Why can ice and trapped air at the same depth have different ages?

An ice core can preserve records in both its solid ice and its enclosed gases. Those records do not automatically refer to the same time, even at the same depth. The solid layer began as snowfall, whereas air could continue exchanging with the atmosphere before it became trapped.

Compacted snow passes through a porous stage called firn. Connected spaces between grains permit air movement and diffusion. As the firn becomes denser, those pathways gradually close and bubbles become isolated. The air then enclosed is generally younger than the surrounding ice, which has already spent time being buried and compacted.

Researchers therefore distinguish an ice chronology from a gas chronology. The ice-minus-gas age difference is often called Δage. It depends on conditions such as temperature, snowfall accumulation and firn transport, rather than being a single correction valid for every core or depth.

Our fictional layer is 1000 years old relative to a shared reference year. Its supplied difference is 200 years, so the representative mean gas age is 800 years. The subtraction compares ages before that same reference; it does not mean that air molecules themselves were created 800 years ago.

Real gas samples contain a distribution of atmospheric ages because exchange and closure are gradual. This can smooth a changing atmospheric signal. Comparing an ice-based climate record with a gas-based record therefore requires their separate dating evidence and uncertainties, not simply matching their positions in the core.