Editor’s comments

Oxygen isotope ratios (Delta oxygen-18) measured from conodont (extinct eel-like creatures) fossils is a reliable proxy for ancient seawater temperatures. Conodonts were widespread throughout the Permian-Triassic period allowing continuous sampling of the same species of conodonts over multimillion year intervals. The authors measured oxygen isotope ratios from conodonts found in the Nanpanjiang Basin, South China, to reconstruct Late Permian to Middle Triassic equatorial seawater temperatures. By analyzing different species of conodonts, some of which lived in very shallow and others in deeper waters. they were able to estimate upper water column temperatures (~70 m water depth) as well as extremely shallow water to estimate sea surface temperatures (SSTs).

Ma = million years ago

Calculation of seawater temperatures from oxygen isotope ratios revealed rapid warming across the Permian-Triassic boundary of 21° to 36°C over 800,000 years reaching a temperature maximum within the Griesbachian followed by cooling in the Dienerian. A second rise to high temperatures is seen in the late Smithian, followed by relatively stable temperatures in the Spathian, cooling at the end of this stage and stabilization in the early Middle Triassic.

The late Smithian Thermal Maximum (LSTM) marks the hottest interval of entire Early Triassic, when upper water column temperatures approached 38°C with SSTs possibly exceeding 40°C. The entire Early Triassic record shows temperatures consistently in excess of modern equatorial annual SSTs. These results suggest that equatorial temperatures may have exceeded a tolerable threshold both in the oceans and on land. Few plants can survive temperatures consistently above 40°C. Similarly, for animals, temperatures in excess of 45°C cause protein damage. However, for most marine animals, the critical temperature is much lower.

Evidence in the fossil record

Extreme equatorial warmth would have left a distinct signature in the Early Triassic fossil record.

• The fossil fish record is exceptionally good in the Early Triassic, with many well-preserved faunas known from locations such as Madagascar,Greenland, and British Columbia. However, the authors’ compilation of fish occurrences reveals that they are very rare in equatorial locales, especially during the late Griesbachian and the Smithian, despite being common at higher latitudes at these times. The general absence of fish fossils in equatorial regions coincides with the temperature maxima reconstructed from the oxygen isotope record, and the authors interpret this coincidence as equatorial exclusion because of inhospitably high temperatures.
• During the same period invertebrates remain common, especially low mobility mollusks with a metabolism that allows them to cope with stresses of high temperature and low oxygen.
• Early Triassic marine reptiles are also not found in equatorial waters until the middle-late Spathian, ~1 to 2 My after their first appearance in higher latitudes during the Smithian. Calcareous algae are also absent from equatorial oceans thoughout the entire end-Permian to early-Spathian interval although they are present in higher latitudes.
• Critically high temperatures may also have excluded terrestrial animal life from equatorial Pangea, and with SSTs approaching 40°C the land temperatures are likely to have fluctuated to even higher levels. The authors’ compilation of tetrapod (four-limbed vertebrates) fossil occurrences reveals them to be generally absent between 30°N and 40°S in the Early Triassic. This is a stark contrast to Middle and Late Triassic occurrences, when they occur at all latitudes.
• There is also a global “coal gap” that indicates the loss of peat swamps during the end-Permian to Middle Triassic .
• A known effect of temperature increase include smaller adult size which will produce a fossil record dominated by small individuals. This is a well-known phenomenon in the Early Triassic marine fossil record and is called the Lilliput effect. The authors cite data from equatorial marine fossils where small body sizes are seen during the high temperature intervals.

The relation between global warming and extinction can be examined in the Early Triassic. The loss of many Permian holdover organisms later in the early Triassic (for example, conodonts and others) may reflect the effect of lethal temperatures.

The clearest temperature extinction link is with the LSTM and the end-Smithian event that saw major losses among many marine groups, including bivalves, conodonts,and ammonoids. Losses among tetrapods on land at the same time suggest that high temperatures affected a broad diversity of ecosystems.

The ultimate driving factor behind the end- Permian warming has been attributed to greenhouse gas emissions, either from volcanoes or massive coal/gas fires. Both are expected to leave evidence in the carbon-13 isotope record, and this is the case for both the end Permian–Griesbachian and Smithian interval, although it has yet to be demonstrated that a second burst of Siberian volcanism occurred in the Smithian.

To maintain high temperatures for about five million years of the Early Triassic requires strong and persistent greenhouse conditions. In the absence of measured atmospheric CO2 for this entire period the authors hypothesize that high temperatures would enhance the activity of fungi and bacteria releasing large amounts of terrestrial light carbon into the atmosphere and consequently forming humus-poor soils as observed in Early Triassic soils of Australia and Antarctica. Together with the absence of peat formation, higher decomposition rates may have led to a reduction in organic carbon burial on land further contributing to higher atmospheric CO2 levels.

Science 19 October 2012:Vol. 338 no. 6105 pp. 366-370