Sea surface temperature during the Quaternary at ODP Sites 976 and 975 (western Mediterranean)

Based on the relative abundances of the planktonic foraminifera species found in Quaternary samples from ODP Sites 976 (Alboran Sea) and 975 (Menorca Rise), and using a modern analog technique, winter and summer sea surface temperatures were estimated. Frequency distributions of temperatures are asymmetrical, more so at ODP Site 976, where there is a greater abundance of samples from the warm part of the temperature range. During the Quaternary, there is a general tendency for temperatures to fall, with an overall diminution of 1°C for the winters and of more than 2°C for the summers. Overall mean temperatures and those of the biostratigraphic intervals identified are higher at ODP Site 975 than at 976. When differences are statistically significant, the Q1 interval (i.e. the oldest in the Quaternary) is colder than the UPl (i.e. terminal Pliocene) and the Q2 interval; the Q3 interval is also colder than the Q2 interval and the Q4 interval (i.e. the most recent interval). Differences between intervals, general trends and the asymmetries of frequency distributions cannot be explained as linear responses to insolation. The stratigraphic series from ODP Site 976 was dated by correlating the relative maxima and minima of the summer temperature series with those of insolation during the month of July at 65°N. It was found that 26.2% of the variance of the time series of temperatures was concentrated in the harmonics related to precession and obliquity, while 19% was located in those related to eccentricity. There are indications of the need to add lags and leads to the age of temperature spikes with respect to insolation, but such delays and leads cannot be quantified precisely.