DETECTION, DISTRIBUTIONS, AND BIOGEOCHEMICAL IMPLICATIONS OF FOSSIL ARSENOLIPIDS IN MARINE AND LACUSTRINE SEDIMENTS

Abstract

Arsenolipids are a class of arsenic-containing lipids that occur widely in modern aquatic environments and are produced by phylogenetically diverse microorganisms through a conserved arsenic-methylation pathway. Their potential as molecular biomarkers in the geological record, however, has been limited by analytical, distributional, and interpretive uncertainties. This dissertation establishes fossil arsenolipids—including both arsenic-containing hydrocarbons (AsHCs, bearing an O=As(CH₃)₂– head group) and thioxoarsenolipids—as a class of element-specific molecular fossils, systematically characterizing their analytical detection, sedimentary distribution, and stratigraphic variability. Chapter 2 establishes an analytical framework for fossil arsenolipid detection that combines silica-gel solid-phase extraction (SPE), normal-phase liquid chromatography (NPLC), and dual-energy collision-induced dissociation (CID) mass spectrometry. The framework recovers arsenolipid signals from matrices ranging from algal biomass to ancient sedimentary rocks for which conventional reversed-phase methods do not yield arsenolipid detection. Chapter 3 demonstrates the broad distribution and long-term preservation of arsenolipids through a survey of fourteen samples comprising eight modern aquatic settings (spanning algal biomass, coastal and lacustrine sites) and six fossil sedimentary samples spanning the Quaternary to the mid- to late Cenomanian (~94–100 Ma). At least one arsenolipid class was detected in every sample, but the two classes differed in range: AsHCs were recovered from twelve of the fourteen samples and thioxoarsenolipids from thirteen, across organic-rich shales, lacustrine deposits, and carbonate-rich pelagic chalks. AsHCs were detected in fossil sediments as old as the Late Cretaceous (~90 Ma), whereas thioxoarsenolipids persisted into the older OAE2 black-shale interval (~94–100 Ma); together these extend the preservation record across at least ~100 Myr of sedimentary burial and indicate that the two classes differ in long-term preservation. The pelagic chalk recovery further extends the lithological range beyond the high-TOC anoxic settings of classical biomarker work. Chapter 4 presents the first stratigraphically resolved record of two arsenolipid compound classes across thirteen samples from the Holocene Black Sea sapropel sequence (GGC18, 11.7–0.6 ka). The two compound classes follow contrasting stratigraphic patterns involving distinct biogeochemical controls (producer-side for AsHCs and sulfide-mediated formation for thioxoarsenolipids), but both trace to the same upstream environmental transition (the development of photic-zone euxinia). The ratio of AsHCs to thioxoarsenolipids (the AsHC/thioxoarsenolipid ratio) integrates both responses into a composite metric whose approximately eleven-fold response across the Unit III/II boundary supports its use as an indicator of major regime transitions in sedimentary archives. Together, these results characterize the analytical detection, distribution, and time-resolved environmental response of arsenolipids and support their further development as a paleoenvironmental tool.