Friday, 6 May 2016

Investigating Climate Change in Eastern Australia...by Melanie Leng

Melanie Leng is the head of the Stable Isotope Facility
at the British Geological Survey
In the stable Isotope Facility at the British Geological survey we spend most of our time collaborating with UK universities and research institutes. However, every now and again we get an opportunity that’s too good to be true… One such opportunity came a few of years ago when an email popped into my inbox from Australia. Dr John Tibby and Dr Cameron Barr (from the University of Adelaide) explained that in Australia they have a particular problem in that there are relatively few geological archives of climate change, so researchers into past climate tend to rely on short timescale corals (which can be related to seawater salinity and temperatures) or tree rings (a proxy for rainfall amount). However, both corals and trees tend to only live for a few hundred years, so they were keen to develop new records of Australian climate… 

Cameron Barr sampling leaves from the
paperbark tree on Fraser Island, Queensland
John and Cam hypothesised that the broad-leaved paperbark tree (Melaleuca quinquenervia for those botanically minded) might contain a signal of the amount of rainfall as it’s long been known that the geochemistry (specifically the carbon isotope composition which could be measured within the Stable Isotope Facility at the BGS) of some leaves change in response to water stress. The drier the climate the less the leaves evapotranspirate (similar to sweating), meaning that the “pores” on a leaf are more closed and restrict the plant’s ability to use CO2 from the atmosphere. When conditions are wet, the pores open and the plants can use more CO2 from the atmopshere. This difference in the carbon utilised is ultimately recorded in the leaf carbon as the leaves grow under different climate conditions. With this relationship in mind, we sampled paperbark tree leaves, experimenting with leaves from different parts of the tree, as well as obtaining leaves that had been collected over an eleven year period. We sampled the leaves and compared the carbon to the amount of rainfall for each year, which on North Stradbroke Island SE Queensland, was directly related to water levels and therefore how wet the years were.
One of the lakes on North Stradbroke Island surrounded
by paperbark trees

Overall, we have shown that there is a significant relationship between the leaf carbon and rainfall (averaged over the life span of the leaf). This finding will now allow us to collect much longer records of climate in Australia because we looked in the lakes of Fraser and North Stradbroke Islands which have paperbark trees growing on their shores and found that their leaves were preserved in some of the lake sediments. The leaves we have collected go back thousands of years, and thus we now will be able to investigate aridity in Eastern Australia as well as other places where these trees grow, such as Papua New Guinea and New Caledonia, so there is more to come!

Our paper detailing these initial findings has recently been published, please see: Tibby, J., Barr, C., McInerney, F.A., Henderson, A.C.G., Leng, M.J., Greenway, M., Marshall, J.C., McGregor, G.B., Tyler, J.J., McNeil, V. 2016. Carbon isotope discrimination in leaves of the broad-leaved paperbark tree, Melaleuca quinquenervia, as a tool for quantifying past tropical and subtropical rainfall. Global Change Biology
A core of sediment from the lake showing a sand layer (white)
from an erosion event, and fragments of paperbark tree leaves
 within the lake sediment mud which has accumulated over
thousands of years


Thanks go to all our collaborators (co-authors), but special thanks go to Margaret Greenway from Griffith University who collected leaves every 28 days (even on Boxing Day!) for more than 12 years

Melanie Leng is the head of the Stable Isotope Facility at the British Geological Survey, Twitter @MelJLeng.





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