Chronometric dating relative dating
The ratio depends on two factors, the temperature and the isotopic composition of the seawater from which the organism secreted its shell.
Shells secreted from colder water contain more oxygen-18 relative to oxygen-16 than do shells secreted from warmer water.
Other criteria that have been used to define the Pliocene–Pleistocene include the appearance of humans, the appearance of certain vertebrate fossils in Europe, and the appearance or extinction of certain microfossils in deep-sea sediments.
These criteria continue to be considered locally, and some workers advocate a climatic boundary at about 2.4 million years.
The Pleistocene is subdivided into four ages and their corresponding rock units: the Gelasian (2.6 million to 1.8 million years ago), the Calabrian (1.8 million to 780,000 years ago), the Ionian (780,000 to 126,000 years ago), and the Tarantian (126,000 to 11,700 years ago).
Of these, only the Gelasian and Calabrian are formal intervals, whereas others await ratification by the ICS.
The Calabrian, which was previously known as the early Pleistocene, extends to the Brunhes–Matuyama paleomagnetic boundary at 780,000 years ago.
The Ionian, also known as the middle Pleistocene, extends to the end of the next to the last glaciation at about 130,000 years ago.
Grenda, Jeffrey Homburg, Manuel Palacios-Fest, Steven Shelley, Angela Keller and Davis Maxwell Spatial and Temporal Distributions of Archaeological Heated-Rock Cooking Structures in the Transverse Mountain Ranges: Proposed Markers of Land-Use Shifts since the Early Holocene [SCA Proceedings - PDF].
The boundary occurs near the level of several important marine biostratigraphic events and, more significantly, is just above the position of the magnetic reversal that marks the top of the Olduvai Normal Polarity Subzone, thus allowing worldwide correlation.
Since evidence of Cenozoic glaciation was discovered in rocks laid down earlier than those of the Vrica section, some geologists proposed that the base of the Pleistocene be moved to an earlier time.
This fact precipitated a debate among geologists over the formal start of the Pleistocene, as well as the status of the Quaternary Period, that was not resolved until 2009.
The Pleistocene Epoch (2.6 million to 11,700 years ago) is the epoch of hominin (protohuman) expansion.