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!
I asked Naomi exactly what factors determine whether an old soil (or paleosol) has carbonates in it or not. ‘Well,’ she started, ‘the time element is the cop out. When soil scientists fail to find carbonates in a paleosol, the usual answer is there wasn’t enough time for carbonates to develop. It’s true that when vegetation begins to grow on a layer of sediment, it takes time for the carbonates to form as the plant roots emit carbon dioxide into the soil.’
(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!)
Naomi continued: “But think about it…the calcium carbonate nodules also need calcium to form. It’s possible that diatomite, which is so prevalent at Olorgesailie – and consists of silica – doesn’t provide enough calcium to form CaCO2 nodules. So, the amount of calcium in the ground can also affect whether or not carbonate nodules form.
‘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.’
Naomi and I discussed all of this while standing in front of that most ancient soil in the Olorgesailie region - - a soil that makes Naomi happy because of all the carbonate found in it – at just the right level, where plant roots grew more than a million years ago. She collected samples – and we all look forward to the results back in the lab!