5.3 Geographical Isotope Analysis
Isotope ratios are not just influenced by an individual's diet. This lesson discusses how geographically linked isotopes, particularly strontium, oxygen, and lead, can be used to investigate mobility and geographical origin in the past. This method is even more intensive than dietary isotope analysis, and requires significant background understanding of earth sciences, geology, and taphonomic processes to wield effectively. This lesson will cover the core principles of the method, as well as touch on the requirements to conduct it and its many uses.
Isotope Geology
In the same way that carbon and nitrogen isotopes are carried along the food webA complex network of feeding relationships between organisms, used in isotope studies to trace dietary inputs. from consumer to consumer, so do isotopes associated with geology and climate. Oxygen is the most obvious of these, a fundamental element required for countless biological processes.
Oxygen is predominately incorporated into plants and animals through water, and travels through the food web before becoming stored in hard tissueThe mineralised tissues in the body, such as bone, enamel, cementum, and dentine, which survive in the archaeological record.. Oxygen is linked to the climate, rainfall, and temperature of the area it was ingested in, as different isotopes fractionate throughout the water cycle and become more or less abundant in different regions. Strontium and lead are more complex.
Strontium is elementally similar to calcium, a fundamental element for constructing the mineral component of our bodies among other things. This means that strontium can follow and replace calcium through the body, ultimately becoming stored in its stead. Strontium is directly linked to the bedrock of geographical regions, entering the soil through cycles of weathering and erosionThe process by which rocks and soil are worn away, which contributes to the geological incorporation of isotopes like strontium and lead. before being incorporated into plants, then the animals that eat them.
Lead behaves somewhat similarly to this, and can also be weathered and eroded into soil from rocks. From there it is incorporated into human tissue equivalent to the amount of exposure, with natural lead only present in trace amounts and the advent of metallurgy and then the industrial revolution creating huge turning points in lead exposure due to its abundance.
One notable departure from dietary isotope analysis is the substantial effects of taphonomic processes on geological isotopes. Due to the abundance of oxygen, strontium, and sometimes lead in burial environments, the ratios from an individual's life can be overwritten, averaged out, or completely changed by diagenesisThe physical and chemical changes that occur in bone or teeth after burial, often altering isotope values. after death. This is because bone and dentineThe dense, bony tissue forming the bulk of a tooth beneath the enamel, used in isotope and DNA studies. are relatively porous and collagenous. The lower the mineral component of a hard tissue, the less resistant it is to most taphonomic changes. This allows oxygen, strontium, and lead from burial soils to leech into tissue and display ratios that do not represent life. Due to this, enamelThe hardest tissue in the body, forming the outer layer of teeth, resistant to diagenetic change and ideal for isotope and DNA analysis. is the preferred tissue to measure for geographical isotope analysis, providing data that is least likely to have undergone alteration.