A modified Rock-Eval approach to track organics in (bio)carbonates

Abstract

Biominerals, notably carbonates, serve as valuable biosignatures for identifying past or present life in terrestrial environments. However, distinguishing between biogenic and abiotic minerals usually required multiple high-resolution techniques, challenging their application for the in situ search for extraterrestrial life in space missions with limited analytical capabilities. This study investigated the potential of gas profiles (i.e., carbon dioxide CO${}_2$, carbon monoxide CO, and sulfur dioxide SO${}_2$) generated by Rock-Eval purified air combustion (in the range 50–700 °C) and dinitrogen pyrolysis (in the range 700–1000 °C) of 66 natural and laboratory carbonates to detect organics associated with these carbonate minerals that could hold clues to their origin (either abiotic or biogenic) and formation process. For bio-related and organo-carbonates containing Ca and Ca/Mg, CO and SO${}_2$ emissions detected below 700 °C were the product of combustion and associated thermal cracking of organic compounds initially coating mineral grain surfaces, while those detected above 700 °C, during and after the thermal decomposition of Ca- and Ca/Mg-carbonates, were derived from the thermal cracking of organic compounds trapped within carbonate crystals, suggesting the carbonates formed in the presence of organic compounds of biological or abiotic origin. For hydrated Mg-carbonates, the interpretation is more challenging due to multiple phase transitions that overlap with the combustion and thermal cracking of surface and trapped organic compounds in the same temperature range. Overall, this study highlights the potential of our modified Rock-Eval approach as a valuable technique for rapidly identifying and characterizing potential biosignatures in Ca- and Ca/Mg-carbonate at the bulk sample scale, an approach that can be reasonably implemented on in situ space instruments for the search for present or past extraterrestrial life.