Global Patterns in Wetland Biomarker Stable Carbon Isotopic Compositions and Implications for Past Reconstructions of Methane Cycling
Abstract
Wetlands are the largest natural source of atmospheric methane and play an essential role in the global carbon cycle. The stable carbon isotopic composition (δ13C) of bacterial and plant lipids has been used to study modern and past wetland biogeochemistry. However, due to the small number of modern wetlands that have been studied, a lack of consistency between target compounds and the narrow range of wetland diversity sampled, this approach lacks a rigorous modern-day framework. Here, we undertake an unprecedented global survey of bacterial and plant lipid δ13C values in wetlands to generate a modern-day reference dataset and probe drivers of isotopic variability.
Within our global dataset, the C3 carbon fixation pathway predominantly determines leaf wax (n-alkane) δ13C values; however, mid-chain n-alkanes can also incorporate CH4-derived CO2, especially in the mid-to-high-latitudes where Sphagnum-dominated wetlands are widespread. Bacterial-derived C31 hopane δ13C values track those of leaf waxes but are relatively enriched (0 to 10‰), indicating a heterotrophic ecology and preferential consumption of 13C-enriched substrates (e.g. carbohydrates). In contrast, bacterial-derived ≤ C30 hopanoids can be strongly 13C-depleted (up to -46‰) and up to 15‰ depleted relative to co-occurring long-chain n-alkanes. This provides clear evidence for the incorporation of isotopically light methane into the bacterial community (especially at near neutral pH) and confirms that ≤ C30 hopanoids are suitable candidates for tracking changes in wetland methane cycling. These observations also indicate a strong source decoupling between ≤ C30 and > C31 hopanoids (of which, the latter are derived exclusively from bacteriohopanepolyols). Previous analyses of ancient sediments have suggested such decoupling as have analyses of modern cyanobacterial mats and cultures; however, our work suggests a more profound and globally widespread decoupling in wetlands that has significant implications for all future hopanoid work Taken together, our results indicate that lipid biomarkers are important tools for evaluating modern and ancient biogeochemical processes and could potentially provide insights into terrestrial methane cycling during the Cenozoic and Mesozoic.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP23A..04I
- Keywords:
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- 0424 Biosignatures and proxies;
- BIOGEOSCIENCESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHYDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHY