Rising productivity drove marine euxinia during the Late Devonian mass extinctions

The relationship of oceanic anoxia to changes in marine productivity during the biocrises of the Late Devonian remains uncertain. We present zinc isotope (δ66Zn) data from the euxinic Upper Devonian Chattanooga Shale in the Dupont GHS core (Tennessee, USA), reflecting the Zn isotopic composition of Late Devonian seawater. From the lower Frasnian to the upper Famennian, δ66Zn values gradually increase, indicating a progressive rise in marine productivity. Strong correlations of this shift with redox proxies (UEF, MoEF, Corg/P; │r│ 0.88; p 0.001; EF—enrichment factor) suggest that increased productivity was a key cause of contemporaneous marine euxinia and heightened ecological stress. Biomarker data record increasing inputs of terrestrial organic matter from higher plants, closely paralleling the shift in seawater Zn isotopes (r = +0.73; p 0.005). These collectively suggest that the enhanced terrestrial phosphorus flux associated with the expansion of terrestrial higher plants was the primary driver for the observed δ66Zn patterns (r = +0.88, root mean square error = 0.11‰). Our study highlights the potential role of rising marine primary productivity in triggering Late Devonian ecological crises (e.g., the Dasberg Event), providing a cautionary parallel to the expansion of modern coastal dead zones.