Lipid biomarker stratigraphic records through the Late Devonian Frasnian/Famennian boundary: Comparison of high- and low-latitude epicontinental marine settings

The pervasiveness of black shale preservation in association with Late Devonian biological crises suggests marine anoxia played a major role in driving ecological perturbations. However, Devonian black shale deposition is still mechanistically poorly understood. We have compiled detailed biomarker lipid chemostratigraphic records for 83 different rock samples using molecular constituents of bitumens of Upper Kellwasser equivalent black shales from two foreland basins: from the low paleolatitude Appalachian Basin (New York State) and from the high paleolatitude Madre de Dios Basin (Bolivia), in order to better understand local environmental conditions and organic source inputs during this depositional event. Despite strong indications from stable nitrogen isotopic signatures for fixed nitrogen nutrient limitation, the biomarker assemblages with consistently low-moderate hopane/sterane ratios (< 0.8) indicate that algae were major marine primary producers in both basins throughout the Frasnian/Famennian (F/F) stratigraphic coverage. Consistently higher C28/C29 sterane ratios at higher paleolatitude in the more nutrient-replete Madre de Dios Basin suggest prasinophyte microalgae flourished in this setting in accordance with palynological evidence for high contributions of Tasmanites cysts in these strata. All samples contain only very low absolute amounts of aryl isoprenoids (with 2,3,6-trimethyl substitution) and other aromatic carotenoids, up to several orders of magnitude lower than concentrations reported from other Phanerozoic euxinic basins. These data are consistent with local marine paleo-redox models for both basins lacking a persistently shallow sulfidic aquatic zone and demonstrate that temporally persistent or spatially pervasive photic zone euxinia is not necessarily associated with all black shale sequences in the Late Devonian.