SCIENCE & TECHNOLOGY ON-SITE Soil analysis is shedding light on ancient activities around a residential patio (shown) and a ceremonial plaza at the Palmarejo site in Honduras.
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DIGGING DEEPLY INTO THE PAST Chemistry is increasingly helping archaeologists answer questions of what, where, and when CELIA HENRY ARNAUD, C&EN WASHINGTON
M
O D E R N ARCHAEOLOGISTS ARE
detectives trying t o solve t h e mysteries of the past. T h e word archaeologist conjures u p t h e image of someone in the field painstakingly uncovering the foundations of ancient buildings or sifting through dirt to find rare artifacts. Such a notion of archaeology may seem more like history than science, but chemistry is now a key implement in the archaeologist's toolbox, from the radioactive carbon-14 used to estimate the age of objects to the trace elements used t o figure out the origins of artifacts. Archaeologists want to understand how people lived in earlier times. Scholars studying modern history can rely on the written record, supplemented by objects, t o illum i n a t e people's lives. For archaeologists, the written record is limited, if it exists at all. I n m a n y cases, t h e physical artifacts are all archaeologists have t o g o on, a n d they m u s t reconstruct t h e past from t h e objects themselves and their present-day surroundings.
Chemistry can answer a number of questions that can help with this reconstruction: H o w old is an artifact? W h a t is it made of? W h e r e did it come from? A t the American Chemical Society national meeting held last m o n t h in Atlanta, a symposium o n archaeological chemistry addressed these very questions as speakers described a variety of projects. T h e symposium, cosponsored by t h e Divisions of Nuclear Chemistry & Technology and of the History of Chemistry, was the 11th in a series going back more t h a n 50 years. T h e symposia a r e held a p p r o x i m a t e l y every five years. A major way that chemistry contributes to archaeology is in dating artifacts. In radioactive methods, the decay rate of a radioactive element in a sample and that element's half-life are used t o estimate the age of the sample. T h e most prevalent technique by far uses carbon-14 and also requires a calibration based on the amount of that isotope in the atmosphere at any given point in the past, as measured in tree rings. Previously, the radio-
active decay of carbon-14 was measured, but now the amount of carbon-14 can be measured directly by mass spectrometry. Carbon-14's 5,730-year half-life makes it suitable only for dating of objects younger than 5 0 , 0 0 0 years, or about nine half-lives, because after that point too little carbon-14 remains to be measured. O t h e r radioactive m e t h o d s such as potassium-argon dating are suitable for samples t h a t are millions of years old. T h a t leaves a substantial time span in between. A t the ACS meeting, Anne R. Skinner, a senior lecturer at Williams College in Williamstown, Mass., described electron spin resonance (ESR) dating, one of the methods suitable for this intermediate age range. ESR dating measures the radicals generated by exposure to environmental radiation, typically from uranium and other radioactive elements and cosmic radiation. T h e age is then calculated by dividing the measured amount of radicals in the sample by the assumed annual dose, which is based on the environmental history of the surroundings. THE AGE RANGE for w h i c h E S R d a t i n g works depends o n the amount of time required to build up a detectable level of radicals. Typically, ESR dating can't be used for objects younger than a few thousand years. A t the other end of the age range, samples older than 5 million to 8 million years tend to become saturated, and they can no longer be reliably dated by ESR. ESR dating could help w i t h one of the questions t h a t archaeologists and paleontologists are trying t o answer in Australia: Did the arrival of early humans in Australia drive large marsupial mammals such as the giant kangaroo and diprotodon, a bearlike animal, t o extinction? T h e E S R dates of diprotodon teeth found in Australia's Lake Eyre Basin indicate that these marsupials still lived 4 3 , 0 0 0 years ago, while the first horninids arrived in Australia about 50,000 years ago, according to Skinner. T h e teeth h a d b e e n deposited by water, so t h e site contained "a jumble of t e e t h of different ages," Skinner said. Therefore, she could only estimate a m a x i m u m age. N o n e t h e less, t h e t e e t h establish "an overlapping time between marsupials and horninids,"
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she said. "One possible explanation is that [the marsupials} were hunted down." S k i n n e r measures c a r b o n a t e radicals trapped within the hydroxyapatite of tooth enamel. "Teeth are the hardest substance in t h e body," Skinner said. Inside t h e hydroxyapatite crystals, she added, t h e free radicals are protected and can last for billions of years. Sometimes, however, dating methods are needed for objects that are only a few hundred years old. Christopher M . Stevenson, an archaeologist at t h e Virginia D e p a r t ment of Historic Resources, in Richmond, described a method for dating glass objects based on the diffusion of water into their surfaces. H e selected high-calcium glasses u s e d in w i n e b o t t l e s because t h e y w e r e among the first to be brought to America and c o n t i n u e d t o b e used u n t i l t h e 19th century. H e focused on b r o k e n edges of the glass because he assumes that the time the bottle was broken corresponds to when the bottle was discarded. Otherwise, wine bottles of that era were reused repeatedly, so the age of the glass itself would not help date the deposit in which it was found. T o use t h e bottles as date markers, he first needed to establish a calibration curve for the rate at which water diffuses into glass containing different amounts of calcium. H e measured the water diffusion and alkali exchange by infrared spectroscopy and secondary ion mass spectrometry. H e and his coworkers t h e n tested t h e calibration by determining the ages of bottles found in a trash deposit near the site of slave cabins at Monticello, T h o m a s Jefferson's plantation near Charlottesville, Va. T h e y already knew t h a t these were t o r n d o w n a r o u n d 1800. T h e estimated ages of the glass bottles determined with the calibration fell into that time period. Right now, the method is only precise enough to give t h e m dates within a 50-year interval. DESPITE THESE other techniques, carbon14 dating is still the workhorse technique for archaeologists. Being able to use carbon-14 dating requires that the material contain at least some organic component. Graduate student K a t h r y n Duffy and her coworkers are investigating t h e pigment Maya blue, which is m a d e by mixing t h e organic dye indigo with inorganic palygorskite clay, as a possible material for radiocarbon dating. Scientists have generally considered Maya blue from an inorganic perspective, but Duffy wants to isolate the indigo so that it can be used for radiocarbon dating of murals. A t the ACS meeting, she described a procedure for extracting the indigo by using hydrofluoric acid to etch away the mineral
portion. T h e indigo is then irreversibly oxidized w i t h nitric acid to isatin before it is radiocarbon dated. She and her colleagues demonstrated the extraction technique with m o c k samples of plaster w i t h laboratorymade Maya blue on the surface. Joseph B. Lambert, a professor at Northwestern University w h o has long been involved with archaeological chemistry, was i n t r i g u e d by Duffy's talk. "It h a d never occurred to me that you could extract out t h e organic material and t h e n carbon-14 date it," he told C & E N . "Maya blue as it appears on a wall painting or piece of pottery is never considered a source of organic materials. You can carbon date very small
IN THE DARK The black coating over a cave painting of a shaman figure at Little Lost River Cave in Idaho is probably made of humic and fulvic acids from soil rather than cooking residues. amounts. I'll be interested to see how the indigo project proceeds." Age estimates are only meaningful in an archaeological context, however, if the artifacts can be connected with human activity. Ruth A n n Armitage, a chemistry professor at Eastern Michigan University in Ypsilanti, and her graduate student coworkers have been working to identify the black coating on rock paintings in Little Lost River Cave in Idaho. Archaeologists had obtained carbon-14 ages of t h e coating because it was plentiful and expendable, whereas the underlying painting was small. Archaeologists had theorized t h a t t h e coating might be some sort of cooking residue. If the coating indeed results from such a human activity, its age can be used as a m i n i m u m age for the underlying paintings. H o w e v e r , t h e r e have b e e n questions about w h a t the coating actually is. I n At-
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in West Africa. From the ratios of the four lanta, Armitage's grad students Jamie A. lead isotopes, Fenn suspects that much of Brown and Reshmi Perumplavil described the copper came from North Africa, even the work they have done to nail down the though there are copper deposits true origin of the residue. They | in West Africa. He is using this analyzed samples of the coating j information to trace trade routes by using X-ray photoelectron | across the Sahara. spectroscopy and two different Sometimes, human remains sample preparation methods for i are the best source of data for gas chromatography/mass spec§ answering certain questions. trometry. Of all the comparison Kelly J. Knudson, an assistant samples they tested, the coating professor and director of the Arleast resembled their experimenchaeological Chemistry Laboratal cooking residues. tory at Arizona State University, Instead, they found that the described efforts to identify the coating most closely resembled origin of human heads found at the common soil components sites used for rituals at the anhumic and fulvic acids, suggestcient Wari city of Conchopata, ing that it was deposited by water BITE DOWN in the central Peruvian Andes. dripping through the soil above ESR dating the cave. The problem with try- reveals that giant Tiffiny A. Tung, an archaeologist at Vanderbilt University, ing to use humic and fulvic acids marsupials, like excavated and reconstructed the to date the coating is that those the diprotodon trophy heads, which the Wari, components can come from any this tooth who preceded the Incas, had layer of the soil, which could be came from, prepared by drilling a hole in older or younger than the under- and hominids the top and then smashing the lying painting. "That's why you overlapped in skull. Archaeologists aren't sure can't really date those if you're Australia. whether the trophy heads came trying to date human activity," from the Wari's revered ancestors or from Armitage said. She is not ready to rule out victims of warfare or raiding. an anthropogenic origin for some of the carbon that may be under the coating, but Determining the geographic origins of the evidence strongly supports an environthe trophy heads could provide one piece mental origin for the coating itself. of evidence to identify the people. Knudson and her colleagues do this by measuring In addition to ages of objects, archaethe isotope ratios of strontium in bone and ologists also want to know the geographic tooth enamel. "Strontium origins of objects that may wind up far from where 1 gives us a really nice signathey started. The compo2 ture that allows us to track sition of objects can help IE populations from different identify the source of the S geologic zones," Knudmaterial. Knowing where S son told C&EN. "It's not something came from and perfect, but it's the first where it ended up can protechnique we've had that vide clues to human migraallows us to look at human tion or trade patterns. At migration in the people the ACS symposium, such themselves instead of lookobjects ranged from coins ing at other artifacts." and pottery to trophy huThe assumption is that man heads. strontium ratios reflect the ratios found in the surT h o m a s R. Fenn, an rounding geologic formaanthropology graduate tions and that those ratios student at the University don't change as they are of Arizona, uses lead isoincorporated into food and tope ratios to identify the move up the food chain. source of copper artifacts USE YOUR HEAD By Because tooth enamel is found in Western sub-Sa- analyzing the ratio of set when a tooth forms, haran Africa. (Even al- strontium isotopes, the strontium ratios in most pure copper artifacts Knudson and coworkers contain traces of lead car- concluded that trophy heads teeth reflect a person's environment from 10 weeks ried through the smelt- excavated at Conchopata, before birth to about 15 ing process from the ore.) Peru, probably came from years of age, depending The artifacts were found people captured during on the tooth. In contrast, at four archaeological sites raids on other villages.
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