Heterogeneous oxygenation coupled with low phosphorus bio-availability delayed eukaryotic diversification in Mesoproterozoic oceans: Evidence from the ca 1.46 Ga Hongshuizhuang Formation of North China

Abstract

The Mesoproterozoic Era is characterized by low atmospheric oxygen levels, pervasive ocean anoxia, and long stasis in biotic evolution. As to the ocean oxygenation patterns, timing and their potential effects on the early eukaryotic evolution, however, opinions are controversial. To further constrain the redox evolution of Mesoproterozoic oceans, an integrated study was conducted on the ~1.46 Ga Hongshuizhuang Formation of the North China Platform using multiple techniques. The results show that the black shales were largely deposited in the environments below storm-wave base in an intra-shelf basin. The concentrations of redox sensitive trace metals measured from the shales are at nearly upper continent crust level, with average Mo, U, and V enrichment factors of 6.65, 1.99, and 1.24, respectively, suggesting predominantly suboxic to weekly oxygenated conditions. Short anoxic conditions with intermittent pore-water euxinia existed only in the middle part of the formation. No framboidal pyrite was observed through the studied intervals, consistent with the low degree of pyritization (DOPT, 0.01 ± 0.02), S/Fe (0.02 ± 0.04) and Fe/Al (0.37 ± 0.10) values indicative of non-euxinic water-column conditions. Nitrogen isotope compositions show exclusively positive value (δ15N = 2.43 ± 0.54‰), invoking aerobic N cycling and a relatively stable nitrate pool in contemporary seawaters, which are also supported by the presence of low organic carbon to phosphorus (Corg/P) ratios and negative Ce anomaly. Multiple-proxy correlation across the North China Platform shows a spatio-temporal heterogeneity in redox landscape during the Hongshuizhuang deposition. Relatively constant euxinic conditions developed in northern part of the basin, which became waning and replaced by anoxic–suboxic conditions towards south. Low total organic carbon (TOC), total nitrogen (TN) and P contents in this formation point to oligotrophic water-mass in the basin. It seems that the nutrient limitation, coupled with heterogeneous oxygen concentrations, have depressed the diversification of early eukaryotes. Given that the Hongshuizhuang Formation was largely deposited under more or less oxygenated seawaters with moderate nitrate availability, the absence of eukaryotic records and their biomarkers may have been caused by nutrient limitation, especially phosphorus, in the contemporary seawater.