Also of interest to earth scientists is the Sr isotopic composition of seawater, which has varied through geologic time. In this case, variability is set up by the competing rates of sea-floor production, and weathering of continental landmasses. Marine organisms which produce calcium carbonate from seawater will inherit the Sr isotopic composition of the global ocean at that point in time and can be useful for dating sediment from the sea floor.
Archeologists use the isotope ratios of strontium to determine residential origins and migration patterns of ancestral humans. The human body incorporates Sr by way of diet. Since Sr isotope ratios in soils, rocks, and waters vary widely in nature, and are not appreciatively fractionated by biologic processes, the assumption is that the isotope values for strontium in bone and tooth enamel will reflect those in the portion of the biosphere in which an individual lived.
Thus, strontium isotope composition provides links to the land where food was grown or grazed. Depending on the needs of the researcher, our lab extracts Sr by acid digestion; we can digest materials completely, or we can leach readily available material via partial extraction.
Stable isotope analysis is a scientific technique which is used by archaeologists and other scholars to collect information from an animal's bones to identify the photosynthesis process of the plants it consumed during its lifetime. That information is enormously useful in a wide number of applications, from determining the dietary habits of ancient hominid ancestors to tracing the agricultural origins of seized cocaine and illegally poached rhinoceros horn.
All of the earth and its atmosphere is made up of atoms of different elements, such as oxygen, carbon, and nitrogen. Each of these elements has several forms, based on their atomic weight the number of neutrons in each atom. For example, 99 percent of all carbon in our atmosphere exists in the form called Carbon; but the remaining one percent carbon is made up of two several slightly different forms of carbon, called Carbon and Carbon Carbon abbreviated 12C has an atomic weight of 12, which is made up of 6 protons, 6 neutrons, and 6 electrons—the 6 electrons don't add anything to the atomic weight.
Carbon 13C still has 6 protons and 6 electrons, but it has 7 neutrons. Carbon 14C has 6 protons and 8 neutrons, which is too heavy to hold together in a stable way, and it emits energy to get rid of the excess, which is why scientists call it " radioactive.
All three forms react the exact same way—if you combine carbon with oxygen you always get carbon dioxide , no matter how many neutrons there are. There are always one hundred 12C atoms to one 13C atom. But, the ratio of the forms of carbon gets altered as part of the photosynthesis process. Plants that live in regions with lots of sun and little water have relatively fewer 12C atoms in their cells compared to 13C than do plants that live in forests or wetlands.
Scientists categorize plants by the version of photosynthesis they use into groups called C3, C4, and CAM. In other words, if you can determine the ratio of 12C to 13C that is stored in an animal's bones, you can figure out whether the plants they ate used C4, C3, or CAM processes, and therefore, what the environment of the plants was like. In other words, assuming you eat locally, where you live is hardwired into your bones by what you eat. That measuring is accomplished by mass spectrometer analysis.
Carbon is not by a long shot the only element used by stable isotope researchers. Currently, researchers are looking at measuring the ratios of stable isotopes of oxygen, nitrogen, strontium, hydrogen, sulfur, lead, and many other elements that are processed by plants and animals. That research has led to a simply incredible diversity of human and animal dietary information. The very first archaeological application of stable isotope research was in the s, by South African archaeologist Nikolaas van der Merwe , who was excavating at the African Iron Age site of Kgopolwe 3, one of several sites in the Transvaal Lowveld of South Africa, called Phalaborwa.
Van de Merwe found a human male skeleton in an ash heap that did not look like the other burials from the village. The skeleton was different, morphologically, from the other inhabitants of Phalaborwa, and he had been buried in a completely different manner than the typical villager.
The man looked like a Khoisan; and Khoisans should not have been at Phalaborwa, who were ancestral Sotho tribesmen. As noted previously, the radiogenic isotope 87 Sr is a product of the radioactive decay of 87 Rb. Weathering releases Sr from bedrock into the local environment where it is taken up by plants and then ingested by animals in their food.
Recently, maps of predicted Sr isotopic variation in the environment have been published for the U. Within humans, Sr can substitute for calcium and is found primarily in bones and teeth.
There is in essence no isotopic fractionation between Sr in source e. Similar to Sr, the lead isotope method [ 70 ] takes advantage of radioactive decay, time, and the relative concentrations of elements specifically Pb, Th, and U in the environment. The radiogenic isotopes Pb, Pb, and Pb are derived from the radioactive decay of U, U, and Th, respectively. These anthropogenic factors must be considered when creating reference samples datasets or Pb isoscapes [ 71 ] to ensure both spatial and temporal comparisons are appropriate for the provenancing questions being asked.
At present there are relatively few published Pb isoscapes, for the U. However, additional reference datasets of Pb isotopic variations in human tissues are available, e. Like Sr, Pb can substitute for calcium in bone and teeth and there is generally no isotopic fractionation between element source and human tissue. In the space available for this article it is impossible for us to provide a detailed explanation of isotope analysis and the available methods, instrumentation, etc.
Instead, we provide a brief overview of some common types of instrumentation and highlight the importance of standards in analysis. The isotope ratios of the bio-elements are typically measured via IRMS. Samples must be converted into simple gases for analysis: i.
Today, most samples are converted to gas immediately before introduction to the mass spectrometer through the use of various peripherals and a carrier gas He in what is known as continuous flow-IRMS CF-IRMS. Sample analysis via CF-IRMS can be divided into four steps: 1 Combustion or thermal conversion of a sample into simple gases using an elemental analyzer EA ; 2 introduction of the gases into the ion source of the mass spectrometer via the interface; 3 ionization of the gas molecules followed by separation and detection of the ions in the mass spectrometer; and 4 evaluation of the raw data.
To ensure all isotopes of an element are counted, multiple masses must be monitored simultaneously — for example, N 2 containing 14 N 14 N mass 28 versus 14 N 15 N mass A simple schematic of the analysis of bio-elements e. Recently a new method for isotope analysis of bio-elements has been introduced: isotope ratio infrared spectroscopy IRIS [ 79—81 ]. The technique relies on the characteristic absorption of light by different isotopologues of gas to measure isotope ratios.
Isotope analysis via MC-ICP-MS involves the ionization of a sample usually in liquid form through the use of a high-energy plasma discharge. However, most analysts include an international reference material, such as SRM, in analytical sequences for quality control purposes — i. Secondary reference materials have been carefully calibrated to the primary materials; these secondary materials are available for purchase by laboratories, but typically in limited quantities to prevent their exhaustion.
For day-to-day operation, analysts will typically develop in-house laboratory standards that are calibrated to the secondary reference materials. Historically normalization was calculated as a one-point or offset correction based on multiple measurements of a single laboratory standard.
Today, recommended good practice for forensic isotope analysis advocates the use of at least two laboratory standards for normalization to generate a stretch-shift or slope-intercept correction [ 75 , 84 , 85 ].
To address this, some recent publications have presented recommended terminology and data presentation guidelines for reporting the results of bio-element isotope analysis.
These include recommendations for forensics [ 84 ], ecology [ 86 ], and archaeology [ 87 , 88 ]. To date, however, we are unaware of any published guidelines for reporting the results of trace metal isotope analysis. While isotope ratios can be measured in all human tissues and bodily fluids, forensic applications are often constrained by the tissues available for sampling as well as the specific needs of a particular case investigation.
Time and funding are additional factors that may constrain the types and extent of isotope analysis completed. However, in unidentified remains cases, hair and nails may not preserve or be retained following autopsy or skeletal analysis, leaving only teeth and bone available for sampling.
Hair and nail keratin and bone collagen are the most commonly sampled proteinaceous tissues, whereas bioapatite from teeth and bone is the most commonly sampled biomineral tissue.
Hair and nails are considered serial recorders in that they provide an incremental record of recent diet and residence. Although various factors such as health and age can influence growth rates of keratin, hair grows on average 0. Sub-samples of hair and nails can be used to reconstruct a travel and dietary history for an unidentified decedent. Individuals who are sedentary may show similar isotope ratios throughout serial sections of hair or nails, whereas a traveler may show values that change regularly.
For incremental studies of hair and nails, it is important to record the directionality of the tissue so that changes in isotopic composition can be examined chronologically. Hair is the preferred sample over nails given that its growth rate and metabolic inputs are better understood.
Bone tissue is constantly remodeled throughout life. However, the remodeling rate is not constant and varies throughout the lifespan, with faster bone turnover times in young adults compared to elderly adults. Bones within the skeleton also remodel at different rates. Collagen is composed of both nonessential and essential amino acids, the latter of which derive from dietary protein [ 93 , 94 ].
Hydroxyapatite, or bioapatite, is the biomineral component of tooth enamel and bone. While both the carbonate and phosphate fractions can be analyzed, the former is preferred due to its straightforward sample preparation procedures. Unlike carbon isotopes of bone collagen, which are biased toward dietary protein, bioapatite forms from dissolved bicarbonate in the blood and provides a record of consumption of carbohydrates, lipids, and protein not used in tissue synthesis of bone collagen i.
In addition, oxygen isotopes of the carbonate in bioapatite reflect residence patterns during the time of tissue formation. Thus, oxygen isotopes of teeth reflect region-of-origin during infancy and childhood, whereas bone provides a record of residence history during the last several years of life.
Bioapatite in bone can potentially produce misleading results due to turnover and mixed residence pattern signals. However, the data will be useful for cases where an individual died recently after migrating or traveling to a new area since their isotope profiles will still reflect their previous residence location.
In context of unidentified human remains cases, diagenesis can be defined as the chemical alteration of biological remains due to the interaction with the environment, including soil and groundwater [ 9 ]. For most cases, diagenesis is not a major concern as the hard tissues of the skeleton are resilient against chemical alteration for short time intervals usually days to months after death.
However, for human remains that have been exposed or buried for years to decades, significant degradation may occur, especially to hair and nails, and to a lesser degree, bone and teeth. Sample quality assessment metrics established for keratin, collagen, and bioapatite provide parameters for including or excluding samples for isotope analysis [ 8 ]. Evaluation of sample quality is critical for medicolegal cases since compromised samples may yield inaccurate and misleading results.
In this section, we present the application of isotope analysis as an investigative tool using three case examples. We emphasise the use of different isotope systems for answering specific questions and for predicting region-of-origin of unidentified human remains cases and deceased undocumented border crossers from the U. In , a long bone shaft fragment was recovered during subsurface testing at a construction site in downtown San Francisco. The bone was examined by a local cultural resource management firm, and an archaeologist identified it as possibly human.
The construction company and medical examiner were concerned that the bone may be Native American in origin, causing potential issues with future development at the site. The bone had been fragmented into eight pieces and had to be reconstructed. Based on overall morphology, the fragments derived from a mammalian long bone, such as a humerus, tibia, or femur. The overall texture and distribution of cortical and cancellous bone compared more favorably with human bone than nonhuman bone.
For example, the boundary between cortical and cancellous bone is poorly defined in humans whereas in nonhuman animals, it is a much more defined boundary [ 95 ]. Although the macroscopic assessment did not permit a definitive identification as human, it was more consistent with being human than nonhuman. In humans, the bone microstructure typically appears as a series of concentric secondary osteons.
In most nonhuman animals, the bone microstructure appears as a series of brick-like structures, characteristic of plexiform bone [ 96 , 97 ]. A small sample of bone was embedded in resin and slides were prepared using standard histological protocols and examined using an Olympus CX41 microscope Olympus, Tokyo, Japan. During the histological examination, both lamellar bone and osteonal bone were noted as well as a lack of osteonal banding, negating a definitive determination of human versus nonhuman.
The degraded nature of the bone was a limiting role in the histological assessment. Given that the macroscopic and microscopic analyses were inconclusive but suggestive of human , the construction firm approved the use of isotope analysis to determine whether or not the bone collagen had carbon and nitrogen isotopic compositions similar to prehistoric Native Americans from the San Francisco Bay Area.
However, macroscopic characteristics can exclude sea mammals as the source of the bone fragment. While the isotope data in itself cannot be used to definitively identify an unknown bone as human, it can provide strong circumstantial evidence. Bivariate plot of bone collagen stable carbon and nitrogen isotope values of prehistoric hunter-gatherers from Central California.
In , a human skull was located in the desert in southern California. The biological profile indicated the remains were those of a young Hispanic female. Despite obtaining a DNA profile and conducting an extensive missing persons search, law enforcement was unable to identify the decedent. In , the remains were submitted to the HIL for preparation and isotope analysis.
Investigators were interested in knowing whether the decedent may have been from Latin America versus a U. Americans than Latin Americans. Given this information, we suggested that the decedent was likely from the continental U. To track her most recent movements, 10—12 hair strands were sampled; they were oriented at the basal root to distal ends. After cleaning, the hair strands were taped at the root end into a clean tinfoil pouch and submitted for measurement of oxygen isotope ratios.
The bundle of hair strands was cut into segments using a razor blade. Segment lengths were measured using calipers. A uniform growth rate of 0. Each month of the year was defined as This enabled an estimation of the time period represented by each hair segment, with time zero being the basal root end of the hair, representing the time closest to death. Segments that were isotopically very similar — indicating little variation in drinking water inputs during the associated time periods — were averaged.
The isotope results in this case was useful in suggesting the decedent was most likely from the U. These data can provide new investigative leads that may ultimately result in personal identification of the decedent. A Region-of-origin prediction map for Case Study 2 using the oxygen isotopic composition of hair. Water base layer data used for region-of-origin prediction from [ 20 ]. These possible regions include areas within the Western U.
B Region-of-origin prediction map for Case Study 2 using the oxygen isotopic composition of hair. Maps created by E. Kipnis, IsoForensics, Inc. A recent increase in migrant border crossings through South Texas has resulted in a rise in the number of migrant deaths along the Texas-Mexico border, which by exceeded the number of migrant deaths along the Arizona-Mexico border [ ]. Migrant deaths are most often caused by heat-related illness and dehydration [ , ], with the majority of fatalities occurring around the Falfurrias checkpoint in Brooks County Texas , located approximately 80 miles north of the border.
In , forensic anthropologists from Baylor University and University of Indianapolis began the process of exhuming human remains of deceased unidentified border crossers UBCs buried at a cemetery in Brooks County. For one of these individuals OpID , a male recovered from Falfurrias, Texas, we sampled a premolar and metatarsal to obtain information on provenance and diet, respectively.
Bone collagen data indicate a diet composed of both C 3 and C 4 resources. Americans Figure Box-and-whisker plot comparison of bone collagen stable carbon isotope data for a sample of UBC remains from South Texas and a sample of U. Americans partial data set derived from [ ], Figure Box represents the interquartile range IQR and whiskers are 1. Americans than other UBC remains. Figure 5 illustrates an isoscape prediction map of the possible locations where the decedent may have obtained his drinking water and food during childhood.
The darkest gray highlighted areas indicate locations where he may have obtained drinking water based on oxygen isotopes. Similarly, the lighter gray highlighted areas indicate locations where he may have obtained food based on strontium isotopes. The red highlighted areas indicate locations where both oxygen and strontium isotopic predictions overlap, representing the most likely origin. Oxygen and strontium isotope data based on tooth enamel provide region-of-origin predictions for several areas, including the Western U.
Based on these isotope data, these areas cannot be excluded as a possible place of origin for the decedent. However, it is a strong possibility that the decedent was U. There are at least three possibilities: 1 the decedent is not a UBC but was treated as such in death; 2 the decedent may have spent his childhood years in the U.
Additional research is needed to more fully address cases such as this that diverge from the expected pattern of other UBC remains analysed to date. Region-of-origin prediction map for Case Study 3, remains of an unidentified border crosser OpID , using the oxygen and strontium isotopic compositions of tooth enamel.
Water base layer data used for region-of-origin prediction from [ 20 ] and [ 60 ]. The darkest gray highlighted areas indicate locations where the individual may have obtained their drinking water based on measured oxygen isotope ratios of tooth enamel. The lighter gray highlighted areas indicate locations where the individual may have obtained their food based on measured strontium isotope ratios of tooth enamel. The red highlighted areas indicate locations where both oxygen and strontium isotopic compositions overlap, representing the most likely regions-of-origin.
These possible regions include several areas within the Western U. Map created by B. Tipple, IsoForensics, Inc. It is important that future applications of isotope analysis consider possible roadblocks to progress in forensic anthropology casework. First and foremost is the paucity of nationally or internationally agreed and validated standard operating procedures SOPs for the preparation and analysis of human remains teeth, bone, hair, and nails.
Unfortunately, funding constraints can limit the basic or applied research necessary to develop SOPs and reference datasets that are fit for purpose. Presenting interpreted data in ways that are most useful to law enforcement is another important consideration when introducing isotope testing results to the legal system. Practitioners may need predictive models describing isotopic variation in materials of forensic interest that do not yet exist or exist only on small scales e.
H isotopes of collagen, Pb isoscapes, etc. Finally, the case studies presented herein highlight the fact that a multi-isotopic profile is almost always beneficial for forensic anthropology casework as opposed to analysis of only one or two isotope systems.
Analysis of other isotope ratios e. Addition of these other isotope systems to forensic anthropology casework will likely require development of new instrumentation — as well as new reference materials. While isotope analysis has been used for several decades to determine whether samples of chemically similar substances — such as drugs, explosives, paints, plastics, or tapes — may share a common source, applications of the technique to unidentified human remains for forensic profiling purposes are continuing to emerge.
Here, we attempted to provide a brief but comprehensive overview of isotope analysis and its utility to medicolegal cases. Although it is impossible to cover all facets of isotope analysis for human remains testing in a single article, the background, case studies, and references included in this work provide a foundation for forensic anthropologists interested in using this scientific tool to provide investigative leads in their own casework.
Daniel Wescott, and Dr. Kate Spradley for their assistance with the UBC samples. Douglas H. Ubelaker for his kind invitation to contribute to this journal issue.
0コメント