Time Scale Controversy: accurate orbital calibration of the late Miocene

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The objective of the project is to investigate potential intrinsic problems with the rock clock synchronization between radio-isotopic and astronomic dating techniques. We hypothesize that the tuning of ash layers in the late Miocene Mediterranean sections (Sorbas, Faneromeni, Messadit, etc.; located in magnetochrons C3An.2n, C3.Ar and C3Bn) used for the synchronization is incorrect by 2-3 precession cycles (40-60 kyr). To test this hypothesis, we are developing a very accurate, astronomically calibrated late Miocene time scale for magnetic polarity Chrons C3An.1n (6 Ma) to C4n.1n (7.5 Ma) (including C3An.2n, C3.Ar and C3Bn) using three ODP/IODP sites outside the Mediterranean (926, U1337, U1338). During the first phase of the project, preliminary interpretation of new magnetostratigraphic data acquired at Site U1337 allowed identification of fourteen geomagnetic reversals, of which eight reversal positions have been located with very high confidence. The remaining six reversal positions, including Chron C3Bn, still need further analyses. The samples for the stable isotope stratigraphy analyzed so far contain sufficient benthic foraminifera to generate an isotopic record suitable for astronomical tuning. The main goal of this extension proposal is to exactly pinpoint the position of six key reversals and extend the high-resolution benthic stable isotope record at Site U1337, and fill gaps located in isotope records from ODP Sites 982 and 1264. All records will be integrated to generate a high-resolution, astronomically tuned benthic stable oxygen isotope stack between 5.2-8.2 Ma to provide a global stratigraphic reference section for the late Tortonian and Messinian. High-resolution paleoceanographical reconstructions based on the envisaged benthic stable oxygen and carbon isotope records will allow resolving of outstanding late Miocene paleoclimate questions. The paleoceanographic reconstructions based on our new data need to be accompanied by investigations evaluating how the primary benthic isotope signal is affected by secondary diagenetic alterations, and analytical and natural sample variability.