Climate–carbon feedbacks from the Arctic to carbon removal

Arctic permafrost stores around 1,300 billion tonnes of organic carbon—roughly one and a half times the amount currently present in the atmosphere. Thaw can expose this ancient carbon to microbial degradation and greenhouse-gas release, but the strength of this feedback depends on how bioavailable the carbon is and whether associations with minerals protect it.

My research investigates how mineral interactions control the stability and bioavailability of permafrost carbon. Using geochemical analyses, mineral-selective extractions, Mössbauer spectroscopy and incubation experiments, we found that 33–74% of the organic carbon in Pleistocene permafrost was associated with fine-grained minerals. Reactive iron minerals provided particularly effective protection and reduced microbial CO₂ production after thawing (Martens et al., 2023).

Recent collaborative studies have further examined how interactions among minerals, enzymes and microbial communities regulate soil carbon stabilization. This work shows that mineral–enzyme interactions can promote organic carbon accumulation in permafrost soils (Zhu et al., 2025), while temperature-dependent interactions between iron minerals and microbial enzymes influence carbon stabilization across climatic gradients (Guo et al., 2026). Understanding how these coupled mineral and biological processes respond to warming is essential for predicting the permafrost-carbon climate feedback.

Arctic warming destabilizes high-latitude soils and permafrost, mobilizing terrestrial carbon through rivers and coastal erosion into the Arctic Ocean. However, large uncertainties remain regarding how much carbon is released, where it originates and what happens to it during transport. To address these questions, I contributed to an international effort to create the Circum-Arctic Sediment Carbon DatabasE (CASCADE; Martens et al., 2021). The database combines observations of organic carbon, stable and radiocarbon isotopes, and terrestrial biomarkers in Arctic Ocean sediments, using the ocean as a receptor system for tracing carbon mobilization from land.

Using CASCADE together with spatial analysis and dual-isotope source apportionment, I investigated how terrestrial carbon release, transport and remineralization vary across the circum-Arctic drainage basin. This work revealed strong regional differences in the mobilization of carbon from soils, peatlands and permafrost, as well as in its transport to and transformation within the Arctic Ocean (Martens et al., 2022). More recently, I contributed to a synthesis of the land–ocean Arctic carbon cycle that connects carbon release on land with its transport, transformation, greenhouse-gas production and burial in the ocean (Vonk et al., 2025).

The Arctic Ocean carbon cycle is influenced by both the uptake of atmospheric CO₂ and the transfer of terrestrial carbon from rivers, coastal erosion and thawing permafrost. Once this terrestrial organic carbon enters the ocean, it can be transported, buried or decomposed by microorganisms, releasing additional CO₂ and contributing to ocean acidification. Changes in sea ice, ocean circulation and biological productivity further affect how carbon is transformed and stored.

My research examines how these interacting processes have changed through time using geochemical signals preserved in marine sediments. I use boron isotopes in foraminifera to reconstruct past seawater pH and investigate how ocean circulation and carbon storage responded to glacial–interglacial climate change. In a recent collaborative study, boron isotopes revealed lower deep-ocean pH and reduced ventilation of the Arctic Ocean during the last glacial period (Farmer et al., 2026).

I also study glycerol dialkyl glycerol tetraethers, or GDGTs—microbial membrane lipids that provide information about biological activity and past ocean conditions. My recent work showed that GDGT distributions in central Arctic Ocean sediments reflect a combination of lateral transport, terrestrial inputs and microbial production within the sediments. These processes complicate their use as temperature proxies, but some GDGT groups may provide information about past changes in Arctic biological productivity (Martens et al., 2026). Together, these approaches help reveal how atmospheric forcing, land–ocean carbon transfer and internal ocean processes shape the Arctic carbon cycle.

The focus of my work as a PhD student was to study past changes in carbon release from terrestrial systems to understand their potential for future perturbations in the global carbon cycle. As part of this research, I analyzed sediment archives to reconstruct the climate sensitivity of Arctic permafrost and organic carbon release to the Arctic Ocean during past warming events, such as the last glacial termination. This work resulted in two studies published in Global Biogeochemical Cycles (Martens et al., 2019) and Science Advances (Martens et al., 2020), in which we studied carbon isotopes (δ¹³C, Δ¹⁴C) and terrestrial biomarkers (such as plant lipids, e.g. n-alkanes and n-alkanoic acids, as well as lignin phenols) in marine sediment cores from the Arctic Ocean. The results revealed massive permafrost carbon release during three warming events documented in Greenland ice cores, suggesting that a few degrees Celsius of climate warming may be sufficient to trigger large-scale permafrost thaw. This work provides a paleo-reference for the potential future carbon release from permafrost in response to anthropogenic climate change.

The Greenland Ice Sheet holds a significant amount of freshwater, equivalent to approximately 7.4 meters of potential global sea level rise. Recent research highlights its sensitivity to climate warming and the resulting impact on sea levels and ocean circulations. However, data limitations hinder our understanding of the ice sheet's response to major climate changes, restricting our ability to predict future sea level rise and climate feedback. To address this, I am participating in Expedition 400 of the International Ocean Discovery Program (IODP) to NW Greenland (Baffin Bay) aboard the JOIDES Resolution from August 13th to October 13th, 2023. Expedition 400 aims to recover sediment archives with high temporal resolution, documenting the climate history and variability of the Greenland Ice Sheet over the past 30 million years. Subsequent post-cruise research on these sediment cores will involve utilizing geochemical proxies to trace temperature and precipitation. Additionally, analysis of microfossils (foraminifera) will uncover connections between the Arctic Ocean, North Atlantic, and the Greenland Ice Sheet. Our findings will shed light on the factors influencing the Greenland Ice Sheet, thus contributing to enhanced predictions of global sea level rise amidst ongoing Arctic and global warming.