Climate secrets beneath the sea

The Earth cooled rapidly at the Eocene-Oligocene climate transition, leading to the formation of the Antarctic ice-cap. However, the extent of climatic change in the northern hemisphere during this early Cenozoic 'greenhouse-to-icehouse' transition is unclear. Dr Helen Coxall looks into this.

Around 34 million years ago the last ‘natural’ CO2-induced greenhouse climate period ended and the Earth cooled rapidly, leading to the formation of the Antarctic ice-cap, a geographical feature that remains a major influence on the global climate today. The marine record of this transition has been extensively studied but much still remains to be learned about the extent and wider effects of these changes.

Dr Helen Coxall is a palaeontologist based at Stockholm University in Sweden. “I am the coordinator of an EU-funded project which focuses on a deep-sea bore-hole in the southern Labrador Sea. We aim to extract climate records from the Eocene-Oligocene transition,” she says. Research into the ancient geological past requires scientists to look beyond palaeoclimate data stored in tree rings and ice cores, towards the archives recorded in ocean sediment. However, gathering data of this kind isn't easy, particularly in areas like the southern Labrador Sea where seas are often rough.

“In 1985 the Ocean Drilling Program ship JOIDES Resolution lowered the drillstring through about 3.5 kilometres of water at the chosen drill site. At the sea floor drilling commenced to a maximum depth of about 700 metres. The interval I’m looking at is located at around 300 below sea floor,” continues Dr Coxall. “Sediment accumulates across the world’s oceans, but the thickness of it varies; to access 34 million year-old rocks or sediments we have to locate regions where the overlying layers are thin and the target interval is within reach of drilling technology”.

Historical records like these hold important insights into how climate-modes formed, and through using a tool kit of geochemical indicators, sediment characteristics, and fossil assemblages, palaeoclimate scientists can reconstruct the evolution of our climate. Much of this evolution has been marked by incremental change over timescales measured in millions of years; however, the Eocene-Oligocene climate transition marks a major jump to a new mode.

“The global climate seems to have gradually cooled over a period of several million years prior to the Eocene-Oligocene climate transition, this was probably related to gradually decreasing levels of atmospheric carbon dioxide. But 33.7 million years ago there was a very abrupt change that resulted in the growth of the Antarctic ice cap from small temporary glaciers to continent-wide ice caps,” explains Dr Coxall. Data gathered from both land and sea-based sources show that this ice growth happened over a relatively short time in geological terms – around 400,000 years – although Dr Coxall says improved temperature reconstructions will require the extraction of further data from new ocean sites. “Ice cores have been taken from Antarctica, but they only go back a million years at most. So far it hasn’t been possible to drill any further on Antarctica, so the surrounding oceans – and other areas of the global oceans – are the best sources of data on ancient time periods,” she stresses. “While we have a fairly clear idea of what things were happening on Antarctica and when, there’s still a lot of controversy over what was going on in the northern hemisphere at this time. However, there’s been a decided lack of suitable locations where we could study this.”

The southern Labrador Sea is a prime location for this kind of research. Carbonate fossils are extremely useful for climate reconstructions, so their presence is important to scientists. “I extract microscopic marine fossils called foraminifera. These organisms have a calcium carbonate shell, from which we can extract clues about the make-up of the ocean waters, the amount of ice stored on land, and the physical conditions that these organisms lived under. We do this by analysing the chemical make-up of their shells,” explains Dr Coxall. Foraminifera and other microfossils are spectacularly well-preserved in the southern Labrador Sea, allowing scientists to reconstruct the temperature of the ocean to a high level of accuracy; technically demanding work where it's crucial to take the organism’s position in the water column into account. “We’re looking for free-floating ‘planktonic’ foraminifera and their bottom-living counterparts – benthic foraminifera,” continues Dr Coxall.

“We use our knowledge of microfossil ecology and evolution to select appropriate markers. One goal is to reconstruct the surface temperature of the ocean, and so need to ensure that we choose a species of planktonic foraminifera that lived in the surface waters. Using oxygen stable isotope analysis combined with measurements of trace amounts of magnesium and calcium it is possible to estimate temperatures for the upper part of the water column. Accurate measurements of water temperature changes close to the surface give us a good indication of average atmospheric temperature changes in that region and help us separate the amount of cooling from the amount of global ice-sheet growth. This will be compared to similar data extracted from the bottom living fossils, which reveal deep ocean changes, as well as other Atlantic data to build a picture of climatic gradients 34 million years ago – that’s what we can use this type of approach for. Temperature and other parameters, eg. carbon dioxide concentration, can be estimated using alternative methods but our analytical approach of using oxygen stable isotopes is extremely well-established and we need only small amounts of fossil material to generate quite a lot of data.”

These data hold real significance in terms of our understanding of the evolution of global climate. Existing models suggest that reduced levels of CO2 were the primary cause of the Eocene-Oligocene transition; however, there are other hypotheses. “Around that time some tectonic changes occurred, which resulted in passageways between South America and Antarctica – the Drake passage , and Australia and Antarctica – the Tasmanian Gateway, widening. The Antarctic Circumpolar Current was establishing itself at this time, and to this day it plays an important role in isolating Antarctica and keeping it cold. This contrasts with the northern hemisphere, where the North Atlantic Current brings warms ocean water from low latitudes, which then warms western Europe and feeds into the North Atlantic thermohaline circulation,” outlines Dr Coxall.

Modelling studies of the Eocene-Oligocene transition show that the northern hemisphere’s ice-sheet system of glaciers has a different sensitivity to changing CO2 levels than those in the southern hemisphere, a finding of real importance in the context of modern concerns about climate change. Analysis of ancient greenhouse periods does give a useful perspective on contemporary concerns. At the Eocene-Oligocene boundary carbon dioxide decreased and the first massive Antarctic ice cap was established, so useful parallels can certainly be drawn.

However, while acknowledging the wider relevance of her work, Dr Coxall is keen to stress that the project’s focus is on fundamental research. “The Eocene-Oligocene interval contains many important lessons in terms of understanding climate – we plan to pursue further research into the relationship between CO2 levels, temperature change and ice volume change, for which there is currently a bare minimum of data,” she says. “We will need to refine our methods to do this, while we would also like to drill further in the southern Labrador Sea. The existing site has already yielded important material, but there are gaps in it which we need to fill. We also need to look at other important time periods so that we can establish a continuous timeline of climate evolution between the Eocene-Oligocene interval and the present day.”

For more information on the research, contact Helen Coxall at helen.coxall@geo.su.se

Published: Tuesday, 2nd March 2010 by Tom Freeman

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