LAUSR

Abstract Benthic oxygen oases linked to photosynthetic mats have been reported in modern anoxic aquatic systems. Benthic macroalgal blooms were common in stratified, anoxic Neoproterozoic oceans, leading us to hypothesize the existence of benthic oxygen oases at that time. This hypothesis has significant implications regarding the bioavailability of transition metals (e.g., Cu, Zn, Ni, Mo, V) and the distribution of aerobic eukaryotes in these oceans. However, little research has been directed toward testing the benthic oxygen oasis hypothesis in ancient anoxic marine systems. In this contribution, we report on an integrated investigation of iron speciation, trace elements, and carbon–sulfur-δ34Spy for the Tonian Longfengshan Biota-bearing versus non-fossil-bearing shales of the Luotuoling Formation in the Longfengshan section of the Yanshan Basin (North China). Typical marine B/Ga values (6.5–7.3) suggest that the Yanshan Basin was fully marine and well connected with the open ocean. The low FeHR/FeT ratios of the non-fossiliferous intervals (mean 0.14) suggest oxic water-column conditions during their deposition, and their FeHR pool contains significant amounts of pyrite and siderite (up to 44%), implying their formation in underlying reducing sedimentary porewaters. In contrast, the fossil-bearing layers show high and oscillating FeHR/FeT ratios (mean 0.39; up to 0.51) and systematically low Fepy/FeHR (mean ~0), suggesting dominantly ferruginous water-column conditions with redox oscillation. Furthermore, lack of redox sensitive trace-metal and sulfide enrichments and dominance of the FeHR pool by ferric oxide and magnetite (96–100%) indicate substantially oxidative power in the reducing water-column and underlying surface sediments. In combination with coexisting sedimentological and paleontological data, these conflicting and contrasting redox signals can be best interpreted by the development of benthic oxygen oases in deeper reducing waters, regulated by the Neoproterozoic benthic macroalga Longfengshaniaceae most likely in association with microbial/cyanobacterial mats, which contributed the oxidative power in the reducing environments. Our findings provide new insights into early Neoproterozoic oceanic redox conditions, iron cycling and the evolution of early aerobic eukaryotes, including animals.