Using light to describe the ancient world

Reflectance spectroscopy has shown that penguin populations expand and thin on Ardley Island, Antarctica over time. Photo from eliduke

Understanding how climate change affects an animal species first requires an understanding of animal populations over time. A population might naturally wane in response to a temporary food shortage, and could potentially bounce back. It is important to separate natural population dynamics from the results of human activities. For the impact of climate change on Antarctica, penguins are an obvious candidate for environmental monitoring. Xiaodong Liu and colleagues recently published a record of penguin population dynamics recorded in a sediment core representing the last 3000 years of history at Ardley Island, Antarctica.

Specular reflectance is a simple enough concept: a beam of light illuminates a spot on a sample, and a detector is placed in a certain position to collect the light that is reflected. The intensity of the incoming light is known, and the intensity of the reflected light is measured. A reflectance spectrum is created from the reflection intensities of one wavelength at a time, across a range of wavelengths. Liu and colleagues used wavelengths ranging from 200 nm (ultraviolet) to 2600 nm (near infrared).

Liu and colleagues were able to detect “ornithogenic” (bird-made) soil from phosphorus signals in their reflectance spectra. Two sediment cores were collected, and subsamples were extracted, ground, and analysed. Using pure guano and bird-free soil as guides, Liu and colleagues were able to show which levels of the sediment core had an ornithogenic component, and which levels were bird free. Further, the intensity of, err, soil production, from the penguins could be interpreted from the reflectance spectra (with the help of principal component analysis). This in turn was compared to an earlier estimate of population trend, which matched up very nicely. No mention is made as to which penguin species was periodically living on Ardley Island, and one might wonder at the phosphate production of Adélie penguins compared to emperors, although Adélie, chinstrap and gentoo penguins live there today. Regardless, the two cores suggest that Ardley Island has featured substantial changes in penguin population over time.

So, reflectance spectroscopy can be used to understand the natural thinning and expanding that penguin populations undergo.

Liu X, Sun J, Sun L, Liu W, Wang Y. 2011. Reflectance spectroscopy: a new approach for reconstructing penguin population size from Antarctic ornithogenic sediments. Journal of Paleolimnology 45:213–222. DOI 10.1007/s10933-010-9493-6

Image from eliduke

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