Sea-level and surface-water change in the western North Atlantic across the Oligocene–Miocene Transition: A palynological perspective from IODP Site U1406 (Newfoundland margin)

Abstract

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.

Temperate Oligocene surface ocean conditions offshore Cape Adare, Ross Sea, Antarctica

“…Present-day distribution of dinoflagellates and their cysts depends mostly on surface water temperature, but also 185 on nutrient availability, salinity, bottom water oxygen, primary productivity and sea-ice cover (Dale, 1996;Prebble et al, 2013;Zonneveld et al, 2013). Based on the notion that habitat affinities and feeding strategies of most dinoflagellates can be extrapolated to the fossil assemblages, while keeping in mind that most Oligocene dinocyst species and genera are extant, we can utilize 'deep-time' dinocysts assemblages as a paleoceanographic proxy (Sluijs et al, 2005;Bijl et al, 2013;Prebble, 2013;Crouch et al, 2014;Egger et al, 2018;Kulhanek et al, 190 2019). We link modern ecological affinities to Oligocene dinocyst taxa in the Pacific sector of the Southern Ocean, following Prebble et al (2013) and separate the dinocysts assemblages into Gonyaulacoids (G-cyst) and…”

Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 2: Insights from Oligocene–Miocene dinoflagellate cyst assemblages

“…3). Supporting evidence for the ecologic affinities of the dinocyst groups comes from empirical data, such as correlation of abundances with other sediment properties or proxies (Sluijs et al, 2005;Egger et al, 2018), for instance with regard to the affinities of Nematosphaeropsis labyrinthus, Operculodinium spp., Pyxidinopsis cpx. (this includes Corrudinium spp.…”

Astronomical tunings of the Oligocene–Miocene transition from Pacific Ocean Site U1334 and implications for the carbon cycle

“…Reliable reconstructions of CO 2 are rare across the OMT (www.p-co2.org) and the OMT does not seem associated with a large change in the depth of the Pacific calcite compensation depth (Pälike et al, 2012). Therefore, additional constraints on atmospheric CO 2 concentrations and burial fluxes are required to better understand the climatic and oceanographic mechanisms associated with the increased phase lag (Egger et al, 2018).…”