The answer to the title’s question will likely cause many readers to ask themselves, “Why didn’t I think of that?” The reason is that the answer is pretty simple.
Some authors, including myself, have noted that Egyptian radiocarbon dates in Upper Egypt (uphill, in the south) tend to appear approximately 120 years younger than contemporaneous organic material from Lower Egypt (in the north). The problem is even worse than this discrepancy suggests. The dates scientists have assigned to northern radiocarbon-dated materials are through Bayesian statistical modeling, whereas the apparent age discrepancy of many northern items compared with southern Egyptian materials appears to be much greater than merely 120 years. What causes these differences, and which region yields dates more closely aligned with the Radiocarbon International Calibration Curve?
The simple part of the answer is “soil.” The Calibration Curve’s backbone for the second millennium BCE is primarily radiocarbon-dated German oak tree rings. Every site has offsets due to regional and local factors. Carbon sinks absorb carbon dioxide and release it after decades or centuries. Because carbon-14 is an unstable isotope, the tiny fraction of the gas with this isotope, rather than carbon-12 or carbon-13, becomes even more miniscule over time. Therefore, when the oceans and seas absorb this gas and gradually release it later, the amount of carbon-14 in the carbon dioxide has diminished. The same is true of soil, another carbon sink.
Plants absorb carbon dioxide from the air and convert it to sugar through photosynthesis. Then, they use that sugar for fuel and to make other things, such as cellulose, a primary structural component. When leaves and branches on oak trees die, they fall to the ground and deteriorate. Insects and other small animals, as well as microorganisms, break down those organic materials through aerobic respiration (with oxygen), releasing the waste product carbon dioxide into the air above the soil. Thus, the carbon dioxide the trees use for photosynthesis is not solely from the atmosphere. The CO2 contribution from the soil is a small but significant factor. The average time the released carbon has been in the soil can be several decades or even centuries. Consequently, the carbon dioxide in the air just above the soil can have significantly less carbon-14 than the gas not far above the forest canopy.
How does this situation in German forests compare with plants grown in the Nile River Valley of ancient Upper Egypt? The source of the organic material carried in the annual Nile flood season was the tropics. The warmth and moisture of those tropical areas cause rapid plant growth and the speedy breakdown of organic detritus since the animals and microorganisms that break down dead plants thrive in warm, moist conditions. When the heavy summer monsoon rains arrive, it washes soil and partially decomposed organic material into the headwaters of the Nile River. As the river water velocity diminished in the Nile Valley of Upper Egypt, it deposited a thick layer of soil.
When the flood waters dissipated, farmers plowed the soil, allowing abundant oxygen to reach the microorganisms in the soil. The plowed, rich soil and warm weather allowed intense agricultural practices with the crop plants growing close together. Consequently, the rapid decomposition of organic material in the soil caused the ground’s contribution of carbon dioxide to be significantly higher than in the German forests. Moreover, the carbon sink residency in the tropical biomass, including soil, is extremely short compared with temperate forests. Thus, the carbon-14 in the soil’s organic materials has diminished only slightly from the quantity in the atmosphere. These reasons account for the higher amounts of carbon-14 in Upper Egyptian organic samples that appear around a century younger than the Calibration Curve’s standard. What about Lower Egypt?
Each year, Nile flooding carried tropical soil to Egypt, but it also mixed it with soil picked up from the Nile Valley deposited in previous years, decades, and centuries. That mixture of organic materials in the north has carbon-14 quantities equal to or even lower than the German oaks, especially if sequestered in a dead-end area such as the Faiyum reservoir. Thus, Lower Egyptian radiocarbon-dated materials can approximate the International Calibration Curve, but many samples can appear centuries older.
One comment