Challenges and Future Developments in Alkenone Paleoceanography Revealed by Cretaceous Studies in High Latitudes of the Southern Hemisphere

Abstract

Although C40 alkenones are known from the Cretaceous, the simultaneous detection of di- and tri-unsaturated molecules had not been reported. Hasegawa and Goto (2024) continuously detected these molecules for the first time from the Cretaceous (~94 Ma) and discussed the cooling associated with organic carbon burial during Oceanic Anoxic Event 2. In geological samples, isomers where one of the Z-unsaturated sites becomes E can exist, causing di-unsaturated isomers to overlap with tri-unsaturated molecules on GC, making it difficult to evaluate their content or even recognize them. However, it was found that this issue can be resolved by applying a medium-polarity column. The usefulness of trace analysis using GCMS also emerged. SIM mode analysis enabled the detection of trace alkenones that could not be detected previously, yielding results including the world's first detection from the Santonian of the Cretaceous and the Paleocene. Moving forward, we aim to establish a quantification method through trial and error using chemical ionization and GCMSMS. If analysis becomes possible with 1/100 of the conventional amount, the application range of the alkenone paleothermometer will increase dramatically, potentially revolutionizing paleoceanography.