Mitigation strategies in practice
Existing decontamination methods are unified, not replaced: each performs the same two moves — isolate the foreign component on age-independent grounds, then validate out-of-sample. Radiocarbon climbs ABA → ABOX-SC → ultrafiltration → hydroxyproline AMS; zircon climbs air abrasion → CA-ID-TIMS, CL-guided dating, ²⁰⁴Pb common-Pb, concordance. Every rung has a stated ceiling; none converts an Axis-B problem into an Axis-A guarantee.
Existing decontamination methods are not replaced but unified: each is the same two moves—isolate the foreign component on age-independent grounds, then validate out-of-sample. Radiocarbon climbs ABA → ABOX-SC → ultrafiltration (>30 kD) → compound-specific hydroxyproline AMS, plus ΔR calibration and Bayesian outlier models; zircon climbs air abrasion → CA-ID-TIMS, CL-guided spot dating, ²⁰⁴Pb common-Pb, and concordance. Every rung has a stated ceiling, and none converts an Axis-B problem into an Axis-A guarantee.
This protocol does not replace the decontamination methods each field already uses; it names the logic they share and adds out-of-sample validation as a standing requirement. Both communities have evolved a ladder of mitigations (Table 4). Each rung removes more of the foreign component on grounds independent of the age being measured, and each has a documented ceiling. Reading them as instances of one strategy is the contribution; the reagents are not new.
| Clock | Method | What it isolates / removes | Ceiling (where it fails) |
|---|---|---|---|
| ¹⁴C | ABA → ABOX-SC (charcoal) | acid/base-soluble + oxidisable contaminants | recalcitrant cross-linked carbon |
| ¹⁴C | ultrafiltration of collagen[20] | >30 kD degraded gelatin/humics | cannot remove <30 kD or cross-linked/glue carbon |
| ¹⁴C | compound-specific (hydroxyproline) AMS[21,22] | a bone-specific amino acid only | collagen-based glue; needs more material |
| ¹⁴C | ΔR databases + Marine20/IntCal20[15,16] | known reservoir offset (curve) | offset transferability is locality-dependent |
| ¹⁴C | Bayesian age models + outlier detection[24] | statistically discordant dates (priors) | garbage-in if priors/associations wrong |
| U–Pb | air abrasion[26] → CA-ID-TIMS[25] | outer / radiation-damaged Pb-loss domains | whole-grain inheritance (needs single-grain) |
| U–Pb | CL/BSE imaging + spot dating (SIMS, LA-ICP-MS) | specific growth zones (rim vs core) | ambiguous textures ⇒ circular assignment |
| U–Pb | ²⁰⁴Pb / Tera–Wasserburg common-Pb | non-radiogenic Pb component | leverage grows for young, low-radiogenic grains |
| U–Pb | concordance + cross-method (⁴⁰Ar/³⁹Ar, (U–Th)/He, astrochronology) | internal + external consilience | no written-record ground truth in deep time |
Radiocarbon. The pretreatment ladder runs from acid–base–acid, through wet-oxidation stepped combustion (ABOX-SC) for charcoal, to ultrafiltration of bone collagen, which retains the high-molecular-weight gelatin fraction (>30 kD) and discards low-weight degradation products and humics[20]. [F] Its ceiling is explicit and was reached at Vindija (§9): ultrafiltration cannot remove contaminants below the filter cut-off or those chemically cross-linked to collagen. The next rung, compound-specific dating, isolates a single amino acid (hydroxyproline, ~10% of collagen and rare elsewhere in nature) so that essentially every contaminant except collagen-based glue is excluded, at the cost of requiring more bone[21,22]. [F] Crucially, each rung selects a chemically or molecularly defined fraction—an age-independent criterion—never “the part that looks the right age.” For the reservoir problem, the field maintains regional ΔR databases and the Marine20/IntCal20/SHCal20 calibration curves[15,16]; but reservoir-offset transferability remains locality-dependent, the same caveat our leave-one-out test surfaced quantitatively (§6 unequal gains). A statistical layer—Bayesian age–depth modelling with explicit outlier detection (OxCal and similar)—down-weights discordant dates using stratigraphic priors[24], a model-level realisation of “screen, then report honest uncertainty.” [I]
Zircon. Physical and chemical removal of compromised material progressed from mechanical air abrasion of grain exteriors[26] to chemical abrasion (CA-ID-TIMS), which anneals radiation damage and then partially dissolves the damaged, Pb-loss domains, leaving residual closed-system zircon and effectively eliminating Pb-loss discordance[25]. [F] Imaging-guided microsampling—cathodoluminescence or back-scatter imaging followed by spot dating with SIMS or LA-ICP-MS—targets individual growth zones, supplying the rim-versus-core classifier the protocol depends on. Isotopic screening removes the common-Pb component through measured ²⁰⁴Pb or the Tera–Wasserburg intercept, with the honest caveat (§11) that its leverage grows for young, low-radiogenic grains. Finally, concordance is a built-in self-check, and cross-method agreement (⁴⁰Ar/³⁹Ar sanidine, (U–Th)/He) together with astrochronological tuning supplies the consilience that substitutes for written-record ground truth where none can exist. [I]
Where this protocol sits. Every method above is an instance of the same two moves: (i) isolate or avoid the foreign component on grounds independent of the age one is trying to measure—chemistry, molecular identity, crystallography, stratigraphy—and (ii) validate out-of-sample or by cross-method consilience. The contribution of this paper is not a new reagent but a single accounting that makes those two moves explicit, comparable across chronometers, and auditable per claim through the [F]/[I]/[A] tags. [I] And there is a limit no rung removes: no mitigation converts an Axis-B problem into an Axis-A guarantee. Where the foreign component cannot be characterised—an unknown reservoir, an ambiguous CL texture—the correct output is a wider, explicitly stated uncertainty, not a precise number. [A]