Science · Daily Learning Experience · 20 August 2026

The First Day of the Cenozoic

You are 30 kilometres offshore of Progreso, Yucatán, on the deck of a jack-up boat called L/B Myrtle. A steel pipe is dragging up the story of a single day — a day sixty-six million years ago that lasted about twenty-four hours and rearranged the planet forever.

Hole M0077A · 21°27′00″N 89°57′00″W · water depth 19.8 m · core reaching 1,335 m below the sea floor

You've been sent to read a book that took one day to write

It is spring 2016. Two dozen scientists are packed onto a lift-boat above the buried rim of the Chicxulub crater. A rotating steel pipe drives ~1.3 km straight down through Cenozoic limestone and, at 616 m, punches into a shattered layer that piled up in a single terrible day. From 747 m to 1,335 m is the crater's peak ring: pink granite from ~10 km down, dragged upward and outward by the impact in about ten minutes and now, at last, in your gloved hands.

"About 130 m of impact rock, from crystalline melt at the base to a fining-upward suevite at the top capped by charcoal-bearing sediments, was deposited on the first day of the Cenozoic." — Gulick et al., The First Day of the Cenozoic, PNAS 2019

Every dark band, every crushed pink crystal, every splinter of fossil charcoal is a wristwatch. Together they tell you what happened between minute zero and roughly hour twenty-four. Below is the core. Above is the day. Read them both.

1. The Core — thirteen hundred metres in your gloves

Drag the slider or click a layer. The right column is the core, drawn to relative scale from Hole M0077A. The left column reads out what that depth is.

Depth
505 m
Layer
K-Pg boundary clay
66.043 Ma
Moment on the day
Weeks to years after the impact — global iridium fallout settles.
505 m ↑ younger · older ↓ 1335 m

2. The First Day Clock — one day compressed into a slider

Scrub across twenty-four hours. Time zero is the moment of impact. Every scene is drawn live in code.

T − 30 s T + 0 s T + 24 h
A Late-Cretaceous morning over the shallow Yucatán sea. A bright new star has been getting brighter for two seconds.

3. The Impact Bench — throw a rock at the Earth

Choose an impactor. The energy comes out live in joules and in megatons of TNT — the crater diameter is a simple Collins-style scaling. The white dashed lines are the real Chicxulub values.

4.4×10²³ JKinetic energy
105 millionMegatons of TNT
183 kmCrater diameter
Hiroshima × 7 billionComparison

The real Chicxulub impactor is estimated at ≈10 km, ≈20 km/s, at an angle of 45–60° — the "deadliest possible" angle for spraying vaporised rock into the stratosphere. Try dropping a comet at 60 km/s and watch the crater triple.

4. Tanis — a fish that swallowed the sky

Two thousand kilometres north of the impact, in a river in what is now North Dakota, a paddlefish is filter-feeding. About one hour after the impact, a wall of water from a distant lake pushes upriver while glass droplets — microtektites, cooled ejecta from the crater — rain down from the sky. The fish inhales them. Click a gill to see what a scanning electron microscope sees today.

The paddlefish is alive. It has been alive for the last thing it will ever be alive for.

5. The Missing Sulfur — what the core does NOT contain

The Yucatán target rock was a kilometre of anhydrite (calcium sulfate) sitting on top of limestone. If the impact simply piled the target rock back into the hole, the core should be one-third sulfate. It isn't. Anhydrite is absent. All of that sulfur left the crater and flew into the stratosphere as ≈325 gigatons of sulfate aerosol — a curtain that shut off sunlight for years and turned a bad day into a bad decade.

The bar on the left is what the target rock was made of. The bar on the right is what the core actually preserved. The missing wedge is the murder weapon.

Six things worth carrying home

66.043 MaDate of impact, ± 43,000 years — the second-most-precisely-dated moment in deep time
10 minTime for the peak-ring granite to rise from ~10 km down and then collapse outward
130 mThickness of one day's worth of rock in Hole M0077A
≈325 GtSulfur that left the target rock as aerosol — the real killer of most everything
≈75%Fraction of species on Earth that did not see the second week of the Cenozoic
SpringSeason of the impact, read from tree rings of growth in fish bones at Tanis (DePalma et al., Nature, 2022)

Four honest wobbles

"About one day." The 130 m of impact rock actually stacks in hours-to-days; "the first day" is a good approximation, not a Swiss stopwatch. The soot cap arrived weeks to months later.
Impact energy. Published estimates run from ≈10²³ J to ≈10²⁴ J; "100 million megatons" is the round number people say aloud. The bench here uses the middle of the road.
Tanis timing. The DePalma 13–90 min ejecta-arrival window is a real calculation from spherule velocities — but Tanis itself is contested by some researchers, and the tightest claims (down to specific hours) go beyond what the fossils can prove.
The impactor. A 2024 paper argues a carbonaceous chondrite carried less sulfur than a comet or S-type asteroid would have — meaning most of the killing sulfur came from the target, not the traveller.

Timeline — how we found out

Examination — Expedition 364 crew certificate

Home experiment — the Layered Bottle

Fill a tall clear jar or drinks bottle with these, in order, without shaking: 1 cm of pebbles (peak-ring granite), 1 cm of coarse gravel (impact-melt breccia), 2 cm of coarse sand (suevite), 2 cm of fine sand (fining-upward tail of the surge), and a pinch of crushed charcoal from your grill (wildfire cap). Now tape a paper strip alongside marked 0, 10 min, 1 hour, 1 day, weeks. You have just built the first six hundred metres of Hole M0077A, in the order it was deposited, on a scale where 1 cm ≈ 25 m. Take a photo. It is the same photo the drilling ship took, just less muddy.

Bonus round: sprinkle a teaspoon of table salt on top and shake gently. Watch the salt vanish downward as water joins it. That is roughly the way the sulfate did NOT stay behind at Chicxulub — except at continental scale, and with sunlight instead of water.

The closing question

If a single day can be preserved in a hundred and thirty metres of rock and read sixty-six million years later — how carefully do you think the day you are having right now is being written down, somewhere, by something you have not yet noticed?