Award: OCE-2023687

Our project sought to address the question of where long-lived organic molecules that are dissolved in the oceans come from, in particular molecules containing sulfur. Our approach was to measure the relative abundance of two stable sulfur isotopes (S-32 and S-34) in these molecules, which is technically very difficult due to the presence of million-fold higher sulfate ions in seawater. We developed a new preparatory chemistry to adequately isolate these organic molecules, and a new elemental analyzer/mass spectrometry method to measure their isotope abundances with high precision at trace levels. We conducted these S isotope measurements on 100 samples of dissolved organic matter (DOM) that had been previously collected by our collaborators from around the world; we also collected 2 dozen new samples from oceanographic stations in the Pacific and Atlantic oceans. Our data show that DOM molecules have (34S/32S) isotope ratios that are entirely consistent with being formed from ocean sulfate, and inconsistent with being formed by reactions of hydrogen sulfide in anoxic porewaters. This result negates one of the leading hypotheses for how long-lived DOM forms, i.e. by reactions in anoxic sediments. Instead, this sharpens our focus on understanding how relatively short-lived biomolecules in the surface ocean get transformed into long-lived DOM molecules. Last Modified: 01/02/2024 Submitted by: AlexSessions