Radiocarbon dating and the Hallstatt plateau
About this app
A radiocarbon age is not a calendar date: the measurement bell runs across the real IntCal20 calibration curve and falls onto the time axis as a calendar density. On the Hallstatt plateau a lab error of ±20 years becomes a span of 272 years, on the steep section around 790 BC only 29. Wiggle matching with a chain of tree-ring samples snaps into place on the curve.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
Radiocarbon years are not calendar years
Radiocarbon dating measures how much radioactive carbon (¹⁴C) is left in organic material. The lab reports a conventional radiocarbon age in years BP, counted from 1950 and computed with the Libby half-life of 5568 years. Because the ¹⁴C content of the atmosphere varied over time, this age is not a calendar date. Only the calibration curve translates it. The current curve for the Northern Hemisphere is IntCal20 (Reimer et al. 2020); for the last few millennia it is built from tree rings whose calendar year is known.
The Hallstatt plateau
Between about 800 and 400 BC the calibration curve runs almost flat at around 2450 BP. Wood from 700 BC and wood from 650 BC have almost the same radiocarbon age there. A measurement of 2450 ± 20 BP therefore fits many calendar years: with 95.4% probability the date lies between 749 and 415 BC, spread over five parts totalling 272 years. The same precision at 2600 BP, where the curve drops steeply, gives only 805 to 777 BC, that is 29 years. It is the curve, not the lab, that blurs the date.
What the app shows and what you can try
The measurement appears as a bell curve on the radiocarbon axis. A band runs across the real IntCal20 curve, and where it meets the curve a curtain of light falls to the calendar axis. Below it the probability density forms, with the ranges for 68.3% and 95.4%. Drag the bell up and down, change the lab error and add a comparison measurement on the steep section. In the wiggle match you place a chain of tree-ring samples with fixed spacing on the curve and see where it snaps in and how much the span shrinks.
Frequently asked questions
- What is the Hallstatt plateau in radiocarbon dating?
- It is the part of the calibration curve between about 800 and 400 BC where the radiocarbon age barely decreases with calendar age and stays around 2450 BP. Samples from these four centuries can therefore be told apart only roughly by radiocarbon alone, even when the lab measures very precisely.
- What does BP mean in a radiocarbon age?
- BP stands for “before present” and means years before 1950. A conventional radiocarbon age in BP is computed with the Libby half-life of 5568 years and is not yet calibrated. Only the calibration curve, today IntCal20, assigns calendar years to it, often as a range in several parts with probabilities.
- What is wiggle matching?
- In wiggle matching several samples from one piece of wood are dated, with a known number of tree rings between them. The whole sequence is fitted to the calibration curve at once. Because the spacing is fixed, it fits in only a few places; at kinks in the curve this narrows the date considerably, on flat sections less so.
- What can I try in the app?
- You move a radiocarbon measurement along the axis, change the lab error and immediately see which calendar years fit and how large the span is. A comparison measurement shows the difference between the plateau and the steep section. In the wiggle match you let a chain of samples snap into place on the real IntCal20 curve.
Subject: History
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