Fish-based diets can cause headaches for archaeologists

Fish-based diets may be good for your health, but they can cause headaches for archaeologists. While archaeologists are aware fish-based diets could cause inaccuracies in Carbon-14 dating, they were not aware the anomalies could go up to 500 to 2,100 years. "I had not anticipated an error of up to 2000 years ... The implications of this discovery are fairly frightening, because it is crucial for archaeology to have a reliable dating procedure," Felix Riede, an archaeologist at Aarhus University who uses Carbon-14 dating in his work, said in an article posted on PastHorizonsPR.com. He added that while an error of a few hundred years is acceptable in dating Palaeolithic finds, an error of 2,000 years "is of great importance, even for the oldest periods.” Riede said there is a need to look at more reliable dates such as Carbon-14 dates of short-lived terrestrial plants or twigs, then compare them to the dates from cooking pots. But fish diets are not the only sources of error. PastHorizonsPR.com cited charcoal from a camp fire may also be one - "where the charcoal dated may be from the innermost ring of a 500-year-old tree which was felled 100 years before it finally ended up in the camp fire." Reservoir effect PastHorizonsPR.com cited an incident some years back in Northern Germany, when archaeologist Sönke Hartz carried out excavations at a prehistoric campsite of the Ertebølle culture near the river Trave. Hartz discovered an ancient pottery sherd that held remnants of burnt food, and sent the pot sherd for carbon-14 dating. He was amazed when the lab dated it at 5200 BC. While he at first thought it was an "archaeological sensation" since it appeared "many hundreds of years older" than all the pottery previously found in Northern Germany, he began to have doubts. He reasoned that since he found the pot by the river, the food crust could possibly consist of fish- and "there were dating problems with freshwater fish, which could give misleading ages.“ To get a radiocarbon date, one measures the amount of remaining Carbon-14 atoms in a sample. The less Carbon-14 left, the older the sample. "Hard water contains less Carbon-14 than the atmosphere, because dissolved carbonates are Carbon-14 free. A fish caught in hard water has thus a higher Carbon-14 age than contemporaneous terrestrial samples. If such a fish is then cooked in a ceramic pot, the radiocarbon age of the food crust will be higher than if a terrestrial animal was cooked in the pot," PastHorizonsPR.com said. It said this is called the “reservoir effect” because "the fish’s carbon actually comes from another 'reservoir' than the carbon in terrestrial animals from the surrounding area." PastHorizonsPR.com said an examination of freshly caught fish from the River Trave showed a large reservoir effect, and a variance from between 500 to 2,100 years. "In effect, this means that some of the fish swimming in the Trave today seem to be over 2,000 years old, when radiocarbon dated," it said. New test Researchers tested the new knowledge from The Aarhus AMS 14C Dating Centre by cooking fish stew in ceramic pots - boiling a freshly caught fish with a Carbon-14 age of 700 years. They burned the meal onto the pot fabric, which was then taken for dating. "Even though the food crust was made only weeks before, Carbon-14 dating returned a 14th century date and thus provided evidence that food crusts on pottery take on the same age as the ingredients," PastHorizonsPR said. "These results reveal that freshwater reservoir effects have to be seriously considered and understood whenever residues on prehistoric pottery is radiocarbon dated. The same also applies to the bones of humans who had eaten significant amounts of freshwater fish. This is real food for thought," it added. — LBG, GMA News