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From Greenhouse to Icehouse

An Expedition 318 scientist observes an iceberg.

An Expedition 318 scientist observes an iceberg. Credit: John Beck, IODP-USIO

By Kevin Welsh

An ambitious drilling expedition off the coast of East Antarctica is looking for evidence linking declining carbon dioxide levels with the glaciation of subtropical Antarctica 34 million years ago.

In early 2010 I was fortunate to take part in an ocean drilling expedition on the Antarctic margin south of Australia, with the aim of unravelling some of the changes in regional and global climate that happened when Antarctica progressively changed from a warm, forested greenhouse land 40 million years ago to a glaciated wilderness soon afterward. I was aboard Expedition 318 of the Integrated Ocean Drilling Program (IODP), which was designed to take thousands of metres of sediment cores from the continental margin. These cores, recovered from the IODP drill ship JOIDES Resolution, would hold vital information about changes in the climate, the nature of the ocean, and the organisms living on land and in the ocean as far back as the warm Eocene Epoch, which ended 34 million years ago.

Remote, cold and desolate, the polar latitudes hold a special place in the collective imagination. The coldest, highest (on average) and driest place in the world, Antarctica is impossibly inhospitable, with ice sheets up to 4 km thick.

At a time when global temperatures are predicted to rise appreciably over the next century or so, one might consider that the already warm tropics are of greater importance in our quest to understand the effects of climatic change. However, there are several reasons why the higher latitudes are worthy of investigation. These regions are important cogs in the global climate system for three specific reasons.

First, the reflective nature of ice means that they are one of the main causes of planetary albedo, which is the ability of the Earth to reflect solar radiation, and therefore heat, back into space.

Second, they are where we find water that is cold enough and saline enough to form dense water capable of sinking to the bottom of the ocean. This causes the world’s oceans to overturn, and in doing so sequestering or releasing carbon from deep ocean sinks and changing the distribution of heat around the globe.

Finally, and probably most importantly, the higher latitudes are repositories of the vast majority of the worlds’ freshwater, locking up more than 100 metres worth of sea level rise in their bulky and apparently immovable mass.

East Antarctica, the largest potential source of sea level rise, is arguably the least understood. It behoves us to understand the drivers of glaciation and sensitivity of this system.

It might seem reasonable to expect that if there is a large land mass over the southern pole then we should expect to see ice forming there. However, this is not always the case. East Antarctica has held a more or less polar position since approximately 325 million years ago, and although it is thought to have occasionally experienced glaciations, by the Eocene epoch 56–34 million years ago it was experiencing warm and subtropical climates with evidence of the existence of subtropical rainforests along its margin.

Ice first appeared relatively suddenly on East Antarctica at the end of the Eocene period, but what caused changes in Antarctica’s climate to bring about glaciation?

The first efforts to understand this conundrum focused upon long-term palaeo­geographic changes that caused the opening of gateways around Antarctica, such as the Drake Passage and the Tasman Gateway south of Tasmania. Opening and deepening of these gateways allowed the establishment of the Antarctic Circumpolar Current, which flows around Antarctica’s coast. It is thought that this constantly circulating body of water prevents the southward movement of warm water that forms in the equatorial limbs of rotating water masses known as gyres in the Pacific, Atlantic and Indian oceans. Thus, when the current first formed, Antarctica’s coast was cut off, leaving the southern continent in what is described as “thermal isolation”.

This presumed tectonic influence on the creation of Antarctic ice sheets, based largely on scientific ocean drilling in the two gateways, was the prevailing wisdom for many years.

However, a number of scientific studies were uncovering some of the surprising rhythms to this Cenozoic cooling. One of the main tools that has been useful here are time series of the ratio of isotopes of oxygen found in the carbonate shells of deep sea-dwelling microorganisms called foraminifera.

This isotopic ratio is largely controlled by deep sea temperatures and global ice volume. Water containing lighter isotopes of oxygen (with fewer neutrons) evaporates more easily from the ocean, and continental ice has an isotopic signature that is very depleted in heavier oxygen isotopes. We can therefore track the waxing and waning of continental ice masses using this proxy.

Studies using this proxy have shown the decline in oceanic temperature during the past 50 million years and increase in ice volume over the past 34 million years. However, it has also shown sharp transitions in ice volume and transient warmings superimposed upon the overall cooling trend, indicating that the dynamics of southern continental glaciations were anything but simple and slowly evolving.

Specifically, a sharp increase in the relative proportion of oxygen-18 at the beginning of the Oligocene 34 million years ago shows a relatively abrupt ice sheet expansion. This, combined with some apparent reversals in the volume of continental ice sheets, began to sow doubts about the notion of glaciations controlled by the progressive opening of oceanic gateways around Antarctica.

Further work utilising the presence of non-endemic species of microfossils to reconstruct oceanic circulation has indicated that there is only limited evidence that warm waters from the tropics bathed the coast of Antarctica before the Antarctic gateways opened.

If this is so, then what other mechanisms could account for the initiation of glaciations? One possibility is the changing level of CO2 in the atmosphere.

It is well-known that CO2 is a “greenhouse gas” that absorbs more radiation in the infrared thermal range than it re-emits, helping to keep our planet relatively warm and habitable. It is generally accepted that the level of CO2 in the atmosphere has been decreasing from several thousand parts per million in the global atmosphere of the Eocene to several hundred parts per million in the present atmosphere, and there is evidence to suggest that transient warmings during the Eocene are associated with significant excursions in CO2.

It has been suggested that the build-up of ice on Antarctica could be associated with this trend in reduced CO2, with modelling results showing that, over a particular threshold change in CO2, ice cap expansion would occur very rapidly – in less than a million years.

Recent results from sediment cores collected in Tanzania indicate a link between global ice volume and atmospheric CO2 at the Eocene/Oligocene boundary. A model of CO2 influence on continental ice volume could also explain transient changes in ice volume if this correlated with similarly transient variations in global atmospheric CO2.

The question still remains as to why the initial sudden decrease in CO2 occurred at the same time that the Tasman Gateway opened south of Australia, as has been proven by earlier ocean drilling.

To test the hypothesis that atmospheric CO2 levels are the dominant driver of East Antarctic ice sheet dynamics we require high quality reconstructions of ice sheet volume and dynamics, and of palaeoclimate, which can only be obtained from the margin of East Antarctica. This data must extend back over the past 56 million years so that periods preceding the arrival of the ice and periods of the greatest ice volume may be sampled, as well as those all-important periods of transition from one state to another. Because most of Antarctica is covered in ice, drilling the strata containing its climate history is much more feasible at sea than on land.

Currently there are very few of these types of sedimentary records in existence. In fact, only five ocean drilling expeditions in the past 40 years have been able to sample these sorts of records along the 10,000 km long coast of East Antarctica. This is because there are great technical problems that are associated with the drilling of glaciated margins, not the least of which are the climatic conditions and the ever-present danger of icebergs. In addition to this there are few research vessels with the capability of collecting such sediments for research purposes, and the great expense of these types of expeditions makes it difficult for any one country to mount an expedition on their own.

Fortunately, Australia is a member country of the IODP, a consortium of countries including the US, Japan, Europe, Australia, New Zealand, Korea, India and China that commits money and scientific expertise to drilling the ocean floor to recover these sorts of records using the JOIDES Resolution.

In early 2010 an IODP expedition was mounted to the Wilkes Land margin, which is more than 2000 km south of Australia, to drill sediment cores from water depths between 500 metres and 4000 metres to collect the material needed to resolve these and other important questions regarding the dynamics of the East Antarctic ice sheet and associated sea-ice. During the expedition, which lasted 2 months, the ships’ crew of 130 worked round the clock while braving 10-metre waves and avoiding rogue icebergs to collect nearly 2 km of sediment core from seven sites and a number of different deep sea environments.

Prior to the expedition, the age and nature of the subsurface geology was only inferred from subsurface profiles recorded by seismic data surveys, so one of the first tasks aboard ship was to collect data on the age and type of sediment recovered. This data allowed the chief scientists to quickly alter the mission as needed to ensure that its scientific aims were achieved. Thirty-three scientists representing 12 different countries were aboard, including myself, and together we conducted this initial analysis.

The first scientists to access the cores were the micropalaeontologists, who were able to provide an estimate of the age of the rock by studying the microfossils preserved within the sediments. This age data was then refined by palaeomagneticists who looked for subtle changes in the preserved magnetic orientation of minerals in the sediments to indicate well-dated reversals in the Earth’s magnetic field.

Geophysical testing of the cores allowed the scientists on board to understand density changes in the subsurface rock, which can be used to calibrate seismic surveys of the subsurface. Micropalaeontological and sedimentological analysis defined the types of environments that were sampled and how they had changed with the opening and deepening of the Tasman Gateway, and the likely palaeoclimatic conditions.

Onboard geochemical analysis assessed the safety for the ship of any gaseous sediments being drilled, and the sediments were even sampled for their living microfaunal communities by the ship’s microbiologist in an attempt to understand the types of life that are flourishing below the ocean floor.

The initial results of the expedition were extremely promising as high quality sedimentary cores were collected in deep sea and shelf environments from a number of highly significant time periods. Importantly the expedition was able to collect sediments documenting the first arrival of ice on the Wilkes Land margin at the beginning of the Oligocene, where the presence of large “drop stones” in deep sea sediments confirmed that icebergs must have been carving rock from the continent itself and depositing large fragments far from land in the deep ocean.

Geochemical evidence also revealed a change from humid subtropical weathering of Antarctic rocks to sediments that are mechanically produced by the action of glacial weathering.

The oldest recovered sediments were dated to the early and middle Eocene, when the climate of Antarctica was hot and subtropical and the Tasman Gateway was relatively narrow and shallow. These cores will provide an excellent insight into the unglaciated Wilkes Land margin and are likely to contain evidence of warming events known as hyperthermals. Investigation of these extreme warming events provides insights into drivers of short-term climatic variation.

An unprecedented record of the waxing and waning of ice sheets dating from the late Oligocene through to the upper Miocene (~34–17 million years ago) was observed in deep ocean cores. Apparently cyclic variations in the oxygen content of oceanic bottom waters detected in these sediments give a tantalising insight into the beginning of the formation of deep water on the Wilkes Land margin.

Additionally, superb records were obtained through the Pliocene period (5.3–2.6 million years ago). This is also of special importance since the early Pliocene has been suggested as a potential analogue for future climatic scenarios, since global mean temperatures are thought to have been several degrees warmer and mean sea levels up to 25 metres higher than present.

Finally, an extraordinary record of recent climate of the East Antarctic margin has been gathered from diatom oozes that accumulate in deep basins scoured into the continental shelf by giant tongues of ice. These deposits are up to 250 metres thick, and cover the past 10,000 years. So high is the accumulation rate of sediment in this basin that sampling can be done on essentially seasonal changes, much like tree rings or coral archives. These records can be matched with ice core records from the continent itself to disentangle the relationship between the atmosphere and the ocean, thus providing an understanding of how these important keystones in the modern climatic system affect one another in one of the most remote yet most important regions of the world.

The shipboard science party is currently analysing the material collected during the cruise, employing a wide variety of geochemical, palaeontological and geophysical tools to unravel the history of the East Antarctic ice sheet to determine its dynamics and driving forces, and most importantly to provide the tools for predicting the future state of this important feature of the global climate system.

Kevin Welsh is an associate lecturer at the University of Queensland’s School of Earth Sciences. He would like to thank the expedition Co-chiefs Carlota Escutia (Universidad de Granada), Henk Brinkhuis (Utrecht University), Rob Dunbar (Stanford University), Staff Scientist Adam Klaus (Texas A&M University) and the EXP318 Scientists. The initial results of the expedition are now published on the IODP website (http://www.publications.iodp.org/preliminary_report/318/).