Sahara provides nutrients for the Amazon

Absolutely fascinating paper, but what does it mean?

Abstract. About 40 million tons of dust are transported annually from the Sahara to the Amazon basin. Saharan dust has been proposed to be the main mineral source that fertilizes the Amazon basin, generating a dependence of the health and productivity of the rain forest on dust supply from the Sahara. Here we show that about half of the annual dust supply to the Amazon basin is emitted from a single source: the Bodélé depression located northeast of Lake Chad, approximately 0.5% of the size of the Amazon or 0.2% of the Sahara. Placed in a narrow path between two mountain chains that direct and accelerate the surface winds over the depression, the Bodélé emits dust on 40% of the winter days, averaging more than 0.7 million tons of dust per day…

3. Discussion

Using satellite data and reanalysis wind fields we have identified a remarkable connection between the Amazon forest and a single dust source in the Sahara: the Bodélé depression and its wind regime. A unique combination of global wind pattern and topography forms a vigorous dust source that emits an average of more than 0.7 Tg of dust per emission day and is active mostly during the winter–spring, which is different from most other Saharan dust sources. We estimate that between November and March, the Bodélé depression sends more than half of the dust that is deposited annually in the Amazon forest. Our direct measurements are consistent with a recent modelling study showing that the Bodélé is responsible for > 40% of dust optical depth over the Amazon in the winter season [25].

The soil of the Amazon tropical rainforest is shallow, poor in nutrients and almost without soluble minerals. Heavy rains have washed away the nutrients in the soil obtained from weathered rocks. The rainforest has a short nutrient cycle, and due to the heavy washout, a stable supply of minerals is required to keep the delicate nutrient balance [26]. Kimmins [27] showed that any change in the nutrient supply will convert tropical forests to `wet deserts’.

Despite the insight we gained into the role of the Bodélé in fertilization of the Amazon forest, some key questions remain open. What is the relationship of the mineralogical content of the dust to the local surface mineralogy? What is the size of the reservoir of mineral dust there? And since when has the Bodélé emitted such a huge amount of dust, and for how long will it continue to do so?…

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