Local depositional and diagenetic conditions control the positive δ34S excursion during the Ordovician-Silurian transition: Evidence from the Wufeng-Longmaxi formations in Sichuan Basin

Abstract

The Ordovician-Silurian (OS) transition was characterized by significant changes in global climate, marine redox conditions, and biological evolution, which include global seawater euxinia, the onset of the Hirnantian glaciation, and large-scale extinctions. During this time, the marine sulfur cycle was disrupted, and the pyrite sulfur isotope (δ34Spy) showed a large positive excursion. This excursion is closely tied to the Hirnantian glaciation, with δ34Spy increasing at the start of and decreasing at the end of the glaciation. Although it is thought that variations in marine sulfate concentration and primary productivity are responsible for the positive excursion in δ34Spy, contemporary marine sediment δ34Spy data suggest that sedimentary processes can also lead to significant fluctuations in δ34Spy. To assess the impact of local changes in sedimentary conditions on this positive δ34Spy excursion event, we conducted detailed depositional and geochemical analyses of sedimentary pyrite found in black shales from the Wufeng-Longmaxi formations in the Weiyuan area of the Sichuan Basin, China. These sedimentary pyrites include three types: framboidal, euhedral, and laminated pyrite. Bulk sample sulfur isotope analyses revealed that δ34Spy values vary from −16.2‰ to 15.8‰ (mean = −3.8‰, n = 22). In situ analyses of δ34Spy show that different pyrite crystals exhibit a range of intragranular δ34Spy values that are likely indicators of diagenesis. Variations in bulk sample δ34Spy are strongly and negatively correlated with the proportion of framboidal pyrite, i.e., lower levels of framboidal pyrite content resulted in a higher bulk sample δ34Spy value, suggesting diagenetic alteration controls on δ34Spy. In addition, the δ34Spy excursion in the Wufeng-Longmaxi formations also appears to be compatible with lithological changes related to sedimentation rate. The δ34Spy values from the Guanyinqiao Bed, which consists of calcareous shales, are higher than those from black shales with a relatively low sedimentation rate. Our results suggest that the positive δ34Spy excursion during the OS transition during the Hirnantian glaciation is controlled not only by diagenetic alterations but also by local sedimentation rate.