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1. How teeth record and attenuate seasonal signals

   https://doi.org/10.1016/j.jas.2024.106148  

   Green, Daniel R; Smith, Tanya M; Olack, Gerry; Williams, Ian S; Tafforeau, Paul; Colman, Albert S; Uno, Kevin T (January 2025, Journal of Archaeological Science)

   Variability of oxygen isotopes in environmental water is recorded in tooth enamel, providing a record of seasonal change, dietary variability, and mobility. Physiology dampens this variability, however, as oxygen passes from environmental sources into blood and forming teeth. We showcase two methods of high resolution, 2-dimensional enamel sampling, and conduct modeling, to report why and how environmental oxygen isotope variability is reduced in animal bodies and teeth. First, using two modern experimental sheep, we introduce a sampling method, die-saw dicing, that provides high-resolution physical samples (n = 109 and 111 sample locations per tooth) for use in conventional stable isotope and molecular measurement protocols. Second, we use an ion microprobe to sample innermost enamel in an experimental sheep (n = 156 measurements), and in a Pleistocene orangutan (n = 176 measurements). Synchrotron and conventional μCT scans reveal innermost enamel thicknesses averaging 18 and 21 μm in width. Experimental data in sheep show that compared to drinking water, oxygen isotope variability in blood is reduced to 70–90 %; inner and innermost enamel retain between 36 and 48 % of likely drinking water stable isotope range, but this recovery declines to 28–34 % in outer enamel. 2D isotope sampling suggests that declines in isotopic variability, and shifted isotopic oscillations throughout enamel, result from the angle of secretory hydroxyapatite deposition and its overprinting by maturation. This overprinting occurs at all locations including innermost enamel, and is greatest in outer enamel. These findings confirm that all regions of enamel undergo maturation to varying degrees and confirm that inner and innermost enamel preserve more environmental variability than other regions. We further show how the resolution of isotope sampling — not only the spatial resolution within teeth, but also the temporal resolution of water in the environment — impacts our estimate of how much variation teeth recover from the environment. We suggest inverse methods, or multiplication by standard factors determined by ecology, taxon, and sampling strategy, to reconstruct the full scale of seasonal environmental variability. We advocate for combined inverse modeling and high-resolution sampling informed by the spatiotemporal pattern of enamel formation, and at the inner or innermost enamel when possible, to recover seasonal records from teeth. 

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2. Extreme isotopic heterogeneity in Samoan clinopyroxenes constrains sediment recycling

   https://doi.org/10.1038/s41467-021-21416-9  

   Adams, Jenna V.; Jackson, Matthew G.; Spera, Frank J.; Price, Allison A.; Byerly, Benjamin L.; Seward, Gareth; Cottle, John M. (December 2021, Nature Communications)

   null (Ed.)

   Abstract Lavas erupted at hotspot volcanoes provide evidence of mantle heterogeneity. Samoan Island lavas with high 87 Sr/ 86 Sr (>0.706) typify a mantle source incorporating ancient subducted sediments. To further characterize this source, we target a single high 87 Sr/ 86 Sr lava from Savai’i Island, Samoa for detailed analyses of 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotopes and major and trace elements on individual magmatic clinopyroxenes. We show the clinopyroxenes exhibit a remarkable range of 87 Sr/ 86 Sr—including the highest observed in an oceanic hotspot lava—encompassing ~30% of the oceanic mantle’s total variability. These new isotopic data, data from other Samoan lavas, and magma mixing calculations are consistent with clinopyroxene 87 Sr/ 86 Sr variability resulting from magma mixing between a high silica, high 87 Sr/ 86 Sr (up to 0.7316) magma, and a low silica, low 87 Sr/ 86 Sr magma. Results provide insight into the composition of magmas derived from a sediment-infiltrated mantle source and document the fate of sediment recycled into Earth’s mantle. 

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3. The significance of enamel thickness in the teeth of Alligator mississippiensis and its diversity among crocodyliforms: Enamel thickness in crocodyliforms

   https://doi.org/10.1111/jzo.12707  

   Sellers, K. C.; Schmiegelow, A. B.; Holliday, C. M. (July 2019, Journal of Zoology)

   Enamel is the hardest tissue in the vertebrate body. Although variation in enamel microstructure is often linked with diet, the gross proportions of the tissues that compose vertebrate teeth remain relatively unexplored in reptiles. To investigate the patterns of enamel thickness in crocodyliforms, we used micro‐computed tomography scanning to evaluate enamel thickness in teeth of Alligator mississippiensis from rostral, intermediate and caudal locations in the tooth row from an ontogenetic range of animals. We also evaluated enamel thickness in the derived teeth of several extinct crocodyliforms with disparate craniodental morphologies. Our data show that enamel thickness scales isometrically with skull length. We also show that enamel is relatively thicker in caudal teeth than teeth in more rostral positions, concordant with the higher bite forces they experience during feeding. We compared our data with existing enamel thickness data reported from dinosaurs and mammalian taxa to find that archosaurs have markedly thinner enamel than most mammals. These findings serve as a basis for future investigations into the diversity and function of the proportions of dental tissues. 

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4. Oxygen isotope analyses of mammalian tooth enamel confirm low seasonality of rainfall contributed to the African Humid Period in Somalia

   Reid, R.E.B.; Jones, M.B.; Brandt, S.A.; Bunn, H.T.; Marshall, F. (January 2019, Palaeogeography palaeoclimatology palaeoecology)

   During the African Humid Period (AHP), between ~14 and 5 ka, north and eastern Africa were much wetter and greener than they are today. Although the AHP has long been attributed to an increase in rainfall driven by orbital forcing, many details regarding the timing, pacing, and contributing moisture sources remain to be determined, especially for eastern Africa. Recent research suggests that both Atlantic and Indian Ocean moisture contributed to the AHP in eastern Africa. Large mammalian faunas from two Late Pleistocene/Holocene rockshelter sites in southern Somalia, Gogoshiis Qabe and Guli Waabayo, provide an unusual opportunity to investigate the AHP in an area of eastern Africa that is orographically isolated from Atlantic Ocean moisture. To track changes in aridity at these sites, we used the oxygen isotope aridity index, which exploits the difference in tooth enamel oxygen isotope (δ18O) values between evaporation insensitive obligate drinkers and evaporation sensitive non-obligate drinkers to calculate a water deficit value. Water deficit values calculated from dik-dik (Madoqua spp.; non-obligate drinkers) and warthog (Phacochoerus spp.; obligate drinkers) tooth enamel δ18O values at Gogoshiis Qabe and Guli Waabayo are in agreement and progressively increase toward the present. Additionally, two of three direct dated serially-sampled warthog teeth (ID 4032 and 4033) from Guli Waabayo demonstrate low seasonality of rainfall during much of the AHP (range in δ18O ≤ 1.8‰). One tooth (ID 4035) with high amplitude variability in δ18O values (3.1‰) dates to 8470 ± 66 cal yr B.P., a period identified as a lowstand in several lake level records. Our results suggest that regions isolated from Atlantic Ocean moisture likely experienced a less-pronounced AHP than those receiving moisture from multiple sources but indicate less seasonal variability than present. Our findings also support the presence of climate variability within the AHP. 

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5. Biologically available Pb: A method for ancient human sourcing using Pb isotopes from prehistoric animal tooth enamel

   https://doi.org/10.1016/j.jas.2020.105079  

   Samuelsen, John R.; Potra, Adriana (March 2020, Journal of archaeological science)

   This study analyzes Pb isotopes combining biological (ancient human and prehistoric animal teeth) and geological (soil leachate, whole rock, and rock leachate) samples to determine the origins of prehistoric skeletal elements. It exemplifies how the biologically available Pb method assesses the early lifetime locations of ancient human populations using prehistoric animal teeth and the multivariate/linear nature of Pb isotope data. Lead isotopes provide a valuable technique, in part due to the correlation between their six stable isotope ratios. Other studies have used Pb isotopes for similar purposes, but no clear method for determining a local range has yet been formally defined and tested. The biologically available Pb method uses many prehistoric animal tooth enamel samples to establish a baseline for local ratios in the region, then compares their ratios’ linear patterning to human remains to test if they are non-local. The case study compares Pb isotopes from prehistoric animal teeth, human teeth, and whole rocks from southwest Arkansas. These results are compared to animal samples from Louisiana and Mississippi and human data from Illinois and New Mexico. Soil leachates, Pb concentrations of tooth enamel, and trace element analysis are used to assess contamination. Comparisons to southwest Arkansas whole rock Pb isotope ratios suggest they are too variable to be used for direct comparison to ancient human remains, illustrating that prehistoric animal teeth are more appropriate for direct comparison to prehistoric human teeth. The biologically available Pb method provides a key analysis tool needed for studies of ancient human sourcing. 

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