The 'COPSE' (Carbon, Oxygen, Phosphorus, Sulphur and Evolution) biogeochemical model predicts the coupled histories and controls on atmospheric O 2 , CO 2 and ocean composition over Phanerozoic time. The forwards modelling approach utilized in COPSE makes it a useful tool for testing mechanistic hypotheses against geochemical data and it has been extended and altered a number of times since being published in 2004. Here we undertake a wholesale revision of the model, incorporating: (1) elaboration and updating of the external forcing factors; (2) improved representation of existing processes, including plant effects on weathering and ocean anoxia; (3) inclusion of additional processes and tracers, including seafloor weathering, volcanic rock weathering and 87 Sr/ 86 Sr; (4) updating of the present-day baseline fluxes; and (5) a more efficient and robust numerical scheme. A key aim is to explore how sensitive predictions of atmospheric CO 2 , 2 O 2 and ocean composition are to model updates and ongoing uncertainties. The revised model reasonably captures the long-term trends in Phanerozoic geochemical proxies for atmospheric pCO 2 , pO 2 , ocean [SO 4 ], carbonate δ 13 C, sulphate δ 34 S and carbonate 87 Sr/ 86 Sr. It predicts a two-phase drawdown of atmospheric CO 2 with the rise of land plants and associated cooling phases in the Late Ordovician and Devonian-early Carboniferous, followed by broad peaks of atmospheric CO 2 and temperature in the Triassic and mid-Cretaceous -although some of the structure in the CO 2 proxy record is missed. The model robustly predicts a mid-Paleozoic oxygenation event due to the earliest land plants, with O 2 rising from ~5% to >17% of the atmosphere and oxygenating the ocean. Thereafter, atmospheric O 2 is effectively regulated with remaining fluctuations being a Carboniferous-Permian O 2 peak ~26% linked to burial of terrestrial organic matter in coal swamps, a Triassic-Jurassic O 2 minimum ~21% linked to low uplift, a Cretaceous O 2 peak ~26% linked to high degassing and weathering fluxes, and a Cenozoic O 2 decline.
The progressive oxygenation of the Earth's atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O 2 ) 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 O 2 >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 burialthe net long-term source of O 2 . 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 (δ 13 C) record by ∼445 Ma, and predict a corresponding rise in O 2 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 O 2 levels.oxygen | Paleozoic | phosphorus | plants | weathering
Ocean acidification triggered by Siberian Trap volcanism has been implicated as a kill18 mechanism for the Permo-Triassic mass extinction, but evidence for an acidification event 19 remains inconclusive. To address this, we present a high resolution seawater pH record across 20 this interval, utilizing boron isotope data combined with a quantitative modeling approach. In the 21 latest Permian, the alkalinity of the ocean increased, priming the Earth system with a low level of 22 atmospheric CO 2 and a high ocean buffering capacity. The first phase of extinction was 23 2 coincident with a slow injection of isotopically light carbon into the atmosphere-ocean, but the 24 ocean was well-buffered such that ocean pH remained stable. During the second extinction pulse, 25 however, a rapid and large injection of carbon overwhelmed the buffering capacity of the ocean, 26causing an abrupt and short-lived acidification event that drove the preferential loss of heavily 27 calcified marine biota. kyrs (2) and can be resolved into two distinct marine extinction pulses, with the respective kill 37 mechanisms appearing to be ecologically selective (3). The first occurred in the latest Permian 38 (Extinction Pulse 1; EP1) and was followed by an interval of temporary recovery before the 39 second pulse (EP2) which occurred in the earliest Triassic. The direct cause of the mass 40 extinction is widely debated with a diverse range of overlapping mechanisms proposed, 41 including widespread water column anoxia (4), euxinia (5), global warming (6) and ocean 42 acidification (7). 43Models of PTB ocean acidification suggest that a massive, and rapid, release of CO 2 from 44 Siberian Trap volcanism, acidified the ocean (7). Indirect evidence for acidification comes from 45 the interpretation of faunal turnover records (3, 8), potential dissolution surfaces (9) and Ca 46 3 isotope data (7). A rapid input of carbon is also potentially recorded in the negative carbon 47 isotope excursion (CIE) that characterizes the PTB (10, 11) . The interpretation of these records 48 is, however, debated (12), and of great importance to understanding the current threat of 49 anthropogenically-driven ocean acidification (11). 50Here, we test the ocean acidification hypothesis by presenting a novel proxy record of 51 ocean pH across the PTB, using the boron isotope composition of marine carbonates ( 11 additional counterbalancing alkalinity flux. This is consistent with independent proxy data (6). 129The alkalinity source may have been further increased through soil loss (26) carbon to the atmosphere, yet remarkably, the acidification event occurs after the decline in 13 C, 139when 13 C has rebounded somewhat and is essentially stable (Fig. 2). 140Unlike the first carbon injection, the lack of change in 13 C at this time rules out very 141 13 C-depleted carbon sources, because no counterbalancing strongly 13 C-enriched source exists.
It is unclear why atmospheric oxygen remained trapped at low levels for more than 1.5 billion years following the Paleoproterozoic Great Oxidation Event. Here, we use models for erosion, weathering and biogeochemical cycling to show that this can be explained by the tectonic recycling of previously accumulated sedimentary organic carbon, combined with the oxygen sensitivity of oxidative weathering. Our results indicate a strong negative feedback regime when atmospheric oxygen concentration is of order pO2∼0.1 PAL (present atmospheric level), but that stability is lost at pO2<0.01 PAL. Within these limits, the carbonate carbon isotope (δ13C) record becomes insensitive to changes in organic carbon burial rate, due to counterbalancing changes in the weathering of isotopically light organic carbon. This can explain the lack of secular trend in the Precambrian δ13C record, and reopens the possibility that increased biological productivity and resultant organic carbon burial drove the Great Oxidation Event.
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