Transient deep-water oxygenation recorded by rare Mesoproterozoic phosphorites, South Urals

The Mesoproterozoic deep ocean is thought to have been largely anoxic, dominated by ferruginous conditions. Phosphate was scavenged from the water column iron oxyhydroxides, and dissolved nitrate rapidly reduced along redoxcline. Primary productivity therefore generally low, eukaryotic life restricted oxic marine margins. This model supported new nitrogen isotope data Lower Riphean (ca. 1.55 Ga) strata of South Ural Mountains, where we find evidence for strong redox stratification, coupled with low organic carbon content. In contrast, unconformably overlying siliciclastic sedimentary rocks Middle age 1.33 preserve a rare occurrence apatite concretions, associated relatively high concentrations (up 4 wt%). Pb-Pb isochron concretions Zigazino-Komarovo Formation 1330 ± 20 Ma (MSWD = 3.7). We measured ratios across this interval found δ15N values up + 7.6‰, which are similar those in modern upwelling zones indicative significant reservoir. These observations most parsimoniously explained phosphate- nitrate-bearing waters, suggesting that at least regionally temporarily oxygenated. lack redox-sensitive trace element enrichment these general scarcity accumulations record suggest deep-waters were not global persistent phenomenon, but our results confirm oxic, nutrient-rich refugia existed oceans perhaps important radiation early eukaryotes.