Using stable isotopes and radiocarbon to extract climate information from grey mangroves with non-annual growth rings
Mr Matthew Goodwin1, DR Danielle Verdon-Kidd1, Dr Quan Hua2, Dr Nathan B English3, Dr Heather Haines4,5, Dr Kathryn Allen5,6,7
1School of Environmental and Life Science, University of Newcastle, Callaghan , Australia, 2Australian Nuclear Science and Technology Organisation , Lucas Heights , Australia , 3School of Health, Medical and Applied Sciences, Flora, Fauna and Freshwater Research, Central Queensland University, Townsville , Australia , 4Chronos 14Carbon Cycle Facility, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, Australia, 5ARC Centre of Excellence for Australian Biodiversity and Heritage, University of New South Wales, Sydney, Australia , 6School of Geography and Spatial Science, University of Tasmania, Sandy Bay, Australia, 7School of Ecosystem and Forest Sciences, University of Melbourne, Richmond, Australia
Hydroclimate variability in Australia is not well understood prior to the commencement of instrumental climate records in the mid-20th Century. Instrumental climate records can be extended further back in time using proxy data obtained from annual ring forming trees using dendrochronology. However, aside from several exceptions (e.g., Callitris spp.), suitable trees are rare in mainland Australia. Novel techniques such as bomb-pulse radiocarbon dating and stable isotope analysis have made it possible to obtain climate information from trees that do not form annual growth rings. Grey mangroves (Avicennia marina) are the most common mangrove species in NSW, but their growth layers are non-annual. However, grey mangroves are highly sensitive to climate-related variation in freshwater availability and soil salinity. In this study we demonstrate that radiocarbon-based time series of δ18O and δ13C measured from grey mangroves can be used as hydroclimate proxies.
Four grey mangrove stem sections were sampled from dead mangrove trees in the Myall and Hunter River estuaries in NSW, Australia in 2018 and analysed layer-by-layer for δ18O and δ13C using isotope ratio mass spectrometry. Four of the growth layers in each stem including the pith, the outermost layer and two other layers spaced evenly along the selected measurement radius were dated using bomb-pulse radiocarbon dating. A simple age / growth model was prepared for each stem assuming linear growth between the dated layers. Age estimates for all growth layers were truncated to integer calendar years allowing isotope data from the four stems to be averaged into composite δ18O and δ13C series covering the 1962-2016 period. The composite δ18O and δ13C time series were then assessed for similarity with a range of relevant climate variables using Spearman correlation analysis.
Significant correlations were found between δ18O and rainfall, days rain, sea level, vapour pressure, Palmer Drought Severity Index (PDSI) and the El Niño Southern Oscillation (ENSO). Grey mangrove δ18O values appear to reflect the relative proportions of assimilated sea water (δ18O ≈ 0‰ VSMOW) and 18O-depleted fresh water entering mangrove wetlands as rainfall and runoff. Higher δ18O values were observed during known droughts in the 1960’s and during the millennium drought, whilst lower δ18O values occurred at the same time as La Niña events in 2010-12, 2007-08, 1998-2001, 1988-89 and 1973-76. The composite δ13C series was positively correlated with temperature, vapour pressure and evapotranspiration, suggesting that grey mangrove δ13C values were primarily influenced by atmospheric moisture demand. The most significant positive δ13C peak occurred at the same time as the intense El Niño drought of 1982/83, and the most significant negative δ13C peak occurred at the same time as the La Niña of 2010-12 that was the wettest 24-month period on record in Australia.
These results demonstrate that stable carbon and oxygen isotope ratios in grey mangroves yield valuable hydroclimate information. Grey mangroves can live for up to 800 years, are widespread along northern Australian and tropical coastlines and could provide important information regarding pre-instrumental climate in regions currently lacking high-resolution centennial scale climate proxy records.
Matthew Goodwin is a PhD candidate at the University of Newcastle investigating pre-instrumental hydroclimate variability in New South Wales using a multi-proxy approach to extract climate information from mangroves and speleothems.