Interdisciplinary studies | Zheng Isotope Geochemistry Laboratory

Natural stable calcium isotope ratios: a new gold standard for bone balance?

Natural stable calcium (Ca) isotope ratios are novel measures of bone calcium balance, based on the phenomenon of isotope fractionation where lighter isotopes are preferentially incorporated into bone. Calcium has many stable isotopes, the most abundant of which is ⁴⁰Ca, comprising ≈97% of calcium found in nature. ⁴⁴Ca is the next most abundant at ≈2%, and the other 4 stable isotopes, ⁴²Ca, ⁴³Ca, ⁴⁶Ca, and ⁴⁸Ca, comprise the remaining 1% of calcium found in nature. Shown herein, the change in the calcium isotope ratio δ^44/42Ca measured in a simple serum or urine sample reflects bone calcium balance, where higher δ^44/42Ca values indicate balance tipping toward net bone calcium gain, and lower δ^44/42Ca values indicate balance tipping toward net bone calcium loss. This reflects bone balance of the whole skeleton. Bone calcium balance by δ^44/42Ca does not specifically reflect differences in bone matrix mineralization, bone turnover, or bone volume, which are assessed by histomorphometric analyses of bone biopsy specimens.

Applying strontium (87Sr/86Sr) isotope analyses in collaboration with Indigenous communities to advance repatriation efforts and engage in scholarship that is relevant to these communities

Strontium (87Sr/86Sr) and oxygen (δ18O) isotopes preserved within plants and animals reflect the geographic region where they originated. Isotope analyses could therefore be innovatively used to help repatriate ancestors and associated funerary objects and items of cultural patrimony made from biological materials under the Native American Graves Protection and Repatriation Act (NAGPRA). This ability would be valuable in cases where information on the location of their removal (provenance) is missing. Estimating cultural affiliation without provenance is difficult and can delay or prevent repatriation. My research aims to address this challenge by modeling baseline isotope values – an isoscape - within Minnesota to link isotope values preserved in biological materials with possible regions of origin. This approach reframes the aim of archaeological research toward restorative practices and away from extractive practices.

This research will develop an 87Sr/86Sr isoscape of Minnesota and combine it with a pre-existing δ18O isoscape to produce a model that can estimate the geographic origin of archaeological organic remains that lack specific provenance information. Proportions of strontium isotopes will vary across the landscape according to differences in the age and type of bedrock, and proportions of oxygen isotopes will vary according to differences in precipitation patterns. Strontium and oxygen isotopes are both incorporated into plants, animals, and humans as a byproduct of their subsistence, and thus reflect the organism’s geographic origin. Estimating geographic origin based on 87Sr/86Sr and δ18O can only be applied in situations where there is a pre-existing model of a region’s natural isotope variability, or isoscape. Estimates of geographic origin are more accurate and specific if they consider two isotope systems – strontium and oxygen – both of which reveal different kinds of information about potential geographic origins. For that reason, this project combines the strontium isoscape developed through vegetation sampling with a pre-existing oxygen isoscape into a single model that can predict geographic origin using a bivariate probability density function. The model will then be evaluated based on its ability to accurately estimate the geographic origin of archaeological faunal samples of known geographic origin.