Sample Selection Recommendations for Biogenic Apatite
Biogenic Apatite (CaPO4) is a molecule made up of calcium, phosphorus and oxygen, which is present in both soft and hard tissues in organisms. For paleoclimatology, paleontology and archaeology research, bones and teeth are the tissues more commonly used due to their preservation potential and commonality within the fossil and archaeological records.
During bone diagenesis, the organic (e.g. collagen) portion of bones and teeth is usually chemically and/or physically altered whilst the phosphate portion often remains unaltered – making it an ideal candidate for isotopic analysis. However, this is not always the case – some samples are more optimal than others.
Burial Environment
Generally, bones and teeth are found in a variety of heated states based on circumstances following death of the individual; ranging from best to worst preserved: non-heated, partially heated and cremated. Those with limited heating are more optimal due to the conservation of the physical structure.
Environmental conditions during burial also greatly impact the quality of the bone, particularly the ambient temperature and humidity of the burial environment – as well as soil pH, hydrology and oxygen. Bones which are deposited in cool and dry environments tend to keep an in-tact structure (limited cracking, surface details in-tact) compared to those which are found in warm and wet environments (bone surface rough and/or not identifiable, structure is falling apart) (Matthiesen et al. 2021). The porosity of the soil is important in preservation – where higher porosity soils (which allow more water and air throughflow) preserve bone structure less effectively than low porosity soils (Hedges and Millard, 1995). Furthermore, studies have shown that bones deposited within more basic soil (e.g. pH < 7) tend to have better preservation than those found in more acidic soils (Pokines and Baker, 2014; other study). Such conditions vary depending on the depth of the bone deposition - those found below the water table are often restricted in water availability and oxygen (the former fueling microbial degradation) and often in a higher preservation state (Surabian, 2012).
Subsampling
Different structures in the human body are developed at different times in our life. For example, teeth and many fundamental bones are developed in the womb or as a baby, and generally remain the same throughout our lifetime; whereas some bones are constantly regenerated throughout one’s lifetime. As a result, analyzing oxygen isotopes in different parts of the body can provide environmental information relevant to the time when they were developed. For example, due to the biomineralization of teeth – they tend to represent discrete units of time. For example, subsampling along the growth axis of vertebrate teeth can provide information on seasonal / annual variation in climate, diet and migration (Zazzo et al. 2012). On the other hand, bones are reworked and therefore tend to present an average over the individual’s lifespan. However, this is not always the case. For example, marine turtles retain regular growth layers within their humerus bones (Snover et al., 2010), providing information on early-life stages through stable isotopes analysis (Tomaszeqics et al. 2015). This type of analysis is known as skeletochronology – where individual growth layers are identified for a time series analysis (Zug et al., 2002).
Thin sections of dodo hindlimb bones demonstrating ontogenetic growth series (adapted from Angst et al., 2017; https://www.nature.com/articles/s41598-017-08536-3)
Bones (when compared to teeth), have higher porosity, higher organic content and smaller crystallite size making them more susceptible to recrystallization and isotopic alteration. Within sample types themselves subsampling is important, different types of bone have different porosities with ‘cancellous (spongy) bone’ being more porous than ‘cortical (compact) bone’. It’s best to collect good cortical bone fragments from larger bones since they’re preserved well. Other larger bones including femur, tibia, upper arm bone, skull plate and jaw are optimal choices. For teeth preferred samples are single complete incisors, canines or molars with all four roots attached.
Contact us today to discuss using biogenic apatite in your research. When you’re ready to submit your samples for analysis, visit the sample submission page for further recommendations and details of other isotope services.
References:
Angst, D., Chinsamy, A., Steel, L. and Hume, J.P., 2017. Bone histology sheds new light on the ecology of the dodo (Raphus cucullatus, Aves, Columbiformes). Scientific reports, 7(1), pp.1-10.
Hedges, R.E. and Millard, A.R., 1995. Bones and groundwater: towards the modelling of diagenetic processes. Journal of archaeological science, 22(2), pp.155-164.
Matthiesen, H., Eriksen, A.M.H., Hollesen, J. and Collins, M., 2021. Bone degradation at five Arctic archaeological sites: Quantifying the importance of burial environment and bone characteristics. Journal of Archaeological Science, 125, p.105296.
Pokines, J.T. and Baker, J.E., 2021. Effects of burial environment on osseous remains. In Manual of Forensic Taphonomy (pp. 103-162). CRC Press.
Snover, M.L., Hohn, A.A., Crowder, L.B. and Macko, S.A., 2010. Combining stable isotopes and skeletal growth marks to detect habitat shifts in juvenile loggerhead sea turtles Caretta caretta. Endangered Species Research, 13(1), pp.25-31.
Surabian, D., 2012. Preservation of buried human remains in soil. US Department of agriculture, Natural Resources Conservation Service. Connecticut.
Turner Tomaszewicz, C.N., Seminoff, J.A., Avens, L. and Kurle, C.M., 2016. Methods for sampling sequential annual bone growth layers for stable isotope analysis. Methods in Ecology and Evolution, 7(5), pp.556-564.
Zazzo, A., Bendrey, R., Vella, D., Moloney, A.P., Monahan, F.J. and Schmidt, O., 2012. A refined sampling strategy for intra-tooth stable isotope analysis of mammalian enamel. Geochimica et Cosmochimica Acta, 84, pp.1-13.
Zug, G.R., Balazs, G.H., Wetherall, J.A., Parker, D.M. and Murakawa, S.K., 2002. Age and growth of Hawaiian seaturtles (Chelonia mydas): an analysis based on skeletochronology.