Dating the past: stratigraphy, radiocarbon and how we fix events in time

In our last lesson, we began to explore how archaeologists find and understand the past. We discussed how they excavate sites, carefully recording the context of artifacts and features. We touched on how early archaeologists like Augustus Pitt Rivers developed careful excavation methods and how William Flinders Petrie used seriation to put artifacts in a relative order. Seriation allows us to say that one type of pot is older than another, but it doesn't tell us *how much* older, or give us a specific year. To really build a timeline of human history, we need more precise ways to date things. Today, we will look at two powerful methods that archaeologists use to fix events in time: stratigraphy and radiocarbon dating. These methods allow us to move beyond just knowing what happened in what order, to understanding *when* things happened. Let's begin with stratigraphy. Imagine you are digging a hole in your garden. As you dig, you might notice different layers of soil. Perhaps a dark topsoil layer, then a lighter clay layer beneath it, and perhaps some rocky soil even deeper. Stratigraphy is the study of these layered deposits. In archaeology, these layers are called *strata* (a single layer is a *stratum*). The key idea behind stratigraphy is very simple and logical: in any undisturbed sequence of layers, the oldest layers will be at the bottom, and the youngest layers will be at the top. This is known as the *Law of Superposition*. This principle was first formally described in the 17th century by a Danish scientist named Nicolaus Steno. Steno was an anatomist, someone who studies the structure of living things, but he also became very interested in geology, the study of the Earth's physical structure and substances. He observed how layers of rock and soil were formed, often by water carrying sediments – tiny bits of rock, sand, and mud – which then settled and hardened over time. He realized that the material at the bottom must have been deposited first, and the material on top later. For archaeologists, these layers are not just natural soil. They often contain evidence of human activity. Imagine a village that was built, lived in for a time, and then abandoned. Over the years, dirt and dust would accumulate, burying the old village. Later, new people might build another village on top of the old one. If they continued to live there for a long time, and then that village was also abandoned and covered by natural processes, you would end up with layers. The pottery, tools, and house foundations from the first village would be in the lower, older layer. The artifacts from the second village would be in the upper, younger layer. By carefully digging through these layers, an archaeologist can see the sequence of human occupation, one period on top of another. This is why the careful excavation methods of people like Augustus Pitt Rivers, whom we mentioned in the last lesson, are so important. He understood that archaeologists needed to record not just what they found, but *exactly where* they found it, including which layer. This allows them to build a relative chronology. A *relative chronology* means we can say that event A happened before event B, but we don't know the exact year that A or B occurred. For example, if we find Roman coins in a layer above Greek coins, we know the Roman layer is younger than the Greek layer, even if we don't know the exact dates. Stratification, the creation of these layers, happens in many ways. Sometimes it's natural, like wind blowing sand to form dunes, or a river flooding and depositing mud. But often in archaeological sites, humans cause stratification. When people live in a place, they build structures, they throw away rubbish, they dig pits, and over time, these activities create layers. For example, if a house burns down, a layer of ash and charcoal might form. If new builders then level the site and construct a new building, they create a new layer on top of the ash. Each of these layers, or strata, represents a different period of time or a different event. The records produced by archaeologists are essential for understanding stratigraphy. These include detailed field notes, photographs, and especially *stratigraphic sections*. A stratigraphic section is like a cross-section of the Earth, a drawing or photo that shows the different layers, their colors, textures, and the artifacts found within them. These sections are a primary record of the site's history, documenting the layers as they were uncovered. However, interpreting stratigraphy is not always straightforward. Sometimes, a site is disturbed. For example, later inhabitants might dig a large pit for a well or for burying trash. This pit would cut down through older layers, mixing up artifacts and creating a confusing situation. Archaeologists call these disturbances *intrusive features*. Animal burrowing, tree roots, or even modern construction can also disturb layers. When this happens, the simple rule of "older at the bottom, younger at the top" might not apply perfectly in that specific spot. Expert archaeologists must carefully observe and record these disturbances to understand the true sequence of events. Another challenge is what archaeologists call *taphonomy*. Taphonomy is the study of how organisms decay and become fossilized or preserved, and how objects move or are affected after they are deposited. For example, water flowing through soil can move small artifacts or chemicals. Or changes in soil acidity can destroy organic materials in some layers while preserving them in others. These processes can make the stratigraphic record more complex to read. Despite these challenges, stratigraphy remains a fundamental method. It provides the essential backbone for understanding the relative sequence of events at any archaeological site. Without it, we would just have a jumble of artifacts, without knowing which came first or how they relate to each other in time. But what if we want to know not just the relative order, but the actual *calendar age* of an artifact? This is where *absolute dating methods* come in. And the most famous and widely used absolute dating method for organic materials is radiocarbon dating. Radiocarbon dating is a scientific technique that measures the decay of a specific type of carbon, called carbon-14 (14C), in organic materials. *Organic materials* are those that come from living things, like wood, charcoal, bones, seeds, or textiles. The story of radiocarbon dating begins with carbon itself. Carbon is a basic building block of all life on Earth. Most carbon in the world is a stable form called carbon-12 (12C). However, a small amount of carbon in the atmosphere is carbon-14 (14C), which is radioactive. This means it is unstable and slowly changes into another element over time. How does carbon-14 get into the atmosphere? It's created when cosmic rays, high-energy particles from space, hit the Earth's upper atmosphere. These collisions produce neutrons, tiny particles, which then react with nitrogen-14 (14N), another element found in the atmosphere, to create carbon-14 (14C). This newly formed carbon-14 quickly mixes with oxygen to form carbon dioxide (14CO2), just like regular carbon-12. Plants absorb this carbon dioxide from the atmosphere through a process called photosynthesis. Animals then eat the plants, or eat other animals that ate plants. In this way, carbon-14 enters the food chain and becomes part of all living organisms. While an organism is alive, it continuously takes in new carbon, so the amount of carbon-14 in its body stays roughly constant, reflecting the level of carbon-14 in the atmosphere at that time. But here is the crucial point: when an organism dies, it stops taking in new carbon. From that moment on, the carbon-14 inside its body begins to decay. It slowly transforms back into nitrogen-14 at a constant and predictable rate. This rate is measured by its *half-life*. The *half-life* of a radioactive substance is the time it takes for half of its radioactive atoms to decay. For carbon-14, the half-life is approximately 5,730 years. This means that after 5,730 years, half of the original carbon-14 in a dead organism will have decayed. After another 5,730 years (a total of 11,460 years), half of the *remaining* carbon-14 will have decayed, leaving one-quarter of the original amount. By measuring the amount of carbon-14 still present in a sample of organic material, scientists can calculate how many half-lives have passed since the organism died. This allows them to determine the age of the sample in actual calendar years. The brilliant scientist who developed this revolutionary technique was Willard F. Libby, an American physical chemist. He began his work on radiocarbon dating in the late 1940s, and by the early 1950s, the method was being widely applied. For his groundbreaking achievement, Libby was awarded the Nobel Prize in Chemistry in 1960. Radiocarbon dating completely transformed archaeology and many other fields. Before Libby's discovery, archaeologists often relied on historical texts or stylistic comparisons of artifacts to date things. These methods were often limited to regions with writing or could only provide relative dates. Radiocarbon dating offered an independent, scientific way to get absolute dates for organic materials, pushing our understanding of human history far beyond the reach of written records. It allowed archaeologists to build global chronologies, connecting events across different continents and periods, providing a much clearer picture of human prehistory. However, radiocarbon dating is not without its complexities and challenges. One of the most important issues is *calibration*. When Libby first developed the method, he assumed that the amount of carbon-14 in the atmosphere had been constant throughout history. But we now know this is not true. The atmospheric concentration of carbon-14 has fluctuated over time, due to changes in the Earth's magnetic field, variations in solar activity, and even volcanic eruptions. This means that a raw radiocarbon date, expressed in "radiocarbon years Before Present" (where "Present" is defined as the year 1950 AD), doesn't always exactly match a calendar year. To solve this, scientists use *calibration curves*. These curves are created by dating samples of known age using other independent methods. The most important of these is *dendrochronology*, or tree-ring dating. Trees grow a new ring each year, and the width and characteristics of these rings are influenced by climate. By finding very old trees (or pieces of old wood) and matching their ring patterns, scientists can build a continuous chronological sequence of tree rings going back many thousands of years. If you then radiocarbon date a specific tree ring from a known calendar year, you can compare that radiocarbon date to its actual calendar age. By doing this for many different years, scientists have created detailed calibration curves that convert raw radiocarbon dates into much more accurate calendar dates. Another challenge is *sample contamination*. If a sample is contaminated with younger carbon (for example, from modern roots growing into an ancient burial) or older carbon (perhaps from fossil fuels in the soil), the radiocarbon date will be inaccurate. Archaeologists and scientists take extreme care to prevent contamination during excavation and in the laboratory. There is also the *reservoir effect*. In certain environments, like oceans or large freshwater lakes, organisms can absorb carbon that is older than the atmosphere. For example, marine organisms might take in carbon from deep ocean waters that has been out of contact with the atmosphere for a long time, and is therefore depleted in carbon-14. If you radiocarbon date a marine shell, its date might appear older than the actual time the organism lived. Scientists have developed ways to account for these reservoir effects, but they require careful consideration. Radiocarbon dates are also *statistical*. They are always given with a range of uncertainty, for example, "2000 ± 50 BP" (Before Present). This means there is a certain probability (usually 68% or 95%) that the true date falls within that range. It's not a single, exact year, but a window of time. Finally, there's the "old wood problem." If archaeologists date a piece of charcoal from a hearth, it tells them when the tree died, not necessarily when the wood was burned. If people cut down a living tree and immediately burned it, the dates would be very close. But if they used old timber from a building, or gathered driftwood, the radiocarbon date would reflect the age of the wood itself, which could be much older than the archaeological event being studied. This is why archaeologists prefer to date short-lived organic materials like seeds or annual plants whenever possible, as these provide a date very close to the actual event. Despite these considerations, radiocarbon dating remains an incredibly powerful tool. It has allowed us to date everything from the earliest human settlements in the Americas to the development of agriculture, to the construction of ancient monuments, with a level of precision that was unimaginable just a century ago. In summary, stratigraphy provides the essential relative framework for an archaeological site, telling us which layers and events came before or after others. Radiocarbon dating provides the absolute calendar dates for organic materials within those layers, allowing us to pinpoint events in specific years or centuries. Together, these two methods are fundamental to how we build our understanding of the vast human past, moving beyond just finding artifacts to truly fixing human events in time. Next, we will begin to explore what life was like in the very earliest periods of human existence, looking at the tools and environments of our distant ancestors.