Triggers and consequences of glacial expansion across the Eocene - Oligocene Transition

Abstract

The results described in this thesis provide a rather complex picture of climatic, environmental and biotic changes preceding and arising from the onset of Antarctic glaciation. This period is commonly known as the greenhouse to icehouse transition across the Eocene-Oligocene Transition (EOT, 34-33 million years ago). The studies, based on fossil remains of algae and micro-organisms, do now for the first time constrain the timing and magnitude of cooling and ice-growth across the EOT. Environmental change occurred in two steps with the first, the EOT-1 shift (33.9 million years ago), representing cooling, bottom water production and ephemeral ice sheet-development, to be followed ~300.000 year later by abrupt ice sheet expansion towards a continent-scale size at the Oi-1 shift. Seen in the light of the reconstructed late Eocene evolution of the Southern Ocean, it is proposed that a series of intimately coupled positive feedbacks became active before glacial expansion across the Oi-1 shift. Enhanced west-ward circum-Antarctic circulation acted to support cooling of Antarctica, through 'thermal isolation', leading to increased latitudinal temperature gradients. The enhanced gradients enhanced polar wind-fields that spurred the westward circulation. Associated with this circulation-regime, bottom water was produced and enhanced vertical mixing supported biological productivity, which eventually may have acted to sequester CO2 from the atmosphere. This all may have resulted in the accelerated cooling during the initial phases of the EOT. With orbital preconditioning, the Antarctic ice-sheets could expand rapidly to eventually reach a continent-scale across the Oi-1 shift. Consequences of the Oi-1 glaciation were profound. Low-latitude shelf biota are notably affected by cooling and sea-level change during the initial phases of the EOT. The most profound reorganization is recorded in the Southern Ocean, where the onset of modern plankton ecosystems responds to the inception of seasonal sea-ice conditions along Antarctica's margins during the Oi-1. In contrary to what was intuitively assumed, sea-level variations act strongly non-linear. Close to Antarctica, the sea-level increased by ~100 m. This provides a containing feedback for the newly established ice-sheets. The origin of the present-day Antarctic ice-sheets provides an example of a strongly non-linear process, with a series of positive feedbacks leading to relatively rapid and abrupt glacial expansion. Once established, stabilizing feedbacks (e.g., albedo, circum-Antarctic sea-level change) prevent a return to the initial ice-free state. The evaluation of these processes using numerical climate models will yield crucial insight in the role of the climate-ocean-atmosphere systems with regard to ice-sheet stability. This is important because ice-sheets are already affected at observable rate by anthropogenic carbon dioxide emissions and elevated earth surface temperatures