Jannik Martens
I am an earth and environmental scientist with interests in climate change and carbon cycling. My experience and area of expertise include marine geology, paleoclimate research, isotope geochemistry and molecular analysis of organic matter. Some of my key interests and contributions are summarized below.
Arctic Ocean acidification during past and present climate change
The increasing uptake of CO2 by the Arctic Ocean leads to ocean acidification and may cause ecosystem and socio-economic stress within and beyond the Arctic region. Ocean acidification in the Arctic is commonly attributed to the rise in atmospheric CO2. However, Arctic warming is expected to trigger massive release of terrestrial organic carbon from rivers and collapsing permafrost, which may exacerbate ocean acidification by additional supply of CO2 from mineralized terrestrial carbon. In this project we investigate Arctic Ocean acidification during past climate change, e.g. during the last deglaciation or over past glacial-interglacial cycles. Past changes in ocean pH and pCO2 are studied using the boron isotope proxy (11B) in foraminifera, as well as through microbial biomarkers. With this work we hope to expand our understanding of the natural variability of the Arctic Ocean carbonate system during changing climate, and study the contribution of terrestrial processes to Arctic Ocean acidification.
Permafrost carbon release during past warming events
Center to my work as a PhD student was to study past changes in carbon release from terrestrial systems, to learn about its potential for carbon cycle perturbations in the near-future. As part of this work, I studied sediment archives to reconstruct the climate sensitivity of Arctic permafrost and OC 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 (13C, 14C) and terrestrial biomarkers (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 reveal massive permafrost carbon release during three warming events documented in Greenland ice cores, which suggests that climate warming by a few °C might be sufficient to trigger large-scale permafrost thaw. This work provides a paleo-reference for the potential of future carbon release from permafrost in response to anthropogenic climate change.
Circum-Arctic carbon cycling today
Arctic warming causes destabilization of high-latitude soils and permafrost deposits, yet large uncertainties exist regarding the dynamics and processes of carbon release from these systems. Over the past years, I was part of an international team of scientists that created a new database of biogeochemical observations in Arctic Ocean sediments, the Circum-Arctic Sediment Carbon DatabasE (CASCADE; incl. a paper in Earth System Science Data; Martens et al., 2021). For this work, we collected and investigated data of carbon isotopes (13C, 14C) and terrestrial biomarkers (plant lipids, e.g. n-alkanes and n-alkanoic acids; as well as lignin phenols) in sediments to deduce carbon release and re-mineralization through the perspective of the shelf receptor.
Based on the creation of the CASCADE and a number of application studies, we also study the spatial patterns and differences of carbon release and re-mineralization in the vast circum-Arctic drainage basin. For this work, we use spatial data analysis tools and statistical dual-isotope (13C, 14C) source apportionment to estimate terrestrial carbon accumulation in the circum-Arctic shelf seas and study the lateral carbon mobilization from different Arctic carbon reservoirs (e.g. permafrost and peatlands). This work was recently published in Nature Communications (Martens et al. 2022).