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28The Oligocene epoch represents a somewhat neglected chapter in paleoclimate and 29 paleoceanographic history, which is at least partially due to the scarcity of complete

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Understanding the stability of the early Antarctic ice cap in the geological past is of societal interest because present-day atmospheric CO 2 concentrations have reached values comparable to those estimated for the Oligocene and the early Miocene epochs. Here we analyze a new high-resolution deep-sea oxygen isotope (δ 18 O) record from the South Atlantic Ocean spanning an interval between 30.1 and 17.1 Myr ago. The record displays major oscillations in deep-sea temperature and Antarctic ice volume in response to the ~110-kyr eccentricity-modulation of precession. Conservative minimum ice volume estimates show that waxing and waning of at least ~85 to 110% the volume of the present East Antarctic Ice Sheet is required to explain many of the ~110-kyr cycles. Antarctic ice sheets were typically largest during repeated glacial cycles of the 'mid' Oligocene (~28.0 to ~26.3 Myr ago) and across the Oligocene-Miocene Transition (~23.0 Myr ago). Yet, the high-amplitude glacial-interglacial cycles of the mid Oligocene are highly symmetrical, indicating a more direct response to eccentricity-modulation of precession than their early Miocene counterparts, which are distinctly asymmetricalindicative of prolonged ice build up and delayed, but rapid, glacial terminations. We hypothesize that the long-term transition to a warmer climate state with sawtoothed shaped glacial cycles in the early Miocene was brought about by subsidence and glacial erosion in West Antarctica during the late Oligocene and/or a change in the variability of atmospheric CO 2 levels on astronomical time scales that is not yet captured in existing proxy reconstructions.

Ordovician limestone-marl alternations in the Oslo-Asker District have been interpreted as signaling glacio-eustatic lowstands, which would support a prolonged “Early Palaeozoic Icehouse”. However, these rhythmites could alternatively reflect differential diagenesis, without sedimentary trigger. Here, we test both hypotheses through one Darriwilian and three Katian sections. Our methodology consists of a bed-by-bed analysis of palynological (chitinozoan) and geochemical (XRF) data, to evaluate whether the limestone/marl couplets reflect an original cyclic signal. The results reveal similar palynomorph assemblages in limestones and marls. Exceptions, which could be interpreted as reflecting palaeoclimatological fluctuations, exist at the species level: Ancyrochitina bornholmensis seems to be more abundant in the marl samples from the lower Frognerkilen Formation on Nakkholmen Island. However, these rare cases where chitinozoans differ between limestone/marl facies are deemed insufficient for the identification of original cyclicity. The geochemical data show a near-perfect correlation between insoluble elements in the limestone and the marls, which indicates a similar composition of the potential precursor sediment, also in the Frognerkilen Formation. This is consistent with the palynological data. Although an original cyclic pattern could still be recorded by other, uninvestigated parameters, our palaeontological and geochemical data combined do not support the presence of such a signal.