BIOMARKERS AS A TOOL TO CONSTRAIN ANCIENT BIOGEOCHEMICAL CYCLES

Marine microbes serve important roles in many biogeochemical cycles. However, the molecular fossils of microbial life (i.e. biomarkers) retained in the sedimentary archives remain underutilized to constrain the spatiotemporal evolution of elemental fluxes (e.g. C, N, S, and many micronutrient metals) in deep time. Here, a major carbon cycle perturbation, oceanic anoxic event 2 (OAE 2), was selected to assess the potential biomarkers possess to aid investigations of ancient biogeochemical cycles. The positive carbon isotope excursion (+CIE) used to demarcate OAE 2, interpreted to represent enhanced global organic carbon (OC) burial, remains somewhat enigmatic as the principal forcing (productivity vs. preservation) remains elusive. We report a diverse biomarker inventory, produced by a novel liquid chromatography-quadrupole time of flight-mass spectrometry method, capturing a holistic perspective of the marine microbial ecology before, during, and after OAE 2 in the southern North Atlantic (ODP Site 1258, Demerara Rise). Biomarkers in this study represent depth-specific microbial communities, hailing from the euphotic, mesopelagic, and benthic zones, enabling disentanglement of potentially differential microbial responses, based on residence depth, to climatic shifts coeval with OAE 2. Prior to the +CIE, primary productivity (PP) by photoautotrophs and chemoautotrophs spiked, leading to a sustained period (~100 kyr) of enhanced OC production at Demerara Rise which likely occurred at other similar paleogeographic settings. Oxygen consumption via OC remineralization led to oxygen minimum zone expansion and intensification regionally, inducing favorable conditions for rapid OC burial that led to the abrupt +CIE. During OAE 2, vertical advance of euxinic waters and density stratification inhibited efficient transfer of bioessential nutrients (e.g. fixed N) to the upper photic zone. PP was adversely affected, as expanding mesopelagic and benthic communities thrived and fundamentally altered N cycling. Thus, the +CIE was initiated after a prolonged period of heightened PP and perpetuated by enhanced preservation, which also imparted observable alterations to N cycling as shown by biomarker distributions. It is clear biomarkers contribute valuable context to clarify the processes producing the +CIE, as well as other coeval biogeochemical isotopic records. Future studies of biogeochemical nutrient cycling in deep time should consider the boundary conditions biomarkers can provide using this new method.