Atmospheric oxygen regulation at low Proterozoic levels by incomplete oxidative weathering of sedimentary organic carbon

May combine two claims.

The long-term steady-state isotopic composition of the ocean and δ13Ccarb is independent of the burial rate of 'new' organic carbon, as oxygen level adjusts such that net input and output fluxes of δ13C to/from the ocean are equal.

“The long-term steady-state isotopic composition of the ocean and δ13Ccarb is therefore independent of the burial rate of ‘new' organic carbon (Fig. 5a), as oxygen level adjusts such that net input and output fluxes of δ13C to/from the ocean are equal39.”

• cites [39] Bekker, A. in Encyclopedia of Astrobiology (eds Gargaud, M. et al.) (Sringer-Verlag, 2014).Not checked yet

  Not checked in the free run.

May combine two claims.

After ∼10 Myr, atmospheric oxygen and oxidative weathering decrease to a new steady state, leaving δ13Ccarb unchanged from its initial value, with both the net input and output fluxes of δ13C to/from the ocean isotopically heavier than they were.

“However, after ∼10 Myr, atmospheric oxygen and oxidative weathering decrease to a new steady state, leaving δ13Ccarb unchanged from its initial value, with both the net input and output fluxes of δ13C to/from the ocean isotopically heavier than they were (because isotopically-light organic carbon input and output fluxes have declined relative to isotopically heavier inorganic carbon fluxes).”

May combine two claims.

As pO2 declines, the oxidative weathering flux declines and becomes more sensitive to pO2 variations, because oxidative weathering becomes kinetically-limited in a wider range of regions with progressively lower erosion rates.

“As pO2 declines, the oxidative weathering flux declines and becomes more sensitive to pO2 variations, because oxidative weathering becomes kinetically-limited in a wider range of regions with progressively lower erosion rates (Fig. 2a,b).”

May combine two claims.

At pO2∼0.1 PAL, there is still a significant oxidative weathering flux which is a strong function of oxygen concentration, giving the potential to provide strong negative feedback on pO2.

“At pO2∼0.1 PAL, there is still a significant oxidative weathering flux which is a strong function of oxygen concentration (Fig. 2a,b), giving the potential to provide strong negative feedback on pO2.”

May combine two claims.

Combining this with minimum estimates for volcanic and metamorphic inputs, the model predicts pO2 ∼0.1 PAL with oxygen stabilized by the negative feedback from oxidative weathering.

“Combining this with minimum estimates for volcanic and metamorphic inputs, the model predicts pO2 ∼0.1 PAL (Fig. 3a) with oxygen stabilized by the negative feedback from oxidative weathering.”

May combine two claims.

There is a negative feedback dependence of oxygen removal on pO2 near ∼1 PAL, but it is fairly weak.

“It confirms that there is a negative feedback dependence of oxygen removal on pO2 near ∼1 PAL, but it is fairly weak.”

May combine two claims.

Organic carbon (kerogen) is completely oxidized at pO2=1 PAL for low and intermediate erosion rates, but is incompletely oxidized at the highest erosion rates today; 50 cm kyr−1 and 25 cm kyr−1.

“The results (Fig. 2) show that organic carbon (kerogen) is completely oxidized at pO2=1 PAL for low and intermediate erosion rates, but is incompletely oxidized at the highest erosion rates today; 50 cm kyr−1 (dominated by large islands of the Western Pacific) and 25 cm kyr−1 (dominated by the Himalayas) (Fig. 2a, see ‘Methods' section).”

May combine two claims.

Sensitivity analysis increasing shale oxygen diffusivity and removing the contributions of high-uplift regions to global sediment discharge rates can weaken the sensitivity of the global oxidative weathering flux to oxygen variation near pO2=1 PAL and shifts the region of strong negative feedback to somewhat lower pO2.

“This can weaken the sensitivity of the global oxidative weathering flux to oxygen variation near pO2=1 PAL and shifts the region of strong negative feedback to somewhat lower pO2.”

May combine two claims.

In this case thermogenic methane scales with the organic carbon burial rate and hence has a greatly reduced influence in the Precambrian, although it still reduces the stability domain of Proterozoic pO2 with respect to short-timescale perturbations.

“In this case it scales with the organic carbon burial rate and hence has a greatly reduced influence in the Precambrian (Fig. 3b, black line), although it still reduces the stability domain of Proterozoic pO2 with respect to short-timescale perturbations (green line).”

May combine two claims.

Varying the distribution of continental erosion rates between plausible limits causes pO2 to vary by a factor of ∼2, with low global erosion and/or increased shale porosity, and therefore more complete oxidation, producing lower pO2.

“Varying the distribution of continental erosion rates between plausible limits then causes pO2 to vary by a factor of ∼2, with low global erosion and/or increased shale porosity, and therefore more complete oxidation, producing lower pO2 (Fig. 3a).”

May combine two claims.

An increase in organic carbon burial from lower values to >25% of present is sufficient to trigger the Great Oxidation Event in the model, as the net biological source of oxygen exceeds the volcanic input of reduced matter.

“Conversely, an increase in organic carbon burial from lower values to >25% of present is sufficient to trigger the Great Oxidation Event in the model, as the net biological source of oxygen exceeds the volcanic input of reduced matter.”

May combine two claims.

This would drop after 2.5 Ga to a net oxidation rate of ≈1.6 × 1011 mol O2 eq yr−1, which is small relative to sediment and metamorphic redox transformations.

“This would drop after 2.5 Ga to a net oxidation rate of ≈1.6 × 1011 mol O2 eq yr−1, which is small relative to sediment and metamorphic redox transformations (Table 1).”

May combine two claims.

If total organic carbon burial was <25% of present, corresponding to pO2 ∼0.01 PAL, our model predicts a reversal of the Great Oxidation Event, because although the oxidative weathering of ancient organic carbon would have become negligible, the oxygen source to the atmosphere would have been insufficient to counterbalance the volcanic/metamorphic input of reduced matter.

“However, if total organic carbon burial was <25% of present, corresponding to pO2 ∼0.01 PAL, our model predicts a reversal of the Great Oxidation Event, because although the oxidative weathering of ancient organic carbon would have become negligible, the oxygen source to the atmosphere would have been insufficient to counterbalance the volcanic/metamorphic input of reduced matter.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Assuming crust oxidation (resulting in a present Fe3+ excess of ≈2 × 1021 mol O2 eq) is linked to continental growth, an 'episodic continental growth' model with rapid growth from 10 to 80% of present area 3.2-2.5 Ga would imply a net oxidation rate of ≈2 × 1012 mol O2 eq yr−1.

“For example, assuming crust oxidation (resulting in a present Fe3+ excess54 of ≈2 × 1021 mol O2 eq) is linked to continental growth, an ‘episodic continental growth' model55 with rapid growth from 10 to 80% of present area 3.2–2.5 Ga would imply a net oxidation rate of ≈2 × 1012 mol O2 eq yr−1 (balanced by a combination of hydrogen escape and organic carbon accumulation).”

• cites [54] Catling, D. C., Zahnle, K. J. & McKay, C. P. Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth. Science 293, 839–843 (2001). sourceNot checked yet

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• cites [55] McLennan, S. M. & Taylor, S. R. Geochemical constraints on the growth of the continental crust. J. Geol. 90, 347–361 (1982). sourceNot checked yet

  Not checked in the free run.

• AssumptionThe authors state this as an assumption: Crust oxidation is linked to continental growth.

May combine two claims.

If the reaction front reaches the surface, organic matter oxidation is incomplete and detrital organic matter is eroded.

“If the reaction front reaches the surface, organic matter oxidation is incomplete and detrital organic matter is eroded.”

May combine two claims.

Oxygen levels were sufficient to oxygenate some deep ocean basins and deposit gypsum during the Lomagundi carbon isotope excursion ∼2.22-2.06 Ga.

“Oxygen levels were sufficient to oxygenate some deep ocean basins23 and deposit gypsum45 during the ‘Lomagundi' carbon isotope excursion234567 ∼2.22–2.06 Ga.”

• cites [23] Scott, A. C. & Glasspool, I. J. The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration. Proc. Natl Acad. Sci. 103, 10861–10865 (2006). sourceNot checked yet

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• cites [45] Chang, S. & Berner, R. A. Coal weathering and the geochemical carbon cycle. Geochim. Cosmochim. Acta. 63, 3301–3310 (1999). sourceNot checked yet

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• cites 234567Not checked yet

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Classified as probably not a substantial claim.

Paleosol oxidation state at ∼1.85 Ga and ∼1.1 Ga indicates pO2>0.01 PAL.

“Subsequently, paleosol oxidation state8 at ∼1.85 Ga and ∼1.1 Ga indicates pO2>0.01 PAL”

• cites [8] Rye, R. & Holland, H. D. Paleosols and the evolution of atmospheric oxygen: A critical review. Am. J. Sci. 298, 621–672 (1998). sourceNot checked yet

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Classified as probably not a substantial claim.

Atmospheric oxygen remained trapped at low levels for more than 1.5 billion years following the Paleoproterozoic Great Oxidation Event.

“It is unclear why atmospheric oxygen remained trapped at low levels for more than 1.5 billion years following the Paleoproterozoic Great Oxidation Event.”

May combine two claims.

The oxygen source required to trigger the Great Oxidation Event need only have been slightly greater than the supply of reduced gases to the atmosphere and only a fraction of the tectonic supply of reductant in uplifted sediment.

“Therefore, the oxygen source required to trigger it need only have been slightly greater than the supply of reduced gases to the atmosphere and only a fraction of the tectonic supply of reductant in uplifted sediment.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Metamorphic fluxes plausibly scale proportional to global heat flux (Q) and continental area, hence would be ∼1.2 of present.

“Metamorphic fluxes plausibly scale proportional to global heat flux (Q) and continental area, hence would be ∼1.2 of present (increasing both chemical weathering of phosphorus hence oxygen source, and reductant input hence oxygen sink).”

May combine two claims.

Models for Phanerozoic oxygen regulation tend to focus on strong negative feedbacks on organic carbon burial, and any negative feedback on oxidative weathering is estimated to be relatively weak (or non-existent).

“Models for Phanerozoic oxygen regulation1128 tend to focus on strong negative feedbacks on organic carbon burial29 (that is, that the production rate of oxygen decreases as oxygen rises), and any negative feedback on oxidative weathering is estimated to be relatively weak3031 (or non-existent2832).”

• cites [1128]Not checked yet

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• cites [29] Lenton, T. M. & Watson, A. J. Redfield revisited: 2. What regulates the oxygen content of the atmosphere? Global Biogeochem. Cycles 14, 249–268 (2000). sourceNot checked yet

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• cites [3031]Not checked yet

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May combine two claims.

In the modern oxygenated environment, much of the sedimentary organic carbon that is uplifted is oxidized, with the remainder returning to the sediments as 'detrital' organic carbon.

“In the modern oxygenated environment (Fig. 1d), much of the sedimentary organic carbon that is uplifted is oxidized32, with the remainder returning to the sediments as ‘detrital' organic carbon37”

• cites [32] Bolton, E. W., Berner, R. A. & Petsch, S. T. The weathering of sedimentary organic matter as a control on atmospheric O2: II. Theoretical modeling. Am. J. Sci. 306, 575–615 (2006). sourceNot checked yet

  Not checked in the free run.

• cites [37] Galy, V., Peucker-Ehrenbrink, B. & Eglinton, T. Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207 (2015). sourceNot checked yet

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May combine two claims.

This mechanism makes the carbonate carbon isotope (δ13C) record insensitive to changes in organic carbon burial rate, thus explaining the lack of secular trend in the Precambrian δ13C record.

“Furthermore, we show that this mechanism makes the carbonate carbon isotope (δ13C) record insensitive to changes in organic carbon burial rate, thus explaining the lack of secular trend in the Precambrian δ13C record.”

May combine two claims.

Oxidation of sulfides and ferrous iron in sediments and the upper crust are smaller sinks at present, because less than half of the sedimentary iron and sulfur are in reduced form.

“Oxidation of sulfides and ferrous iron in sediments and the upper crust are smaller sinks at present, because less than half of the sedimentary iron and sulfur are in reduced form.”

May combine two claims.

The balance of mechanisms that regulated atmospheric oxygen at much lower Proterozoic levels was quite different, and incomplete oxidative weathering played a key role.

“Here, we argue that the balance of mechanisms that regulated atmospheric oxygen at much lower Proterozoic levels was quite different, and that incomplete oxidative weathering played a key role.”

May combine two claims.

The second transition represents a fundamental change in oxygen regulation regime, in which reductant burial exceeds the reductant supply via sediment recycling and the predominant negative feedback control shifts from the oxygen sink to the oxygen source.

“This second transition is not as abrupt, but still represents a fundamental change in oxygen regulation regime, in which reductant burial exceeds the reductant supply via sediment recycling and the predominant negative feedback control shifts from the oxygen sink to the oxygen source.”

May combine two claims.

Prior to the Great Oxidation Event under pO2<10−5 PAL, the oxidation of reduced sedimentary organic carbon would have been negligible, with oxygen stabilized instead by the oxygen sensitivity of reactions with reduced gases.

“Prior to the Great Oxidation Event under pO2<10−5 PAL (Fig. 1b), the oxidation of reduced sedimentary organic carbon would have been negligible, with oxygen stabilized instead by the oxygen sensitivity of reactions with reduced gases.”

May combine two claims.

Near modern pO2 (∼1 PAL), uplifted organic carbon is completely oxidized in slowly eroding soil environments, meaning oxidative weathering is transport-controlled by the supply of reduced material, rather than kinetically controlled by the pO2 level.

“This is because near modern pO2 (∼1 PAL), uplifted organic carbon is completely oxidized in slowly eroding soil environments32, meaning oxidative weathering is transport-controlled by the supply of reduced material, rather than kinetically controlled by the pO2 level.”

• cites [32] Bolton, E. W., Berner, R. A. & Petsch, S. T. The weathering of sedimentary organic matter as a control on atmospheric O2: II. Theoretical modeling. Am. J. Sci. 306, 575–615 (2006). sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Mantle inputs scale as Q2 increasing both CO2 input and the seafloor hydrothermal oxygen sink (primarily serpentinisation in the low-sulfate Precambrian).

“Mantle inputs scale as Q2 increasing both CO2 input and the seafloor hydrothermal oxygen sink (primarily serpentinisation in the low-sulfate Precambrian48).”

• cites [48] Sleep, N. H. in Metal Ions in Biological Systems, Biogeochemical Cycles of Elements, vol. 43 (eds Sigel, A. et al.) 49–73 (Taylor & Francis, 2005). sourceNot checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing).

Secular changes in tectonic (and solar) forcing will modify this picture.

“Secular changes in tectonic (and solar) forcing will modify this picture40.”

• cites [40] Claire, M. W., Catling, D. C. & Zahnle, K. J. Biogeochemical modelling of the rise in atmospheric oxygen. Geobiology 4, 239–269 (2006). sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Atmospheric oxygen eventually approached modern levels during ∼0.6-0.4 Ga, with the first compelling evidence for pO2≥0.6 PAL from the appearance of Palaeozoic fossil charcoal at ∼0.4 Ga.

“Atmospheric oxygen eventually approached modern levels611 during ∼0.6–0.4 Ga, with the first compelling evidence for pO2≥0.6 PAL from the appearance of Palaeozoic fossil charcoal23 at ∼0.4 Ga.”

• cites [611]Not checked yet

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• cites [23] Scott, A. C. & Glasspool, I. J. The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration. Proc. Natl Acad. Sci. 103, 10861–10865 (2006). sourceNot checked yet

  Not checked in the free run.

May combine two claims.

At high erosion rates, oxidative weathering is incomplete and detrital kerogen is preserved in marine sediments, meaning that the global oxidative weathering flux is somewhat sensitive to pO2 variations even at the present day.

“However, at high erosion rates, oxidative weathering is incomplete and detrital kerogen is preserved in marine sediments333435, meaning that the global oxidative weathering flux is somewhat sensitive to pO2 variations even at the present day3031.”

• cites 333435Not checked yet

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• cites [3031]Not checked yet

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May combine two claims.

In the Phanerozoic, the major oxygen source is organic carbon burial (derived from oxygenic photosynthesis) and the largest sink is oxidative weathering of uplifted sedimentary organic carbon (kerogen).

“In the Phanerozoic, the major oxygen source is organic carbon burial (derived from oxygenic photosynthesis) and the largest sink is oxidative weathering of uplifted sedimentary organic carbon (kerogen).”

May combine two claims.

In this regime, redox balance requires that oxidative weathering of uplifted sediment is incomplete (limited by global oxygen supply), and the atmospheric oxygen level is determined by the land-surface integrated oxygen-sensitive kinetics of oxidative weathering.

“In this regime, redox balance requires that oxidative weathering of uplifted sediment is incomplete (limited by global oxygen supply), and the atmospheric oxygen level is determined by the land-surface integrated oxygen-sensitive kinetics of oxidative weathering.”

Classified as probably not a substantial claim.

Assuming negligible burial of terrestrial organic matter in the Proterozoic and assuming modern ocean nutrient levels, marine organic carbon burial could have been comparable to the modern level of ∼2.5 × 1012 mol C yr−1.

“Assuming negligible burial of terrestrial organic matter in the Proterozoic, which comprises ∼50% of total burial today111346, and assuming modern ocean nutrient levels, marine organic carbon burial could have been comparable to the modern level of ∼2.5 × 1012 mol C yr−1.”

• cites 111346Not checked yet

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• AssumptionThe authors state this as an assumption: Burial of terrestrial organic matter in the Proterozoic was negligible.
• AssumptionThe authors state this as an assumption: Ocean nutrient levels in the Proterozoic were modern.

Classified as probably not a substantial claim.

Conventionally the constancy of δ13Ccarb is taken to imply a constant 'f-ratio' of 'new' organic to inorganic carbon burial.

“Conventionally the constancy of δ13Ccarb is taken to imply a constant ‘f-ratio' of ‘new' organic to inorganic carbon burial495051.”

• cites 495051Not checked yet

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May combine two claims.

The one-box model behind the very low pO2 inferences can be questioned because it assumes manganese can be oxidized in the weathering environment imparting Cr-isotope fractionation above pO2∼0.001 PAL, whereas the soil is predicted to be anoxic and reducing up to pO2∼0.1 PAL.

“Again, the one-box model behind these very low pO2 inferences14 can be questioned because it assumes that manganese can be oxidized in the weathering environment imparting Cr-isotope fractionation above pO2∼0.001 PAL, when instead the soil is predicted to be anoxic and reducing up to pO2∼0.1 PAL (Fig. 6).”

• cites [14] Planavsky, N. J. et al. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science 346, 635–638 (2014). sourceNot checked yet

  Not checked in the free run.

May combine two claims.

This in turn requires at least ∼40-50% of modern ocean nutrient concentrations, or if they were lower, a more efficient biological carbon pump and/or more efficient sedimentary preservation of organic carbon.

“This in turn requires at least ∼40–50% of modern ocean nutrient concentrations, or if they were lower, a more efficient biological carbon pump and/or more efficient sedimentary preservation of organic carbon.”

May combine two claims.

The iron oxidation state of shales only changes after the Great Oxidation Event, suggesting significant ongoing oxidization of crustal iron.

“However, the iron oxidation state of shales only changes after the Great Oxidation Event, suggesting significant ongoing oxidization of crustal iron6.”

• cites [6] Bekker, A. & Holland, H. D. Oxygen overshoot and recovery during the early Paleoproterozoic. Earth Planet. Sci. Lett. 317–318, 295–304 (2012). sourceNot checked yet

  Not checked in the free run.

Classified as probably not a substantial claim.

Model parameters are taken from the default parameter set of Bolton et al., which generate a reaction front at depth ∼10 m for oxidative weathering of organic carbon at global-mean values for TOC (1 wt%) and erosion rate (5 cm kyr−1).

“Model parameters (summarized in Table 2) are taken from the default parameter set of Bolton et al.32 (their Table 1 and Figs 3–5), which generate a reaction front at depth ∼10 m for oxidative weathering of organic carbon at global-mean values for TOC (1 wt%) and erosion rate (5 cm kyr−1) (Fig. 6).”

• cites [32] Bolton, E. W., Berner, R. A. & Petsch, S. T. The weathering of sedimentary organic matter as a control on atmospheric O2: II. Theoretical modeling. Am. J. Sci. 306, 575–615 (2006). sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Burial of reduced carbon is nutrient (rather than carbon) limited whereas the burial of reduced sulfur only requires a redox gradient generated by the much larger carbon export production flux.

“This burial of reduced carbon is nutrient (rather than carbon) limited whereas the burial of reduced sulfur only requires a redox gradient generated by the much larger carbon export production flux.”

May combine two claims.

The Precambrian sulfur cycle was dominated by a reduced sedimentary reservoir (pyrite), which after surface oxidation in weathering was soon reduced again and buried, whereas the carbon cycle consists of a primarily oxidized reservoir that is available for biotic burial of the reduced form.

“This is because the Precambrian sulfur cycle was dominated by a reduced sedimentary reservoir (pyrite), which after surface oxidation in weathering was soon reduced again and buried, whereas the carbon cycle consists of a primarily oxidized reservoir that is available for biotic burial of the reduced form.”

May combine two claims.

The rise of oxygen to present levels required an increase in organic carbon burial rate, which may have begun in the Neoproterozoic Era but in this model was not completed until the mid-Palaeozoic Era with the colonization of the land by plants and fungi.

“The rise of oxygen to present levels simply required an increase in organic carbon burial rate, which may have begun in the Neoproterozoic Era10, but in our model was not completed until the mid-Palaeozoic Era with the colonization of the land by plants and fungi, liberating nutrient phosphorus from rocks and producing high C/P material for burial1113.”

• cites [10] Lenton, T. M., Boyle, R. A., Poulton, S. W., Shields-Zhou, G. A. & Butterfield, N. J. Co-evolution of eukaryotes and ocean oxygenation in the Neoproterozoic era. Nat. Geosci. 7, 257–265 (2014). sourceNot checked yet

  Not checked in the free run.

• cites [11] Bergman, N. M., Lenton, T. M. & Watson, A. J. COPSE: a new model of biogeochemical cycling over Phanerozoic time. Am. J. Sci. 304, 397 (2004). sourceNot checked yet

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• cites [13] Lenton, T. M. et al. Earliest land plants created modern levels of atmospheric oxygen. Proc. Natl Acad. Sci. 113, 9704–9709 (2016). sourceNot checked yet

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May combine two claims.

The conventional interpretation of the long-term carbon isotope record in terms of changing proportions of new organic to inorganic carbon burial is only appropriate in a high oxygen world where oxidative weathering of organic carbon is fairly complete and the δ13C value of carbon inputs remains fairly stable.

“The conventional interpretation of the long-term carbon isotope record (in terms of changing proportions of new organic to inorganic carbon burial) is only appropriate in a high oxygen world where oxidative weathering of organic carbon is fairly complete, and therefore the δ13C value of carbon inputs remains fairly stable.”

May combine two claims.

The sulfur cycle could not have provided an equivalent negative feedback on pO2 at lower levels, even though oxidation and subsequent reduction of sulfur were more important fluxes in the Proterozoic surface redox budget than they are today.

“The sulfur cycle could not have provided an equivalent negative feedback on pO2 at lower levels, even though oxidation and subsequent reduction of sulfur were more important fluxes in the Proterozoic surface redox budget than they are today15.”

• cites [15] Canfield, D. E. in Treatise on Geochemistry 2nd edn, Vol. 6 (eds Holland, H. D. & Turekian, K. K.) 197–216 (Elsevier, 2014). sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

The spatially-variable mixing of detrital and marine sources implies that the δ13Corg signature will be location-dependent.

“Furthermore, the spatially-variable mixing of detrital and marine sources implies that the δ13Corg signature will be location-dependent.”

May combine two claims.

There could have been large Precambrian changes in new organic carbon burial that may not show up in long-term δ13Ccarb, reopening the possibility that the Great Oxidation Event was driven by an increase in new organic carbon burial.

“Instead there could have been large Precambrian changes in new organic carbon burial that may not show up in long-term δ13Ccarb, reopening the possibility that, for example, the Great Oxidation Event was driven by an increase in new organic carbon burial.”

May combine two claims.

The oxidative weathering model has been calibrated against laboratory measurements constraining local kerogen and pyrite oxidation kinetics, and a field study constraining oxygen transport through porous shale.

“The model has been calibrated against laboratory measurements constraining local kerogen45 and pyrite3263 oxidation kinetics, and a field study constraining oxygen transport through porous shale43.”

• cites [45] Chang, S. & Berner, R. A. Coal weathering and the geochemical carbon cycle. Geochim. Cosmochim. Acta. 63, 3301–3310 (1999). sourceNot checked yet

  Not checked in the free run.

• cites [32] Bolton, E. W., Berner, R. A. & Petsch, S. T. The weathering of sedimentary organic matter as a control on atmospheric O2: II. Theoretical modeling. Am. J. Sci. 306, 575–615 (2006). sourceNot checked yet

  Not checked in the free run.

• cites [63] Williamson, M. A. & Rimstidt, J. D. The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation. Geochim. Cosmochim. Acta 58, 5443–5454 (1994). sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Recycling of sedimentary organic carbon and its kinetically limited oxidation could have sustained low Proterozoic atmospheric oxygen levels, albeit not as low as has recently been inferred.

“Recycling of sedimentary organic carbon and its kinetically limited oxidation could have sustained low Proterozoic atmospheric oxygen levels—albeit not as low as has recently been inferred14.”

• cites [14] Planavsky, N. J. et al. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science 346, 635–638 (2014). sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

The carbon isotope decoupling observed in the Neoproterozoic δ13C record, where δ13Ccarb varies but δ13Corg is near constant, can be explained by the recycling of detrital organic carbon.

“The carbon isotope decoupling observed in the Neoproterozoic δ13C record61 (where δ13Ccarb varies but δ13Corg is near constant) can, as previously proposed496162, be explained by the recycling of detrital organic carbon.”

• cites [49] Derry, L. A. in Treatise on Geochemistry 2nd edn, Vol. 12 (eds Holland, H. D. & Turekian, K. K.) 239–249 (Elsevier, 2014). sourceNot checked yet

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• cites [61] Swanson-Hysell, N. L. et al. Cryogenian glaciation and the onset of carbon-isotope decoupling. Science 328, 608–611 (2010). sourceNot checked yet

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• cites [62] Johnston, D. T., Macdonald, F. A., Gill, B. C., Hoffman, P. F. & Schrag, D. P. Uncovering the Neoproterozoic carbon cycle. Nature 483, 320–323 (2012). sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

If the oxygen source increased a lot, for example, in the Lomagundi event, then the oxidative weathering sink might have been temporarily overwhelmed leading to high oxygen levels.

“We have assumed that any increase in oxygen source triggering the Great Oxidation Event was modest, but if the oxygen source increased a lot, for example, in the Lomagundi event, then the oxidative weathering sink might have been temporarily overwhelmed leading to high oxygen levels234567.”

• cites 234567Not checked yet

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• AssumptionThe authors state this as an assumption: Any increase in oxygen source triggering the Great Oxidation Event was modest.

Classified as probably not a substantial claim.

The shale model was run repeatedly to span the present-day global distribution of erosion rates across the continental surface and the results weighted by the observed fractional areal contributions of these different erosion rates to obtain a global oxidative weathering flux.

“The shale model was run repeatedly to span the present-day global distribution of erosion rates across the continental surface44 and the results weighted by the observed fractional areal contributions of these different erosion rates to obtain a global oxidative weathering flux.”

• cites [44] Milliman, J. & Meade, R. World-wide delivery of river sediment to the oceans. J. Geol. 91, 1–21 (1983). sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

Long-timescale results of interest are insensitive to the detailed form of the partitioning of atmospheric CO2 as proportional to the square of the total amount of carbon in the ocean and atmosphere.

“Atmospheric CO2 was made proportional to the square of the total amount of carbon in the ocean and atmosphere. Long-timescale results of interest here are insensitive to the detailed form of this partitioning.”

Classified as probably not a substantial claim. May combine two claims.

A sensitivity analysis examined how the oxygen dependence of global organic carbon oxidation varied with removing the contribution to global sediment discharge rates of the highest-uplift regions (oceanic islands), or those plus the next highest uplift regions (SE Asia and the Himalayas), and with increasing shale oxygen diffusivity (n*=1.5), designed to establish a lower limit at which this O2-­­

“In a sensitivity analysis (Fig. 7) we examined how the oxygen dependence of global organic carbon oxidation varied with removing the contribution to global sediment discharge rates44 of regions with the highest uplift (oceanic islands), or those plus the next highest uplift regions (SE Asia and the Himalayas), and with increasing shale oxygen diffusivity (n*=1.5).”

• cites [44] Milliman, J. & Meade, R. World-wide delivery of river sediment to the oceans. J. Geol. 91, 1–21 (1983). sourceNot checked yet

  Not checked in the free run.