Tibetan uplift: looking a gift horse in the mouth

The old saying stems from it being possible to tell the age of a horse, indeed that of a number of herbivores, from the number of dark and light bands that show on the worn surface of its teeth. Because grasses contain abrasive material, such animals’ teeth grow throughout their lives, different coloured material being laid down depending on the time of year. But there is a great deal more to this annual layering, from a chemical standpoint. By looking at various isotopes that are incorporated into enamel and dentine, it is possible to say where a horse – or a human for that matter – once lived (from variations in strontium-isotopes proportions for instance), and what it ate. The second forensic sign can be worked out from the carbon isotopes that a tooth has picked up during growth. Grasses have different proportions of carbon isotopes than those of other kinds of plans, such as shrubs and trees, the one depending on the so-called C3 type of photosynthesis and grasses on the C4 process. Each takes up carbon isotopes in measurably different proportion (d 13C in grasses is significantly lower than it is in C3 plants). Using carbon isotopes from teeth of fossil vegetarian animals is therefore a useful way of checking on the past proportions of grasses and other plants – often controlled in some way by climate. Neogene sediments of the Tibetan side of the High Himalaya contain abundant vertebrate faunas, and in view of the controversy over when the Tibetan Plateau began rapidly to rise (see When did Tibet Rise? in March 2006 issue of EPN) their dental geochemistry is a potentially useful approach to take. New results are somewhat at odds with those from other methods (Wang et al. 2006. Ancient diets indicate significant uplift of southern Tibet after ca. 7Ma. Geology, v. 34, p. 309-312).

Previous work using another approach (see When did Tibet Rise? in March 2006 issue of EPN) strongly suggests that southern Tibet was above 4 km elevation as far back as the Middle Eocene (40 Ma). Carbon isotopes in the teeth of Late Miocene Tibetan horses and rhinoceroses show that they ate a great deal of grass, unlike the modern yaks and wild herbivores that have to browse C3 plants. Wang and co-authors interpret this to signify that the southern Tibetan Plateau was considerably warmer than today, and also much lower: maybe around 2.5-3.5 km rather than the present 4 km or more. For elevation to change by 1-2 km in 7 million years suggests remarkably rapid uplift late in the evolution of the Plateau and adjoining Himalaya. Grasses, however, depend on both higher temperature and greater rainfall, but also on reduced CO2 in the atmosphere. They increased in their global cover only since about 8 Ma ago, when CO2 began to decline and climate cooled globally. Would it be possible for changes in the Asian monsoon to have had an effect on Tibetan vegetation, thereby explaining to dental evidence? Tibet is as dry as it is, because the monsoons now lose all their moisture in rising over the high Himalaya. If moist air and therefore cloud found its way into Tibet during the Miocene, maybe it would have been warmer too.

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