One mechanism, two detectabilities
The dominant attribution failure is identical on both clocks: foreign older material in ⇒ measured age biased old (radiocarbon reservoir / dead-carbon; zircon inheritance). They differ only in detectability — atomically homogeneous and self-concealing in radiocarbon, granular and separable in zircon. Daughter loss biases the other way, so screening must guard both directions.
The dominant attribution failure is identical on both clocks: foreign older material in ⇒ measured age biased old (radiocarbon reservoir/dead-carbon; zircon inheritance and antecrysts). The clocks differ only in detectability—atomically homogeneous and self-concealing in radiocarbon (needs external ΔR correction), but granular and separable in zircon (excluded from the data). Daughter loss biases the other way, so accuracy must guard both directions.
The central observation of this paper is that the dominant Axis-B failure is identical across the two chronometers in mechanism and in sign:
Unified mechanism. Incorporation of foreign, older material into the dated sample biases the measured age toward greater age. In radiocarbon this is the reservoir / hard-water / dead-carbon effect and old-carbon contamination; in zircon it is inheritance, antecrysts, xenocrysts and detrital recycling.
The mechanism is bidirectional in its failure modes. Loss of the daughter product biases ages younger: episodic Pb loss in radiation-damaged zircon, and modern-carbon contamination in radiocarbon. Accuracy therefore requires guarding both directions.
Where the two methods diverge is not in the mechanism but in its detectability, and this single distinction explains why zircon is comparatively robust to the contamination both methods suffer (Table 1).
| Radiocarbon (reservoir / dead C) | Zircon U–Pb (inheritance) | |
|---|---|---|
| What enters | old carbon atoms | old whole crystals |
| Sign of bias | older | older |
| Mixing scale | atomic — homogeneous | granular — discrete grains |
| Detectable in data? | No (self-concealing) | Yes: bimodal ages + concordance |
| Handling | external correction (ΔR) | exclusion from the data |
Because the radiocarbon contaminant is chemically identical to the sample carbon and mixed atom-for-atom, it cannot be separated—only subtracted using an externally estimated reservoir offset, which varies with locality and context. Because zircon inheritance arrives as separate crystals carrying their own concordant ages, it presents as distinct age modes that can be identified and removed. [I] This is the precise sense in which a single zircon date is reliable for the grain analysed but requires context to be trusted as the age of a rock-forming event: the date is robust; the attribution is the weak link.