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Day 35 (July 30, 2011): The Life of a Soil
July 30, 2011
Naomi and I took the opportunity today to talk about ancient soils and what they can tell us. The best way to do it was to take an excursion to the oldest soil we know of in the Olorgesailie region. It’s at the very bottom of what we call Member 1 of the Olorgesailie Formation, and it’s very likely close to the oldest dated volcanic ash in the record of sediments – about 1.2 million years old. As you may recall, Naomi is an isotope chemist and geologist, and her research goal here is to find carbonates formed within old soils. The carbonates are formed by precipitation in the zone of a soil where plant roots grow. The chemistry of the carbonates reflects the woody or grassy vegetation, and thus also the amount of shade, that covered past landscapes. That kind of information was valuable to the animals that lived and consumed plants on those landscapes – and undoubtedly to the early human toolmakers. And so, learning from the soils and carbonates is important to us, too!
(The CO2 reflects the photosynthetic pathway of the plant. And so the precipitation of calcium carbonate nodules at the root level is an indicator of whether the plants represent grass or trees/shrubs. It’s amazing – and kudos to whoever figured that out!)
‘And another thing… nodules are harder to precipitate in a soil that formed on a slope rather than flat ground. And, of course, the amount of rainfall and moisture in the soil is really important. You just don’t get carbonates forming in soils that received more than 1500 mm of rainfall a year. Ancient soils without carbonate in them may reflect wet times, though soils throughout East African don’t usually get that much rainfall.’