The early Middle Devonian Choteč Event in northern Spain: a brief anoxic pulse driven by enhanced weathering and eutrophication

Abstract

The (Basal) Choteč Event, first recognised in the 1980s in Czechia, is a globally widespread anoxic pulse associated with transgression and eutrophication just above the Emsian-Eifelian (Early-Middle Devonian) boundary (cycle 1c of the Johnson et al. [1985] Devonian eustatic sea-level curve). Despite being one of several anoxia-driven faunal turnovers during the Devonian, the Choteč Event remains poorly understood. The global reach, intensity, and duration of anoxia is not constrained and nor is it clear whether eutrophication had its “roots” in contemporary floral developments on land (as suggested for younger Devonian anoxic events). We present a geochemical (carbon isotopes; trace metals as proxies for redox and productivity; and major elements for the Chemical Index of Alteration [CIA] as a weathering proxy) and palynological study of the Cabonera section (León, Spain). This succession is part of an extensive Devonian sequence developed around isolated islands in the Armorican Terrane Assemblage that was located between the supercontinents Laurussia and Gondwana. Here, limestones of the Emsian-Eifelian Santa Lucia Formation are abruptly overlain by siltstones and shales belonging to the Eifelian Huergas Formation. This conodont- and brachiopod-constrained manifestation of the Choteč Event sees the onset of a gradual 4‰ negative δ13Ccarb excursion (CIE) consistent with records in other regions. The lower part of the Huergas Formation (Cabornera Bed) records a brief interval of anoxia (low Th/U, elevated V/Al and U/Al) at the same level that sees the onset of the negative CIE. This appears to have been accompanied by, or was perhaps driven by, greatly enhanced primary productivity, with enrichment factors (EFs) of Ba, and particularly Ni, Zn and P, all >>1. This brief burst of productivity and anoxia soon ended, with EFs falling <<1 through the remainder of the Huergas Formation (which are also well below the values recorded in the Santa Lucia Formation). The CIA values are high throughout the succession, suggesting the prevalence of strong weathering under a warm, humid climate during the Eifelian. The highest CIA values are seen immediately above the level of high productivity and oxygen restriction, suggesting that weathering enhanced further still at that time. A less commonly employed weathering proxy, Rb/Sr, also suggests the onset of enhanced weathering in the Choteč Event beds, where the ratio increases by two orders of magnitude although we cannot rule out the role of facies changes in this. Acritarch, chitinozoan and spore assemblages are somewhat impoverished and have a moderate-high degree of endemism, which is unsurprising considering their isolated location. More broadly, the Eifelian saw significant diversification in several plant groups. It is possible that these changes on land drove the Choteč Event through enhanced pedogenesis and runoff, but there is a mismatch between the temporal scale of land plant evolution and the brevity of the eutrophic anoxic pulse recorded at Cabornera. Instead, we suggest that the Choteč Event in the Armorican Terrane saw a brief phase of extreme productivity – driven by rapidly enhanced weathering of unknown origins – that resulted in anoxia and the shutdown of the carbonate factory.