Before Present

A living thing borrows its carbon from the air, and a small share of that carbon is radioactive, and the borrowing stops at death. What remains decays on a schedule that nothing — heat, depth, prayer — has ever been shown to alter. That is my whole science. The dead keep a residue, and the residue keeps time.

I keep the register of the chronometry section, at the Survey’s diggings on the drowned levels, where the old river fans into the sea. The Asphalt Horizon lies a little under five thousand years below us: a band of glass, stoneware, corroded steel, and the black road-tar that gives it its name. Last spring the excavators opened a chamber beneath a fallen tower — a cellar, sealed first by the tower’s collapse and then by estuary clay, airless the whole time. Wood survives in a place like that. So does cloth. So, as it turned out, does a problem.

S-4406 · structural timber, reused · apparent age 7,140 ± 45 · exceeds the stratum by nineteen centuries. Old wood? — noted, R.

Old wood is the first excuse a dater reaches for: a beam can be centuries older than the building it holds up, and that beam had been reused at least once. But the excuse would not stretch to cover what followed. Everything from the chamber came back too old, and not uniformly too old — the excess grew as we worked down through the horizon, smoothly, like a floor tilting underfoot. We checked for conservation resins in the samples. We checked the bones for a fish-heavy diet, which borrows old carbon from the sea and ages a skeleton politely; the nitrogen said these people ate from the land. We checked for hard-water effects. Then we dated a sealed jar of charred grain from a shelf in the chamber, and stopped checking. A grain of wheat lives one summer. There is no old-wood problem in a grain of wheat. The jar returned an apparent age eleven centuries older than the clay that sealed it.

“The whole century is leaning,” Teo said. He meant it as a joke. I wrote it in the register, because it was the most accurate thing anyone had said all month.

There were two graves in the chamber floor, cut side by side. We lifted the northern burial first: a woman who had died somewhere in her seventh or eighth decade, laid out straight, wrapped in a plain weave that the clay had kept.

S-4471 · woman, seventh–eighth decade · rib collagen · apparent age 6,080 ± 40

S-4471 · the same woman · enamel series, eleven points along the tooth row · apparent ages 4,360 → 620, descending in order of formation

Read that again, as I did. Her ribs were a thousand years older than the clay above her. Her teeth were younger than her ribs by as much as five and a half millennia — and not scattered, which is what error looks like, but ordered. Teeth are laid down on a timetable, the first molars in infancy, the last in youth, and enamel, once formed, never remodels. Sampled in the order her body had made them, her teeth grew steadily younger in apparent age, step by step, like a staircase.

“Someone has salted the trench,” Teo said.

“She was articulated. The clay over her hadn’t been cut since it settled.”

“Then the counters are drifting.”

“Twice? Eleven points, in formation order?”

Error is noise. This was sequence. Her jaw was a recording, and what it recorded was the air itself changing under her, fast, in one direction, throughout her childhood. And the direction was the impossible one. The last-formed enamel was not merely young; its activity was above equilibrium — above what any atmosphere we knew could have lent her. Nothing a body does can make it more radioactive than the air it fed on. Enrichment has no honest source. So the air was the dishonest party. In the space of the ten years her teeth were forming, the radiocarbon in the air she breathed had nearly doubled.

We know what natural variation looks like; it is written in our own tree rings. The great flares move the ratio by a hundredth over a year or two. Her decade moved it by three-quarters. Only one thing in the record makes carbon at that rate, and it is not the sun.

Every section ever cut through the Asphalt Horizon, on every continent, carries the same thin film near its base: a fallout of split heavy metal, spread over the whole earth at once, thin as breath on glass. We had always used it as a marker bed without asking it what it meant. It meant neutrons. It meant that for roughly twenty years the people of that age burned their weapons in the open sky, and the sky, obligingly, made new carbon out of its own nitrogen, and the new carbon went into the wheat, and the milk, and the children, and every tooth formed on earth in those years took the year into itself and shut the door.

After that, the work went quickly. The spike rose over about a decade and fell over about a generation, as the oceans drank the excess down. Fit a person’s enamel series to the rising limb and you have their birth year, give or take one. Set their enamel against their bone — bone forgets, remodelling itself every couple of decades; enamel is testimony — and you have their death, give or take a few. Within a season we had done this for forty-one individuals from nine sites. For one generation of that world, and no other before or since, we can do what we cannot do for kings, for cities, for whole dynasties of that same civilisation: we can say when.

The spike explained her teeth. It did not explain the grain.

The grain, the timbers, the leaning floor of the whole horizon — the excess deadness — needed the opposite of a spike. It needed a vast and growing source of carbon with no activity in it at all, dead carbon, and there is only one store of that on earth, and they were famous for it. We call it the Asphalt Horizon for the roads. The roads were the least of it. They found the deep carbon — three hundred million years dead, not a whisper of a count left in it — and they burned it, more each year for two centuries, until the air’s living carbon was thinned like milk cut with water. Everything alive equilibrated with the smoke. Wheat came up out of the ground already wearing four hundred years. By their last decades, a newborn measured a millennium. They were ageing themselves in advance, the whole biosphere at once, and the more they burned the older they became, so that their final century is not merely blurred to us but dealt out into other people’s centuries, shuffled in among the honest dead of earlier ages, and no measurement will ever deal it back.

I should say, because it is the part strangers find hardest to believe: this was, so far as we can tell, the best-recorded age there has ever been. They stamped the hour on everything they did, to the second, billions of times a day. But they wrote in charge and in magnetism; charge leaks, magnetism fades, and their stones are terse. The age that counted everything kept its accounts in the one medium the air was busy falsifying — the bodies of its own dead.

We needed a zero that the horizon itself would honour, and there was only one candidate. We took the peak of the spike and called it the Datum, and re-based the register for the whole horizon: so many years before the burning, so many after. Every present is provisional, ours included, but a zero must be put somewhere. If their own daters ever faced a version of this problem, they will have chosen a zero of their own; I sometimes wonder where they put it.

The woman of S-4471 was born nine years before the Datum, give or take one. She died about seventy years after it, in the years when the air’s count, falling from the spike, was already sinking past its old level on its way down into the smoke. She was a child under the falling ash, and it is the ash that lets me write her line in the register with a confidence no one else in that chamber will ever be given.

The second grave had been cut beside hers with such care that it shaved the edge of her cut without touching her. A man in his middle thirties, wrapped in the same weave. On his wrist there was a small machine for keeping time, corroded into a single piece. We could not read it. The bone sequencers could read him, though: along the mother’s line and across the rest of it, he was her daughter’s son.

His teeth held only the tail of the spike, a whisper above the ordinary weather of the old air. His bones were smoke. We fitted everything we had, and the honest interval was four centuries wide. He was born too late for the sky to sign him.

There is a column in the register for basis. For most of the people of his generation and after — the most counted, most stamped, most thoroughly recorded people who have ever lived — that column will never say isotopes. Where we can fill it in at all, it will say what I wrote for him.

S-4472 · man, fourth decade · kin to S-4471: her daughter’s son · isotopes: an interval of four centuries · date: with S-4471 · basis: the grave