Environmental and climatic impact of the eruption of large igneous provinces
Paul Wignall,
School of Earth and Environment,
University of Leeds,
Leeds LS2 9JT
September 9, 2004
Introduction
That large igneous province eruptions may be capable of causing environmental and climatic effects was first mooted in a serious way when the debate on the origin of the end-Cretaceous mass extinction first began in 1980. The Deccan Traps rapidly became, and still remain, the principle “other contender” for this extinction event, although it is fair to say that it has always remained in second place to meteorite impact. Rampino and Stothers (1988) deserve the credit for being the first to suggest that all flood basalt province eruptions have caused mass extinctions, and various super-optimists continue to propose that all large igneous provinces are capable of doing some sort of environmental damage (Courtillot 1999; Wignall 2001; Courtillot and Renne 2003).
In general, LIP eruptions seem to be associated with some or all of the following climatic/environmental effects
- Phases of rapid global warming
- Oceanic anoxia and/or increased oceanic fertilisation
- Calcification crises
- Mass extinction
- Release of gas hydrates (as interpreted from rapid negative shifts of d13C)
It is probably fair to say that only the Karoo-Ferrar eruptions show all of these features, although the Siberian Traps come close, but the other LIP eruptions show several of them. Volcanogenic cooling has also been proposed for several extinction events but, in this reviewer’s opinion at least, the evidence is insubstantial. The preponderance of evidence for warming strongly suggests that carbon dioxide emissions do all the damage (knock-on effects then include the release of gas hydrates, acidification of ocean surface waters, elevation of the oceanic CCD), but herein lies the core of the problem. As identified some time ago, the amount of CO2 released during the eruption of even the largest individual flood basalt flow is unlikely to have exceeded current annual anthropogenic pollution rates (Caldeira and Rampino 1990; Wignall 2001), and we are certainly not even close to recreating the conditions of these ancient catastrophes. Perhaps the volcanic CO2 eruptions merely served as a trigger to something else, such as the release of methane from clathrates. However, the tell-tale evidence for methane release (the rapid negative shifts of d13C) is seen much less frequently than the evidence for warming.
Rather than speculate and generalise further it is probably best to consider the environmental effects of LIPs on a case-by-case basis.
The Emeishan Traps
It is very early days in the study of the environmental effects of this province. It was only a little more than ten years ago that it was first appreciated that there was a major mass extinction at the end of the Middle Permian. Many of the losses at this boundary had previously been subsumed into the end-Permian mass extinction (Hallam and Wignall 1997). Much of the evidence comes from South China, and it remains to be established if this extinction had any effect outside of the palaeotropics. The victims of the extinction are last seen in the top of the Maokou Limestone and, over large areas of SW China, this unit is overlain by the flood basalts of the Emeishan province (He et al., 2003). Plenty of people have now linked the two phenomena but of course it is no use trying to date an extinction horizon in sections where the highest occurrence of fossils is constrained by lavas. You won’t find a fossil record in a basalt! Therefore, the time correlation requires comparing the highest fossil occurrences in continuous marine sections and relating this level back to the lava pile. The magnetostratigraphic work of Ali et al. (2002) goes a long way to doing this and reveals, rather surprisingly, that the extinctions may have occurred during the waning phases of the eruptions when the more explosive (but less voluminous) phase of volcanism dominated.
There is rather a dearth of extinction mechanisms for the end-Middle Permian (end-Guadalupian) mass extinction. Hallam and Wignall (1997) related it to the extreme lowstand of sea level at this time whilst there is tentative evidence for a sharp, negative d13C anomaly coincident with extinction, but really it is case of “watch this space” for this extinction.
The Siberian Traps
Undoubtedly the most celebrated link between a LIP and a mass extinction is that between the Siberian Traps and the end-Permian mass extinction. Earlier cause-and-effect relationships, proposed around 10 years ago, tended to focus on the likelihood of severe volcanogenic cooling/ice age triggering etc. as a cause of this greatest mass extinction of all time (e.g. Campbell et al. 1992). However, the evidence for a severe phase of global warming during the Permian-Triassic transition is now overwhelming (e.g. Retallack 1999) which of course points the blame at CO2 eruptions. Many workers now see the catastrophic release of methane from gas hydrates, presumably triggered by the CO2 emissions, as the cause of this warming. See Ryskin (2003) for a methane-related extinction mechanism which is probably a little too catastrophic, and Retallack et al. (2003) for one which is only a little less so. In the oceans the principle effect of warming seems to have been to create one of the most severe anoxic events of the Phanerozoic (Wignall and Twitchett 2002). Kidder and Worsley (2004) provide an interesting discussion on the nature of the Early Triassic super-greenhouse climate. Marine anoxia can of course cause extinctions in the oceans but not on land and the possibility exists that the terrestrial extinctions were not the product of warming per se. Sulphate aerosols and acid rain are possible kill mechanism on land, but is it possible for explosive volcanism in western Siberia to have a lethal effect in the southern hemisphere? In the short time they’re in the stratosphere, how could the aerosols cross from the northern to the southern hemisphere? More thought is needed on the nature of the terrestrial extinctions.
Of course the Siberian Traps do not have it all their own way in end-Permian debate and many will have probably seen the recent reports of an enormous impact crater offshore of NW Australia – The Bedout crater – claimed to be of end-Permian age (Becker et al. 2004). This reviewer doubts both its age and impact origin but, once again, watch this space (and see Kerr’s comments in the same issue of Science that the paper appeared in).
Central Atlantic Magmatic Province (CAMP)
The realisation that the Central Atlantic Magmatic Province was both very large, and of the right age (see Marzoli et al. 1999), to be implicated in the end-Triassic mass extinction came as something of a relief to mass extinction workers. Hitherto this event had proved rather intractable to study, primarily because of a dearth of complete marine boundary sections and a consequent lack of study. At least now we have a culprit!
There are still various problems associated with a CAMP-extinction link. Firstly, there is the detailed timing. The detailed sedimentary record of the Newark Basin contains both evidence for the terrestrial extinctions and a flood basalt record; but the first basalt occurs somewhat above the extinction horizon (Fowell and Olsen 1993). Of course it could be argued that the Newark Basin basalts are only the “feather-edge” of the CAMP and their lowest occurrence in this basin could considerably post-date the actual onset of eruptions in the province as a whole. Certainly the available radiometric ages suggest a close temporal link between the onset of eruptions and the end of the Triassic (Pálfy et al. 2002). Unfortunately, it is not at all clear that the end of the Triassic was marked by a mass extinction. As recent reviews have shown, extinction rates may have been high throughout the last few million years leading up to the end of the Triassic (Hallam 2002; Tanner et al. 2004). Ammonoids were extremely diverse in the Late Triassic and their losses contribute substantially to the extinction peak seen in Sepkoski’s compilations. The majority of these losses occurred in the late Norian, the penultimate stage of the Triassic, and only a handful trickled on into the final Rhaetian Stage.
If the end-Triassic mass extinction “event” was in fact a prolonged phase of extinction losses beginning in the Norian then the crisis was well underway long before the CAMP eruptions. In this case the volcanism becomes an irrelevance. In contrast to this, the radiolarian record reported by Ward et al. (2004) provides a very different story. Their data, which comes from a section in the Queen Charlotte Islands of Canada, show that an exceptionally diverse radiolarian population was abruptly eliminated at a level they take to mark the end of the Triassic. If this story can be replicated in other locations, then it is likely that there was a sudden productivity collapse at the end of the Triassic and we can start pondering the environmental effects of the CAMP eruptions once again.
Perhaps because the basic ground work on the timing of the end-Triassic mass extinction still requires a lot of work, proposed end-Triassic extinction mechanisms are still very much in their infancy and rather vague. Hallam and Wignall (1999) note that the extinctions are related to a rapid, short-lived regression at the end of the Triassic (perhaps a regional doming effect associated with the CAMP plume head). This may at least have been responsible for the emergence and consequent demise of reefs in the Tethyan Ocean. Hesselbo et al. (2002) talk of perturbations of the carbon cycle and global warming at the end of the Triassic which they attribute to CAMP CO2 emissions. Cohen and Coe’s (2002) Os isotope data suggests that the CAMP volcanism may have begun around the T/J boundary based upon increases in the concentrations of both Re and Os and a decline of 187Os/188Os ratios. However, this interpretation should be treated with caution because the data comes from the T/J boundary sections of SW England. Here the latest Triassic succession has long been known to be quasi-marine, with a restricted fauna that only became normal marine in the earliest Jurassic as sea-level rose and allowed proper connections with the world’s oceans. The Re and Os data rather nicely follows this trend of increasing “marineness” and it could alternatively be interpreted as a proxy for the sea-level rise rather than volcanism.
Karoo-Ferrar Traps
As noted above, the link between the Karoo-Ferrar eruptions and contemporaneous environmental/climatic changes is well developed. Good radiometric dates indicate that the eruptions began in the early Toarcian Stage of the Early Jurassic (Pálfy and Smith 2000). This coincides with a well-documented, oceanic anoxic event, a warming trend, a calcification crisis in equatorial latitudes and marine mass extinction (Wignall 2001; Jenkyns et al. 2002; Bailey et al. 2003; Erba 2004). Major carbon isotope perturbations at this time include a sharp negative excursion, roughly coincident with the onset of ocean anoxia, that is interpreted as the product of a catastrophic release of methane from gas hydrate reservoirs (Hesselbo et al. 2000; Beerling et al. 2002). At the acme of the warming, the increase of continental weathering rates appears to have been reflected by a decline in 187Os/188Os ratios (Cohen et al. 2004). The blame for all these environmental woes is generally laid at the door of volcanic CO2 emissions.
Paraná-Etendeka Province
Given the context of the environmental damage wrought by the Karoo-Ferrar Province perhaps the most remarkable aspect of the next youngest LIP was just how little environmental change it appears to be associated with. This was the Paraná-Etendeka Province which, like the Karoo-Ferrar Province, was erupted in southern Gondwana. Radiometric dating indicates a latest Valanginian (Early Cretaceous) age and, until recently, the oceanic record has suggested that little of interest happened at this time. However, recent studies have revealed an extremely watered-down version of the effects seen during the Toarcian and end-Permian crises. Thus, ODP cores have revealed a thin, late Valanginian black shale event indicating anoxic deposition during an episode recently named the Weissert oceanic anoxic event (Erba et al. 2004). The calcareous nannoplankton fossil record indicates a contemporaneous calcification crisis that may reflect oceanic fertilisation (calc. nannoplankton are thought to prefer low nutrient conditions) and/or acidification by volcanogenic CO2 emissions (Erba 2004). This crisis did not cause extinctions and in fact proved something of a spur to evolution, as there is significant plankton radiation immediately after the Weissert event.
Unlike other intervals marked by LIP eruptions, it is unclear if there were any substantial global temperature changes at this time. A case can be made for cooling in the last stages of the anoxic event but this is likely to reflect CO2-drawdown due to organic matter burial during the event (Erba et al. 2004), rather than be a volcanic cause. There is no negative carbon isotope anomaly associated with this event and so no volcanogenic warming-triggered gas hydrate release has been invoked.
Ontong-Java (and Kergulen) Plateau
It is clear that a substantial part of the vast Ontong Java Plateau was erupted around the Barremian/Aptian boundary of the Early Cretaceous. Some radiometric dates from the almost-as-vast Kerguelen Plateau also indicate a similar age (Courtillot and Renne 2003). This great volume of oceanic volcanism slightly predates the oceanic anoxic event, known as the Selli Event (Larson and Erba 1999). However just prior to this event there was a “nannoconid crisis” in which some very small, calcareous, plankton that had previously occurred in rock-forming abundances in the oceans suddenly became rather rare. This could reflect a calcification crisis, due to volcanogenic CO2 input, or a fertilisation crisis that did not favour the oligotrophic-adapted nannoconids (Erba 2004). Erba further suggests the direct warming of the ocean water by the lava pile may have contributed to the break-down of ocean stratification and expansion of the oxygen minimum zone. Both the warming and the fertilisation may have contributed to the anoxic event but this effect was curiously delayed relative to the eruptions. The reestablishment of normal oceanic conditions after the Selli Event saw the reappearance of the missing nannoconids and indicates that the crisis was not an extinction event.
Jahren (2002) has proposed that the doming of the ocean floor, prior to eruption of the oceanic plateau lavas, may have destabilised large amounts of gas hydrates and released large volumes of methane into the atmosphere and thus stimulated warming. A sharp, negative d13C anomaly is seen in the early Aptian, just after the onset of the nannoconid crisis and shortly before the Selli Event, but this probably post-dates the doming and is after the onset of the main phase of eruptions. However, as with all the modelling concerning Aptian events, the precise dating of the oceanic volcanism will provide a key to testing many of these scenarios. Does anyone want to do the Os isotope work?
Caribbean-Colombian (and Kerguelen) Plateau
The Cenomanian-Turonian (C-T) boundary is marked by the type example of the Cretaceous oceanic anoxic events, the culmination of Cretaceous greenhouse warming (and sea-level rise) and a minor extinction event in the marine fossil record (Hallam and Wignall 1997; Jenkyns 1999). It thus has many of the hallmarks of other volcanogenic events, although evidence for gas hydrate release is lacking and there is only weak evidence for a calcification crisis (Erba 2004). All of these events essentially coincide with the eruption of the Caribbean-Columbian LIP and probably part of the Kerguelen LIP, and the Madagascan flood basalts too (Courtillot and Renne 2003). For what it’s worth, a lot of kimberlite pipes also date to around the C-T boundary so there is a lot of volcanic culprits to choose from at this time.
Kill mechanisms for the C-T extinction may include trace metal poisoning in the oceans (Erba 2004b), but this is a proposition that is rather difficult to test. The anoxic event itself provides the most obvious cause of the marine extinctions and the contribution of volcanism to global warming and fertilisation of the oceans provides a route to link volcanism and extinctions (Sinton and Duncan 1997). Both warming (directly by the oceanic lavas) and possible oceanic acidification (by volcanic SO2 release) would have released CO2 to the atmosphere thus exacerbating an already established trend of global warming (Kerr 1998).
The Deccan Traps
Climate change during the K/T transition has been intensively studied and it is probably for this reason that climate history at this time appears rather complex, however a reasonable consensus has been reached in recent years. The mid-Maastrichtian was a rather cool interval but this was replaced by a rapid phase of warming that began around 400 kyr before the K/T boundary in the final part of the C30n chron (Wilf et al., 2003). This was reversed by a rapid cooling trend around 100 kyr before the boundary in which the 4-5°C temperature gain was lost. The cooling coincides with a sharp sea-level fall (a signature of glaciation perhaps?) and a lowstand was reached shortly before the K/T boundary and began rising again across the boundary (Hallam and Wignall 1999).
Despite these substantial oscillations in climate and eustasy it is does not appear that they caused much in the way of extinctions. Keller (2003) has shown that the latest Maastrichtian warming pulse was associated with a destabilisation of planktonic foraminiferal populations and brief-lived blooms of opportunist taxa such as Guembelitria. According to Keller these may reflect the expansion and intensification of the mid-water oxygen minimum zone. However, interesting though they are, these fluctuations are rather trivial compared with the near-total and abrupt mass extinction of planktonic foraminifera (and various other groups) at the K/T boundary.
The possibility that the Deccan Trap eruptions were implicated in some, or perhaps all of these changes, has of course been known for some time. However, only recently has it been possible to infer the timing of the eruptions relative to these changes, thanks primarily to the Os isotope work of Ravizza and Peucker-Ehrenbrink (2003). These key data suggest that the main eruptive phase coincided with the late Maastrichtian warm pulse. Thus, like other LIP eruptions, volcanic CO2 release emerges as the most likely driver of environmental change. The modelling work of Dessert et al. (2001), based primarily on Sr isotope fluxes, goes a long way to replicating the environmental effects of the Deccan Traps. The initial flux of volcanic CO2 is seen to cause a calcification crisis in the oceans and global warming of the order of 4°C. After approximately a million years chemical weathering of the expanse of flood basalt draws down the atmospheric levels and achieves a steady state that is slightly cooler than before the eruptions (Dessert et al. 2001).
Like the other LIP eruptions of the Cretaceous the Deccan Trap eruptions appear to have caused perceptible/significant climatic effects, but only modest biotic effects, perhaps because the oceanic system did not become anoxic. It has been argued that the biosphere was already rather stressed at the point of meteorite impact, but without that impact one suspects the K/T boundary event would only have ranked alongside other minor Cretaceous events such as the Selli and Weissert events.
North Atlantic Igneous (Brito-Arctic) Province
The climatic events around the Palaeocene-Eocene boundary have received ample study and are reasonably well understood. Thus, a sharp negative d13C excursion is generally taken as the signature of gas hydrate release which in turn is held responsible for the contemporaneous and brief (120 kyr) warming pulse at this boundary (Kennett and Stott 1991; Norris and Röhl 1999). Contemporary changes in the oceans include a caclification crisis and the development of deep-water dysoxia. This dysoxia not surprisingly had a bad effect on the creatures living there (Kennett and Stott 1991; Speijer 1994). However, this was not a time of mass extinction by any stretch of the imagination, in fact extinction rates were some of the lowest ever recorded at this time; the P-E boundary was a good time to be alive.
These climatic/oceanic changes are all highly comparable to the changes observed during Cretaceous LIP eruptions and in this case they may coincide with the eruption of the North Atlantic Igneous Province. However, this seems to have been erupted in two, discrete pulses with the younger pulse coinciding with the Palaeocene-Eocene thermal maximum at 55 Ma, but with the older eruptive phase coinciding with a rather cool interval (Courtillot and Renne 2003). In a valuable recent study of three marine P-E boundary sections, Schmitz et al. (2004) noted that the thermal maximum coincides with the onset of an unusual phase of explosive basaltic volcanism. They suggest that this followed on from a preceeding phase of voluminous, subaerial flood basalt volcanism seen primarily in East Greenland. Thus the timing of the volcanism appears right.
Final thoughts
Perhaps the most intriguing question arising from the link between LIPs and environmental changes is the remarkably different magnitudes of the supposed volcanogenic effects. Thus, the Toarcian climatic and environmental changes are remarkably similar to those proposed for the end-Permian crisis, only the calcification crisis was not seen during the older event. The Toarcian and Permian events are also very similar to the changes seen during the Palaeocene-Eocene thermal maximum, only a mass extinction event is missing from this younger event which, perhaps significantly, was of much briefer duration. It appears LIPs can cause changes that range from the interesting-but-benign (Palaeocene-Eocene boundary), to severely damaging (Toarcian) to utterly catastrophic (end-Permian). Partial solution to this problem may come from modelling work, such as that of Dessert et al. (1991), which reveals that factors such as pre-eruption atmospheric CO2 levels, and the rate of eruption are especially key variables in any climatic changes.
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