Nils Keno Lünsdorf

Testing the preservation of biomarkers during experimental maturation of an immature kerogen

Lipid biomarkers have been extensively applied for tracing organisms and evolutionary processes through Earths history. They have become especially important for the reconstruction of early life on Earth and, potentially, for the detection of life in the extraterrestrial realm. However, it is not always clear how exactly biomarkers reflect a paleoecosystem as their preservation may be influenced by increasing temperatures (T) and pressures (P) during burial. While a number of biomarker indices reflecting thermal maturity have been established, it is often less well constrained to which extent biomarker ratios used for paleoreconstruction are compromised by T and P processes. In this study we conducted hydrous pyrolysis of Green River Shale (GRS) kerogen in gold capsules for 2–2400 h at 300°C to assess the maturation behaviour of several compounds used as life tracers and for the reconstruction of paleoenvironments ( n -alkanes, pristane, phytane, gammacerane, steranes, hopanes and cheilanthanes). Lignite samples were maturated in parallel with the GRS kerogen to obtain exact vitrinite reflectance data at every sampling point. Our experiment confirms the applicability of biomarker-based indices and ratios as maturity indicators (e.g. total cheilanthanes/hopanes ratio; sterane and hopane isomerization indices). However, several biomarker ratios that are commonly used for paleoreconstructions (e.g. pristane/phytane, pristane/ n -C 17 , phytane/ n -C 18 and total steranes/hopanes) were considerably affected by differences in the thermal degradation behaviour of the respective compounds. Short-term experiments (48 h) performed at 400°C also revealed that biomarkers >C 15 (especially steranes and hopanes) and ‘biological’ chain length preferences for n -alkanes are vanished at a vitrinite reflectance between 1.38 and 1.83% R O . Our data highlight that ‘thermal taphonomy’ effects have to be carefully considered in the interpretation of biomarkers in ancient rocks and, potentially, extraterrestrial materials.

Tracing ultrahigh-pressure metamorphism at the catchment scale

Finding traces of ultrahigh-pressure (UHP) metamorphism in the geological record has huge implications for unravelling Earth’s geodynamic evolution, such as the onset of deep subduction. Usually, UHP rocks are identified by specific mineral inclusions like coesite and characteristic petrographic features resulting from its (partial) transformation to the lower-pressure polymorph quartz in thin sections of crystalline rocks. This approach relies on very small sample size and is thus limited to a few points within large regions. Here we present the first findings of coesite inclusions in detrital mineral grains. The intact monomineralic inclusions were detected in garnets from a modern sand sample from the Western Gneiss Region, SW Norway. They represent the first known intact monomineralic coesite inclusions in the Western Gneiss Region, and their presence is suggested to indicate the erosion of UHP rocks in the sampled catchment area. The novel approach introduced here allows for tracing UHP metamorphic rocks and their erosional products at the catchment scale instead of being limited to outcrops of crystalline rocks. It opens new avenues for the prospective exploration of UHP metamorphism in Earth’s geological record.