Ice core dating is an independent method of absolute dating based on counting individual annual layers in large ice sheets.
The official boundary between the Pleistocene and Holocene can be observed well in ice core records. This boundary marks the very beginning of warmer climates.
The boundary is precisely 10 cm to the left of the break in the middle. It is only 1-2 cm thick and marks the last year of the last ice age. It also marks the end of the last minor glacier advance, the Younger Dryas, which lasted a little more than 1000 years. In the years following this year, temperatures in Greenland rose by 16 C over 20 years, dust concentrations dropped by a factor of 10 over fewer than 10 years, and the annual snowfall more than doubled over three years.

In 1783, the Icelandic volcano Laki erupted. The eruption lasted eight months and killed a fifth of Iceland’s population. The gas cloud from the eruption deposited a noticeable layer of sulphuric acid on the ice sheet, leading to falling temperatures in Europe and affecting harvests in the following years. This layer is not visible to the naked eye, but rather in the Electrical Conductivity Measurements (ECM). The ECM spike appears in the left portion of the ice core presented in this image.
In 1789, women walked to Versailles in France to protest the lack of bread and high food prices. This was one of the events that culminated in the French Revolution.

The ice cores show warmer temperatures in Greenland around 1000 CE, which may explain the Norse peoples' settlement there at that time. The year 1000 CE was 1 degree warmer than 1970—in Greenland—and 1850 was the coldest decade since the ice age. The Scandinavian Stone Age was 2.5 degrees warmer than it was in 1970.
On arrival in an uninhabited southern Greenland, Erik the Red named the land Greenland because it was green and lush along its fjords in contrast to the northeast of Iceland from which he came. The Norse peoples lived in Greenland for 400 years, after which the Little Ice Age occurred.

Due to ice's high thermal capacity and low thermal conductivity, every meter of ice core still "remembers" the temperature it had while it was at the surface.
Prof Dorthe Dahl-Jensen from the Niels Bohr Institute took advantage of this peculiar physical characteristic of ice and measured temperatures in the GRIP borehole after drilling, meter by meter, through 3 km of ice with a thermometer with a precision of 0.01 degrees. The temperature in 1970 CE was -31.7 C, in 1000 CE -30.6 C, in 1650 CE -32.3 C (when the Swedes took over Skåne, Halland and Blekinge), from 3000 to 6000 BCE -29.2 C (Holocene optimum) and from 21,000 BCE to 30,000 BCE -58 C.
These are all Greenland temperatures. While Greenland was 26 degrees colder 25,000 years ago, Kenya and Tanzania were only about 2.5 degrees colder (lake sediment studies), suggesting that the difference in global average between 1970 and 1000 could amount to only a fraction of a degree.

This ca. 15 cm long ice core fragment with visible air bubbles represents one year in ice core records.

In countries that celebrate Christmas and lie on latitudes where snow falls in winter, the big question leading up to Christmas is always, “Will it be a White Christmas this year?” The first Christmas snow can, in fact, be found in the ice cores — the snow which fell the year Jesus was born, the year from which the dating of the Christian era begins. In the NGRIP ice core from central Greenland, this Christmas snow can be found approximately 380 metres under the ice cap. The ice cap is like a huge sandwich, with each year represented as a layer in the ice.

Time before 450,000 years ago cannot be read in the Greenlandic Ice Sheet, since ice-core drilling reaches the bedrock and there is no more ice to measure. Using a stone drill, DNA from a forested southern Greenland has been found under the ice, presumed to be half a million years old. From the found DNA, we can reconstruct the landscape as it looked then, with plants, bushes, and insects, but we do not know precisely how old these finds are.
The latest research project, green2ice, funded by the Horizon Europe ERC Synergy grant, is trying to answer this question.
This unique project brings together researchers specialised in various complementary fields. Dorthe Dahl-Jensen brings her expertise in geophysics and modelling, François Fripiat in biogeochemistry on basal ice and frozen sediments, Pierre-Henri Blard in geochemistry and geochronology, and Anders Svensson in ice core measurements and stratigraphy.

The Vedde ash layer is 0.5 cm thick and approximately 10 cm from the left edge of this 55 cm long section. All the other layers visible in this ice core section are so-called 'cloudy bands', which are visible due to the high dust content in springtime snow during the ice age. One can almost count annual layers in this section. The annual layers are 4-2 cm thick.
When an ice core is bored all the way to the bottom of the Greenlandic Ice Sheet, only three visible ash layers, which stem from large volcanic eruptions, are found: the Saksurnavatn volcanic eruption ash layer from the early Holocene, some 10,000 years ago; the Vedde ash layer; and the NAAZ II ash layer.
There are about 20 layers of crypto-tephras, invisible to the naked eye but containing ash particles. One of these is the Alaskan Okmok eruption of 43 BCE (the Caesar volcano); this eruption has been fingerprinted by tiny ash particles.
The Vedde ash layer is one of the most noticeable and stems from a large, unnamed Icelandic volcano that scattered ash across Greenland, the North Atlantic, and Europe. Vedde ash was first found in sediments near Vedde in Zealand and Denmark, hence the name. The discovery of the ash layer in ice cores allows the Vedde layer in Zealand and elsewhere to be dated with very precise precision.