Evaluating cause and effect between Large Igneous Provinces and

The temporal coincidence between the occurrences of LIPs and biotic crises in the Phanerozoic is a known fact (Bond and Wignall, 2014; Courtillot, 2003; Courtillot and Renne, 2003) (Fig. 1.3) Consequently, LIPs have been widely considered as the main driving force of environmental collapse and key biotic turnovers in the Phanerozoic (Bond and Grasby, 2017; Bond and Wignall, 2014; Courtillot, 2003; Wignall, 2005, 2001). All of the most significant Phanerozoic mass extinctions share similarities such as: (i) high atmospheric CO2 concentrations, leading to fluctuations of the carbon cycle and the global bioproductivity; (ii) periods of global warming triggered by high pCO2 in the atmosphere;

(iii) periods of global cooling due to of CO2 drawdown by, e.g., absorption into biomass, or weathering of large masses of flood basalts from LIPs, or from volcanic SO2 emissions;

(iv) oceanic acidification through volcanic gases; (v) ocean anoxia leading to black shale deposition (Fig. 1.4), with the latter not being a typical feature of the end-Cretaceous extinction (Bond and Grasby, 2017).

Figure 1.3 – Age correlation between the Phanerozoic Large Igneous Provinces and mass extinction events.

Taken from Courtillot and Renne (2003)

It is widely accepted that LIPs are the main driving force for the majority of mass extinction events because of their impression frequency of association (Fig. 1.3 & Fig. 1.4).

Even though the temporal link between the two is compelling, open questions still remain.

For instance, for much the Phanerozoic it is a known fact that the extinction losses occur at a much shorter timescale (<100 kyr) when compared to the average duration of LIPs eruption and emplacement (~500-800 kyr). Evidence for magmatism before, during, and after extinctions events are widespread. To illustrate, in the case of the end-Triassic extinction, the first occurrences of the CAMP start ca. 100 kyr before the onset of extinction (Blackburn et al., 2013; Davies et al., 2017). In Blackburn et al., (2013) the extinction event and the biotic recovery period are both coincident with CAMP occurrences. A similar scenario is postulated for the end-Cretaceous extinction, which has been related to a second of three main pulses of the Deccan Traps (Schoene et al., 2019). The main extinction event is constrained to ca. 50-100 kyr with the recovery period to have started no later than 71 kyr after the main extinction event (Clyde et al., 2016). Therefore, extinction and recovery

usually occur within 2-3 orders of magnitudes less time than of a LIP lifespan. In the case of the Siberian Traps, the magmatism precedes the extinction by ca. 300 kyr and continues ca.500 kyr after the extinction interval (Burgess and Bowring, 2015). This begs the question as to why there is a mismatch between the timescales of important evolutionary turnovers and LIPs. To reconcile this scenarios, it has been suggested that CO2 injections are not continuous throughout LIP’s lifespan and that biotic catastrophes are linked to specific short-lived pulses. Admittedly, in the case of the Deccan Traps the magmatic pulse temporally related to the extinction accounts for 80% of the Deccan volcanism (Schoene et al., 2019). In the case of the Siberian Traps, an intermediate dyke phase is believed to be the main trigger (Burgess et al., 2017). Nevertheless, why certain pulses are contemporaneous with mass extinctions and others are not is still a major limitation to invoking LIPs as the main driving force of extinction and environmental change. Another important drawback is that only a small number of LIPs seem to be linked to a specific mass extinction event, with the Columbia Flood Basalts, North Atlantic Igneous Province, having no link to any significant biological or environmental collapse of any sort.

Figure 1.4 – Illustration between Large Igneous Provinces, extinction, kill mechanisms, and environmental impacts.

Taken from Bond and Grasby (2017)

1.3.3.1 Potential deleterious effects of LIPs

Volcanic CO2 greenhouse: The single most important feature among of all of the major mass extinction events is the increased greenhouse effect from volcanic CO2, which is believed to be the trigger of a series of cascading effects in Earth’s biosphere (Fig. 1.4). By far the most important greenhouse gases that cause global warming are CO2 and CH4

delivered into the atmosphere-ocean system. The continuous and prolonged injection of these gases into the atmosphere is believed to have caused global temperatures to rise above global averages, up to +4 to +10^oC (Beerling and Berner, 2002; McElwain, 1999;

McElwain et al., 2005; Royer et al., 2001; Steinthorsdottir Margret et al., 2011). The increase in atmospheric CO2 levels is coincident with significant δ¹³C excursion towards negative values (e.g., 4‰ to -7‰) in the inorganic and organic carbon isotope reservoirs, which usually overlaps with the extinction intervals. Since the global carbon isotope composition is shifted towards negative values, the injected carbon driving global warming is believed to have negative δ¹³C values, albeit the exact source and its isotopic composition is unknown. An important positive feedback loop to increased atmospheric CO2 is ocean acidification by ocean CO2 uptake forming carbonic acid (H2CO3) (Kump et al., 2000).

Ocean acidification of shallow marine water leads to calcification crisis directly affecting the global carbonate and organic carbon production (van de Schootbrugge et al., 2007). The rate at which CO2 is injected into the atmosphere is what ultimately controls the rate of change in ocean chemistry, notably ocean acidification (Grasby et al., 2013; Honisch et al., 2012; Lau et al., 2016). Smaller protracted injections of CO2 to the atmosphere will result in a small build-up and will generate smaller perturbations. Rain acidification also holds a positive feedback loop with increasing atmospheric CO2 levels as well as SO2 (Black et al., 2014; Bond et al., 2010b). Apart from the direct deleterious effects of acidification of rain to life on earth, the acidification of rain also facilitates the chemical breakdown of minerals during continental weathering. Furthermore, enhanced continental weathering is directly related to increased atmospheric CO2 levels, but as negative feedback loop as enhanced continental weathering constitutes an efficient mechanism of CO2 sink counteracting the increased pCO2 (Kump et al., 2000, 1999). Continental weathering, in turn also triggers enhanced productivity by increased delivery of nutrients from the continents to the oceans by riverine input; notably, N and P (Kump et al., 2000). Global warming also leads to oxygen deficiency in the oceans (Meyer and Kump, 2008). As increasing seawater

temperature decreases the solubility of oxygen in surface ocean water leading to ocean anoxia (Calvert and Pedersen, 1993; Kerr, 1998). The depletion of oxygen in the deep ocean decreases the efficiency of organic matter oxidation in the deep ocean, which effectively shuts down the biological pump. This creates the condition for the enhanced preservation of organic matter, thus increasing the fraction of organic carbon being buried, which crate impact the carbon cycle (Kump and Arthur, 1999). Additionally, under ocean anoxia phosphate is rarely buried and is put back into the water column creating feeding a positive feedback loop with enhanced productivity maintaining the production of organic matter in the shallow ocean and its export to the deep ocean (Meyer and Kump, 2008). If the system continues for long enough, euxinia might ensue, and the appearance of sulphur-reducing organisms become widespread (Lyons et al., 2009; Meyer and Kump, 2008). The release of methane is potentially even worse than that of CO2 for global warming. Global warming can induce the melting of marine methane hydrates (clathrates) that can significantly amplify the effects of global warming (Beerling et al., 2002; Hesselbo et al., 2000).

Volcanic SO2 icehouse: SO2 is also believed to compose a major part of the volatile budget of volcanic eruptions. Contrary to CO2, SO2 is believed to cause global cooling at least on a 2-3 yr timescale (Self et al., 2006, 2006). The rapid conversion of SO2 to sulphate aerosols such as H2SO4 and H2O (Zhao et al., 1995) is viewed as an important cooling mechanism (Jones et al., 2016; Robock, 2000); however, for this mechanism to sustain global cooling for a prolonged time, it is necessary that SO2 is continuously delivered to the atmosphere for a protracted period because of the short residence time of sulphur in the atmosphere. Such a scenario might increase planetary albedo if SO2 can reach the stratosphere (Robock, 2000); a positive feedback loop might trigger an expansion of the polar icecaps (Macdonald and Wordsworth, 2017).

Halogens: Other deleterious effects to marine life arise from the halogen (Cl, F) and toxic metal (Hg, Zn, Cu, Ni, Pb, As, Cd) poisoning delivered directly by volcanism (Font et al., 2016; Grasby et al., 2016; Sanei et al., 2012; Sial et al., 2013), which also contributing to acid rain formation and ozone depletion (Black et al., 2014). In the terrestrial realm, ozone depletion, wildfires, and acid rain may have been important killing mechanisms (Belcher et al., 2010; He et al., 2012; Pausas and Keeley, 2014) although the majority of mass extinctions are limited to marine realms.

The long-term temperature regulation on Earth is controlled by CO2 balance between several sources: volcanic input, metamorphic degassing, silicate weathering, primary productivity, and organic burial (Kump, 1989; Kump and Arthur, 1999). It is thought that the intrusive and extrusive members of LIPs are capable of injecting massive volumes of CO2 into the atmosphere, mainly from two sources: 1) volatiles of magmatic origin (Self et al., 2014 and references therein); 2) so-called thermogenic volatiles, where the injection of CO2 into the atmosphere would be the result of the contact metamorphism between LIP intrusions and organic-rich shales within the surrounded country rock (Svensen et al., 2009). The interaction of the LIP dyke complexes with sediments would induce the sediments to degas carbon-based species (CO2, CO, CH4) into the atmosphere via venting pipes. The sill-dyke complexes of many LIPs cross-cut organic-rich strata or source rocks in sedimentary basins. For instance, the CAMP has dykes and sills that intruded into the Amazonas basin, in Brazil (Milani and Zalan, 1999); the Karoo LIP into the Karoo Basin in South Africa (Aarnes et al., 2011); the Siberian Traps intruded into the coal-rich deposits of the Tunguska basin (Retallack and Jahren, 2008; Rothman et al., 2014; Svensen et al., 2009). The thermogenic release model conditions the release of CO2 into the atmosphere to the nature of the surrounding crust onto which the LIP is emplaced, and thus the LIPs is thought to be merely a mediator of the green-house gases and not their actual source.

Implied in this model is the randomness of an LIP to encounter carbon-rich sediments while penetrating the crust, which would explain why some LIPs are connected to biotic and/or environmental crises and others are not. Furthermore, the carbon isotopic composition of the thermogenic released gas would be highly negative, facilitating the shift of global carbon budget to negative values.

1.3.3.2 Tracing LIP activity in the marine record

One of the central limitations to fully comprehending the relationship between LIPs severe climatic changes, extinctions, drastic changes to ocean chemistry, and variations to global temperatures has been the lack of a direct geochemical proxy for volcanism in marine record where these palaeonenvironmental changes are recorded. This has left the connection only made on a first-order temporal bases. Nevertheless, temporal connections do not imply causality. The lack of a geochemical proxy for volcanism that could be tied with δ¹³C changes in the same stratigraphic section, allowing cause and effect of volcanism

and climate change to be assessed accurately. In the past decade, the use of Hg/TOC as a geochemical proxy for LIP activity in the marine record has shown considerable promise in some biotic and environmental crises (Grasby et al., 2016; Percival et al., 2017, 2015;

Sanei et al., 2012; Thibodeau et al., 2016). Since Hg is put into the environment via volcanic emissions, the unusual enrichment of Hg in the marine record coeval with other shits for environmental factors was considered as a strong proxy of LIP activity. However, the long-term enrichment of Hg in the marine environment is possible by other pathways than solely via oxidation of volcanic Hg⁰ delivered via rainfall. Volcanic Hg is readily absorbed and accumulated into the terrestrial reservoir (biomass and soils) and is a viable source of recycled Hg into the marine environment during environmental crises (Grasby et al., 2017;

Them et al., 2019). Furthermore, Percival et al., (2018) suggest that local sedimentary processes could bias the Hg/TOC signal, such as abrupt changes to the organic matter input into the sedimentary environment could mute the Hg/TOC ratio. It has also been suggested that different marine environments preferentially record different sources of Hg, with proximal marine facies preferentially recording a higher proportion of remobilized terrestrial Hg, whereas distal marine environments preferentially recording volcanic Hg (Grasby et al., 2017). Alternatively, ¹⁸⁷Os/¹⁸⁸Os(i) has been shown to be a much more reliable proxy in recording the effect of LIP volcanism than Hg/TOC in the marine record (Fantasia et al., 2019; Percival et al., 2017; Schoene et al., 2019). Another issue with the Hg/TOC proxy is that anomalies are much shorter than the LIP life span, usually coeval with extinction intervals and climate fluctuation and do not last the entirety of the LIP eruption. Therefore, the relationship between LIPs and environmental changes continues to be elusive, albeit for some cases the connection seems to be fairly well demonstrated.

1.3.3.3 The importance of high-resolution timescales

The degree to which LIPs can impact the Earth’s ecosystems remains to be fully understood. Changes to biodiversity and environmental equilibrium involve a series of complex processes in a series of positive and negative feedback loops that operate at different timescales. For instance, wildfires may operate on some months to years timescale; cooling due to sulphate aerosol degassing, acid rain, and ozone depletion operate on a scale of years to tens of years; ocean acidification, ocean anoxia, global warming due to increased pCO2 operate on a scale of hundreds of thousands of years. Therefore, sorting

out the sequence and rate at which these events take place relative to each other requires adequate time constraints to each of these processes on a case-by-case bases. To illustrate the importance of high-precision timescales, prior to the development of high-precision U-Pb geochronology the duration of extinction events was thought to be close to 1 Ma (Wignall, 2001). Under these circumstances, invoking, for instance, ocean acidification as a viable killing mechanism to extinction events would be unreasonable because of the disparity of the rate at which these processes operate. However, recently, extinction crises have been constrained to the 100 kyr timescale. Furthermore, high-precision U-Pb geochronology has revolutionised our understanding of the timescales of magmatic processes, constraining the duration of LIPs to a few hundred thousands of years, rather than a few millions of years via other geochronological methodologies. Additionally, an accurate picture of the rate of eruption of LIPs is vital to understand the efficiency of volatile volcanic release. This would help establish the temporal relationship between the rate of change of climate proxies such as δ¹³C and δ¹⁸O to the rate of change of Hg/TOC and rate of LIP emplacement. It is unlikely that mass extinctions are caused by a single mechanism, but rather by a combination of direct drivers and feedback loops working in consort. However, as the cause and effect of the many killing mechanisms and the drivers of mass extinctions remain largely unresolved, constraining their durations and their ages is vital to figure out which of the driving mechanisms are the most relevant to causing mass extinctions events. Testing their synchronicity will require the continued effort to overcome the issue hindering evermore precision and accuracy of high-precision U-Pb dates.