Mammalian oxygen isotope ratios: environment indicators


Mammalian oxygen isotope ratios: environment indicators

Steve Down

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The oxygen-18 isotope ratios of animals can be related to the precipitation in their local environment, leading to the possibility of studying climate, the animal region of origin and migration patterns. This well-established relationship has been used to great effect in the fields of palaeontology, archaeology and forensic science.

The most common tissues used to measure the isotope ratios are teeth or bone. More precisely, it is the bioapatite within these structures that is the target for analysis. Bioapatite is a type of calcium phosphate mineral in which measurable oxygen is present in phosphate (PO4) and carbonate (PCO3) forms, the former accounting for about 92% of the total oxygen present.

There is an established relationship between the oxygen isotope ratios of the PO4 and PCO3 types but it differs between species and cannot be extrapolated from one creature to another. It appears that a new relationship must be established for each new species.

Scientists in the UK noted that it is not always easy to perform oxygen isotope ratio measurements since teeth or bones are required for analysis. It would be far better if a more readily available type of tissue could be used to give reliable results. This team decided to measure the oxygen isotope ratio in hair to see if consistent results were obtained and compare them with the data for bioapatite.

Macaque measurements: bone and hair isotope ratios compared

Carolyn Chenery and Angela Lamb from the British Geological Survey, Keyworth, Hannah O'Regan from the Liverpool John Moores University and Sarah Elton from the University of Hull worked with rhesus macaques that had been caught in the wild and were now preserved in the Natural History Museum, London, UK.

The macaques had been living in different locations in Asia, from western India to northern Vietnam, representing a range of climates which might have an effect on their oxygen isotope ratios.

Hair from the pelts and bone from the mandibles were collected and processed before isotope analysis by isotope ratio mass spectrometry.

The oxygen isotope ratios fell into three clear ranges for hair, PO4 and PCO3. Hair had the lowest values at +7.4 to +13.6 ppt, followed by PO4 at +15.3 to +22.0 ppt and PCO3 at +23.9 to +31.3 ppt.

However, the key finding was the relationship between the values for each matrix. Good linear agreement was confirmed between hair and PO4, hair and PCO3, and PO4 and PCO3, with correlation factors of 0.79, 0.87 and 0.91, respectively.

In effect, this means that any one of the three tissue types can be substituted for another when the results from different tissues are being compared. In particular, the team noted that replacement of PCO3 by hair would permit drinking water calculations to be made when bioapatite was unavailable for analysis.

Although it is a valuable relationship to characterise, this measurement by proxy only applies to this one species. There is no published evidence to suggest that hair from one species can be used in place of bioapatite for another species. At this stage, it appears that such a relationship needs to be established for each species.

Environmental implications

The biological implications of the results were trickier to extract, due to the relatively small size of the data set, with only 15 macaques being tested. However, some basic conclusions could be drawn.

There was some correlation between the values for macaques from the same location. For instance, the oxygen isotope ratios for macaques from The Dangs area in India were the highest for the three matrices and those from northern Vietnam were the lowest.

Between the two tropical locations represented, The Dangs and Burma, the expected decrease in the hair oxygen isotope ratio with distance from the source of precipitation was observed. This fall results from preferential loss of the heavier 18O isotopes compared with 16O over distance.

This effect was reversed for wet/dry subtropical climates, for which the oxygen isotope ratio increased with distance from precipitation and with altitude, due to the evaporation of free water and transpiration from plants during the dry season.

It was clear that precipitation quantity and source, altitude and climate all had an influence on the oxygen isotope ratios, but far more data will be required to determine precise relationships.

This study has provided the first data on oxygen isotope relationships between hair, PO4 and PCO3 in a non-human primate, proving that the three are interchangeable for cross tissue comparisons within one species.