Early Pleistocene

Early Pleistocene (also known as Lower Pleistocene, or Calabrian) is a subdivision of the Pleistocene Epoch of the Geologic time scale. The beginning of the stage is defined at 1.806 ± 0.005 Ma (million years ago). The end of the stage is defined by the last magnetic pole reversal 781,000 ± 5,000 years ago. Originally the Calabrian was a European faunal stage primarily based on mollusk fossils. It has been subsumed into, and become equivalent to, the geologic stage called the Early Pleistocene. However, many of the mammalian faunal assemblages of the Early Pleistocene start in the Gelasian (last stage of the Pliocene). For example, the Platygonus and other Blancan fauna appear first in the Gelasian.

History of the definition of the Calabrian
Because sea shells are much more abundant as fossils, Nineteenth and early Twentieth Century geo-scientists used the plentiful and well-differentiatable Mollusca (mollusks) and Brachiopods to identify stratigraphic boundaries. Thus the Calabrian was originally defined as an assemblage of mollusk fossils, most brachiopods being extinct by then. Efforts were then made to find the best representation of that assemblage in a stratigraphic section. By 1948 scientists used the initial appearance of cool-water (northern) invertebrate faunas in Mediterranean marine sediments as the beginning marker for the Calabrian. The 18th International Geological Congress in London (1948) placed the base of the Pleistocene at the base of the marine strata of the Calabrian Faunal Stage, and denominated a type section in southern Italy. However, it was discovered that the original type section was discontinuous at that point and that the base of the Calabrian Stage as defined by fauna assemblages extended to earlier levels within the Pleistocene. A new type section was chosen, several miles from the original one, at Vrica, 4 km south of Crotone in Calabria, southern Italy. Analysis of strontium and oxygen isotopes as well as of planktonic foraminifera has confirmed the viability of the current type section. The 27th International Geological Congress in Moscow in 1984 formally ratified the type section. The starting date was originally thought to be about 1.65 million years ago, but has been recalculated as 1.806 Mya.

Present formal definition
The Global Boundary Stratotype Section and Point, GSSP, for the start of the Pleistocene is in a reference section at Vrica, 4 km south of Crotone in Calabria, Southern Italy, a location whose exact dating has recently been confirmed by analysis of strontium and oxygen isotopes as well as by planktonic foraminifera.

The beginning of the Early Pleistocene hence is defined as: Just above top of magnetic polarity chronozone C2n (Olduvai) and the extinction level of calcareous nannofossil Discoaster brouweri (base Zone CN13). Above the boundary are the lowest occurrence of calcareous nannofossil medium Gephyrocapsa spp. and the extinction level of the planktonic foraminifer Globigerinoides extremus.

The end of the Early Pleistocene is defined as the Brunhes-Matuyama magnetic reversal event.

Nomenclature problems
It is clearer to refer to the currently defined Early Pleistocene as the Calabrian, both because the type section is Calabrian, but more importantly because the lower boundary of the Pleistocene is currently under discussion. Most climatologists and geologists would prefer a start date of around 2.58 mya as more representative of the break between the Pliocene and the Pleistocene.