What do scientists use to mark boundaries in the geologic time scale?

Era is used to mark the boundaries of geological time scale by using radio carbon dating technique.

Explanation:

Geological time starts with Precambrian time.

Geologically, time is divided between the Precambrian era and present time into era.

The three eras are Paleozoic era, Triassic period and Cenozoic era.

Era is further divided into periods.

Scientists use radio carbon dating to study fossils and rock formation. With this study they were able to divide them in exact geological scale.

Geologic scale is the record of events in earth's history and evolution.