Australia: The Land Where Time Began

A biography of the Australian continent 

The Carnian Humid Episode, Late Triassic – A Review

A series of papers were published outlining the evidence for a brief episode of climate change from arid conditions to humid then back again that occurred during the Carnian Stage of the Late Triassic. During this period of time comparisons were made with marine and terrestrial biotic changes, mainly extinction, followed by radiation of flora and fauna. This episode was termed, incorrectly, the Carnian Pluvial Event (CPE) by successive authors. Interest in this episode of climate change has been gradually increasing, with new evidence being published, as well as several challenges to the theory. The exact nature of this humid event has remained equivocal, whether it reflects widespread precipitation or effects on a more local scale, as well as its ultimate cause. It has been shown by bed-to-bed sampling of the Carnian in the Southern Alps (Dolomites) that at the initiation of the episode there was a negative carbon isotope excursion persisting for only part of 1 ammonoid zone (A. austriacum). A significantly longer period is represented by the Carnian Humid Episode, however, both environmentally and biotically, which is, according to Ruffell et al. irrefutable. The evidence in the European, Middle Eastern, Himalayan, North American and Japanese successions is the strongest, though not always so clear in South America, Antarctica and Australia. Causes for the humid episode that have been suggested are the eruption of the Wrangellia Large Igneous Province (LIP) and global warming, which caused evaporation to increase in the Tethyan Ocean and the Panthallasic Ocean.

A prevailing view that throughout much of the Triassic Period the climate was predominantly arid or semiarid, at least across Europa and North America, has long been held, based on evidence of ephemeral lakes, rivers, and extreme seasonality, and abundant red-bed facies, with evaporites and aeolian sands (Crowley, 1994; Lucas & Orchard, 2013). In 1989 Simms & Ruffell collated evidence from published sources, as well as observations of Ruffell et al., for a significant increase in humidity that occurred early in the Julian to Tuvalian of the Carnian Age. It appears the onset and cessation of this humid episode was broadly synchronous with significant biotic changes, extinctions as well as diversifications (Simms & Ruffell, 1989). This hypothesis was not immediately embraced by the geological community, arousing significant opposition among some in Europe (Visscher et al., 1994) and North America (P. Olsen, pers. comm. to Ruffell et al., 1995), in spite of increased interest in palaeoclimatic studies throughout the 1990s. There has been a growing body of evidence that has been found over the 2 following decades (which is summarised in this paper) supporting the original hypothesis that the climatic conditions during the Carnian was significantly more humid than that at of the times during the Triassic.

Conclusions

The existence of a significant episode of climatic change, from arid to humid, during the Carnian stage of the Triassic was established by the original investigations more than 2 decades ago (Simms & Ruffell, 1989, 1990)  and identified evidence for this in marine as well as non-marine successions across Europe and eastern North America. According to Ruffell et al., these papers were a catalyst for others to confirm or refute the conclusions reached in the papers based on new data from the area that was originally covered by Ruffell et al. as well as from distant locations further afield. In general there has been strong support for the suggestion of climate change  in Carnian time from diverse evidence sources, across a much larger area than had originally been identified, and possibly with some consensus on its possible cause. The widespread occurrence of siliciclastic units interrupting the predominantly the carbonate successions that are mostly marine, is one of the main lines of evidence from many of the locations that have been newly documented, as in the original papers (Simms & Ruffell, 1989, 1990), with aspects of their clay mineralogy also used as evidence for climate humidity (Roghi et al., 2010; Haas et al., 2014). This can be attributed to weathering and erosion in most instances, and there is an increase in terrigenous runoff that results, though in one instance the switch to clays from deep-water limestones has been attributed to the effects of acidification of the ocean and rise in the CCD (Rigo et al., 2007). Since 1994 it has been indicated by data that has been published, that excepting certain locations, specifically Malaysia and parts of South America and Antarctica, it appears that evidence for the Carnian Humid Episode is more or less global in extent. Though this can seem to contradict observations of the present where climate belts generally shift to the north and south during climate change episodes. In the world of the Triassic, however, the configuration of land and ocean was very different from the present and it is possible that if some trigger affected Panthalassa and Tethys this could have caused a climate change on a global scale. It was considered (Price, 1999) that climate changes of the ice house type are mostly trends in temperature, while changes of Greenhouse type are from arid to humid. The peak of the Greenhouse following the Permian is represented by the Carnian, such that extreme climate changes might be expected at this time.

Not much had been published on the isotope record of the Triassic at the time of the original work of Simms & Ruffell, though the thesis of increased humidity during the Carnian appeared to be supported by the available isotope data. Carbon isotope curves for several locations have been published in recent years. A significant negative carbon isotope excursion has been indicated by these in the mid Julian that interrupted a positive isotope trend through the Carnian (?Dal Corso et al., 2012), with negative carbon isotope excursions being recorded in sections of the Southern Hemisphere (Graphite Peak of Retallack, Veevers & Morante, 1996) Austria (Hornung, 2008) and Himalayas (Hornung, Krystyn & Brandner, 2007). In the Southern Alps (Dal Corso et al., 2012) the carbon isotope excursion lasted for a much shorter period of time than the Carnian Humid Episode, being confined to the Palynological Assemblage B of the earliest Austrotrachyceras austriacum Zone. Contrasting with this the sedimentological evidence for the Carnian Humid Episode here extends into the Tuvalian dilleri and subblatus zones from the base of the Austrotrachyceras austriacum Zone. When comparison is made with a comprehensive review of the Triassic of the North Sea (McKie, 2014) (especially his figs 18 and 21C) an intriguing aspect of the double negative carbon isotope excursion (Retallack, Veevers & Morante, 1996) becomes apparent. A humid episode in the Late Ladinian was identified (McKie, 2014) and then the Carnian Humid Episode discussed here. Ruffell et al. ask if the earlier carbon isotope shift (Retallack, Veevers & Morante, 1996) and the Campil Event (Roghi et al., 2010) could be reflecting these 2 episodes. If this is the case then why did the earlier (?Ladinian) humid episode of the North Sea not affect the biota as dramatically as did the Carnian Humid Episode? Ruffell et al. suggest the answer may be in McKie’s (2014) fig, 18, where the oxygen isotope data (Korte, Kozur & Veizer, 2005) indicates that warmer temperatures and wetter conditions for the Carnian were coincidental, yet there was no similar warming for the earlier Ladinian.

For the possible cause of the Carnian Humid Episode Ruffell et al. alluded to the possible role of volcanism in their original papers (Simms & Ruffell, 1989, 1990); Simms, Ruffell & Johnson, 1994), though they were not able to identify occurrences that were of an appropriate scale or age. As well as the localised occurrences that they identified, volcanic rocks have been identified in subsequent publications that were associated with the Carnian succession in Indonesia, South America and Iberia, though none of these were on a scale commensurate with the Carnian Humid Episode. It has been noted, however, (Xu et al., 2014) that the synchroneity between the onset of the Carnian Humid Episode and the eruption of the Wrangellia LIP in the northeastern Pacific region (Greene, Scoates & Weis, 2008), while it was specifically linked to the carbon isotope excursion, and subsequent period of climate change, to this same event (?del Corso et al., 2012). The global negative carbon isotope excursion, suggestion of acidification of the ocean and subsequent period of global warming, could, according to Ruffell et al., be ascribed to the effects of flood volcanism. Evaporation from the Tethyan and Panthallasic Oceans may have been increased by the episode of global warming and anticyclonic storms generated which fed on moisture-laden air across Pangaea. In some areas of the world the apparent absence of indicators of a humid climate, such as parts of South America, Antarctica, China and northern Siberia, could possibly reflect a failure of this moisture laden air to penetrate these regions. Ruffell et al. suggest the Carnian Humid Episode may therefore be a global event, though its affects are masked in some locations and in other areas may have yet to be resolved.

Sources & Further reading

RUFFELL, A., M. J. SIMMS and P. B. WIGNALL (2015). "The Carnian Humid Episode of the late Triassic: a review." Geological Magazine FirstView: 1-14.