A boundary, operated
A thin K-Pg event layer carries meteoritic iridium from Gubbio to the Chicxulub crater. Read its profile, total the fallout, and discover why the same mass balance permits more than one impactor size.
The pale line is not a photograph. It marks the event horizon this page lets you read.
The line is narrow enough to miss with a thumb. At Contessa near Gubbio, the red boundary clay held 9.1 ppb iridium, more than 30 times the cited adjacent background. Forty-one years later, a near-chondritic iridium-bearing horizon was reported inside the Chicxulub peak-ring core. Put the cursor on each.
Layer 1: the anomaly
The cursor moves only across observations the sources support. The drawing does not invent a continuous measured profile between them.
Three source positions: below, boundary, above. Off-boundary marks show the cited 0.3 ppb background cap, not transcribed sample points.
Drag to the middle position.
Boundary clay
9.1 ppb
9.1 / 0.3 = 30.33 times the cited background cap.
observed anomalyEqual background makes excess zero by definition. The readout names that state; it is not evidence.
What is locked: Alvarez et al. report 9.1 ppb for the acid-insoluble red boundary clay and an enrichment of more than 30. The 6.3 ppb whole-rock result is a different preparation and is not mixed into this comparison.
Layer 2: the conditional object
A fluence is iridium mass per area. A diameter appears only after four further choices and a spherical-body model.
Global Ir in model
4.081 × 1010 g
s times declared Earth area
Impactor mass M
3.710 × 1017 g
conditional on f and eta
Spherical diameter D
6.854 km
conditional on rho
Diameter is stubborn. Doubling a numerator multiplies it by 1.260. Doubling a denominator multiplies it by 0.794.
2 x fluence
1.260 x D
2 x Ir fraction
0.794 x D
2 x eta
0.794 x D
2 x density
0.794 x D
The paper's rounded intermediate is kept separate from the declared-area calculation above.
printed preliminary
7.4 × 1016 g
from the paper
divide by 0.22
3.4 × 1017 g
3.3636 × 1017 g before rounding
sphere at 2.2 g/cm³
6.6 km
6.634 km before rounding
Using `s = 8 ng/cm²`, `f = 0.5 ppm`, and an Earth area derived from the declared 6371 km radius gives a preliminary mass of 8.161 × 1016 g, 10.28% above the paper's printed preliminary. The exact route gives 3.710 × 1017 g and 6.854 km. The discrepancy is shown, not tuned away.
The evidence rail
The layer and the core establish an impact horizon and link it to Chicxulub. They do not compute how much extinction was caused by each stressor.
22 May and 6 June 1980
Smit and Hertogen published Caravaca iridium and osmium results before the Alvarez paper appeared. The Alvarez paper reported Italy, Denmark, and New Zealand enrichments of about 30, 160, and 20 times background.
2010 synthesis
Schulte and 40 coauthors synthesized the global ejecta layer, fossil timing, and modeled perturbations, concluding that Chicxulub triggered the mass extinction.
2021 crater core
Four laboratories reported a positive anomaly in M0077. The highest-enrichment interval, 616.60 to 616.55 m below seafloor, carries near-chondritic highly siderophile-element ratios. The full-profile fluence is about 20 ng/cm².
Two clocks, 2019
The U-Pb study resolved four high-volume eruptive periods, including one beginning before the boundary. The Ar-Ar study placed more than 90% of mapped lava within less than 1 million years and about 75% after the boundary.
2020 and 2023 models
Hull and coauthors inferred that major outgassing ended before impact. Cox and Keller recovered volcanic-scale emissions and no clear volatile impulse at the boundary.
2024 record, 2026 review
A terrestrial record found about 3 C warming and less than 10 kyr of about 5 C cooling around 30 kyr before the boundary, with recovery before extinction. The 2026 review says lava rate and volatile release are decoupled and key emission and stratospheric-injection constraints remain missing.
No score is computed. Lava volume is not converted into gas flux. Timing evidence can constrain causal stories, but this instrument does not turn chronology into a percentage of extinction.
These are the numbers the offline verifier independently recomputes. Published inputs remain labelled as inputs.
9.1 ppb is the published acid-insoluble red-clay value. The 0.3 ppb comparison is the cited background cap. The separate whole-rock comparison is 6.3 ppb.
This is a conservative floor made from the paper's textual band, dry density, and background. The paper's complete profile gives about 20 ng/cm²; that published total is not re-derived here.
The 6371 km radius is a declared round-number choice. It is not fitted to the 1980 result.
Run the independent check: node research/chicxulub-iridium/verify-chicxulub-iridium.mjs.
The globally distributed event layer, the ejecta record, and the near-chondritic horizon preserved in Chicxulub establish a major impact at the boundary and directly connect the crater-forming event to the K-Pg horizon. The 2010 synthesis concluded that Chicxulub triggered the mass extinction. This page does not claim that the 9.1 ppb measurement alone ended the dispute.
The layer does not identify every kill mechanism, compute Deccan's contribution, or turn impact occurrence into a percentage of extinction. It also does not precisely measure impactor diameter. The diameter is a conditional result with a particularly weak 1980 deposition-efficiency input.
The source text provides the anchor observations and integrated fluences, but the candidate materials do not include a reusable point-by-point table for either section. The Gubbio view therefore uses the boundary value and cited background cap. The M0077 view uses published interval statements. A smooth curve would look more geological and be less true.