Foreign-Material Incorporation as a Cross-Chronometer Accuracy Limit: Extending Matrix-Informed Radiocarbon Correction to Zircon U–Pb Geochronology

This working draft addresses accuracy in radiocarbon and zircon U–Pb geochronology at the
level of the individual sample — accuracy set by sample contamination and event attribution,
not by the decay physics. The decay clocks themselves are taken as established and are not
disputed.

Its central thesis is that the dominant accuracy failure shared by both chronometers has a
single structure: incorporation of foreign, older material biases a measured age toward
greater age (the radiocarbon reservoir / dead-carbon effect; zircon inheritance, antecrysts,
and detrital recycling). The two methods differ only in detectability — in radiocarbon the
old carbon is mixed atom-for-atom and is therefore homogeneous, inseparable, and hidden, and
must be corrected externally; in zircon the old material resides in discrete grains and so
appears as separable age populations that can be excluded from the data itself.

The paper formalises this as a two-axis decomposition (internal clock integrity vs. event
attribution), unifies the underlying physics through the Dodson closure / Fourier number, and
defines a single protocol — screen → classify → correct-where-characterisable → cross-check →
report honest uncertainty — in which zircon populations are classified on age-independent
grounds (crystal position, texture, common-Pb content) rather than by selecting the youngest age.

The protocol is demonstrated on real data: a leave-one-out reservoir correction on published
radiocarbon pairs (RMSE 418→141 yr and 739→453 yr); the canonical Lava Creek Tuff zircon
dataset (Matthews et al., 2015), where classification by crystal position reproduces the
published rim age (crystal faces, 626.5 ka) and isolates the older antecrystic cores (668.8 ka),
consistent with the ~631 ka eruption age and an independent Marine Isotope Stage tie; and
open-access SHRIMP-RG and detrital-zircon data (common-Pb correction; youngest-grain maximum
depositional-age constraint).

The work is explicit about the working range of each method (both the young and the extreme-old
ends degrade), about the assumptions each correction depends on, and about one irreducible
epistemic asymmetry: in the deep-time interval, accuracy is established by consilience —
concordance, cross-method agreement, astrochronology — not by direct comparison with written
records. Every quantitative claim is tagged as directly confirmed [F], inferential [I], or
assumption/open [A].

Companion to "Matrix-Informed Correction and Exclusion Protocol for Biogenic Radiocarbon
Dating" (doi:10.5281/zenodo.17718893), which this paper extends to a second, mechanistically
different chronometer.

Contents of this record: the paper as PDF and as a self-contained HTML version (with in-page
search); the LaTeX source with figures; and a lightweight reproducibility bundle (Python
scripts) that regenerates the zircon analyses from the cited public data.