22-9 Use of Laser Ablation ICP-MS of microvertebrate assemblages for log Ba/Ca vs log Sr/Ca trophic-level indicators allows for evaluation of paleoecosystems in deep-time

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives

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Abstract:

Trophic level proxies for paleontological-aged material is a goal of paleontologists investigating paleoecosystems through time. However, traditional trophic-level indicators such as N-isotopes are impractical due to collagen’s instability during fossilization beyond ~10,000 years. Revolutionary methods such as the oxidation-denitrification method of N-isotope analysis and Ca-isotopes require significant material, are expensive, or require extensive lab preparation times and techniques. Microvertebrate assemblages (MVAs - vertebrate assemblages of material <2 mm in size) can be used to remedy sample size issues, containing many specimens and taxa within a single assemblage, however their small individual sizes limit their utility for some new proxy methods. The use of log Ba/Ca vs log Sr/Ca is a method used in modern and archeological-aged studies that utilizes the decrease in Ba/Ca and Sr/Ca ratios in the digestive processes at higher trophic levels and has recently been successfully applied to Mesozoic-aged material using solution-based ICP-MS or ICP-ES. Here, we combine these tools and materials by using laser ablation (LA) ICP-MS to analyze log Ba/Ca vs log Sr/Ca of MVAs. An advantage of LA-ICP-MS is that analysis is easy, fast, and the rare earth elements (REEs) can be analyzed from the same laser ablation pit and used for assessment of diagenesis. We tested two vertebrate microassemblages as a pilot study from the Cenomanian Mussentuchit Member of the Cedar Mountain Formation and the Turonian Smokey Hollow Member of the Straight Cliffs Formation. Depth profiles were created for each sample and only stable signals of Ba, Ca, and Sr within the lowest REE concentration profile were integrated. We find that samples with less than SREE_N concentrations < ~1000 ppm reveal hypothesized trophic relationships. Within the Mussentuchit Member, faunivores like Siats and tyrannosaur-morphotype teeth plot as secondary consumers while organisms such as Iani and Eolambia plot as primary consumers. Organisms with unknown diet such as the theropod tooth-morphotype Paranychodon plot as primary consumers. These results are broadly consistent with trophic patterns observed from earlier solution ICP-MS and ICP-ES analyses of vertebrates from the Oldman Formation. Additional comparison of samples also analyzed via ICP-ES and solution ICP-MS, as well as with new proxies like Ca-isotopes, will reveal if this technique is a valid, fast, and affordable method for assessing trophic structure in deep-time.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10534

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