Enhanced Continental Weathering as a Trigger for the End‐Devonian Hangenberg Crisis.

The Hangenberg Crisis coincided with a large decline of biodiversity and widespread anoxia in the end‐Devonian ocean. Previous research attributed marine anoxia to the spread of deeply‐rooted plants and/or increased volcanism on the continents, but crucial links have not been thoroughly explored. Herein, we propose enhanced weathering as a key trigger, as evidenced by a negative shift (∼8‰) in lithium isotopes and a coupled response in carbon isotopes of marine carbonates in South China. Our findings imply that rapid weathering of crustal rocks increased nutrient delivery to the ocean, as indicated by an increase in the carbonate‐associated phosphate levels, contributing to oceanic eutrophication. In the absence of massive volcanic emissions and intense orogeny, the cause of enhanced continental weathering was likely the expansion of the terrestrial rhizosphere, highlighting the potential for land plant evolution to initiate weathering changes of sufficient severity to trigger a major bio/environmental crisis in the Earth system. Plain Language Summary: The colonization of land plants during the Devonian is believed to have played a key role in regulating Earth's climate. The initially rapid expansion of seed plants into unvegetated or sparsely vegetated uplands is considered to have caused enhanced rock dissolution relative to clay formation on end‐Devonian continents. As a consequence, larger amounts of isotopically‐light lithium were transported to the ocean via rivers, resulting in a distinct negative excursion in seawater lithium isotopes. Concurrently, increased riverine nutrient delivery stimulated marine primary production and thus facilitated the expansion of oceanic anoxia. As a result, drawdown of atmospheric CO2 resulted in climatic cooling, which together with widespread seafloor anoxia, triggered the major biotic turnover known as the "Hangenberg Crisis". Key Points: Negative shift in carbonate lithium isotopes records major enhancement of continental weathering during the end‐DevonianEnhanced chemical weathering increased riverine nutrient delivery and contributed to widespread marine anoxia and the Hangenberg CrisisRapid expansion of seed plants in sparsely vegetated uplands may account for weathering‐driven climatic cooling and oceanic changes [ABSTRACT FROM AUTHOR]