Holocene Climate Optimum and Last Glacial Maximum in the Mediterranean: the marine oxygen isotope record

Reconstructions with comprehensive estimates of confidence intervals are presented of changes in the W–E stable oxygen isotope gradient in Mediterranean surface waters between the Holocene Climate Optimum and the Present, and between the Last Glacial Maximum and the Present. Rigorous statistical assessment is made of the significances of the mean geographic trends observed in these reconstructions. Firstly, it is concluded that any reconstruction should strictly be based on values obtained by analyses of one single foraminiferal species throughout the basin, as different species are found to respond with isotopic variations of different amplitudes to climatic/hydrographic change. This difference is tentatively related to differences between the habitats and seasons of growth of the various species. Secondly, a significant increase of roughly a factor 3 is found in the Mediterranean W–E oxygen isotope gradient during the Last Glacial Maximum, relative to the Present. This difference is almost entirely due to increased glacial values in the Levantine Sea, which are considered to be a result of a combination of increased evaporation rates and/or somewhat cooler than anticipated surface water conditions. Thirdly, an eastward increase of roughly half the present-day magnitude is found for the W–E oxygen isotope gradient during the Holocene Climate Optimum. Values in the Levantine Sea appear to have undergone up to 0.3‰ more depletion than those elsewhere in the eastern Mediterranean. However, no significant trends are found between the eastern and western parts of the Levantine Sea, nor between values near the Nile delta and those from elsewhere in the Levantine Sea. The Holocene Climate Optimums eastward increase in the Mediterranean oxygen isotope gradient, although weaker than the Present, suggests that the Mediterranean continued to function as a concentration basin, albeit in a less vigorous way than today. Finally, simple mixing arguments are used to argue that inferred oxygen isotope ratios of surface waters may not be used as an indication of conservative property (e.g. salinity) distribution on geological timescales, but instead show amplitudes of response to climatic/hydrographic changes that likely are 2 times larger than the corresponding amplitudes of response for truly conservative properties.