RAPID OSCILLATIONS OF GLOBAL MARINE REDOX CONDITIONS AT THE DEVONIAN-CARBONIFEROUS TRANSITION

The Devonian-Carboniferous transition (DCT) was a key phase in the shift from an Early Paleozoic greenhouse climate to a Late Paleozoic icehouse climate. It coincided with a mass extinction event that eliminated ~21% of marine invertebrate genera. Although the DCT has been linked to changes in marine redox conditions, key aspects of this redox event (e.g., timing, duration and extent) and its causal relationship to coeval biotic changes remain unresolved. Here, we report a high-resolution upper Devonian to lower Carboniferous uranium isotope (δ²³⁸U) record of marine carbonates from the Long’an section (South China) and investigate the nature of ocean-redox variations during the DCT. The δ²³⁸U values vary from −0.82 ‰ to +0.32 ‰, with three negative excursions (N1, N2, and N3) and two positive excursions (P1 and P2). The negative excursions occur at the Lower/Middle Siphonodella praesulcata zone boundary (N1), in the mid-Upper Si. praesulcata Zone (N2), and in the lower Si. sulcata-Si. duplicata Zone (N3). The first positive δ²³⁸U excursion corresponds to high Mo, Zn, and Fe contents and elevated U/Al and Fe/Al ratios, suggesting this positive excursion might have been overprinted by local watermass anoxia associated with an extremely rapid expansion of marine anoxia, thus enriching the sediment in isotopically heavier ²³⁸U(IV). The relatively stable values of Mo, Zn, U/Al and Fe/Al across N2, P2 and N3 suggest that local water anoxia did not influence these rapid fluctuations in δ²³⁸U, and small variations of traditional carbonate diagenetic indicators (e.g. Mn and Sr contents, Mn/Sr ratios, δ¹⁸O) also suggest that these excursions are not systematically altered by post-depositional diagenesis. We argue that a rapid expansion of marine anoxia is typically associated with a temporally short-lived positive δ²³⁸U excursion before shifting to low δ²³⁸U values, leasing to a “S” shape curve in the δ²³⁸U records. This has also been observed at the Permian-Triassic boundary (e.g., Zhang et al., 2018). Uranium isotope mass balance modeling suggests episodes N2 and N3 represent a rapid expansion of marine anoxia across the DCT. Although our study is limited to one locality in S. China, we expect similar δ²³⁸U trends from other DCT sections around the globe, and we encourage studies from other coeval sections to confirm our trends.