Onset of the Mid-Miocene Climatic Optimum: inferences from stable isotopes on benthic foraminifera at ODP Site 1264 in the southern Atlantic

Abstract

In 2011, stable isotope records of benthic foraminifera from Ocean Drilling Program Site 1264 have been presented. To gain more knowledge about high resolution early to middle Miocene climate changes, these Oligocene-Miocene records have been extended in this paper. The extended records span the interval from ~18.91 to 16.97 Ma. The δ18O record exhibits a sharp decrease around 17 Ma, identified as the onset of the Mid Miocene Climatic Optimum (MMCO). The cause of the MMCO is not fully understood, but recent research indicates that it is related to a sharp increase in the atmospheric CO2 level. The carbon isotope record shows a significant increase from ~17.8 to 16.97 Ma, which is the result of the Monterey event, a period of global CO2 drawdown. By applying spectral- and wavelet analysis to stable oxygen and carbon isotope records, a very strong, continual imprint of the long term (~400 kyr) eccentricity cycle on the amplitudes of the oxygen and carbon isotope records has been identified. Besides the long term eccentricity cycle imprint, a much weaker imprint of short term (~100 kyr) eccentricity cycle on the amplitudes of the carbon and oxygen isotopes record has been recorded. This data is compared to the stable oxygen and carbon isotope records from the Site 926 and Site 1090. As a result, the Miller events Mi-1ab and Mi-1b have been identified within this interval. The changes in the eccentricity cycle, the oxygen and carbon isotope records of Site 1264 match well, although a phase lag ~43 kyr is identified for the long term eccentricity amplitudes variations of the carbon isotope record compared to the oxygen isotope record. This suggests a coupling between climatic changes and changes in the carbon and oxygen isotope records.