Time Scale Controversy: accurate orbital calibration of the late Miocene
Final Report Abstract
First and foremost, this project intended to resolve the late Miocene timescale controversy revolving around the accurate age of the Fish Canyon sanadine (FC) radio-isotopic dating standard by testing the ‘rock clock synchronization’ and thus the late Miocene Mediterranean tuning. This was achieved by producing highly accurate astronomical ages of all magnetochrons between 8.3 and 6.0 Ma at a suitable independent succession outside the Mediterranean, namely at IODP Site U1337 in the equatorial Pacific retrieved by IODP Exp. 321. As part of this project, we generated astronomically tuned, high-resolution benthic foraminiferal stable isotope records and a new magnetostratigraphy, which represents a new deep-sea late Miocene stable isotope and magnetic polarity reference section for the 8.3-6.0 Ma interval. Our unequalled record additionally provides unprecedented insight into late Miocene climate evolution. The fidelity of this new deep-sea record was verified by comparison too an improved record from Atlantic ODP Site 926, which has a highly accurate astrochronology developed independently from the benthic stable isotope stratigraphy. With our new late Miocene reference section from Site U1337, we were able to test the ‘rockclock synchronisation’ and reconcile previously contradictory ages for the FC age. According to the ‘rock-clock’ synchronisation the FC standard age was initially set to 28.201±0.046 Ma and later refined to 28.173±0.028 Ma. Our results point to a younger FC standard age of 28.083±0.045 Ma, which is more consistent with the 28.10 Ma age based on tuned Eocene sections and a recently revised Fish Canyon sanadine age of 28.126±0.019 Ma, obtained by a new generation of multicollector mass spectrometers enabling far more accurate 40Ar/39Ar dating than before. Despite our reconciliation of the FC standard age, achieving irrefutable integration of astrochronological, magnetostratigraphical and radio-isotopic dating techniques will require analysis of a section with the potential to recover accurate chronologies using all three techniques. To date, this has not been achieved, as a section with the potential to generate these records is yet to be found. However, our study provides the first step towards achieving full integration, as we generated a complete and robust magnetostratigraphy at a single deep-sea location, where previously the signal was deemed too weak to achieve significant results. Following the successful completion of the main objective of the project, the second major outcome of this project was the generation of a robust, high-resolution benthic δ18O stack between 8.00-5.33 Ma, representing the first benthic isotope reference section for the late Tortonian and Messinian stages. Establishing accurate deep-sea age control for the 8-6 Ma interval is crucial to understand patterns in late Tortonian-early Messinian benthic foraminiferal stable isotope records, characterized by the late Miocene carbon isotope shift (LMCIS) and a long-term benthic δ18O decrease with distinct short-term cycles superimposed. At the start of this project, the highresolution benthic δ18O records available for the late Miocene were severely limited; at the project conclusion, we have achieved a fourfold increase in the number of high-resolution benthic stable isotope records for the late Miocene. We generated a robust late Miocene Pacific benthic δ18O stack for the 8.30-5.33 Ma interval, which has been tied into the GPTS, by integrating the new U1337 chemo-magnetostratigraphy with a high-resolution benthic δ18O record from U1338. This was then combined with highly accurate, high-resolution stable isotope stratigraphies from the North (ODP 982), equatorial (ODP 926) and South (ODP 1264) Atlantic. Our new stacked benthic δ18O record will be key in constraining the role of glacio-eustatic change in the Messinian Salinity Crisis by determining the exact timing and extent of the large Late Miocene glacial cycles, and help establish the feasibility of the onset of Northern Hemisphere glaciation during the late Miocene in future studies. Finally, using the global correlation of benthic δ18O records, we independently compared the δ13C records from the major oceanic basins. Our global δ13C compilation allows us to redefine the Late Miocene Carbon Isotope Shift as a four-stage event, potentially responding to a strong 400 kyr forcing. This indicates that periodic increases in CO2 uptake by the oceans or periodic organic matter input from the continental regions may be playing a major role in late Miocene carbon cycle dynamics, which will be tested using carbon modelling in the future.
Publications
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2017, ‘Expedition 363 Preliminary Report: Western Pacific Warm Pool’, Proceedings International Ocean Discovery Program
Rosenthal, Y., A. Holbourn, D. Kulhanek, and Expedition 363 Scientists
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2017: Revisiting the Ceara Rise, equatorial Atlantic Ocean: isotope stratigraphy of ODP Leg 154 from 0 to 5 Ma, Climate of the Past, 13, 779-793
Wilkens, R. H., Westerhold, T., Drury, A. J., Lyle, M., Tian, J, and Gorgas, T.
