Strontium and Oxygen Isotope Analysis Reveal Late Cretaceous Sharks Teeth in Iron Age Strata in Southern Levant
Thomas Tütken, Michael Weber, Irit Zohar, Hassan Helmy, Nicolas Bourgon, Omri Lernau, Klaus Peter Jochum and Guy Sisma-Ventura
Summarized by Colton Carrier, who is a senior at University of South Florida studying for his Bachelor of Science in geology.
What data were used: Fossil teeth were found in the City of David, Jerusalem (Figure 1) from two different categories of oceanic fish: bony fish and cartilaginous fish (sharks). The bony fish included Conger conger, a fish residing in deep water at approximately 1000 feet, and Sparus aurata, a coastal water fish. Both fish are extant and currently live in the Mediterranean. What researchers found in another site in Gilead and from City of David, Jerusalem was six Sparus aurata teeth. Researchers also found 10 shark teeth in City of David: teeth of Centrophorus granulosis, a deep-sea shark from the Mediterranean, and Carcharhinus plumbeus, a neustonic (living at the water surface) shark from Red Sea. Because teeth preserve different isotope levels, the extant fish and shark teeth were sampled to construct a database of the strontium and oxygen isotope levels. This data was collected because the shark teeth and other teeth in sediments from the Southern Levant (which is an area of Palestine/Israel) are of different ages and the isotopes preserved can inform us of their true ages. To complement their dataset, they also used a dataset of strontium and oxygen isotope values derived from modern fish: Centophorus granulosis, a deep-sea filter feeding shark, and modern Conger conger, to use as a reference to the shark teeth found.
Methods: First, the researchers identified the teeth and then did isotope analysis on the dental tissue, where they measured the amount of strontium and oxygen isotopes. They then did a screening for diagenetic alteration, or how the teeth were changed through the fossilization process, which consisted of X-ray diffraction, total organic carbon content determination, and LA-ICP-MS which is spectrometry (or shooting lasers at an object and seeing what rebounds back). Using LA-ICP-MS, they were able to perform the isotope analyses. Following this, they did a linear discriminant analysis to match the found fossil teeth to the teeth in the dataset in order to effectively determine the ages of the teeth.
Results: The results determined the age of the shark teeth found in the City of David to be of Late Cretaceous (100mya-65mya). They found the bony fish (Sparus aurata) had a lower apatite crystallinity than the shark teeth samples, meaning they were younger. Fluorapatite was the primary phase of mineral, this indicates diagenetic uptake into tooth tissue, which means the shark teeth had a longer burial time/older age. Total Organic Carbon (TOC) content of the shark teeth is 40 times lower than other pelagic sharks like the Great White Shark, and 8 times lower than the bony fish teeth found, meaning the shark teeth have been decaying longer, also lending evidence for the age of the fossil teeth. Trace elements found using spectrometry analysis were uranium and neodymium, which are typical of fossilized shark teeth. With all of the data, the shark teeth were estimated to be approximately 86.5 to 76.3 million years in age. The S. aurata teeth were similar to modern fish.
Why is this study important: Late Cretaceous shark teeth were found in Iron Age layer, 8th-9th century BCE. There is no clear answer as to how the shark teeth got there, so this raises interesting questions as to how humans may have interacted from fossils during this time.
The big picture: Other animals can interwork fossils into new sediments, just as the authors scientifically assume that humans did to these teeth in the Iron Age. This should be a bias that should be considered in future investigations. This study is also an important interdisciplinary analysis of archaeology and paleontology that may help us begin to learn more about how humans viewed fossils and if they were moved or collected frequently in the past.
Citation: Tütken, T., Weber, M., Zohar, I., Helmy, H., Bourgon, N., Lernau, O., Jochum, K.P., and Sisma-Ventura, G., 2020, Strontium and Oxygen Isotope Analyses Reveal Late Cretaceous Shark Teeth in Iron Age Strata in the Southern Levant: Frontiers, https://www.frontiersin.org/articles/10.3389/fevo.2020.570032/full