipitation we find that, on the century scale, dust deposition is related to precipitation in tropical West Africa until the 17th century. At the beginning of the 19th century, a sharp increase in dust deposition parallels the advent of commercial agriculture in the Sahel region.

I find these millennial-scale records fascinating because they highlight the complex interaction of multiple controls on Earth surface processes and commonly, although not always, reveal the significant impact that human civilization has on these processes.

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* according to authors, ‘dust’ refers to airborne particles mostly within the very fine silt range (<10 microns) with some as coarse as fine sand (~200 microns)

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Mulitza, S., Heslop, D., Pittauerova, D., Fischer, H., Meyer, I., Stuut, J., Zabel, M., Mollenhauer, G., Collins, J., Kuhnert, H., & Schulz, M. (2010). Increase in African dust flux at the onset of commercial agriculture in the Sahel region Nature, 466 (7303), 226-228 DOI: 10.1038/nature09213

image from NASA’s Earth Observatory website [link]