Botuverá Cave, Brazil · one stalagmite · 115,000 years
Water drips from a cave ceiling onto a stone. Each drop leaves a film too thin to see. Keep it up for 115,000 years and you get about 70 centimetres of stone, with the water’s chemistry kept in every layer.
Botuverá Cave, Brazil
Rain soaks through the soil above a cave and picks up carbon dioxide, which lets the water dissolve a little limestone. When a drop falls in the cave, it loses some of that gas and leaves a thin film of calcite on the stone below.
The stone on screen is a stalagmite called BT-2, from Botuverá Cave in southern Brazil. The database lists its top as still forming, dated to the year 2000.
The top of the stone
Between its top and its first date, which is about 2,300 years old, BT-2 grew about 8.3 millimetres in a thousand years. On average that is less than a hundredth of a millimetre a year. The thin layers at the top are drawn at that spacing; this stone’s yearly layers were not measured.
Further down, each colour is a measurement. Oxygen atoms come in a lighter and a heavier kind, and the stone keeps the mix that was in the dripping water. Blue is lighter, orange heavier.
Eighty years
At that rate, a life of 80 years adds 0.67 millimetres. That is thinner than a credit card (0.76 millimetres).
19 millimetres down
A layer cannot tell you its age, so scientists measure it. They take small samples and measure how much thorium has built up from the decay of uranium. The first measured date, 19 millimetres down, is 2,245 years, give or take 130 (a 95% range).
Each date is a small mark on the edge of the stone. Between two dates, the age is worked out from the depth.
14,000 to 12,000 years ago
Each band is coloured by how heavy the oxygen in that piece of stone is. Between 14,000 and 12,000 years ago it gets heavier, one of the biggest shifts in the whole stone.
That is when the last ice age was giving way to the warm period we live in, which began about 11,700 years ago. The study’s summary says the oxygen varies with where the rain comes from and how much falls.
26,000 to 19,000 years ago
When the ice sheets were at their largest, about 26,000 to 19,000 years ago, this stone in Brazil went on growing, about 5 to 6 millimetres every thousand years.
Zoomed out
Zoom out and the colours rise and fall in a slow rhythm. The pattern repeats about every 24,000 years. Chance alone would produce a repeat this clear about once in 70 times.
The study’s authors link these swings to summer sunlight, whose strength shifts as the direction of Earth’s axis slowly turns.
687 millimetres down
The oldest date, 687 millimetres down, is 114,905 years, give or take 5,100. This is the bottom: everything above it, about 70 centimetres, arrived one drop at a time.
Averaged over its life, it grew about 6 millimetres in a thousand years.
Caves around the world
Scientists have gathered about 900 records like this one, from 365 caves, in one shared database. Each dot is a cave; Botuverá is the gold one.
Six caves
Six of the caves are in Romania. How fast a stone grows depends on the cave, the rock and the water, so one stone says little about another.
One stalagmite from Cloșani Cave grew 46 centimetres in about 3,800 years, roughly 15 times faster than BT-2 grew over its last 2,300 years.
What is measured and what is drawn. The depths, the ages, the dates with their margins and the colours are the stone’s own data (689 oxygen measurements; 20 uranium–thorium dates, plus the top, taken as still growing; every “give or take” is a 95% range). The outline, the width, the curved layers, the cave, the drops and the thin layers at the top are drawn. Colours between two measurements are blended, and the colour scale is stretched to the stone’s own spread (the 95th percentile of its distance from its average), so that its swings are easy to see. Ages are years before 1950, which at these scales is “years ago” to within a century. No one measured how often a drop falls in this cave, so the page gives no rate of dripping. The study’s title says 116,000 years; the stone was sampled slightly below its oldest date (687 millimetres, 114,905 ± 5,100 years), and the ages of those last centimetres are estimated from the dates above, so the page says 115,000. The database gives the first date as 2,245 years; the original study’s table gives about 2,350, and the page uses the database.
Your lifetime. The stone stopped growing when it was taken from the cave, so the sentence under your birth year says what a stone like this would have added at its recent rate, 8.3 millimetres per thousand years (between 7.9 and 8.8 if the dates are allowed their full 95% margin). Other stretches of this stone grew between 2.4 and 14 millimetres per thousand years.
The colours and the rhythm. Lighter or heavier oxygen is the quantity measured (δ18O). The study’s summary links it to the source and amount of rain at this site; other caves differ. The shift between 14,000 and 12,000 years ago is 2.2 parts per thousand, comparing 1,000-year means. The 24,000-year repeat is the strongest peak of the stone’s own series; random series with the same slow drift produce a peak that strong about once in 70 times (p = 0.014).