The Marine Redox Change and Nitrogen Cycle in the Early Cryogenian Interglacial Time： Evidence from Nitrogen Isotopes and Mo Contents of the Basal Datangpo Formation, Northeastern Guizhou, South China

Cryogenian Datangpo Formation was deposited during the interglacial time between the Sturtian and Marinoan ice ages. We studied nitrogen isotope compositions and contents of Mo of the black shales from the basal Datangpo Formation in northeastern Guizhou, South China, for an attempt to reconstruct the marine redox change and nitrogen cycle during the interglacial time. Based on lithostratigraphy as well as geochemical profiles, the basal black shales can be divided into four intervals： Interval 1 has the lowest δ~（15）N value（＋5.0‰）; in interval 2, δ~（15）N values vary between ＋6.4‰ and ＋7.4‰（the first peak）; interval 3 records stable values of δ~（15）N around ＋6‰; and interval 4 is characterized by its higher δ~（15）N values, between ＋6.7‰ and ＋7.8‰（the second peak）. The values of enrichment factor of Mo decrease from 56.8 to 2.6 with the ascending stratigraphic trend. It indicated that immediately after the Sturtian glaciations, the marine seawater above the transitional zone between the shelf to slope of the southern margin of the Yangtze Platform was stratified, with shallow seawater being oxic but deep water being sulfidic. Subsequently, high denitrification rates prevailed in expanded suboxic areas in spite of a short emergence of an oxic condition in the surface seawater, and the deep seawaters were still anoxic or even euxinic.