Salt Range (Pakistan)
During four field campaigns between 2007 and 2010, researchers of the Palaeontological Institute and Museum collected numerous invertebrate fossils and rock samples from the Early Triassic of the Salt Range and neighbouring Surghar Range in northwest Pakistan. The Salt Range and Surghar Range are key reference areas for the Permian and Triassic, and the fossiliferous layers of the marine Mianwali Formation have been extensively studied since the mid-19th century. Up to now, a diverse invertebrate fauna and some vertebrate remains (disarticulated chondrichthyan and osteichthyan fossils, trematosauroid 'amphibians') is known. One of the most complete sections of the Mianwali Formation is exposed at Nammal Gorge in the western part of the Salt Range (see photo below, courtesy of Thomas Brühwiler).
Although I did not have the opportunity to join for field work in the Salt Range, I kindly received several bed-controlled rock samples from the Mianwali Formation of Nammal Gorge for oxygen isotope analyses, a method to deduct relative changes of ancient seawater temperature. For this study, we dissolved the carbonate rock samples with acetic acid and then isolated phosphatic microfossils from the residues. The layers are often rich in conodont fossils. Conodonts are small, extinct animals that inhabited the seas between the Cambrian and the Triassic (ca. 540 - 200 million years ago). Usually, only the tooth-like elements of their complex feeding apparatus are preserved. Apart from conodonts, the residues also frequently yield fish teeth and scales. We performed oxygen isotope analyses predominantly on conodonts and some selected actinopterygian teeth.
The results of our isotope analyses were published in the journal Nature Geoscience in 2013. The preservation of the Salt Range conodonts and fish teeth is exceptional (CAI = 1), unlike many other sites, suggesting that the trends in our O-isotope data are robust. Based on our data, we show that seawater temperatures during the Early Triassic were generally relatively warm and subjected to large fluctuations. Most notably, a relatively short cooling phase during the early Smithian interval was interrupted by a pronounced warming (~6°C), yielding peak temperatures during the middle Smithian interval. Another cooling phase (~8°C) is noted across the Smithian-Spathian substage boundary of the Early Triassic. Warmer temperatures are correlated with increased taxonomic turnover rates in ammonoids, followed by a severe extinction event for both ammonoids and conodonts in the late Smithian interval. The trends in the oxygen isotope curve from the Salt Range show a significant correlation with those seen in the global carbon isotope curve. The negative shifts in those records have been linked to bursts of CO2 from the Siberian Traps Large Igneous Province. Our data thus confirm that the Early Triassic was a time of major climatic changes, which had severe consequences for the biota. The following figure from our paper summarizes our results.
In a more recent study, we presented more oxygen isotope data from Nammal for the Smithian-Spathian boundary interval. The conodonts were analysed using SHRIMP. In our study (Goudemand et al. 2019), we showed that the extinction event was preceeded by a prolonged warm interval, but coincided with the onset of a fast cooling event.
As mentioned above, the residues from Nammal also yielded a plethora of ichthyoliths (osteiichthyan and chondrichthyan teeth and scales). A taxonomic study on those fossils is currently in progress.