For about a decade it has been suggested that the Tibetan Plateau, which rises to more than 5000 metres, has a profound effect on climate. This may be partly due to the way such a high and enormous area deflects regional wind patterns, but largely to its profound interconnection with the South Asian monsoon. When such a circulation barrier arose is critical to understanding how it relates to climate evolution in the latter part of the Cenozoic. There are various suggestions, based on aspects of its structural and magmatic evolution. Theory suggests that the southern part came into being in Eocene times, possibly because a segment of the lithosphere beneath broke off to subside into the mantle – there are volcanic rocks whose chemistry does suggest such a mechanism. About 8 Ma ago the southern Plateau began to spread laterally, producing a series of N-S extensional basins, which suggests that by then sufficient gravitational potential had accumulated to make the thickened crust unstable. About that time various signatures arose in foraminifera of the Indian Ocean and sediments derived by erosion, which suggest that the monsoon increased in intensity.
When the Plateau attained sufficient elevation above sea level to start spreading sideways and affect atmospheric circulation largely rests on these theoretical judgements. For the ideas to firm up needs some means of estimating topographic elevation, which is not easy to do. One way is to use plant remains that can give clues, either because the species involved are sensitive to elevation today, or the morphology of their leaves shows signs of physiological adaptation to elevation. The first is ruled out in old sediments, simply because the species present are now extinct.. Plants metabolism is dependent on diffusion of water and CO2 into their leaves during photosynthesis, and features, such as stomata density, give clues to the conditions for such diffusion. Luckily, sediments from southern Tibet do contain well-preserved plants, and a multinational group led by Bob Spicer of the British Open University have attempted to assess palaeo-elevation for the time at which they were deposited (Spicer, R.A. and 7 others 2003. Constant elevation of southern Tibet over the last 15 million years. Nature, v. 421, p. 622-624). Their method relies on linking leaf morphology to a property of the atmosphere, known as moist static energy (MSE), through estimates of atmospheric enthalpy from the leaves. That is not the end of the estimation, because MSE needs to be related to elevation and the only way is to use climatic modelling for the past. Whatever, Spicer and colleagues reckon that 15 Ma ago their sampling site was more or less at the same elevation as today, around 4.5 km above sea level. If true, they have established that the south part of the Plateau was already in existence during the Middle Miocene. Being so convoluted, despite its apparent precision, the leaf analysis method does need independent confirmation. There is a much easier and arguably more reliable method, based on the change in the size of bubbles formed by gas escaping from lavas, according to atmospheric pressure (see Cunning means of estimating uplift in November 2002 issue of Earth Pages News). There are lavas in southern Tibet that date from Cretaceous times, including some about a million years younger than the plant remains.
Precambrian warmth and methane
Methane is a more efficient “greenhouse” gas than CO2, but it soon oxidises in the presence of oxygen. During the Phanerozoic there have been several massive releases of methane, probably from gas hydrates in deep-ocean sediments, which produced warming spikes that decayed away quickly in geological terms. Before there was much, if any, oxygen in the atmosphere, methane could linger and add to the retention of heat by carbon dioxide and water in the atmosphere. One of the longest running disputes in environmental geochemistry concerns when oxygen levels became significant in the Precambrian, and what they were compared with later times. Whether the Earth was warm or cold has a bearing on this. Cosmological theory suggest that stars similar to the Sun progressively grow more energetic with time. Without some kind of greenhouse effect, the Earth would have been condemned to frigidity from its outset. Even today, with a more radiant Sun, only atmospheric retention of solar heat keeps overall temperature from being well below freezing. The further back in time, the greater the “greenhouse” effect would have to have been to stave off complete ice cover and a runaway “icehouse”. Methane almost certainly played a part in this once methane generating organisms evolved, up to about 2200 Ma, when there are signs (continental redbeds and soils rich in iron oxides) that atmospheric oxygen was appreciable. However, warmth prevailed for about 1.5 billion years thereafter, until the plunges into frigid conditions of the so-called “Snowball Earth” period from about 700 to 550 Ma. Somehow, the greenhouse effect lingered.
Alexander Pavlov of the University of Colorado, and colleagues from Pennsylvania State University have addressed the implications of this continued warmth in terms of maximum oxygen levels needed to avoid complete oxidation of methane releases (Pavlov, A.A. et al. 2003. Methane-rich Proterozoic atmosphere? Geology, v. 31, p. 87-90). Today, more than 90% of all methane production beneath the ocean floor is consumed by bacteria, depending on the amount of dissolved oxygen and sulphate ions (for aerobic and anaerobic methanotrophs). There is plenty of evidence that deep Precambrian ocean water was anoxic, so a great deal more methane would have emerged from them. That it was also poor in sulphate ions is shown by their low levels in solid solution with carbonates and Proterozoic sulphur isotopes in marine sediments. The authors argue that this signifies low atmospheric oxygen levels, around 5 to 18 percent of modern concentrations. The scene may have been set for an excess of methane production over its oxidation, thereby keeping the “greenhouse” warming above the levels when glaciation would have been widespread.. If so, something completely upset this balancing act in the Neoproterozoic, to drive down temperatures several times – the “Snowball Earth” events. The trigger may have been a boost in oxygen production and retention in the atmosphere.
El Niño in the Eocene
The oceanographic-climatic phenomenon in the equatorial Pacific, known as the El Niño-Southern Oscillation (ENSO), now seems to be major force in driving climate shifts far afield, such as the current drought in the Horn of Africa. Its cyclicity relieves the suffering brought by El Niño events, yet the processes may well be highly unstable. Some believe that it is only a matter of time before ENSO reverts to a permanent El Niño condition, with disastrous consequences. Such a stabilisation in the past may have resulted in warming at high latitudes that permitted lush vegetation in near-polar regions, during the Cretaceous and the Eocene. The Eocene was much warmer than now, as a result of a massive release of methane from seafloor sediments around 55 Ma. So it makes sense to look at its climate record to check for a permanent El Niño. Matthew Huber and Rodrigo Caballero of the University of Copenhagen have compared climate records from annually layered lake sediments from the Eocene of Germany and Wyoming in the western USA with climate models to test the hypothesis (Huber, M. & Caballero, R. 2003. Eocene El Niño: Evidence for robust tropical dynamics in the “hothouse”. Science, v. 299, p. 877-881). The climate data from the lake sediments (thickness variations in annual layers) show clear signs of a roughly 5-year cycle of climate change, attributed to an Eocene ENSO. This tallies nicely with simulations for the Eocene continent-ocean set-up. Although the authors claim that their findings refute the hypothesis that global warming tends to shut down ENSO, which is a comforting thought, Eocene ocean and air circulation was not the same as now by any means. There have been interglacial periods during the Pliocene to present climate system in which temperatures exceeded those of the Holocene. Surely, annually layered sediments from those times will provide a better test.