Abstract
Abstract. We studied sediments from the ca. 1400 million-year-old Xiamaling Formation from the North China block. The upper unit of this formation (unit 1) deposited mostly below storm wave base and contains alternating black and green-gray shales with very distinct geochemical characteristics. The black shales are enriched in redox-sensitive trace metals, have high concentrations of total organic carbon (TOC), high hydrogen index (HI) and iron speciation indicating deposition under anoxic conditions. In contrast, the green-gray shales show no trace metal enrichments, have low TOC, low HI and iron speciation consistent with an oxygenated depositional setting. Altogether, unit 1 displays alternations between oxic and anoxic depositional environments, driving differences in carbon preservation consistent with observations from the modern ocean. We combined our TOC and HI results to calculate the differences in carbon mineralization and carbon preservation by comparing the oxygenated and anoxic depositional environments. Through comparisons of these results with modern sedimentary environments, and by use of a simple diagenetic model, we conclude that the enhanced carbon mineralization under oxygenated conditions in unit 1 of the Xiamaling Formation required a minimum of 4 to 8 % of present-day atmospheric levels (PAL) of oxygen. These oxygen levels are higher than estimates based on chromium isotopes and reinforce the idea that the environment contained enough oxygen for animals long before their evolution.
Highlights
The Mesoproterozoic Era (1600 to 1000 million years ago, Ma) was a time of profound biological transition
Iron speciation from the core materials shows that elevated ratios of four different pools of highly reactive iron (FeHR) / FeT are generally associated with samples containing high total organic carbon (TOC) (Fig. 3a)
The relationship between TOC and hydrogen index (HI) indicates substantial oxic mineralization of organic matter when sediments deposited in oxygenated water
Summary
The Mesoproterozoic Era (1600 to 1000 million years ago, Ma) was a time of profound biological transition. It witnessed the emergence of nascent eukaryote ecosystems, and more generally, it linked the dominantly prokaryote world of the Paleoproterozoic Era (2500 to 1600 Ma), and before, to the Neoproterozoic Era (1000 to 541 Ma), where eukaryotes greatly diverged and where animals first evolved (Butterfield, 2015; Knoll, 2014). There are few constraints on oxygen levels during the Mesoproterozoic Era. The idea that Mesoproterozoic oceans were largely anoxic below the surface mixed zone generated a model providing maximum oxygen concentrations in the range of 40 % of present-day atmospheric levels (PAL) (Canfield, 1998). Chromium (Cr) associated with Mesoproterozoicaged iron-enriched marine sediments has shown a lack of observable fractionation, suggesting no oxidative weathering
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