Australia: The Land Where Time Began
End-Permian Climate Systematic Swings Resulting from Outgassing of Carbon and Sulphur from the Siberian Traps
The flood basalt magmatism of the Siberian Traps coincided with the mass extinction event at the end of the Permian about 252 Ma. Links have been proposed between magmatism and ecological catastrophe, among which are global warming, global cooling, depletion of ozone and chemistry changes in the ocean. The critical combinations of environmental changes responsible for global mass extinction are, however, undetermined. In this paper Black et al. present the results from global climate simulations of outgassing from flood basalt that account for sulphur chemistry and the microphysics of aerosol coupled with the circulation of the atmosphere and the ocean. Black et al. considered the effects of carbon and sulphur in isolation and in tandem. They found that the climate response to flood basalt-scale outgassing that is prolonged is strongly influenced by coupling with the ocean. They suggest that sulphur and carbon emissions from the Siberian Traps combined to generate systematic swings in temperature, ocean circulation and hydrology within a trend on the longer term towards a greenhouse world in the Early Triassic.
Profound shifts in the cycling of carbon, sea surface temperature (SST), chemistry and circulation of the ocean, and weathering during the Permian/Triassic transition are recorded by geochemical proxies. Equatorial SST, in particular, increased by 5-10oC (Joachimski et al., 2012; Cui, Kump & Ridgwell, 2013; Sun et al., 2012; Chen et al., 2016) in less than 100 ky (Burgess et al., 2014), seawater 87Sr/86Sr, which is a reflection of the weathering of crustal material, underwent the most rapid increase in the Phanerozoic eon (Song et al., 2015; Dudás et al., 2017), sharp negative excursions at the onset of the end-Permian mass extinction (Payne & Clapham, 2012) and a pulse of ocean acidification is recorded by boron isotope data several 10s of thousands of years after the carbon isotope excursion (Clarkson et al., 2015).
On Earth, flood basalt provinces are enormous magmatic foci that occur at intervals of about 107 years (Courtillot & Renne, 2003), when they emplace volumes of magma of 105 to 107 km3, the emplacement of which is accompanied by intense outgassing. In the case of the Siberian Traps about 7-15 x 106 km3 of magmas, that were extrusive and intrusive, were emplaced beneath, into and on top of volatile-rich sedimentary basins (Sunders, 2016). The environmental consequences that have been hypothesised include the release of toxic metals (Sanei, Grasby & Beauchamp, 2012), halogen emissions that deplete ozone (Beerling et al., 2007; Black et al., 2014), enhanced weathering of fresh volcanic rocks (Cox et al., 2016) and perturbations of the climate magmatic and sedimentary carbon due to the outgassing of sulphur (Saunders et al., 2016; Wignall, 2001; Schmidt et al., 2015).
In this study the Community Earth System Model (CESM1), a 3D global coupled climate model was used to investigate the competing effects, on different timescales, of sustained emissions of sulphur and increases in atmospheric CO2. The atmospheric chemistry effects of emissions from the Siberian traps alone were examined in previous 3D modelling (Black et al., 2014). Geography and offline surface temperature calculation (Schmidt et al., 2017), were employed in prior modelling of the climate effects of flood basalt sulphur, which [?]precluded an investigation of the coupled atmosphere-ocean response. This study focuses on the climate consequences that include hydrology and ocean circulation. Latest Permian palaeogeography is incorporated in the simulations of Black et al. and include the Community Aerosol and Radiation Model for Atmospheres (CARMA), which is a submodel of detailed aerosol microphysics within the CESM1 framework, in order to capture growth and sedimentation of sulphur aerosols, and the climate effects of volcanic aerosols are critically influenced by both processes (Schmidt et al., 2015).
Tempo and scale of sulphur and carbon release from the Siberian traps
The tempo and outgassing are major sources of uncertainty. Melt inclusions record Sulphur concentrations in Siberian magmas. Geochemical Evidence from other food basalt provinces has been found that suggests the time-averaged discharge rates of magma vary substantially (Percival et al., 2017). It is suggested by comparisons with recent fissure eruptions (Thordarson & Self, 1993) and Palaeomagnetic data (Pavlov e al., 2011) that during intervals that were especially intense, the flux of magma may have exceeded the longer-term flux that is implied from geochronology (Burgess & Bowring, 2015) by 1-2 orders of magnitude, 103-104 km3 of magma erupting in as little as 100 years. Therefore, intense sustained emissions of sulphur that lasts 10s to 100s of years, is a unique aspect of flood basalt eruptions (Schmidt et al., 2015). Modelling of eruption plumes indicates that columns could reach 13-17 km in height during episodes of very high eruption rates (Glaze et al. 2017). It was considered by Black et al. that the injection of 2,000 TgSO2/year (Teragrams SO2/year) to the upper troposphere-lower stratosphere (UTLS; at an altitude of 12-14 km). Injections of sulphur lasted 10-200 years, with each year of emissions representing about 100 times the SO2 emissions of Mount Pinatubo eruption in 1991. The simulations of Black et al. span 4.5 kyr, which contrasts with the ~800 kyr duration of Siberian magmatism (Burgess & Bowring, 2015). Black et al. suggest stresses due to the most intense episodes of outgassing may be the most relevant to global biota (14), as the effects of sulphur outgassing are cumulative beyond the lifetime of sulphur in the atmosphere (Bond & Wignall, 2014). Therefore, Black et al. focused on the response of the climate to such episodes, which may have been repeated with varying intensity during the overall history of the flood basalt province.
A central quandary for flood basalt carbon is that coincident palaeoclimate and geochemical records are best explained through a massive release of carbon with an isotopic signature that is temporally evolving (Joachimski et al., 2012; Clarkson et al., 2015; Saunders, 2016; Gutjahr et al., 2017), though outgassing of carbon that was related to lavas, magmas that were deep and intrusive, and heating of the surrounding rocks has remained much more challenging to quantify than the emissions of sulphur. In part, this is because saturation with CO2 can occur deep within the magmatic system, and towards the surface of the Earth the solubility of CO2 decreases strongly, which partitions CO2 into an exsolved volatile phase (Saunders, 2016). Comparisons with basalts from Hawaii and Iceland suggest that some flood basalt magmas carry ~1 wt% CO2 (11,26), as well as cryptic degassing from intrusive magmas (McKay et al., 2014) and the release of CO2 through metamorphism and assimilation (Schmidt et al., 2015), though there are few direct petrological constraints on Siberian Traps magmatic carbon. Recent measurements of the release of CO2 during continental rifting (Foley & Fischer, 2017) also point to the continental lithospheric mantle, as a potentially important, though not well known, source of carbon during flood basalt magmatism.
Black et al. considered progressive increases in ΡCO2 going from 710-2,800 and then 5,600 ppmv CO2, motivated by ΡCO2 proxy data and inverse modelling of the carbon cycle (Cui, Kump & Ridgwell, 2013). According to Black et al. the release of 20,000-30,000 Pg of CO2 is required for such increases, which is consistent with the carbon available from the extraction of CO2 from ~105-106 km3 on decamillennial timescales (Saunders, 2016; Gutjahr et al., 2017) or from smaller volumes of magma that also tap crustal carbon reservoirs. Only a fraction of any such interval of eruption is likely to be of sufficient intensity to inject sulphur into UTLS (Glaze et al., 2917). Also, in order to enrich gases more strongly in carbon than in sulphur species requires substantial assimilation or metamorphism of country rocks (Svensen et al., 2009; Black et al., 2014). Therefore, the combined sulphur and carbon simulations in this study encapsulated the effects of UTLS injection of sulphur during the most intense volcanic activity (10-100 years) synchronised with more prolonged intervals, 103-104 years, during which there was a rapid increase of carbon in the atmosphere as a result of volcanism, intrusion and metamorphism. In order to account for uncertainty in the tempo of outgassing from Siberian Traps and the degree to which there is synchronisation of outgassing of carbon and sulphur, Black et al. present simulations in which sulphur and carbon are considered independently prior to considering the combined effects.
Climate response to outgassing from Siberian traps
For the end-Permian interval it is indicated by the climate simulations of Black et al. that as pCO2 reached ~2,800 ppm, the annual averaged equatorial SSTs were 29-30oC ; at 5,600 ppm CO2 the annual averaged equatorial SSTs were ~32oC. On the land surface temperatures were significantly hotter, and reached the annual averaged equatorial SSTs of ~38oC at 2,800 ppm CO2 and almost 45oC at 5,600 ppm CO2. The model reproduced the +8 to 10oC anomaly in the Permian/Triassic SSTs (Joachimski et al., 2012; Chen et al., 2016) under the highest CO2 levels that were considered in this study, after about 3 doublings of the CO2 atmospheric concentrations. The solubility of oxygen is inversely related to salinity and the temperatures of the water, though evidence for the end-Permian anoxia is most prevalent at high latitudes (Bond & Wignall, 2010). Though high latitude SST is lower than tropical SST in all the simulations, increasing pCO2 results in sharper relative decreases in oxygen solubility at high latitudes due to a decreased equator-to-pole temperature gradient. The depletion of oxygen would be exacerbated by more sluggish ventilation under greenhouse conditions, especially in the Tethys region.
Unlike greenhouse gases, sulphate aerosols are rapidly formed from volcanic sulphur emissions which then exert a net cooling effect on the climate (Schmidt et al., 2015). Most sulphate aerosols remain in the Northern Hemisphere (Black et al., 2014), as the Siberian Traps eruptions occurred at ~60oN. A maximum Northern Hemisphere optical depth of ~5 was found for an injection of 2,000 TgSO2/year in the UTLS under baseline greenhouse levels. A global mean temperature decrease of 1.5-3.0oC can result from the increased optical depth after 10 years of emissions, with an annual mean temperature decrease of ~5-15oC on land in the Northern Hemisphere. The global mean surface temperature anomaly was more moderate than has previously been estimated for the cooling of flood basalt, though cooling of landmasses in the Northern Hemisphere is severe, especially at middle to high latitudes. Black et al. attributed this partly to the high palaeolatitude of the Siberian traps and partly to the strong response of the ocean to emissions of carbon and sulphur. They found that changes in the mixed layer density and the equator-to-pole thermal gradient that was due to changes in the surface temperature and hydrology translate to the strength of changes of the overturning in the climate mode that was dominated by sulphur, and in the carbon mode, weaker overturning. It has been demonstrated that a strengthened meridional overturning circulation (MOC) as a result of cooling and diminished runoff at high latitudes following eruptions in the past millennium (Stenchikov et al., 2009). The response of ocean circulation to sulphur emissions from the flood basalts in the simulations of Black et al. is an order of magnitude more pronounced, probably as a result of the prolonged cooling and correspondingly larger effects on the heat content of the ocean. A caveat is that the strength of the response of the MOC to volcanic eruptions has been shown to vary across models (Ding et al., 2014) and to depend on background conditions (Zanchettin et al., 2013). The patterns of surface temperature change are also affected by changes in the circulation of the ocean. An enhanced Panthalassic overturning and polewards transport of heat resulting from prolonged sulphur emissions leads to periods of warming in the Panthalassic Ocean. The maximum optical depth in the Northern Hemisphere decays to less than 0.1 within a year of the cessation of the sulphur emissions. Contrasting with this, the circulation of the ocean takes several decades to recover after emissions wane, which is an indication that changes in the circulation of the ocean, and its heat content, can extend the effects of sulphur emissions from flood basalt well beyond the lifetime of aerosols in the atmosphere.
Finally, it is well known from present that aerosols and CO2 cause competing effects on the hydrological cycle due to the positive Clausius-Clapeyron slope of water vapour, which is an indication that, to the first order, surface temperature increases correspond to increased concentrations of water vapour in the atmosphere. As a consequence, global greenhouse conditions lead to intensification of the hydrological cycle (Allen & Ingram, 2002) on longer time scales. The patterns of precipitation shift in the opposite direction during intervals in which the release of sulphur is vigorous.
Climate swings that are predicted compared with the record of proxies
According to Black et al. the coupled changes in surface temperature, circulation of the ocean and hydrology led them to identify a cooler ‘sulphur mode’ that is a characteristic of intense UTLS sulphur injection and a warmer ‘carbon mode’ that prevails on longer timescales when the injection of sulphur at high altitudes wanes. Repeated swings between these sulphur and carbon climates modes are implied by multiple episodes of intense magmatism. A comparison of the model and proxy data simulations of Black et al., given the idealised forcing and compressed timeline of their simulations, was performed to assess the consistency of magnitude, sign and dynamics, rather than to establish detailed alignment.
The maximum temporal resolution of proxy records that are available from the Permian/Triassic interval (Chen et al., 2016) is ~104 years, which is too coarse to resolve swings reliably on the timescales that are described here. Also, oxygen isotope records from the end-Permian that were available were derived primarily from low-latitude sites, at which it is predicted by the simulations of Black et al. there was a cooling that was less pronounced. In this context the results of these simulations serve as predictions that could be most reliably tested with proxy records that were of higher resolution, ideally from high palaeolatitude sites in the Northern Hemisphere. Significant evidence exists, nevertheless, for repeated shifts in the environmental conditions during the emplacement of the Siberian Traps, as well as other flood basalts. Evidence that has been found for an initial cooling that was followed by warming during other flood basalt eruptions has been attributed to early liberation of sulphur from cratonic lithosphere (Guex et al., 2016), though alternatively; it could reflect differing timescales of sulphur-driven and carbon-driven climate that has been discussed in this paper. Records that are uranium isotope based of marine redox conditions also show marked fluctuations that occurred through the interval of mass extinction at the end of the Permian (Bond & Wignall, 2010; Lau et al., 2016). Variations in sulphur isotope have been interpreted as evidence of variations in the vigour of overturning during this time (Algeo et al., 2008). According to Black et al. such fluctuations are consistent with swings in oxygen solubility, runoff, ventilation and productivity. As is the case with the oxygen isotope records from the end-Permian, the temporal resolution of the uranium isotope data sets that are available (Zhang et al., 2018; Lau et al., 2016) is limited to about 104 years. However, as changes encompassing the deep ocean occur more gradually than those in the atmosphere or ocean surface, shifts in forcing by volcanic activity on centennial timescales could resolve partially in the records of marine chemistry.
Over the long term, the model of Black et al. predicts runoff increases that are induced by warming, especially at high latitudes as there is a diminishing of the equator-to-pole temperature gradient, which provides a mechanism to explain the steep increase in 87Sr/86Sr from the latest Permian to the Early Triassic. The increase in 87Sr/86Sr has been interpreted as a consequence of intense continental weathering at this time (Song et al., 2015; Dudás et al., 2017). The eruption of the Siberian Traps in the Northern Hemisphere emplaced millions of square kilometres of fresh volcaniclastic rocks and early lavas that were enriched in 87Sr, which was probably acquired through interaction with ancient Siberian continental material (Wooden et al., 1993; Black et al., 2015). The consequent fertilisation of the ocean and enhanced productivity may have contributed to ocean anoxia (Winguth & Winguth, 2012).
Punctuated environmental stress from Siberian magmatism
The results the study by Black et al. support strongly several conclusions, even given uncertainties in the tempo of outgassing from flood basalt and the origins of the carbon that caused the excursion of δ13C and elevated pCO2 (Cui, Kump & Ridgwell, 2013) that occurred at the end-Permian. Modelling (Glaze et al., 2017) of flood basalt eruption column demonstrates that injection of sulphur at high altitudes is possible only during episodes that span a small fraction of the overall duration of the extinction interval, and the residence time of sulphur aerosols in the atmosphere are very short relative to CO2 (Wignall, 2001; Schmidt et al., 2015). Black et al. predict, therefore, swings between a cooler climate mode that is driven by sulphur during intervals of intense fire fountaining of the Siberian Traps and a warmer climate mode that is driven by carbon on longer timescales. It is demonstrated by the modelling of Black et al. that these climatic swings encompass ocean circulation, marine oxygen solubility and precipitation and patterns of runoff. The available temperature proxy data from the end-Permian mass extinction (Joachimski et al., 2012; Sun et al., 2012; Chen et al., 2016) reveal warming of the surface on the longer term but do not carry resolution that is high enough to test directly for rapid swings in surface temperature, though fluctuation in ocean circulation (Algeo et al., 2008) and redox (Bond & Wignall, 2010) are recorded by proxies. If the predicted climate swings are not substantiated by future high resolution data from the end-Permian, the implication is that most of the sulphur from the Siberian Traps did not reach the UTLS or that there are fundamental gaps in the understanding of the response of the climate to outgassing from flood basalt.
The environmental changes due to magmatism should be evolving and not monotonic, as has been implied by variations in the rate of magma emplacement, volatile sources, and volatile concentrations over the life cycle of flood basalt provinces, as well as differences in atmospheric lifetime. Extinction depends on the speed of environmental change relative to the capacity of organisms to adapt to the changed conditions (Bürger & Lynch, 1995). The simulations of Black et al. are related to magmatism of the Siberian Traps to the punctuated deterioration of global ecosystems during the mass extinction event at the end of the Permian.
Black, B. A., et al. (2018). "Systemic swings in end-Permian climate from Siberian Traps carbon and sulfur outgassing." Nature Geoscience 11(12): 949-954.
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