Responses of Sr, Nd, and S Isotopes of Seawater to the Volcanic Eruptions During the Early Middle Triassic, South China

Abstract

Abstract The wide distribution of tuff layers, locally named the “green bean rocks” (GBRs) in the Yangtze Block straddling the Early Middle Triassic marine sequence indicates intense volcanic eruption(s). Sr, Nd, and S isotope compositions and trace elements of marine sediments were analyzed spanning the tuff layers to elucidate their responses to the volcanic eruptions and related environmental changes. The Sr isotope compositions of marine sediments are comparable to those of open seawater during the time interval of ca. 245–248 Ma. Sr and Nd isotope compositions of the samples show synchronous increases in the 87Sr/86Sr ratios and εNd(t) values during the deposition of GBRs. The elevated 87Sr/86Sr ratios and εNd(t) values are proposed to be caused by the input of volcanic tephra and increased influx of weathering product of mafic rocks (most likely the Emeishan flood basalts). The S isotope compositions of sulfates exhibit a negative shift in the GBRs, which could possibly be attributed to greater input of lighter 32S from weathering products and volcanic eruptions. The variation of Th/U ratios indicate that the GBRs formed in an anoxic environment, resulting from high marine productivity as a consequence of more nutrients from weathering and volcanic materials. The responses of Sr, Nd, and S isotopes to volcanic eruptions during the Early Middle Triassic indicate this event resulted in adverse effects, namely enhanced eutrophication and low O2 levels, acidic precipitation, toxic components, etc., that could cause ecological destruction both on land and in the sea.

Keywords

• the Early Middle Triassic
• South China
• volcanic eruption
• marine sediments
• Sr
• Nd and S isotopes