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Palaeoclimate - Karoo vertebrate destructive sampling

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ProposalDescription: 

The end Triassic Mass Extinction (ETE) is hypothesized to have been caused by large-scale, volcanically induced perturbations to the global carbon cycle, which caused rapid global climate change. The Central Atlantic Magmatic Province (CAMP), the volcanic event of interest, covered most of northern Africa, eastern North America, northern South America, and parts of western Europe with approximately 3x106 km3 of basalt flows, making the CAMP the most extensive and voluminous volcanic event on Earth. Mathematical estimations suggest that the CAMP emitted between 8,000-9,000 Gt of carbon as CO2 which impacted global climate causing environmental and ecosystem changes worldwide. Of significant importance is understanding the effects of the CAMP on continental ecosystems. A key record of these effects should be preserved within the Elliot Formation of the Karoo Basin. The Elliot Formation (EF), spanning the end Triassic - early Jurassic, is comprised of continental deposits representing fluvial-overbank deposits. The EF also contains the skeletal remains of many terrestrial vertebrate taxa as well as ichnofossils and fossilized wood. Paleoclimate records can be gleaned from various climate-sensitive chemical proxies including vertebrate fossils. Hydroxylapatite, the crystaline mineral form of calcium phosphate with substitution vacancies [(Ca,REE,Sr,U,Th)10(PO4,CO3)6(OH,F,Cl)2] which makes up skeletal tissue, reflects the water and food consumed during an animal’s lifetime in the oxygen and carbon content of the phosphate and carbonate molecule. In the case of herbivores, the C-isotopic composition reflects the dominant floral makeup while the O-isotopic composition reflects metabolism, seasonality, and the isotopic composition of consumed/living water. Because climate controls the isotopic composition of surface water and flora, isotopic shifts in vertebrate mineral isotope composition reflects shifts in climate and environmental conditions over time. Thus, the stable isotopic composition of vertebrates provides a powerful record of changes to the climate and environment during the volatile Triassic to Jurassic transition and end Triassic mass extinction.

Expanded_Motivation: 

Methodology (short): We will be requesting access to teeth, most of which have already been sampled for Ca-isotope analysis. Teeth grow incrementally and are constantly growing. As such, serial sampling of teeth from the enamel-root junction to the tip can give a sense of seasonal variation (w/dry and hot/cold) experienced over the life of an animal. Although teeth are preferred to bone sampling, histological thin sections and accompanying polished billets can be used. By sampling in conjunction with thin sections, we attempt to accurately sample non-diagenetically altered cortical bone. Although it is possible that some of the isotopic signals may not be used due to diagenetic alteration, any cyclical signal should be considered biogenic and can still inform us of seasonal signals. Several specimens from the Evolutionary Studies Institute at the University of Witwatersrand as well as the National Museum Bloemfontein already have been sectioned for histological study and these specimens can be additionally sampled for isotopes given, they are already destructively sampled. Analyzing these specimens would provide new quantitative climatic and ecological context to existing Elliot Formation vertebrate studies (e.g., Chinsamy, 1993; Warren and Damiani, 1999; Barrett, 2004; Bender and Hancox, 2004; McPhee et al., 2018; Bordy et al., 2020; Viglietti et al., 2020a, b; Chapelle et al., 2021; de Cerff et al., 2021). Sample drilling and preparation Sampling will be conducted by Dr. Celina Suarez, associate professor at the University of Arkansas following sample replication. Dr. Suarez has more than 10 years’ experience sampling vertebrate material. A 0.5-0.1 mm drill bit will be used to drill small lines within the bone cortex, generating ~5 mg of powder. For teeth, enamel will be sampled along the surface and care will be taken to not penetrate the enamel-dentine juncture, generating ~500 µg (0.5 mg) of powder for O-isotope analysis of phosphate and ~ 5 mg of powder for C and O-isotopic composition of carbonate (Figure 1). Sampling will take place at the University of Witwatersrand utilizing a microdrill in the Geosciences building or a portable handheld drill with drill bits in the collections room of ESI. Powders will be captured in 7.5 ml microcentrifuge tubes and transported to the United States for analysis. Phosphates: ~ 500 µg (0.5 mg) of powder will be converted to silver phosphate using methods described in Bassett et al. (2007), Suarez et al. (2014), and Vennemann et al. (2001). Silver phosphate samples are analyzed on a high temperature conversion elemental analyzer (TCEA) attached to an isotope ratio mass spectrometer for oxygen isotopes in the phosphate molecule of apatite at the University of Arkansas Stable Isotope Laboratory (UASIL). Carbonates: Along with phosphates, we will also sample carbon and oxygen isotopes within the carbonate molecule that substitutes for phosphate. 2-5 mg of powder will be cleaned of organics and diagenetic calcite using Koch et al. (1997) methods. Though previous studies have drilled for 10 mg (e.g., Rey et al., 2016), 5 mg of powder is enough for this analysis. Samples will be analyzed via phosphoric acid digestion in a gas bench attached to an isotope ratio mass spectrometer. All isotope work will be conducted at the University of Arkansas Stable Isotope Lab (UASIL). We are requesting access to specific taxa that represent contrasting ecological settings. Ideally, we would like to sample evaporative insensitive taxa (I.e., aquatic taxa such as Sarcopterygii and temnospondyls) and contrast them to fully terrestrial taxa such as dinosaurs, cynodonts and crocodylomorphs. We want to sample teeth whenever possible due to their resistance to diagenesis (Trautz, 1967; Blake et al., 1998; Zazzo et al., 2004a, b). Teeth are the standard for analyzing vertebrate bioapatite for biogenic signals, however, we acknowledge that teeth are either not always available, too small, or too valuable to be sampled. In these cases, the bone cortex can be used successfully if careful screening for diagenesis is completed as has been demonstrated by Rey et al. (2016). We would like to sample at least 3 specimens per taxon and to sample each specimen at least 5 times at approximately 5 mg of powder. This will result in sampling of approximately 63 specimens (we are currently listing 54 specimens, with some specimens still needing preparation). This should result in 0.81 g - 1.58 g of powder total for all Elliot Formation specimens. For sampling along histologic sections, the number of samples will be determined on the thickness of the cortex. At a minimum, samples will be samples 5 times and at a maximum ~20 times, both generating ~ 5 mg of powder. Ideally, we will sample a representative Cynodont from the lower and upper Elliot (e.g Scalenodontoides versus Tritylodon), a representative sauropodamorph (e.g. Plateosauravus versus Massospondylus) and a crocodylomorph (as a non-thermoregulating organism) to address changes in seasonality between the two members. This would result in a maximum of 0.6 g of powder (20 samples per six specimens sampled at 5 mg of powder per specimen). In total, we expect to sample approximately 2.5 g of powder for the entire study. Prior to analysis, 3D digital reproductions of specimens can be made using Next Engine Laser Scanning or an equivalent scanner at the University of Witwatersrand as well as traditional molding and casting. Photo documentation will be made by UA personnel (C. Suarez) prior to and throughout the sampling process. Great care will be taken to ensure all specimens sampled will not be completely destroyed or damaged beyond use, so they can be studied by future scientists (see figure 1). We will sample with a microdrill to create a minimum amount of powder necessary for each analysis (e.g. ~ 0.3 mg for phosphate oxygen-isotope analysis and 3-5 mg for carbonate carbon and oxygen isotope analysis). We are requesting access to specimens who have previously been sampled for calcium isotopes as well as those that have associated histological thin sections. Utilizing specimens that have previously been destructively sampled will allow us to integrate this study into existing destructive Karoo Basin vertebrate research (e.g., Rey et al., 2016; Chapelle et al., 2021; de Cerff et al., 2021; Davechand and Choiniere, in progress). Calcium is the major constituent of the mineral phase of bones and teeth and is almost exclusively derived from food among terrestrial vertebrates. As a result, application of calcium-isotopes are used as a dietary and trophic structure indicator. The addition of traditional stable isotopes, specifically δ13C (dietary indicator) and δ18O (metabolic and water source indicator), offers a powerful tool in determining differences in ecological niches. Plotting C, O, and Ca-isotope values together will allow us to evaluate quantitative changes in those niches through the ETE and into the Jurassic (e.g. Cullen et al., 2022). Statement why this study cannot be done in South Africa: Sampling will take place in South Africa, at the Evolutionary Studies Institute at the University of Witwatersrand where advice and recommendations as well as limitations to sampling can be made in conjunction with the Curator (Dr. Choiniere). Only sample powders will be returned to the University of Arkansas, Fayetteville, AR, USA. The PI, Dr. Celina Suarez will prepare samples for analysis in her wet chemistry laboratory. Once prepared, the samples will be analyzed at the University of Arkansas Stable Isotope Laboratory (UASIL) facility, a state-of-the-art stable isotope facility that has all necessary equipment (mass spectrometers) available for analysis (https://isotope.uark.edu/contact/). UASIL is the flagship university for the State of Arkansas and receives over $1 million a year in sample analyses and data published by UASIL is published in many peer-reviewed journals every year. The lab is well prepared to analyze samples generated by the PIs.

ApplicationDate: 

Tuesday, August 23, 2022 - 12:16

CaseID: 

19356

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