The Amazon: rampant, green, wet fertility
The Sahara: dry, dusty & dead
Does that match your pre-conceptions? By the end of this mail you may have different ideas. But first, a couple of personal stories:-
About 30 years ago there was a weather report, and the chap mentioned that our weather was coming from the Sahara. That night there was a light rain. The next morning all the vehicles in the street were covered in a fine, yellow dust. Sahara dust.
About 40 years ago I was near the top of the Yorkshire Wolds, trying to find folks that wanted to have lemonade delivered to their homes. A home-owner told me that Ermine Street (a Roman Road) ran across the bottom of his garden. He said that it was buried under 10 foot of soil. Afterwards, I thought: “where on earth has 10 foot of soil come from?”. Surprisingly, at least in part, the answer is ‘The Sahara’.
The Amazon appears to be so fertile & lush, yet the soil that it grows in is almost dead; almost all the fertility is locked up in the trees. That is why, when modern humans have cut down the forest using “slash & burn” techniques & tried to grow other things, after a short period they have failed. The soil fertility quickly washes out & weeds proliferate. They have then had to move on to the next patch of forest to slash & burn, leaving desert behind them.
The previous paragraph is only underlined by the discovery of Terra Preta (black earth) across some patches of the forest. Those discoveries also have profound implications into the evolution of the Human race from hunters to farmers. The Archeologist Betty Meggers claimed that the best that the Amazon could support was 0.2 hunters per square kilometre (0.52/sq mi). At the time that she said this, there was no point in considering farmers, as the soil was so poor that it could not support farming. Then, recently, it has been estimated that 5 million people may have lived in the Amazon region in AD 1500 (oops). So, who's right?
Both are right, and it is the Terra Preta that makes the point, because where Terra Preta exists the soil is not dead, but fertile. The point is that it is humans that are responsible for the Terra Preta. They have looked after the soil & made it fertile. That is the true meaning of 'farmers'. If you are interested, it is (at least) 2 things that allows dead soil to become Terra Preta soil:- poo & bugs. Yup, poo again (wonderful stuff!). The planet has evolved a strange symbiosis between plants & animals & bugs & soil that allows all to prosper. It used to be plants & dinosaurs & bugs & soil, but the dinos almost all got wiped out by that asteroid & us mammals had to take over.
Once the previous paragraphs were understood there was an obvious question:- (ignoring the Terra Preta) where does the Rainforest fertility come from?
The Amazon is a very large river in Brazil (the northern part of South America) which flows through a basin-shaped area of land. The Amazon Basin is 7 million square km (2,700,000 sq mi), of which 5,500,000 square kilometres (2,100,000 sq mi) are covered by the rainforest. That is reckoned to be 390 billion individual trees divided into 16,000 species. That's a lot of trees!
It is difficult to grasp just how fertile this patch of land is; here are some facts to try to help:
One in ten known species in the world lives in the Amazon rainforest. That constitutes the largest collection of living plants and animal species in the world.
About 2.5 million insect species.
About 2,000 birds and mammals.
To date, at least 40,000 plant species, 2,200 fishes, 1,294 birds, 427 mammals, 428 amphibians, and 378 reptiles have been scientifically classified in the region.
(we could easily go on)
One of the points is that the vast coverage of broad-leaved trees itself helps cause the weather. Yes, the Rainforest itself causes the torrential rain for which it is known. The fact that it is close to the Equator helps, but mainly it is the huge number of trees that causes it all. If they were not there, then the land might well be more like The Sahara, which is also close to the Equator (but in Africa, on the other side of the Atlantic).
The next point is that those torrents of water wash the fertility right out of the soil & into the Amazon River & out into the Atlantic.
A little story about the Amazon River to help make this point:
Some sailors had to take to their lifeboats & tried to make it to the coast of South America. They had little water & knew that they must not succumb to the temptation to drink seawater (they would die in horrible torment if they did). Even so, after many weeks, and days without water, one of the men did drink it, and declared that the sea-water was fresh water. They were 100 miles from the coast, but it *was* fresh water, because their boat was in the stream of water that flowed from the Amazon. That may give a little bit of an idea just how much water flowed off the land in the Amazon Basin.
So, where does that fertility come from? The answer is that a farmer spreads fertiliser on the land. So who is that farmer? Well, it is the wind...
If the classic image of the Amazon is of torrential rain, then the classic image of the Sahara is of drought. Here is a photo to make that point:-
It is certainly true that the Sahara receives very little rain. However, there is another feature that is just as important - the wind blows constantly, from East to West. Those sand-dunes look the way that they do because of the wind (it sculpts & moves them across the land), and where you have wind in a dry place, you have dust.
NASA launched the CALIPSO satellite on 28 April 2006 (“Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation”). Between 2007 and 2013 it observed and measured the dust contents of the wind that blows across the Sahara & then across the Atlantic. The results were published earlier this year, and they are dramatic (all the newspapers reported it - one link at top):-
182m tonnes of dust travels across the West coast of Africa from the Sahara & across the Atlantic.
That is equivalent to the contents of 689,290 large trucks (that's a lot of trucks!).
It is 1,600 miles (2,575km) from the West coast of the Sahara to the East Coast of South America & the Amazon Basin.
132m tonnes of dust remain in the air at the American Coast, and ~28m tonnes falls over the Amazon basin, with the rest being carried further west.
That is really interesting, but by itself would not mean very much. After all, sand-dunes are pretty much dead places, and what's the point if dead dust is dropped on dead soil? No, the whole thing gains another dimension because of the Bodélé Depression in northern Chad - the Dustiest Place in the World.
The Bodélé Depression is the site of the (dried-up) Lake Megachad, a 400,000 km² lake. That is bigger than all the current Great Lakes of America put together, and pretty close to the size of California. That's a big lake! The original lake was full of life, and that life concentrated nutrients within the lake sediment (iron & phosphorous are key ingredients easily measured today).
Bristow's measurements are that up to 4 metres depth of dust have been removed from across the lake-bed in the past 1,000 years. The calculations are that there is probably another 1,000 years of dust remaining in the lake-bed if nothing else changes.
The Lake Megachad dust is not as rich as modern fertiliser, but it is far more nutrient-rich than simple sand. In addition, as those trace-elements were sourced from lake-life they are far more likely to be bio-available than otherwise.
The studies show that there are 2 routes taken by this dust across the Atlantic:- in the summer passing by the Caribbean and in the winter swinging further south to the Amazon.
The final aspect that scientists are considering is the effects on Climate Change, and there are at least 2 points:-
Damn interesting, yes/no?
--------- Alex Kemp