Australia: The Land Where Time Began
The Smithian - Lethally Hot Temperatures in the Early Triassic
It has been believed that global warming has played a part in many of the biotic crises of the past. In the study carried out by the authors1 it was shown that coinciding with the mass extinction at the end of the Permian there was a rapid rise of atmospheric temperatures that were exceptionally high in the Early Triassic, so high in fact that they were inimical to life in equatorial regions to such an extent that recovery following the mass extinction event was suppressed in entire ecosystems. Evidence of this event were the loss of calcareous coral and few fish present in the equatorial regions of the Tethys Ocean. In the Early Triassic most plants and animals were driven from terrestrial ecosystems of the tropics by high temperatures, and these high temperatures are believed to have been a major cause of the end-Smithian crisis (Smithian is the earliest division of the Early Triassic).
The authors1 suggest the current anthropogenic global warming is likely to be contributing to the rapid loss of biodiversity that is presently occurring (Bálint et al., 2011). In the geological past a warming climate has been implicated in severe biotic crises as a corollary to the direct cause of death, as in the case of of the spread of anoxia in the oceans (Hallam & Wignall, 1997). According to the results of this study lethally hot temperatures have been shown to have a direct control on extinction and recovery during the end-Permian mass extinction event and following it. There are several reasons why the aftermath of this event is remarkable, besides the large scale of the losses, including the long recovery time (Payne et al., 2004), the widespread prevalence of small taxa (Twitchett, 2007), and the lack of coal deposits from the Early Triassic (Retallack et al., 1996). As well as these there are several facets of the fossil records from low latitudes, that include the distribution of fish, marine reptiles, and tetrapods, that can be related to extreme temperatures above thresholds of tolerance.
The release of large volumes of carbon dioxide from the flood basalt eruptions of the Siberian Traps and related processes have long been regarded as a potential trigger for the climate warming (Reichow et al., 2009 & Sobolev et al., 2011) that was one of the causes of the end-Permian crisis (Hallam & Wignall, 1997). In the apatite of conodont teeth the oxygen isotope ratio δ18O has proven to be a reliable proxy for the temperature of palaeoseawater (Joachimski et al., 2012), and as conodonts suffered few losses at the genus level at the end of the Permian (10) it has allowed the use of teeth from the same genera over many millions of years (11) to track any changes in the oxygen isotope ratios, and hence the temperature of the oceans over long periods of time. Working with conodont teeth recovered from sections the Nanpanjiang Basin of South China the authors1 used the δ18Oapatite of conodont teeth to reconstruct the sea temperatures of the equatorial regions from the Late Permian to the Middle Triassic. The main record of their study used the genus Neospathodus to monitor the upper water column temperatures, estimated to have been at about 70 m depth, while at extremely shallow water depths taxa, such as Pachycladina or Parachirognathus spp., Platyvillosus spp., were used to obtain sea surface temperatures (SSTs).
The results obtained showed perturbations of both carbon and oxygen isotope ratios that were almost synchronous, with 3 positive excursions being observed in the Dienerian, about 251.5 Ma, the Early Spathian, about 250.5 Ma, and at the Spathian-Anisian boundary, the transition from the Early Triassic to the Middle Triassic about 247.5 Ma. The minima in the δ13Ccarb and δ18Oapatite occur in the Griesbachian, about 252.1 Ma, and the Smithian-Spathian transition about 250.7 Ma. The δ18Oapatite values obtained from taxa of conodont are in accordance with their different water depth habitats:
Neospathodus species have heavier values, about 0.7/ml (‰), than those from Pachycladina/Parachirognathus spp. and Platyvillosus spp. Gondolellids from deeper water were found to have δ18Oapatite values that was even heavier, about 0.4‰, than are found in Neospathodus spp. Oxygen isotope data from the latest Spathian-early Anisian at Bianyang and Guandao are more scattered, and compared with samples from other sections, are up to 1.3‰ heavier. According to the authors1 such an enrichment of 18O towards the interior of the platform is interpreted to be the result of evaporation as occurs in the Bahama Bank of the present (Frank, 2000), and both these locations are near the Great Bank of Guizhou. Most of the data presented were obtained from distal, open-water environments and therefore the palaeotemperature they present is a faithful record.
A rapid warming across the Permian-Triassic boundary is recorded, 21oC to 36oC over about 0.8 My (Joachimski et al., 2012), when seawater temperatures are calculated from δ18O values, with a maximum temperature being reached during the Griesbachian, about 252.1 Ma, which was followed in the Dienerian by cooling. In the late Smithian, about 250.5 Ma, there is another temperature rise to high values, and in the Spathian the temperatures were relatively stable, which was followed by cooling at the end of the end of this stage and in the early Middle Triassic there was stabilisation. The thermal maximum of the late Smithian (LSTM) was the hottest interval of the entire Early Triassic, a time when the temperatures of the upper water column reached as high as 38oC, and with SSTs that possibly exceeded 40oC.
Equatorial SSTs of the present were consistently exceeded throughout the record of entire Early Triassic, which suggests a tolerable threshold of temperatures may have been exceeded in the oceans and on land. In the case of C3 plants when temperatures rise above 35oC photorespiration predominates over photosynthesis (Berry & Bjorkman, 1980), not many plants being capable of surviving temperatures that are consistently above 40oC (Ellis, 2010). In animals, when temperatures exceed 45oC protein is damaged, and this is temporarily alleviated by the production of heat shock proteins (Somero, 1995). The critical temperature is much lower for most marine animals, as at higher temperatures metabolic oxygen demand increases as temperature increases and oxygen dissolved in seawater decreases with increasing temperature (Pörtner, 2002), resulting in hypoxemia and the onset of mitochondrial metabolism that is anaerobic and can be sustained for only a short time (Pörtner, H.O., 2001). Consequently, marine animals don't survive for long when the temperature rises above 35oC, especially those with a high performance and high oxygen demand, such as cephalopods (Pörtner, H.O., 2002).
In this paper the authors1 examine the proposition that in the case of such temperature extremes in the equatorial regions some distinct signature should have been left in the fossil record from the Early Triassic. In the Early Triassic there were many fish that have been well preserved in places such as Madagascar, Greenland and British Columbia, the number of well preserved fish fossils being the result of the widespread distribution of anoxic facies that have been known from this time (Wignall et al., 2002). The authors1 found that the fish from this time, especially in the Griesbachian and the Smithian, were very rare in tropical waters, though they were common at higher latitudes in both these periods of time. The rarity of fish in the Early Triassic appears to have reached an extraordinary degree as indicated by units from the Early Triassic such as the dysoxic-anoxic Daye Formation of South China, and such conditions were widespread yet have failed to yield any fossil fish fauna. The lack of ichthyofauna in equatorial regions from this time coincides with the temperature maxima that have been constructed from the 18Oapatite record, the authors1 interpreting this coincidence as recording exclusion from the equatorial regions as a result of the temperatures being above the threshold for the survival of fish in these latitudes.
A contrasting finding is that invertebrates continued to be common in equatorial regions at this time (Galletti et al., 2008), especially the sessile molluscs that had better adapted metabolism that was oxyconforming allowing them to cope with the synergistic stresses of high temperature and low oxygen concentrations (Pörtner, H.O., 2002; Pörtner, H.O., 2010). Marine reptiles also exhibit high aerobic activity and are suggested to probably have had an oxygen-limited thermal tolerance that was relatively low, not being known from the equatorial fossil record until the middle-late Spathian, about 1-2 My after they first appear in the deposits from higher latitudes during the Smithian (Callaway et al., 1989; Cox et al., 1973). Calcareous algae are missing from the fossil record of the equatorial regions for the entire length of the interval from the end-Permian to the Spathian, though they were present at higher latitudes such as Spitsbergen (Wignall, 1998), being suggested to probably reflect their intolerance of the high temperatures, though the abundance of equatorial calcimicrobial carbonates in shelf waters, one of the stand-out features of the Early Triassic (Knoll et al., 2007) is believed to possibly be the result of the much higher tolerance of higher temperatures of photosynthesis by cyanobacteria (Pörtner, H.O., 2002).
It is suggested that terrestrial animals may have been excluded from the equatorial parts of Pangaea by critically high temperatures with land temperatures probably fluctuating to higher than the SSTs that were approaching 40oC. The study by the authors1 indicates that in the Early Triassic tetrapod fossils were generally absent between 30oN and 40oS, with only rare exceptions (Lucas, 1998; Borsuk-Bialynicka et al., 1999), which contrasts strongly with the Middle and Late Triassic at which time they are found at all latitudes. Suitable strata for the preservation of tetrapod fossils is not a contributing factor in the "tetrapod gap". The Buntsandstein of Europe, one of the best known terrestrial formations from the Early Triassic, that has been investigated intensively, indicates that tetrapods are extremely rare in the Induan (lower part), not becoming common until the middle and upper units that have been dated to the late Early to Middle Triassic (Sues et al., 2010). Coinciding with the tetrapod gap of the equatorial regions of Pangaea was a "coal gap" that extended from the end Permian to the Middle Triassic, indicating the lack of peat swamps (Retallack & Veevers, 1996). The formation of peat, that indicated the high level of plant productivity, wasn't reestablished until the Anisian, and then only in the high latitudes of the Southern Hemisphere (Retallack & Veevers, 1996), though gymnosperm forests had appeared earlier, in the Early Spathian, though only at high latitudes of both hemispheres (Galletti et al., 2007; Schneebeli & Hermann et al., 2012). Forests that were conifer-dominated were not established in equatorial regions of Pangaea until the close of the Spathian, coal formation again appearing in equatorial regions in the Carnian, about 15 Ma after they disappeared at the end of the Permian (Retallack & Veevers, 1996). According to the authors1 it is suggested by these signals that the thermal tolerances of many marine vertebrates were exceeded by temperatures in equatorial regions, at least during 2 thermal maxima, though terrestrial temperatures were severe enough to suppress plant and animal abundance for most of the Early Triassic.
It has been suggested that animals with large body size probably had decreased thermal tolerance (Pörtner & Knust, 2007). The nonlethal effects of increased temperature include such things as smaller body size and increased juvenile mortality at higher temperatures (Angilletta, 2009; Sheridan & Bickford, 2011), resulting in a fossil record that is dominated by small individuals. This is a well-known phenomenon in the marine fossil record from the Early Triassic, termed the Lilliput Effect (Twitchett, 2007). The authors1 suggest this effect is a response to high temperatures and should be obvious in assemblages from the equatorial regions especially in the thermal maxima of the Griesbachian and Smithian. Data from marine fossils of equatorial regions, where assemblages of small body and trace fossils, that are confined to these intervals (Metcalfe et al., 2011; Twitchett, 1999), confirms the prediction. It is suggested by the evidence of the Lilliput Effect being confined to the equatorial regions that this was mainly a phenomenon that was controlled by temperature, as small body size of marine invertebrates can also result from low oxygen levels, and though in the Early Triassic marine dysoxia occurred on a global scale (Wignall & Twitchett, 2002) the Lilliput Effect is seen only among marine invertebrates from the equatorial region.
In the Early Triassic the relationship between extinction and global warming can be examined, the rapid rise across the Permian-Triassic boundary coinciding with a mass extinction event, though at the time of crisis absolute temperatures were modest, being less than 30oC (Joachimski et al., 2012). Together with rising temperature, synergistic factors such a spread of anoxia, may have also had an important role in the marine extinction events Hallam & Wignall, 1997;Wignall & Twitchett, 2002). Later in the Griesbachian many holdover taxa from the Permian were lost, such as conodonts, radiolarians and brachiopods, that is suggested by the authors1 to possibly be the result of lethal temperatures, followed by a recovery that was temporary, and radiation in the Dienerian, which was a cooler time. The clearest link between an extinction event and temperature is seen in the LSTM end-Smithian event during which many groups, such as conodonts, bivalves and ammonoids suffered major losses (Stanley, 2009; Orchard, 2007; Rong et al., 2004), as well as losses among the terrestrial tetrapods (Lucas, 1995), suggests this crisis affected a wide diversity of ecosystems.
The greenhouse gas emissions, from either volcanogenic (Sobolev et al., 2011) or thermogenic sources (Svensen et al., 2009) have long been believed to be the ultimate driving forces of the warming at the end-Permian event. This has been found to be the case, both leaving a negative excursion in the record of δ13C, this being the case for both the end-Permian-Griesbachian and the Smithian intervals, though it has not yet been demonstrated that there was a second pulse of volcanism in Siberia during the Smithian. Strong greenhouse conditions that were persistent would be required to maintain high temperatures for about the first 5 My of the Early Triassic. The activity of decomposers such as fungi and bacteria would be greatly increased at high temperatures that would result in the release of large amounts of terrestrial light carbon into the atmosphere (Stanley, 2010) that would consequently form oligotrophic soils that were humus-pore, that have been observed in Amazon Rainforests of the present and soils of Australia and Antarctica that have been dated to the Early Triassic (Retallack & Krull, 1999). As well as the suspension of peat formation at a global scale, elevated rates of decomposition may have resulted in a significant reduction of the burial of carbon on land, that would contribute further to atmospheric CO2 levels that were higher (Broecker & Peacock, 1999).
In the Early Triassic is is likely that high, oscillating temperatures controlled the rate and nature of recovery after the mass extinction event at the end-Permian, as is shown by an inverse relationship between temperature and changes in biodiversity, the loss of both terrestrial and marine vertebrates, that was temporary, and the reduced size of those that survived. No evidence for the amelioration of a tropical warming by a climate thermostat that redistributes warmth to the poles is provided by SSTs that are derived from δ18O data. Taxa may be forced to move from the tropics progressively by extreme global warming, until they reached high latitudes or went extinct. Marine organisms such as vertebrates, that exhibit low oxygen-dependent thermal tolerance, are the first to leave for cooler climes.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|