Leaf morphology and the timing of the rise of the Tibetan Plateau
The Tibetan Plateau, arguably the earth’s most significant topographic feature, has an average elevation of over 5000 m extending over an area about 1000 by 2500 km. The plateau formed largely as a result of the collision of India into Asia, which started around 55 m.y. ago; but uplift of the plateau has taken place over a fairly long period since then, and appears to have happened in a stepwise manner (Tapponier et al., 2001).
The Himalayan-Tibetan uplift has had a profound effect on the earth’s climate. On the one hand its impact on the rates of silicate-rock weathering is thought by many to have been important in drawdown of atmospheric CO2 and cooling during the Tertiary, and on the other it plays an important role in controlling the Asian Monsoon, the rainfall system that is responsible for feeding almost half of the world’s population. We know that monsoons have been happening for approximately the past 8 million years, but we don’t have a very clear picture of the timing of the uplift of the Tibetan Plateau.
A recent research project, led by geologists from Britain’s Open University, provides some additional data that helps to establish the timing of uplift of part of the plateau. Spicer et al., (2003) have studied Middle Miocene (ca. 15 m.y.) leaves from volcanic ash-rich lake sediments in the Namling region of southern Tibet. The fossiliferous rocks are now situated at 4300 to 4600 m elevation. By careful description of a wide range of leaf morphological features, and comparison with existing data sets, they have determined that the leaves grew in a setting with a mean annual temperature of around 6.8º C. Based on comparisons with Miocene climate models, and with other leaf studies on similar-aged rocks formed at close to sea level in Japan, the authors conclude that the Namling leaves grew at an elevation of close to 4600 m.
Spicer et al. propose that the Namling region, along with most of southern Tibet, has been at an elevation of around 5000 m since at least the Middle Miocene; however they are careful to point out that some parts of the existing plateau may have been elevated earlier, and some parts later.
morphotyping involves description of overall leaf morphology, patterns and
types of venation, and leaf margin characteristics.
These features, when compared with existing data sets from both
modern and ancient leaves, can be used to make inferences about the
climatic conditions in which the leaves grew.
For more information see: http://www.peabody.yale.edu/collections/pb/MLA/
Spicer, R, Harris, N, Widdowson, M, Herman, A, Guo, S, Valdes, P, Wolfe, J, and Kelley, S., 2003, Constant elevation of southern Tibet over the past 15 million years, Nature, V. 421, p. 622-24. (February 2003)
Tapponier, P, Xu, Z, Roger, F, Meyer, B, Arnaud, N, Wittlinger, G and Jingsui, Y, 2001, Oblique stepwise rise and growth of the Tibet Plateau, Science, V. 294, p. 1671-77.
Earle, Malaspina University College, Nanaimo, B.C., 2003. Return to Earth