Early Late Pliocene biochronology and surface water temperature variations in the Mediterranean

Abstract

A high-resolution magnetobiostratigraphy is presented for the early Late Pliocene in Sicily. Paleomagnetic measurements on samples from 100 stratigraphic levels provide an unprecedented high-quality polarity sequence which extends from the upper reversed Gilbert to the upper normal Gauss subchron. The resultant chronology for the early Late Pliocene provides ages for the following principal Mediterranean bioevents: (1) FOD Globorotalia crassaformis at 3.40 Ma (2) prolonged absence of Globorotalia puncticulata between 3.38 and 3.16 Ma (3) LOD Uvigerina rutila at 3.16 Ma (4) LOD Sphaeroidinellopsis seminulina at 3.07 Ma (5) LOD Globoquadrina altispira at 3.04 Ma (6) LOD Cibicides italicus at 2.96 Ma. Summer and winter surface water temperature estimates for the period 3.50 to 2.92 Ma are based on the abundances of Globigerinoides sacculifer and Globorotalia puncticulata in 111 samples with an average resolution of some 5000 years. Short-term variations in temperature of the surface water and in colour and carbonate content of the sediment are primarily controlled by the equinoxal precession. A higher seasonality during deposition of the grey-coloured, carbonate-poor facies suggests that at that time the summer solstice occurred near perihelion, whereas an inverse alignment occurred at the time of deposition of white-coloured, carbonate-rich layers. This phase-relationship between the precession cycle and grey-coloured layers is also valid for the sapropels which began to intercalate in the grey-coloured layers at 2.94 Ma. Long-term changes in surface water temperatures include a warming between 3.38 and 3.18 Ma followed by a cooling from 3.18 to 3.03 Ma. The warming of Mediterranean surface waters at 3.38 Ma is believed to be associated with the final closure of the Isthmus of Panama. Consequent strengthening of the Gulf Stream injected larger volumes of warm surface water into the North Atlantic Drift and eastern boundary current and this caused surface water temperatures in the mid to high-latitude northeast Atlantic and Mediterranean to rise. The subsequent climatic cooling at 3.18 Ma is linked to a first stage of Northern Hemisphere continental ice growth.