Environmental changes during Frasnian‐Famennian transition in south China: A multiproxy approach

Abstract

This study presents high‐resolution multiproxy analyses on two Late Devonian sequences for characterizing the environmental changes during the Frasnian‐Famennian (F‐F) transition in south China. Carbon isotope signatures of the carbonate and organic matter consistently show two positive excursions in the late rhenana zone and around the F‐F boundary, implying two increased organic carbon burial events. These observations are consistent with the records from other continents, providing further support for the documented perturbations of the global carbon cycle. Redox indices, mainly of the Cu/Al and U/Al ratios, and hydrothermal‐volcanic proxies, Al/(Al + Fe) and Zr/Al, indicate reducing conditions and increased hydrothermal‐volcanic activities during these carbon isotope excursions. High‐resolution investigations revealed that the reducing conditions predate the onset of the carbon isotope excursions, suggesting that the two positive carbon isotope shifts are likely caused by the expansion of anoxic conditions. The low Al/(Al + Fe) ratio around the F‐F boundary leads the U/Al and Cu/Al anomalies in timing, implying that this anoxic event might have resulted from a long‐term cumulative effect of intense hydrothermal‐volcanic activities. In the late rhenana zone, enhanced hydrothermal activity also seems responsible for the reducing conditions.