The progressive oxygenation of the Earth’s atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O₂) first approached modern levels (∼21%) remains ...unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435–392 Ma, and the appearance of fossil charcoal indicates O₂ >15–17% by 420–400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burial—the net long-term source of O₂. We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today’s global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2‰ increase in the carbonate carbon isotope (δ13C) record by ∼445 Ma, and predict a corresponding rise in O₂ to present levels by 420–400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O₂ levels.
Paired measurements of bulk carbonate (δ13Ccarb), organic matter (δ13Corg), and their difference (Δ13C) can be used to estimate changes in isotopic fractionation through time as a function of O2/CO2 ...in the atmosphere. However, because local scale processes can also affect Δ13C, it is essential to compare sections from widely separated water masses. Here we present new δ13Corg data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published δ13Ccarb records from these sections and paired δ13C values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician δ13Ccarb excursions and now provide a composite Ordovician δ13Corg record. New Lower Ordovician (Tremadocian Stage) δ13Corg data range from ca. −26 to −28‰, decreasing throughout the Lower–Middle Ordovician (Floian–Dapingian Stages) to ca. −29 to −31‰. δ13Corg values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (Sandbian–Katian) δ13Corg data from North America. Δ13C values from well-preserved intervals generally vary between +26 to +28‰ throughout the Lower to Middle Ordovician (Tremadocian to early Darriwilian), but increase to +31‰ during the mid–late Darriwilian and mid Sandbian, similar to published data from younger Late Ordovician positive δ13C excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Δ13C trend shows a ~3‰ increase throughout the Early–Middle Ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms. Modeling studies predict that pCO2 decreased during this time, suggesting that the effect of pCO2 on Δ13C may have been overwhelmed by other controls, such as an in increase in pO2 or a higher O2/CO2 ratio during this biodiversification event.
•New Early–Middle Ordovician paired carbon isotopic analyses are reported here.•New organic isotope data combined with published data make an Ordovician composite.•A 3‰ increase in Δ13C coincides with the onset of the GOBE and possible ocean cooling.•Cooling or pCO2 changes cannot explain the entire Δ13C increase.•Increased pO2 or atmospheric O2/CO2 ratios may explain the long-term Δ13C increase.
Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies ...it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans.
Pulse of atmospheric oxygen during the late Cambrian Saltzman, Matthew R.; Young, Seth A.; Kump, Lee R. ...
Proceedings of the National Academy of Sciences - PNAS,
03/2011, Letnik:
108, Številka:
10
Journal Article
Recenzirano
Odprti dostop
A rise in atmospheric O₂ has been linked to the Cambrian explosion of life. For the plankton and animal radiation that began some 40 million yr later and continued through much of the Ordovician ...(Great Ordovician Biodiversification Event), the search for an environmental trigger(s) has remained elusive. Here we present a carbon and sulfur isotope mass balance model for the latest Cambrian time interval spanning the globally recognized Steptoean Positive Carbon Isotope Excursion (SPICE) that indicates a major increase in atmospheric O₂. We estimate that this organic arbon and pyrite burial event added approximately 19 × 10¹ɸ moles of O₂ to the atmosphere (i.e., equal to change from an initial starting point for O₂ between 10-18% to a peak of 20-28% O₂) beginning at approximately 500 million years. We further report on new paired carbon isotope results from carbonate and organic matter through the SPICE in North America, Australia, and China that reveal an approximately 2‰ increase in biological fractionation, also consistent with a major increase in atmospheric O₂. The SPICE is followed by an increase in plankton diversity that may relate to changes in macro- and micronutrient abundances in increasingly oxic marine environments, representing a critical initial step in the trophic chain. Ecologically diverse plankton groups could provide new food sources for an animal biota expanding into progressively more ventilated marine habitats during the Ordovician, ultimately establishing complex ecosystems that are a hallmark of the Great Ordovician Biodiversification Event.
Concurrent gaps in the Late Devonian/Mississippian fossil records of insects and tetrapods (i.e. Romer's Gap) have been attributed to physiological suppression by low atmospheric pO₂ Here, updated ...stable isotope inputs inform a reconstruction of Phanerozoic oxygen levels that contradicts the low oxygen hypothesis (and contradicts the purported role of oxygen in the evolution of gigantic insects during the late Palaeozoic), but reconciles isotope-based calculations with other proxies, like charcoal. Furthermore, statistical analysis demonstrates that the gap between the first Devonian insect and earliest diverse insect assemblages of the Pennsylvanian (Bashkirian Stage) requires no special explanation if insects were neither diverse nor abundant prior to the evolution of wings. Rather than tracking physiological constraint, the fossil record may accurately record the transformative evolutionary impact of insect flight.
The geological cycling of carbon ties together the ocean-atmosphere carbon pool, Earth's biosphere, and Earth's sedimentary reservoirs. Perturbations to this coupled system are recorded in the ...carbon-isotopic (δ13C) composition of marine carbonates. Large amplitude δ13C excursions are typically treated as individual events and interpreted accordingly. However, a recent compilation of Phanerozoic carbon isotopic data reveals that δ13C excursions are a ubiquitous feature of the geologic record, and thus should be considered in concert. Analysis indicates that Phanerozoic carbon isotope excursions, as a whole, have characteristic durations of 0.5 to 10 M.yr. and exhibit declining amplitude over time. These commonalities suggest a shared underlying control. Here we demonstrate that sinusoidal modulation of the sensitivity of organic carbon and phosphate burial in a simple numerical model of the geologic carbon cycle results in large, asymmetric δ13C oscillations that exhibit their largest amplitudes in the 0.5 to 10 M.yr. period range. As anoxia is known to strongly modulate the C:P burial ratio of organic matter in sediments, we propose that sea-level oscillations were the primary source of sinusoidal modulation for the geologic carbon cycle, and that their degree of influence on the carbon cycle was determined by the state of oxygenation of bottom waters overlying the continental shelves. When oxygen minimum zones (OMZs) were large, shallow, and prone to expansion, sea-level changes would have had the capacity to drive large changes in the areal extent of OMZs in contact with the sea-floor, resulting in strong leverage on the burial sensitivity of organic carbon and phosphate, and thus on δ13C. Progressive oxygenation of the oceans, which was facilitated by biological innovations, resulted in a decline in the amplitude of δ13C excursions over the Phanerozoic, and the biogeochemical stabilization of the Earth System.
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