A Novel Method for Radiocarbon Dating Greenhouse Gas Emissions from Saltmarsh Soils to Address Key Blue Carbon Challenges

The surficial 10 cm of Scotland’s saltmarshes are estimated to store 1.35 ± 0.33 Mt CO₂ equivalent, approximately 3.37% of Scotland’s national greenhouse gas emissions in 2020^1,2. This is largely achieved through effective preservation of organic matter (OM) in low oxygen, sulphidic soils¹. Saltmarshes gain organic carbon (OC) through in-situ (autochthonous) production by vegetation and benthic microalgae, and the accumulation of marine and terrestrial material during tidal inundation (allochthonous)³.

A key blue carbon challenge is to empirically understand, under current and predicted warmer conditions, the sources of OC accreted into and respired from saltmarshes. This can determine the proportion of the total OC pool which is additional through in-situ sequestration or from increased preservation of allochthonous OM³. Alongside ¹³C and ¹⁵N isotopes, radiocarbon (¹⁴C) analysis/dating can be used to determine the sources of saltmarsh surficial soil OC⁴.

We hypothesise that at ambient temperatures the younger and more labile, and predominantly autochthonous OM, will be preferentially decomposed.  But at elevated temperatures the aged, and predominantly allochthonous, OM pool will increasingly contribute to the respired greenhouse gases.

To test this hypothesis, we collected soil cores and surficial sediment samples from three contrasting Scottish saltmarshes. We analysed them for ¹⁴C to gain an understanding of the age and sources of the autochthonous and allochthonous OM accumulating. We also aerobically incubated sub-samples of the soil in temperature-controlled experiments at 11.1 ± 1°C (ambient) and 20 ± 1°C (elevated). The evolved CO₂ was collected on molecular sieve traps and analysed for ¹⁴C content/age.

Our results will facilitate comparison of the age of the bulk OM and the respired CO₂ to the thermogravimetrically measured reactivity of the OM determined using the recently developed Carbon Reactivity Index⁵. We will present our findings and introduce our ongoing work on anaerobic incubations of the same soils, which includes the novel measurement of the evolved ¹⁴CH₄.

Our research contributes to a growing evidence base for emissions from saltmarshes, and the sources of OC accreting in their soils, which is vital for understanding how they cycle carbon and their ability to mitigate climate change. It will contribute to the creation of saltmarsh carbon cycle models and inform work to include saltmarshes in the UK’s Nationally Determined Contributions.

References

• Smeaton C, Burden A, Ruranska P, et al. Using citizen science to estimate surficial soil Blue Carbon stocks in Great British saltmarshes. Front Mar Sci. 2022;9. Accessed November 28, 2022. https://www.frontiersin.org/articles/10.3389/fmars.2022.959459
• Scottish Goverment. Scottish Greenhouse Gas Statistics 2020.; 2022. https://www.gov.scot/publications/scottish-greenhouse-gas-statistics-2020/
• McTigue ND, Walker QA, Currin CA. Refining Estimates of Greenhouse Gas Emissions From Salt Marsh “Blue Carbon” Erosion and Decomposition. Front Mar Sci. 2021;8. Accessed January 26, 2022. https://www.frontiersin.org/article/10.3389/fmars.2021.661442
• Hajdas I, Ascough P, Garnett MH, et al. Radiocarbon dating. Nat Rev Methods Primer. 2021;1(1):1-26. doi:10.1038/s43586-021-00058-7
• Smeaton C, Austin WEN. Quality Not Quantity: Prioritizing the Management of Sedimentary Organic Matter Across Continental Shelf Seas. Geophys Res Lett. 2022;49(5):e2021GL097481. doi:10.1029/2021GL097481