Biomarker carbon and hydrogen isotopes reveal changing peatland vegetation, hydroclimate and biogeochemical tipping points

Molecular fossils (biomarkers) are frequently used to reconstruct climate and environmental conditions across a range of timescales and in nearly every sedimentary archive. Over the past 25 years, they have been increasingly applied to Quaternary peats (and more ancient lignites), where their concentrations and distributions have been used to reconstruct the peat-forming vegetation, microbial communities, redox and water table depth, temperature and pH. There have been fewer but nonetheless exciting applications of these biomarkers’ carbon and hydrogen isotopic compositions. For example, the carbon isotopic composition of leaf waxes can reveal changes in ecology, including methanotroph symbiosis and submerged growth under elevated water tables. Leaf wax hydrogen isotopic compositions are particularly useful in reconstructing changes in hydroclimate, especially when biomarkers from different plant types are compared. Quaternary peat deposits also provide outstanding archives for interrogating the biogeochemical response of climate change via its impact on peat hydrology, redox and carbon cycling. Bacterial-derived hopanes, in particular, have documented drying-induced changes in carbon cycling – geomicrobial system perturbations – in Quaternary sediments. Here, I explore the biomarker isotope approaches that have been applied to freshwater peat deposits and discuss their limitations and new opportunities. A bias towards the Global North and temperate/boreal bogs has resulted in significant gaps in our understanding of tropical peat isotope dynamics, especially since those systems comprise different plant groups with different chemical compositions. Emerging proxies include the hydrogen isotopic composition of bacterial lipids, which could complement carbon isotope analyses and allow new insights into past climate-environmental-geomicrobial interactions. Crucially, the integration of established and new approaches, when applied to high resolution peat records, will provide new insights into biogeochemical responses and resilience to climate change and associated tipping points.