13C
increase near the Oligocene/Miocene boundary, from ~24.5 to 23 Ma, and the
Monterey Excursion between ~17 and 13.5 Ma. In addition, within the Monterey
Excursion, several 13C
maxima occur with quasi-cyclic periods of ~400 k.y.Variations in stable isotopes from Sites 999 and 1000 are linked to major changes in climate associated with the transition from relative global warmth of the late Oligocene-early Miocene to the Neogene "ice-house" world.
The oxygen-isotope increase between ~15.5 and 14.5 Ma is related to the major ice-sheet growth in Antarctica. This major climatic transition is characterized by a quasi-cyclic internal structure of ~400 k.y. reflecting variations in orbital eccentricity. This points to orbitally driven waxing and waning of Antarctica ice sheets.
The long-term trends in
carbon isotopes are marked by two positive excursions: the 13C
increase near the Oligocene/Miocene boundary, from ~24.5 to 23 Ma, and the
Monterey Excursion between ~17 and 13.5 Ma. In addition, within the Monterey
Excursion, several 13C
maxima occur with quasi-cyclic periods of ~400 k.y.
The increase in 13C
at 17 Ma leads the first major cooling step in Antarctica by 1.5 m.y. However,
all short-term climate excursions as indicated by oxygen isotopes seem to be
linked to shorter term 13C
maxima.
