Proterozoic supercontinent break-up as a driver for oxygenation events and subsequent carbon isotope excursions.

Oxygen and carbon are 2 elements critical for life on Earth. Earth's most dramatic oxygenation events and carbon isotope excursions (CIE) occurred during the Proterozoic, including the Paleoproterozoic Great Oxidation Event and the associated Lomagundi CIE, the Neoproterozoic Oxygenation event, and the Shuram negative CIE during the late Neoproterozoic. A specific pattern of a long-lived positive CIE followed by a negative CIE is observed in association with oxygenation events during the Paleo- and Neo-proterozoic. We present results from a carbon cycle model designed to couple the surface and interior cycling of carbon that reproduce this pattern. The model assumes organic carbon resides in the mantle longer than carbonate, leading to systematic temporal variations in the δ13C of volcanic CO2 emissions. When the model is perturbed by periods of enhanced continental weathering, increased amounts of carbonate and organic carbon are buried. Increased deposition of organic carbon allows O2 accumulation, while positive CIEs are driven by rapid release of subducted carbonate-derived CO2 at arcs. The subsequent negative CIEs are driven by the delayed release of organic C-derived CO2 at ocean islands. Our model reproduces the sequences observed in the Paleo- and Neo-proterozoic, that is oxygenation accompanied by a positive CIE followed by a negative CIE. Periods of enhanced weathering correspond temporally to supercontinent break-up, suggesting an important connection between global tectonics and the evolution of oxygen and carbon on Earth.

Oxygenation, Life, and the Planetary System during Earth's Middle History: An Overview.

The long history of life on Earth has unfolded as a cause-and-effect relationship with the evolving amount of oxygen (O2) in the oceans and atmosphere. Oxygen deficiency characterized our planet's first 2 billion years, yet evidence for biological O2 production and local enrichments in the surface ocean appear long before the first accumulations of O2 in the atmosphere roughly 2.4 to 2.3 billion years ago. Much has been written about this fundamental transition and the related balance between biological O2 production and sinks coupled to deep Earth processes that could buffer against the accumulation of biogenic O2. However, the relationship between complex life (eukaryotes, including animals) and later oxygenation is less clear. Some data suggest O2 was higher but still mostly low for another billion and a half years before increasing again around 800 million years ago, potentially setting a challenging course for complex life during its initial development and ecological expansion. The apparent rise in O2 around 800 million years ago is coincident with major developments in complex life. Multiple geochemical and paleontological records point to a major biogeochemical transition at that time, but whether rising and still dynamic biospheric oxygen triggered or merely followed from innovations in eukaryotic ecology, including the emergence of animals, is still debated. This paper focuses on the geochemical records of Earth's middle history, roughly 1.8 to 0.5 billion years ago, as a backdrop for exploring possible cause-and-effect relationships with biological evolution and the primary controls that may have set its pace, including solid Earth/tectonic processes, nutrient limitation, and their possible linkages. A richer mechanistic understanding of the interplay between coevolving life and Earth surface environments can provide a template for understanding and remotely searching for sustained habitability and even life on distant exoplanets.

Mid-Proterozoic redox evolution and the possibility of transient oxygenation events.

It is often assumed that rising environmental oxygen concentrations played a significant role in the timing of the first appearance of animals and the trajectory of their early proliferation and diversification. The inherent large size and complexity of animals come with large energy requirements - levels of energy that can best, if not only, be acquired through aerobic respiration. There is also abundant geochemical evidence for an increase in ocean-atmosphere O2 concentrations in temporal proximity with the emergence of the group. To adequately test this hypothesis, however, a thorough understanding of the history of environmental oxygenation in the time between the first appearance of eukaryotes and the eventual appearance of animals is necessary. In this review, we summarize the evidence for the prevailing long-term conditions of the Proterozoic Eon prior to the emergence of Metazoa and go on to highlight multiple independent geochemical proxy records that suggest at least two transient oxygenation events - at ca. 1.4 and ca. 1.1 billion years ago (Ga) - during this time. These emerging datasets open the door to an important possibility: while prevailing conditions during much of this time would likely have presented challenges for early animals, there were intervals when oxygenated conditions were more widespread and could have favored yet undetermined advances in eukaryotic innovation, including critical early steps toward animal evolution.

Dynamic oxygen and coupled biological and ecological innovation during the second wave of the Ediacara Biota.

Animal life on Earth is generally accepted to have risen during a period of increasingly well-oxygenated conditions, but direct evidence for that relationship has previously eluded scientists. This gap reflects both the enigmatic nature of the early animal fossil record and the coarse temporal resolution of Precambrian environmental change. Here, we combine paleontological data from the Ediacara Biota, the earliest fossil animals, with geochemical evidence for fluctuating redox conditions. Using morphological and ecological novelties that broadly reflect oxygen demand, we show that the appearance of abundant oxygen-demanding organisms within the Ediacara Biota corresponds with a period of elevated global oxygen concentrations. This correlation suggests that a putative rise in oxygen levels may have provided the necessary environments for the diversification of complex body plans and energetically demanding ecologies. The potential loss of organisms with relatively high oxygen requirements in the latest Ediacaran coupled with an apparent return to low oxygen concentrations further supports the availability of oxygen as a control on early animal evolution. While the advent of animal life was probably the product of a variety of factors, the recognition of a possible connection between changing environmental conditions and the diversification of animal morphologies suggests that the availability of oxygen played a significant role in the evolution of animals on Earth.