Your Life as Planet Earth: A new way to understand the story of the Earth, its climate and our origins. by Howard Lee

Author:Howard Lee [Lee, Howard]
Language: eng
Format: epub
Published: 2014-12-15T08:00:00+00:00

Dating humans

Carbon-dating with 14C

In the most recent past, the time of modern humans, dates are measured using isotopes with relatively short half-lives, like carbon-14 (14C). Carbon dating is the technique most people have heard of and it starts with cosmic rays bombarding our atmosphere. Every day some atmospheric nitrogen is transformed into 14C by cosmic ray damage, and it reacts with oxygen to make CO2, which gets incorporated into plants and then animals as food, or into seashells from carbon dissolved in seawater. 14C decays back into nitrogen with a half-life of about 5,700 years, so provided an uncontaminated sample of wood, tooth, bone, etc. can be found, the amount of 14C that has reverted back to nitrogen can be measured to give a date.

Carbon dating is used extensively in archeology, but its short half-life limits it to samples less than 50,000 years old, which is unfortunately right when modern humans were spreading across the globe. 650 Its cosmic origins mean that a correction must be applied for the variable rate that 14C has was created in the past. To do this, a great many samples have been cross-referenced to dates from stalagmites, varves, 651 corals and tree-tings, creating a calibration curve going back 50,000 years. 636 A bonus that falls out of this wiggly curve is that it tells us about space weather in the past, which I will cover in the next chapter. Carbon dating of marine samples is a lot trickier because it takes time to mix the CO2 from the atmosphere into the oceans, and ocean currents create variable reservoirs of 14C, making marine carbon dates inaccurate. 636 437 This does have a beneficial side-effect – it enables 14C to be used as a tracer for ocean circulation. 437

14C false dates & contamination

Since the 1970s, many 14C dates were measured either on contaminated samples or on samples erroneously assumed to be the same age as nearby human remains. 652 The problem was so widespread that some scientists estimate up to 90% of middle to upper Paleolithic (Old Stone Age) dates are wrong. 652 In a 30,000 year-old bone, 98% of the original 14C has decayed, leaving only vanishingly small levels that are easily overwhelmed by carbon that seeped into the bone from the surrounding soil over the ages. The problem stems mainly from collagen, a tissue in bones, which has an annoying preponderance for soaking up contaminants and throwing off date analyses. This issue is now being addressed by far more rigorous sample preparation to remove contaminated collagen, and scientists are busy reanalyzing specimens to correct their dates. In one example, a human jaw from the UK, which was previously dated at 35,000 years old, has now been shown to be more than 41,000 years old 652. In another example, Spanish Neandertal remains were re-dated and found to be at least 10,000 years older than previously thought, limiting the time Neandertals and modern humans coexisted. 601

Dating Sediba

When it comes to dating the bones of our ancestors like Homo erectus and Australopithecus, we have to reach past the range of 14C.

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