Astronomical climate forcing during the Oligo-Miocene

Abstract

In this thesis newly generated high-resolution Oligo-Miocene climate proxy records from Walvis Ridge ODP Site 1264 (south-eastern Atlantic Ocean) are presented (Chapters 2 and 3). The records are tuned to an eccentricity solution (Chapter 3) and they are compared to published Atlantic and Pacific palaeoclimate chronologies (Chapters 2 and 4). The main research objectives are 1) to identify astronomical pacemakers of global significance and test earlier pacing theories, 2) to describe global climate and oceanographic change on astronomical and tectonic time scales and 3) to test the strong hysteresis in ice sheet models that suggest a very stable Antarctic ice sheet once formed.

Chapter 1 gives a general introduction on the “mid”-to-late Oligocene climatic, oceanographic, geographic and cryospheric settings. Climate evolution and dynamics, together with the major underlying processes are introduced.

In Chapter 2, high-resolution early Miocene stable oxygen and carbon isotope chronologies from Walvis Ridge Site 1264 are presented. The data are analysed on an untuned age model to identify the principal astronomical pacemakers, without introducing power on orbital frequencies. A dominance of variance in all datasets on 100-kyr timescales is found. The ?18O data are used to parameterize a suite of 1D ice sheet models and show that between 20 – 80% (avg. ~50%) of the ?18O signal can be explained by changes in Antarctic ice volume. (This chapter has been published as: D. Liebrand, L. J. Lourens, D. A. Hodell, B. de Boer, R. S. W. van de Wal and H. Pälike. Antarctic ice sheet and oceanographic response to eccentricity forcing during the early Miocene. Climate of the Past, 7, 869–880, 2011)

In Chapter 3, extended stable-isotope records together with X-ray fluorescence core scanning data from Walvis Ridge Site 1264 are presented. The records span an 11-Myr mid Oligocene through early Miocene time interval. Ages are calibrated to eccentricity, are in good agreement with the GTS2012 and independently confirm the Oligo-Miocene time scale to the ~100-kyr level. The ~2.4-Myr long-period eccentricity cycle is identified as the main pacemaker of Oligo-Miocene climate events, as identified in the benthic isotope records, at shorter astronomical (eccentricity) periodicities.

In Chapter 4, the high-resolution Oligo-Miocene benthic stable-isotope chronology from Site 1264 is compared to published records from the Atlantic and Pacific to further identify and explore possible global climate pacemakers. In addition, an investigation of long-term trends and inter-/intra-basin isotopic gradients and their implications for ice volume reconstruction and palaeoceanographic studies are discussed. Methods are explored to quantify the apparent change in geometry of ~100-kyr cycles in our benthic ?18O data and the analyses indicate an increased cycle asymmetry (i.e. sawtooth patterns) throughout the Oligo-Miocene. This change in cycle geometry is interpreted as a measure of changing boundary conditions and used to track the evolution of a threshold response mechanism in Earth’s climate system.

In Chapter 5 the main results of this thesis are summarised, the implications for our understanding of the Oligo-Miocene are discussed and perspectives are given on future work.

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