Late Pliocene to early Pleistocene astronomically forced sea surface productivity and temperature variations in the Mediterranean

Abstract

Principal Component Analysis of late Pliocene to early Pleistocene planktonic foraminiferal abundances from the Mediterranean yielded two significant components which represent first order approximations for sea surface productivity and temperature. This interpretation is confirmed by the good fit between the score plots and sea surface productivity and sea surface temperature proxy records based on selected eutrophic and temperature-sensitive species using present-day habitat characteristics. Spectral analysis revealed significant peaks in the orbital frequency bands of the spectrum suggesting that variations in sea surface productivity and temperature are controlled by the quasi-periodic variations in the Earth's orbit. Precession-forced variations are reflected primarily in sea surface productivity and to a lesser degree in sea surface temperature. Maxima in sea surface productivity and temperature in this frequency band occurred at times of sapropel formation. The sea surface productivity maxima can be explained by an increased input of river-borne nutrients and by the shoaling of the pycnocline and the associated intensification of the Deep Chlorophyll Maximum (DCM) layer. Precession-forced variations in sea surface temperature are not related to glacial cycles since this frequency component is almost completely absent in ice volume proxy records for this time interval. Sapropel-bound sea surface temperature maxima presumably reflect periods of maximum insolation during the Northern Hemisphere summer. The contrasting winter signal corresponds to minimum insolation values which may be reflected by peak abundances of Globorotalia inflata. Obliquity-forced variations are reflected primarily in sea surface temperature and not in sea surface productivity. These temperature fluctuations correspond with ice volume changes registered in the oxygen isotope record at the North Atlantic DSDP Site 607. This shows that obliquity-forced sea surface temperature variations in the Mediterranean are primarily driven by glacial cycles.