Using speleothem SO4 isotopes to elucidate S cycling in a fire prone region
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Date
2019-04-11
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Copernicus GmbH
Abstract
Wildfires are a global hazard that can have catastrophic impacts on communities and ecosystems. A speleothem based wildfire record will allow us to determine long-term natural fire regimes and better understand the relationship between wildfires and climate. Recent research has demonstrated the potential of using S as a speleothem paleofire proxy1 but a full characterisation of S in a fire-prone environment is lacking.
Firstly, we used modern monitoring to quantify the relative contributions of S sources in a cave environment with overlying post-fire forest regrowth. Sulphate concentration and isotopic values (δ34S- SO4 and δ18O-SO4) for rainfall (+18.8hand +8.1h respectively), cave drip water (+20.3hand +3.2h), bedrock (+21.7hand +10.6h) vegetation (δ34S- SO4 +22.1h) and soil (δ34S- SO4 +19.5h) were characterised. Results showed the SO4 rainfall input was from a marine source. A 1-2h fractionation of drip water δ34S- SO4 compared to rainfall δ34S- SO4 revealed there was assimilation of SO4 into vegetation above the cave. δ18O- SO4 indicated biogeochemical cycling of S by mineralisation of organic S compounds to sulphate within the soil. These results demonstrate the necessity of dual isotope analysis of δ34S- SO4 and δ18O- SO4 to correctly identify S sources and biogeochemical cycling prior to incorporation of SO4 into a speleothem. Secondly, the S isotope proxy was applied to a 2-12 ka speleothem record from the same region and forest cover. More than fifty 200 mg samples from a flowstone were analysed for δ34S- SO4 and δ18O- SO4:Mean speleothem δ34S- SO4 was enriched in 34S by >2 h compared to modern rainfall, indicative of vegetation fractionation. LGM mean δ34S- SO4 was more negative than Holocene (+22.1hand +23.2h respectively). Fast growth periods at δ10 and 7ka provided multi-annual resolution and 1.5h variability in δ34S- SO4 was observed over decadal timescales, indicative of changes in vegetation cycling of S, which we interpret as a possible fire record.
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Keywords
Fires, Hazards, Caves, Ecology, Climates, Environment, Forests, Plant growth
Citation
Coleborn, K., Baker, A., Treble, P. C. & Wynn, P. M. (2019). Using speleothem SO4 isotopes to elucidate S cycling in a fire prone region. Paper presented to the EGU General Assembly 2019, Vienna, Austria, 7-12 April 2019. In Geophysical Research Abstracts, Vol. 21, EGU2019-12516-1. Retrieved from: https://meetingorganizer.copernicus.org/EGU2019/EGU2019-12516-1.pdf