Sufficient oxygen for animal respiration 1,400 million years ago

Abstract

The Mesoproterozoic Eon [1,600-1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were <0.1% of present levels, sufficiently low to have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon−oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels >4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves.atmosphere | Mesoproterozoic | oxygen minimum zone | trace metals | biomarkers

Earth’s oxygen cycle and the evolution of animal life

“…Finally, we build on these perspectives to propose a simple conceptual model that frames the links between Earth's oxygen cycle and the early evolution of animal life as a question of habitat viability, highlighting the stability of environmental O 2 levels (e.g., ref. 25) against the backdrop of evolving baseline pO 2 (11,26), rather than the attainment of threshold O 2 levels for a single life history stage (10,11,23). We suggest that this view provides a more realistic and powerful framework for understanding the links between environmental stress and the early evolution of complex metazoan life on Earth.…”

Paleoproterozoic sterol biosynthesis and the rise of oxygen

“…Saturated sterane biomarkers typical of those found in Phanerozoic rocks were close to, or below, the methodological detection limits of ~1 p.p.m. Other biomarker studies of Archean, Paleoproterozoic and Mesoproterozoic sedimentary rocks, conducted in the era of heightened contamination awareness, reveal a similar picture of non-detection of eukaryote-specific 24-alkylated steranes (Blumenberg et al, 2012;Flannery and George, 2014;Hoshino et al, 2015) or their detection at levels than cannot be reliably distinguished from contamination (Luo et al, 2015;Zhang et al, 2016). Finally, new methodologies for evaluating the thermal regime experienced by sedimentary organic matter (Ferralis et al, 2016), the cleaning of contaminated rock samples prior to analysis (Jarrett et al, 2013) and a focus on directed identification and sampling of pockets of well-preserved Proterozoic and Archean sediments (Bruisten et al, 2013) suggest that the pre-Cambrian biomarker record can be elaborated and imbued with greater confidence.…”

Breathing room for early animals

“…Additionally, the Cr isotope data showed a large secular trend with "modern" levels of fractionation beginning ∼800 Ma, and this result breathed fresh air into the idea that rising amounts of O 2 in the atmosphere (above a respiration threshold) and the origin of animals were correlated in time. This is where the new study from Zhang et al (6) comes in. Working in fine-grained marine sedimentary rocks of the Xiamaling Formation on the North China Platform, Zhang et al (6) observed patterns of enrichments of trace elements V, U, and Mo similar to those seen in Phanerozoic strata that imply at least episodic bottom water oxygenation.…”

“…This is where the new study from Zhang et al (6) comes in. Working in fine-grained marine sedimentary rocks of the Xiamaling Formation on the North China Platform, Zhang et al (6) observed patterns of enrichments of trace elements V, U, and Mo similar to those seen in Phanerozoic strata that imply at least episodic bottom water oxygenation. Additionally, these rocks are exceptionally well preserved for their age, and, because they have not been substantially buried and heated, the organic-rich rocks still contain hydrocarbon biomarkers.…”

“…Because of their aerobic physiology, the idea was that O 2 levels were perhaps too low to have supported animals until sometime near the end of Precambrian time, and that rising O 2 levels thereafter provided a quantum evolutionary leap of sorts for aerobic biology culminating in the "late" evolution of animals (3). A recent study of middle Proterozoic sedimentary rocks in China, however, favors a different view-suggesting that O 2 levels capable of supporting animal physiology were present more than 500 million years before the appearance of animals (6).…”