Reconstruction of the Cenozoic evolution of the hydrogen isotopic composition of seawater using C37 alkenones

Abstract

Understanding Earth’s climate system and its response to external forcings is one of the most pressing scientific challenges of our time. Insights from past climate change are essential for improving future climate projections, especially as the world continues to warm. Palaeoceanographic reconstructions provide information on climate dynamics far beyond the instrumental record and help constrain the sensitivity of the Earth system to changes in greenhouse gases, ice volume, and the hydrological cycle. Surface ocean salinity, controlled by the balance between evaporation, precipitation, and runoff, is closely linked to the isotopic composition of seawater and reflects regional and global hydrological changes. Reconstructing past seawater δ²H and δ¹⁸O therefore provides insight into changes in the global water cycle and ocean–atmosphere interactions. Despite their importance, long-term seawater isotope reconstructions remain scarce. This thesis investigates the hydrogen isotopic composition of long-chain alkenones (δ²HC37), lipids produced by haptophyte algae, as a proxy for reconstructing surface seawater hydrogen isotopes (δ²HSSW) during the Cenozoic. Compound-specific isotope analyses were carried out on alkenones extracted from marine sediments spanning climatic intervals from the Eocene–Oligocene Transition (EOT) to Late Quaternary glacial–interglacial cycles. The sediment cores originate from the Equatorial Pacific, North and South Atlantic, and the Mediterranean. The results show that δ²HC37 records captures seawater hydrogen isotopes and hydrological variability. A pronounced glacial–interglacial δ²HSSW shift (~14‰) off the Chilean margin demonstrates the sensitivity of the proxy to surface hydrological changes in productive open-ocean settings. In contrast, a remarkably stable δ²HC37 signal across the Middle Miocene at IODP Site U1318 suggests stable seawater conditions during this interval, despite variable δ¹⁸O-based reconstructions. In the eastern Equatorial Pacific, depleted and variable δ²HC37 values indicate alkenone production in the deep chlorophyll maximum, showing that ecological effects can influence hydrogen isotope fractionation under oligotrophic conditions. At ODP Site 959 in the Equatorial Atlantic, exceptionally enriched δ²HC37 values prior to 21.4 Ma likely reflect ancient alkenone-producing haptophyte communities that differ from modern species. Across the Eocene–Oligocene Transition, the positive δ²HC37 shift parallel changes in δ¹⁸O and deep-sea temperature, reflecting Antarctic glaciation and global seawater isotope reorganization during this major climate transition. Together, these records provide the first long-term Cenozoic seawater isotope reconstructions based on δ²HC37. The results generally parallel established δ¹⁸O trends but also reveal additional variability, particularly where carbonate-based proxies are sparse or uncertain. The proxy performs best in open-ocean settings with limited ecological or terrestrial influence, whereas restricted settings require careful consideration of ecological effects. A complementary study on Mediterranean sapropels further showed that alkenones are more reliable for hydrogen isotope analysis than other biomarker lipids such as phytol, palmitic acid, or C₃₀ diols. Overall, this thesis demonstrates the potential of δ²HC37 to complement existing palaeoclimate proxies and highlights the importance of multiproxy approaches for reconstructing past salinity, hydrology, and seawater isotope evolution.