The reported proxy peaks at sites across four continents form a single isochronous layer dating to approximately 12,835–12,735 cal BP.

A Bayesian model consistent with synchrony versus an audit finding 3 of 29 sites securely dated, with boundary assignment circularity unresolved and the 2026 Baffin retraction demonstrating the failure mode in the hypothesis's newest work. The isochron is asserted, not established.

The synchroneity of the proxy peaks is mixed. Proponent Bayesian models argue for a synchronous window, but independent audits suggest only a fraction of claimed sites possess chronologies precise enough to secure this dating. The strongest opposing consideration is the circularity of defining the boundary horizon by the presence of proxies in sediments with wide temporal uncertainties, which can manufacture apparent simultaneity out of diachronous local deposits.

Kennett et al.’s Bayesian modeling (YDIH‑E020) shows that the available dates are statistically consistent with a ~100‑year synchronous boundary window around 12.8 ka, which gives some positive support. Yet Meltzer et al.’s audit (YDIH‑E019) finds that only 3 of 29 claimed sites have chronologies adequate to place their proxy layer securely at the Younger Dryas onset, and the Baffin Bay retraction illustrates how calibration choices can shift a “boundary” horizon outside the Younger Dryas entirely (YDIH‑E034). The strongest opposing consideration is that boundary assignment and date inclusion were not independent of the hypothesis being tested, so apparent synchrony is to some extent constructed rather than measured.

Kennett and colleagues' 2015 Bayesian model finds dates consistent with a synchronous window of 12,835–12,735 cal BP, while Meltzer and colleagues' 2014 audit found only three of twenty-nine claimed sites had chronology adequate to place their proxies at about 12.8 ka. The 2026 Baffin Bay retraction showed recommended recalibration moving a claimed boundary layer out of the Younger Dryas entirely. The strongest opposing consideration is that if the Bayesian date set is representative rather than selected, consistency with a century-scale isochron would be genuine support. The model inherits its date selections and boundary assignments from the same event framework being tested, which can manufacture apparent synchrony.

The formal state is a standoff: Kennett et al. 2015 report the dates consistent with a ~100-year synchronous window, while Meltzer et al. 2014 found only 3 of 29 claimed sites with chronology adequate to place proxies at the right moment — and the Baffin Bay retraction later demonstrated the failure mode in vivo, recommended recalibration moving that site's "boundary" out of the Younger Dryas entirely. The circularity objection — boundaries assigned within the same framework being tested, every horizon overlapping 12.8 ka labeled "the boundary" — has never been answered with a blinded, preregistered design. Strongest opposing consideration: a formal Bayesian consistency result is not nothing, and the audit and the model disagree about date quality rather than arithmetic; no preregistered synchrony test with blinded boundary assignment has yet dissolved the isochron outright.

The affirmative case is a single Bayesian model whose inputs — which dates enter and which stratum counts as "the boundary" at each site — were not chosen independently of the hypothesis, so "consistent with synchrony" is substantially weaker than demonstrated synchrony. Against it, an independent site-by-site audit found only 3 of 29 localities with dating adequate to place the proxy horizon at ~12.8 ka, and the Baffin Bay retraction showed the failure mode operating in 2025: recommended recalibration moved the claimed layer out of the Younger Dryas entirely. The strongest opposing consideration is that circularity is an objection to the method, not a demonstration of asynchrony — a properly blinded synchrony test (YDIH-R004) has never been run. The claim also inherits the prior problem that the peaks being correlated are themselves contested.

A Bayesian model consistent with synchrony (YDIH-E020) against an audit finding 3 of 29 sites securely dated (YDIH-E019), with circular boundary assignment unresolved; the Baffin Bay retraction (YDIH-E034) exhibited exactly this failure mode in the newest work. The isochron is asserted, not established.

A Bayesian model 'consistent with' synchrony (YDIH-E020, carrying an unreviewed 2024 correction) stands against a site-by-site audit finding only 3 of 29 localities adequately dated (YDIH-E019), with boundary-assignment circularity unresolved and the independent onset age (YDIH-E021) offset from the modelled window. The retraction notices (YDIH-E034, YDIH-E035) document the same age-model and calibration defects in the newest work.

A Bayesian model 'consistent with' synchrony (YDIH-E020, itself carrying an unreviewed 2024 correction) stands against a site-by-site audit finding only 3 of 29 localities adequately dated (YDIH-E019), with boundary-assignment circularity unresolved and the independently dated onset (YDIH-E021) offset from the modelled window. The retraction notices (YDIH-E034, YDIH-E035) document the same age-model and calibration defects in the newest work.

A Bayesian model 'consistent with' synchrony (YDIH-E020, itself carrying an unreviewed 2024 correction) stands against a site-by-site audit finding only 3 of 29 localities adequately dated (YDIH-E019), with the circularity of boundary assignment unresolved and the independent onset age (YDIH-E021) offset from the modelled window. The retraction notices (YDIH-E034, YDIH-E035) show the same calibration and age-model defects in the newest work.

A Bayesian model 'consistent with' synchrony (YDIH-E020, carrying an unreviewed 2024 correction) stands against a site-by-site audit finding only 3 of 29 localities adequately dated (YDIH-E019), with boundary-assignment circularity unresolved and the independent onset age (YDIH-E021) offset from the modelled window. The retraction notices (YDIH-E034, YDIH-E035) document the same age-model and calibration defects in the newest work.

A Bayesian model 'consistent with' synchrony (YDIH-E020, carrying an unreviewed 2024 correction) stands against a site-by-site audit finding only 3 of 29 localities adequately dated (YDIH-E019), with boundary-assignment circularity unresolved and the independent onset age (YDIH-E021) offset from the modelled window. The 2026 retraction notices (YDIH-E034, YDIH-E035) document the same age-model and calibration defects in the newest work.

A 2015 Bayesian analysis supports a ~100-year synchronous window, but an independent 2014 chronological review found only 3 of 29 claimed boundary sites had secure dating, and the 2026 Baffin Bay retraction showed the failure mode in action: recommended recalibration moved that site's "boundary" layer out of the Younger Dryas entirely. Radiocarbon uncertainty, reservoir effects, and after-the-fact boundary assignment can manufacture apparent synchrony.

A comprehensive chronological audit showed that only 3 of 29 claimed boundary sites possessed radiocarbon control sufficient to place their proxy horizons at ~12.8 ka cal BP. The apparent global isochron was constructed by selective date inclusion and circular boundary assignments, leading directly to the 2026 retraction of the flagship shocked-quartz and cometary-dust studies for age-model defects. The strongest opposing consideration is Kennett et al.'s 2015 Bayesian analysis, which concluded that calibrated radiocarbon dates from four continents are statistically consistent with a 12,835–12,735 cal BP boundary.

The record holds two directly opposed analyses: Meltzer et al. 2014's audit found only 3 of 29 claimed sites adequately dated for an isochron, while Kennett et al. 2015's Bayesian model found the data consistent with a synchronous 12,835–12,735 cal BP boundary. The strongest opposing consideration against the synchrony claim is that the Bayesian model's date selection and boundary assignments are contested as partly circular, and the paper carries an unreviewed 2024 correction; against the audit, that a formal synchrony model exists and the dispute over date selection is genuinely open. The claim as stated — a single isochronous layer across four continents — is therefore contested at the methodological level, not settled either way.

YDIH-E019 found that only 3 of 29 claimed sites had adequate dating control to place their proxy horizon near 12.8 ka, directly undermining the asserted four-continent isochron. YDIH-E020 found that selected data were consistent with a common 12,835–12,735 cal BP interval, but its force depends on disputed date selection and boundary assignments that were not independent of the hypothesis. The chronology and incomplete-background defects documented in the 2026 retractions reproduce the same failure mode. The strongest opposing consideration is the formal Bayesian synchrony model, but consistency under hypothesis-dependent inputs is not a secure demonstration of one layer.

Site-by-site audit shows only 3 of 29 claimed localities possess radiocarbon control adequate to place proxies at ~12.8 ka; Bayesian models claiming synchrony rely on contested date selection and boundary assignment that risk circularity, and the 2026 retractions explicitly cite age-model failures. Strongest opposing consideration is the formal consistency of selected dates with a narrow window, but consistency under selective inclusion does not demonstrate an isochron.

Meltzer et al.'s site-by-site audit found radiocarbon control adequate to place the proxy horizon at ~12,800 cal BP at only 3 of 29 claimed localities (E019). Kennett et al.'s Bayesian model finds the data consistent with a 12,835–12,735 cal BP window (E020), but its date selection and boundary assignments were made within the event framework, and the paper carries a 2024 correction whose content was not reviewed. The 2026 retraction notices show the same failure mode in the newest flagship work: a calibration that, done as recommended, put ages outside the Younger Dryas, and an age model that omitted available dates (E034, E035). A synchronous layer also presupposes that its peaks are real, which the replication record leaves weak (C003, C004). Strongest opposing consideration: the Bayesian model is the only formal synchrony test in the ledger and returned a coherent century-scale window, and the audit shows inadequate dating, not asynchrony.

A 2015 Bayesian analysis supports a ~100-year synchronous window, but an independent 2014 chronological review found only 3 of 29 claimed boundary sites had secure dating, and the 2026 Baffin Bay retraction showed the failure mode in action: recommended recalibration moved that site's "boundary" layer out of the Younger Dryas entirely. Radiocarbon uncertainty, reservoir effects, and after-the-fact boundary assignment can manufacture apparent synchrony.