The mid-Pleistocene transition: An Eastern Mediterranean perspective
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A reliable timescale is a critical tool in determining phase relations and causal relations in paleoclimate records. Presented here are oxygen isotope records collected from ODP site 967 located in the Eastern Mediterranean. The studied interval covers the MPT, from 1200 ka to 600 ka. Age control is provided by a Ti/Al timescale and changes that occur in global and local, Mediterranean, climate will be described based on this age model. New insights in the phase relation between orbital forcing and paleoclimate are gained by using this Ti/Al timescale. A comparison of the benthic record based on a Ti/Al age model used in this study to the widely used Lisiecki and Raymo (2005) LR04 benthic stacked record reveals a good match between the two records. Although there are differences between the two benthic records they are in sync with regard to the majorclimatic transitions in the isotope records for most of the studied interval. The assumptions made by Lisiecki and Raymo in the tuning of their record such as lag of ice sheets, with regard to the astronomical cycles which govern their waxing and waning, concur with the Ti/Al age model used to date the isotope record of ODP site 967.The main differences which exist between both records are likely related to regional effects acting on the record of ODP site 967 since they are amplitude differences and usually not phase related. An initial attempt at the establishment of a 100 ky period around 1200 ka was found, a precursor to the establishment of a 100 ky climate cycle. The influence of precession on glacial termination increases at this time. High precession amplitude, at high eccentricity, in combination with high obliquity result in a ~100 ky period early in the MPT. The volume of ice formed during glacials in this early 100 ky regime did not significantly change from glacials ice volume before the MPT. This suggests a change in ice sheet dynamics, which requires a higher insolation maximum to melt completely. Around 900 ka ice volume grows significantly, the changes in ice sheet dynamics may have altered the ice sheets to an extent were they become almost impervious to weak insolation maxima. This is seen in the suppression of obliquity maxima, which started at MIS 33. After the increase in ice volume around 900 ka the skipping of obliquity maxima, instead of suppression, describes the trend seen in our benthic isotope record. A fundamental change in ice sheet dynamics based on the regolith hypothesis is proposed. The increased sensitivity to high precession forcing in combination to obliquityis characteristics for the ice sheets which develop at the start of the MPT, a trend which continues over the MPT. The changes in ice sheet dynamics eventually results in larger glacial ice volume. The greater ice volume becomes even less sensitive to obliquity forcing. Glacial terminations occur only when the ice sheets have grown to an unstable size, over two obliquity cycles at high precession forcing.