BIOMARKER EVIDENCE OF LATE DEVONIAN MARINE ALGAL-MICROBIAL COMMUNITY CHANGES DRIVEN BY RIVERINE NUTRIENT INPUTS (CHATTANOOGA SHALE, TENNESSEE, U.S.A.)

Late Devonian marine algal-microbial communities are thought to have changed in response to increased riverine nutrient fluxes related to the spread of vascular land plants. Here, we generated high-resolution biomarker lipid chemostratigraphic records from a drillcore section of the Upper Devonian (Frasnian-Famennian) Chattanooga Shale from the western margin of the Nashville Dome to investigate algal-microbial community changes on an anoxic marine shelf during the Late Devonian. High trace-metal concentrations at the Frasnian/Famennian boundary (FFB) reflect euxinic conditions, but low concentrations at the Givetian/Frasnian (GFB) and Devonian/Carboniferous (DCB) boundaries imply dominantly suboxic conditions. Aryl isoprenoids, a biomarker of obligate anaerobic green sulfur bacteria indicative of photic-zone euxinia, are abundant at the GFB and FFB. Rapid changes in sterane ratios at the GFB and FFB suggest algal community changes, e.g., increases in C₂₇ and C₂₉ steranes and decreases in C₂₈ steranes imply a sudden loss of prasinophyte microalgae at the FFB, with similar but smaller changes at the GFB. Elevated 3β-methylhopanes at FFB reflect enhanced methane cycling. At the DCB, increased amounts of high-carbon-number alkanes and higher moretane/hopane ratios provide evidence of enhanced inputs from land plants, coupled with an increase in marine productivity. These patterns are consistent with Devonian marine redox and productivity changes having been driven largely by riverine inputs linked to expansion of terrestrial floras.