Reorganisation of Earth’s biogeochemical cycles briefly oxygenated the oceans 520 Myr ago (original) (raw)
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Oxygenation as a driver of the Great Ordovician Biodiversification Event
During the Ordovician period one of the greatest biological radiations of the Phanerozoic took place, when genus-level diversity quadrupled and ecospace utilization increased 1. During this Great Ordovician Biodiversification Event (GOBE), marine communities expanded into new niches such as epifaunal suspension feeding and deep burrowing 2. This new niche space was largely exploited by members of the Palaeozoic Evolutionary Fauna (EF), which includes articulated brachiopods, crinoids, ostracodes, cephalopods, corals, and bryozoans 1. Both the Cambrian EF (trilobites and inarticulate brachiopods) and Modern EF (bivalves, gastropods, fish, and so on) also diversified, but it was the expansion of the Paleozoic EF that drove the GOBE. The causes of the GOBE remain poorly understood and may include both intrinsic biological factors and external environmental drivers (such as global cooling, nutrient delivery from erosion, higher sea levels that expanded habitable platform area, and oxygenation) 1,3–5. Oxygen isotopes from well-preserved conodont apatite provide proxy evidence for high sea surface temperatures (~40 °C) at the onset of the Ordovician that may have inhibited diversification , but global cooling throughout the Early–Middle Ordovician brought temperatures closer to modern conditions and possibly into the tolerance window (27–32 °C) for members of the Palaeozoic EF 6. Cooling oceans could also store more dissolved oxygen and more effectively ventilate subsurface environments, which would in turn create a stronger vertical gradient in carbonate saturation that lowered the metabolic costs of skeletal carbonate biomineralization in surface waters 7. A global increase in atmospheric oxygen 8 and oxygenation of shallow marine environments may have also eased stressful conditions for benthic animal life 9 and expanded the range of habitable ecospace for infaunal burrowers deeper into the sediment 10. A more oxygenated ocean could also have supported more predators in the food chain (fish and cephalopods), setting into motion an evolutionary 'arms race' 11. Ordovician global cooling is generally thought to have been caused by decreasing atmospheric CO 2 (the cause of this drop is itself not well understood, but hypotheses include increased silicate weathering 12 and the advent of land plants 13), but a role for increased atmospheric O 2 is possible via an increase in total atmospheric pressure and the associated inhibition of solar optical depth, scattering incident solar radiation that would have otherwise contributed to the surface latent heat flux 14. These arguments for linking oxygenation to cooling and biodiver-sification, while compelling, are hindered by poorly constrained ocean–atmosphere oxygen records. Existing isotope mass balance models are hampered by coarse time resolution (typically 10 Myr bins) that are not capable of resolving changes in atmospheric O 2 as a cause of the main pulses of biodiversification across the GOBE 15–17 (Supplementary Fig. 1). The absence of charcoal is interpreted to reflect atmospheric O 2 levels below 13–15% until the Late Silurian 18 , but primitive non-vascular land plants only expanded into terrestrial environments by the Middle–Late Ordovician 13 , thus the charcoal record is not well suited to constrain Early Ordovician O 2 levels. Land plant expansion is thought to have increased organic burial rates and oxygenated the atmosphere to near modern levels 13 , but the timing and magnitude of this oxygenation is also poorly resolved. Similarly, although iron-based redox proxies suggest that O 2 levels were between 2% and 21% throughout the Ordovician, their resolution is not yet sufficient to resolve finer temporal trends 19. Here we apply a new approach to reconstruct the changes in atmospheric oxygen with high age-resolution based on the O 2-dependence of carbon isotope fractionation during photosynthesis 20. Effects of atmospheric O 2 on photosynthesis Our estimates for Ordovician atmospheric O 2 are based on the link between photosynthetic fractionation of stable carbon isotopes in primary producers (such as marine phytoplankton) and changing O 2. The carbon fixation enzyme ribulose 1,5-bisphosphate carboxyl-ase–oxygenase (Rubisco) has dual carboxylase/oxygenase functions , which results in variations in photosynthetic fractionation (ε p) as a function of CO 2 and O 2 concentrations inside the cell 20–22. Atmospheric O 2 in the past can thus be reconstructed if ε p can be The largest radiation of Phanerozoic marine animal life quadrupled genus-level diversity towards the end of the Ordovician Period about 450 million years ago. A leading hypothesis for this Great Ordovician Biodiversification Event is that cooling of the Ordovician climate lowered sea surface temperatures into the thermal tolerance window of many animal groups, such as corals. A complementary role for oxygenation of subsurface environments has been inferred based on the increasing abundance of skeletal carbonate, but direct constraints on atmospheric O 2 levels remain elusive. Here, we use high-resolution paired bulk car-bonate and organic carbon isotope records to determine the changes in isotopic fractionation between these phases throughout the Ordovician radiation. These results can be used to reconstruct atmospheric O 2 levels based on the O 2-dependent fraction-ation of carbon isotopes by photosynthesis. We find a strong temporal link between the Great Ordovician Biodiversification Event and rising O 2 concentrations, a pattern that is corroborated by O 2 models that use traditional carbon–sulfur mass balance. We conclude that that oxygen levels probably played an important role in regulating early Palaeozoic biodiversity levels, even after the Cambrian Explosion.
Atmosphere–ocean oxygen and productivity dynamics during early animal radiations
Proceedings of the National Academy of Sciences
The proliferation of large, motile animals 540 to 520 Ma has been linked to both rising and declining O2 levels on Earth. To explore this conundrum, we reconstruct the global extent of seafloor oxygenation at approximately submillion-year resolution based on uranium isotope compositions of 187 marine carbonates samples from China, Siberia, and Morocco, and simulate O2 levels in the atmosphere and surface oceans using a mass balance model constrained by carbon, sulfur, and strontium isotopes in the same sedimentary successions. Our results point to a dynamically viable and highly variable state of atmosphere–ocean oxygenation with 2 massive expansions of seafloor anoxia in the aftermath of a prolonged interval of declining atmospheric pO2 levels. Although animals began diversifying beforehand, there were relatively few new appearances during these dramatic fluctuations in seafloor oxygenation. When O2 levels again rose, it occurred in concert with predicted high rates of photosynthet...
Earth history. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals
Science (New York, N.Y.), 2014
The oxygenation of Earth's surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth's surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.
Interface Focus, 2020
One contribution of 15 to a theme issue 'The origin and rise of complex life: integrating models, geochemical and palaeontological data'. A hypothesized rise in oxygen levels in the Neoproterozoic, dubbed the Neoproterozoic Oxygenation Event, has been repeatedly linked to the origin and rise of animal life. However, a new body of work has emerged over the past decade that questions this narrative. We explore available proxy records of atmospheric and marine oxygenation and, considering the unique systema-tics of each geochemical system, attempt to reconcile the data. We also present new results from a comprehensive COPSE biogeochemical model that combines several recent additions, to create a continuous model record from 850 to 250 Ma. We conclude that oxygen levels were intermediate across the Ediacaran and early Palaeozoic, and highly dynamic. Stable, modern-like conditions were not reached until the Late Palaeozoic. We therefore propose that the terms Neoproterozoic Oxygenation Window and Palaeozoic Oxygen-ation Event are more appropriate descriptors of the rise of oxygen in Earth's atmosphere and oceans.
The case for a Neoproterozoic Oxygenation Event: Geochemical evidence and biological consequences
The Neoproterozoic era marked a turning point in the development of the modern earth system. The irreversible environmental changes of that time were rooted in tectonic upheavals that drove chain reactions between the oceans, atmosphere, climate, and life. Key biological innovations took place amid carbon cycle instability that pushed climate to unprecedented extremes and resulted in the ventilation of the deep ocean. Despite a dearth of supporting evidence, it is commonly presumed that a rise in oxygen triggered the evolution of animals. Although geochemical evidence for oxygenation is now convincing, our understanding of the Neoproterozoic earth system and of early animal evolution has changed apace, revealing an altogether more complicated picture in which the spread of anoxia played an important role. The challenge to future researchers lies in unraveling the complex entanglement of earth system changes during this pivotal episode in Earth’s history.
Nature Geoscience, 2024
A geologically rapid Neoproterozoic oxygenation event is commonly linked to the appearance of marine animal groups in the fossil record. However, there is still debate about what evidence from the sedimentary geochemical record—if any—provides strong support for a persistent shift in surface oxygen immediately preceding the rise of animals. We present statistical learning analyses of a large dataset of geochemical data and associated geological context from the Neoproterozoic and Palaeozoic sedimentary record and then use Earth system modelling to link trends in redox-sensitive trace metal and organic carbon concentrations to the oxygenation of Earth’s oceans and atmosphere. We do not find evidence for the wholesale oxygenation of Earth’s oceans in the late Neoproterozoic era. We do, however, reconstruct a moderate long-term increase in atmospheric oxygen and marine productivity. These changes to the Earth system would have increased dissolved oxygen and food supply in shallow-water habitats during the broad interval of geologic time in which the major animal groups first radiated. This approach provides some of the most direct evidence for potential physiological drivers of the Cambrian radiation, while highlighting the importance of later Palaeozoic oxygenation in the evolution of the modern Earth system.
Possible links between extreme oxygen perturbations and the Cambrian radiation of animals
Nature geoscience, 2019
The role of oxygen as a driver for early animal evolution is widely debated. During the Cambrian explosion, episodic radiations of major animal phyla occurred coincident with repeated carbon isotope fluctuations. However, the driver of these isotope fluctuations and potential links to environmental oxygenation are unclear. Here we report high-resolution carbon and sulfur isotope data for marine carbonates from the southeastern Siberian Platform that document the canonical explosive phase of the Cambrian radiation from ~524 to ~514 Myr ago. These analyses demonstrate a strong positive covariation between carbonate δ 13 C and carbonate-associated sulfate δ 34 S through five isotope cycles. Biogeochemical modelling suggests that this isotopic coupling reflects periodic oscillations in the atmospheric O 2 and the extent of shallow-ocean oxygenation. Episodic maxima in the biodiversity of animal phyla directly coincided with these extreme oxygen perturbations. Conversely, the subsequent Botoman-Toyonian animal extinction events (~514 to ~512 Myr ago) coincided with decoupled isotope records that suggest a shrinking marine sulfate reservoir and expanded shallow marine anoxia. We suggest that fluctuations in oxygen availability in the shallow marine realm exerted a primary control on the timing and tempo of biodiversity radiations at a crucial phase in the early history of animal life.