U/Mo ratios and δ ~(95/95)Mo as local and global redox proxies during mass extinction events

摘要

When used in conjunction with trace metal redox proxies, the use of U/Mo ratios and δ ~(95/95)Mo values can considerably enhance the interpretation of ocean oxygenation trends both locally and globally. This is illustrated with studies from two, biogenic-chert dominated records of mass extinction: the Ordovician-Silurian boundary and the Late Permian in sections from the Yangtze Block in South China. The latest Ordovician mass extinction is well known to coincide with oxygenation episode in the Hirnantian Stage here revealed by trace metal proxies (V/V+Ni, V/Cr, Ni/Co) that show it sandwiched between anoxic/euxinic black shales. The predominantly anoxic/euxinic Ordovician-Silurian strata provide a record of seawater δ ~(95/95)Mo values that are seen to gradually rise during the interval from 0.5 to 1.0‰. The exception to the δ ~(98/95) Mo stability is the datum of the slight negatively δ ~(98/95)Mo, associated with decreasing of V/Cr and V/(V+Ni) ratio suggesting the Hirnantian oxygenation event. The Upper Permian chert succession at Shangsi reveals an overall better ventilated record in which the best oxygenated strata show the highest U/Mo values that is interpreted as relative enrichment of U in suboxic conditions. At a few levels anoxic/euxinic conditions show the δ ~(95/95)Mo values to be ~2‰, close to modern values, implying that latest Permian oceans were as well ventilated as those today. The well-known end-Permian oceanic anoxic event is therefore suggested to only develop at the end of this period.