Using light to describe the ancient world

Plant walls includes 'sunblock' compounds, and plants that live at higher altitudes have higher 'sunblock' concentrations. Barry Lomax and colleagues consider the mountain building implications of this relationship. Photo from Wikimedia Commons. Graph reproduced after Lomax et al. 2012

Plants that live at higher altitudes have higher concentrations of ‘sunblock’ compounds in their walls. Barry Lomax and colleagues consider the mountain building implications of the ‘sunblock’/altitude relationship. Photo from Wikimedia Commons. Graph reproduced after Lomax et al. 2012

How quickly do mountains rise? The Southern Alps have been rapidly rising for the last five million years (…stone giants?), and India started raising the Himalayas about 10 million years ago. We might imagine dramatically different landscapes in these parts of the world before the mountains grew. Mountains have a profound effect on local climate, vegetation and animals, so when we reconstruct ancient environments in mountainous places, we need to know how tall the mountains were back in the day.

Modeling the rate of mountain growth is a difficult business, and many models factor in climate change, which is a science all on its own. Fortunately, infrared spectroscopy might provide an easier method. Barry Lomax and colleagues have recently described a method for calculating paleoaltimetry, that is, how high something was in the past, and suggested it could be useful for figuring out rates of mountain growth. In essence, imagine you are standing next to a deposit of fossil plants, and you know the altitude you are standing at, and the age of the fossil deposit. If you calculate the altitude that those plants were growing at during ancient times, then you can calculate a rate of altitude change. So, how can we calculate paleoaltimetry from a fossil plant?

It all comes down to sunlight. From Lomax et al. 2012 “…UV-B radiation flux increases with altitude…due to the physical properties of the atmosphere…” Consider then, that “…The vast majority of terrestrial land plants require sunlight to drive photosynthesis leading to exposure to high-energy short wavelength UV-B radiation, resulting in damage to plant proteins, membrane lipids and DNA. One mechanism by which plants can mitigate these effects is via the up-regulation of UV-B absorbing compounds (UACs)…” (Lomax et al. 2012). Plants that grow at higher altitudes have more UACs – more sunblock – and hence plants record the altitude of their growth.

The UV-B absorbing compounds include ferulic acid and p-coumaric acid, which very importantly, contain aromatic rings of carbon atoms. An aromatic ring is a group of atoms that are joined together in a closed loop, and which freely share electrons, which is what makes them useful for gobbling up UV light. An infrared spectrum of “…[s]poropollenin, the biopolymer that makes up the exine (outer wall) of spores and pollen…” includes a band at 1520 cm-1, which is attributed to aromaticity (Lomax et al. 2012). Lomax and colleagues describe the relative abundance of UACs in sporopollenin by normalizing the 1520 cm-1 band against the absorption of hydroxyl groups (a band around 3200 cm-1). Lomax and colleagues then show a relationship between UAC abundance and altitude. Sure enough, plants that live at higher altitudes need more sunblock.

The next step is to include fossils. The method of Lomax and colleagues could be applied to well-preserved fossil plants found on a mountainside, to figure out how tall the mountain was when the plants lived.

Lomax BH, Fraser WT, Harrington G, Blackmore S, Sephton MA, Harris NBW. 2012. A novel palaeoaltimetry proxy based on spore and pollen wall chemistry. Earth and Planetary Science Letters 353-354: 22–28.

Top image modified from Wikimedia Commons

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