Dust blown from the Sahara Desert crosses the Atlantic Ocean to fertilize the Amazon rain forest.
Ah, so you’ve heard this one before!
Good because that’s not the interesting part.
Scientists have been stumped as to how it is that dust from one of the most nutrient poor places on Earth was acting as fertilizer.
If I pour sand on my garden it’s going to have a bad time; the Sahara Desert isn’t made of Miracle Grow, afterall.
New satellite data finally cracked this mystery. As it turns out, a tiny, nutrient rich bowl that makes up less than 1/500th of the Sahara Desert produces over half the dust blown to the Amazon.The other 99.8% of the Sahara Desert combined produces less dust than this tiny area.
This is like pouring a bowl of Lucky Charms and getting 99% marshmallows every time you pour a bowl. A miracle in its own right.
(Satellite image of sources of trans-Atlantic dust)
The dust from the other 99.8% of the Sahara Desert is nutrient poor, and would do little to fertilize the Amazon, so how is it that 50% of the dust kicked across the Atlantic comes from just 0.2% of the Sahara Desert?
The winds hitting this area, called the Bodele Depression, are unusually strong jet stream winds that are completely absent in the rest of the Sahara. A strange funneling effect happens on a massive scale as the jet stream gets squeezed through the Tibesti Mountains and Ennedi Massif upon entering the former plains of Mega-Chad Lake.
This causes massive amounts of nutrient rich sediments to get pushed high into the atmosphere and carried thousands of miles across the Atlantic.
The Bodele Depression is a remnant of the world’s largest freshwater lake. Just a few thousand years ago Mega-Chad Lake was ~140,000 square miles; for comparison, Lake Superior, the world’s largest freshwater lake today, is just 32,000 square miles.
Today all that remains of this enormous lake is Lake Chad. Soon even this whisper of a grander past will vanish – Lake Chad has shrunk by 95% in just the last 50 years. This is highly unfortunate as 30 million people rely on its water for drinking and irrigation.
The Bodele Depression’s dried lake bed surface is composed of layer upon layer of the phosphorous, nitrogen, and iron rich remains of plankton. This plankton was deposited long before the lake existed; over 100 million years ago this depression was part of the ocean, and for millions of years plankton lived, died, and sank to the bottom to produce these nutrient rich layers.
There is one more factor that makes this an incredibly serendipitous situation. Not only is the Bodele Depression nutrient rich, but it is rich in the exact nutrients that the Amazon needs.
Plants need numerous nutrients to grow; only by feeding them the select few they are short on can they benefit from it. If a plant is short on Manganese and you give it Iron you’re more likely to harm it, much less help it.
The Amazon rainforest’s soil has very little phosphorous. In fact, it has so little that phosphorous is the limiting factor in plant growth. Even though there are abundant amounts of other “fertilizers” the plants can’t grow once they hit the wall of depleted phosphorous.
Well it just so happens that the dust from the Bodele Depression is particularly rich in phosphorous. As it crosses the Atlantic the phosphorous dissolves into the water droplets that fall to the ground in the Amazon’s frequent downpours. The Amazon’s rain is literally a liquid fertilizer enriched with the exact ratio of nutrients that it needs.
What a tale indeed!
Life living in an ancient ocean stored just the right ratio of nutrients for a rainforest thousands of miles away that wouldn’t exist for another 100 million years. Those nutrients then traverse an entire ocean due to a near impossible funneling of a jet stream through just the right geographic features. An event so improbable that it results in just 0.2% of the Sahara Desert producing more dust than the other 99.8%.