My oh Miocene!

Guest commentary by Sarah Feakins

Our recent study in Nature Geoscience reconstructed conditions at the Antarctic coast during a warm period of Earth’s history. Today the Ross Sea has an ice shelf and the continent is ice covered; but we found the Antarctic coast was covered with tundra vegetation for some periods between 20 million and 15.5 million years ago. These findings are based on the isotopic composition of plant leaf waxes in marine sediments.

That temperatures were warm at that time was not a huge surprise; surprising, was how much warmer things were – up to 11ºC (20ºF) warmer at the Antarctic coast! We expected to see polar amplification, i.e. greater changes towards the poles as the planet warms. This study found those coastal temperatures to be as warm as 7ºC or 45ºF during the summer months. This is a surprise because conventional wisdom has tended to think of Antarctica being getting progressively colder since ice sheets first appeared on Antarctica 34 million years ago (but see Ruddiman (2010) for a good discussion of some of the puzzles).

Where did this record come from?

The ANDRILL program is a multinational collaboration involving scientists from Germany, Italy, New Zealand and the United States to drill through ocean sediments around Antarctica. The drilling effort in the austral summer of 2007 involved a rig perched upon the Ross Ice Shelf, drilling down through the ice, 400m of water below that and then grinding down 1km into the sediments. The sediments are bagged and then transported back to the storage facility in Florida from where they are parcelled out to analysis laboratories across the world.

It can take years to process all this sediment and perform all the compositional, elemental and isotopic analyses that need to be done. Numerous scientists work on getting the most information possible out of the core. One of the early findings was the unexpected discovery of abundant pollen in the Miocene part of the core by Sophie Warny (Warny et al, 2009). The pollen came from types of tundra vegetation and indicated summer temperatures above freezing, which was also inferred from the presence of freshwater algae.

After Sophie found the pollen, I began to search for molecular fossils of those same plants. The waxy coating of plant leaves is remarkable for its resilience in sediments. In addition those leaf wax molecules capture an isotopic record of past rainfall. It is these isotopic signatures that allow quantitative insights into temperature and rainfall.

To extract the leaf waxes we don’t look for visual fossils, instead we use organic solvents to dissolve and extract the leaf waxes out from the sediments. Those organic molecules are then purified by passing through a series of filtering steps in the lab. Ultimately we wind up with a pure concentration of the leaf waxes which can be analyzed by mass spectrometry (see photo).

How are the results interpreted?

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  1. S.J. Feakins, S. Warny, and J. Lee, "Hydrologic cycling over Antarctica during the middle Miocene warming", Nature Geoscience, vol. 5, pp. 557-560, 2012.
  2. W.F. Ruddiman, "A Paleoclimatic Enigma?", Science, vol. 328, pp. 838-839, 2010.
  3. S. Warny, R.A. Askin, M.J. Hannah, B.A. Mohr, J.I. Raine, D.M. Harwood, and F. Florindo, "Palynomorphs from a sediment core reveal a sudden remarkably warm Antarctica during the middle Miocene", Geology, vol. 37, pp. 955-958, 2009.