Signatures of time

Scientists have known since at least the 1950s that certain species of stable isotopes show systematic variations in abundance through time. However, it is only in the past decade that this finding has consolidated into a new branch of geology known as ’stable isotope stratigraphy’. This itself is part of an emerging larger discipline that uses elemental and molecular variations to identify different intervals of the geological record and which is known simply as ’chemostratigraphy’. 

The initial focus on stable isotopes was largely a historical accident generated by the interest in unstable isotopes during World War II and at the start of the Cold War. First to discover these variations was the Nobel laureate and Manhattan Project alumnus Harold Urey. Urey was interested in finding a way of measuring temperatures in the geological past and he reasoned, on the basis of physical kinetics, that proportionally more of the light isotope of oxygen, ¹⁶ O, would be incorporated in shell material with a rise in temperature. This would be due purely to the fact that ¹⁶ O has greater vibrational energy than the heavier isotope ¹⁸ O, and so is more easily incorporated into a calcite lattice such as the shell of a mollusc. 

Urey’s colleague Cesare Emiliani took the idea further and applied it to tiny, single-celled foraminifera that live in the surface waters of the ocean and which accumulate in vast numbers on the sea bed. Over eons these form vast drifts of sediment that can be sampled by deep sea coring. 

The huge advantage of cores (compared with land or ’outcrop’ sections) is that the sediments are well ordered in time. It is therefore possible to be fairly sure that the sequence of events is accurately recorded when going from older to younger sediments. Emiliani was able to start to take Urey’s idea to its ultimate conclusion, the mapping of temperature change (ie climate) through geological time. 

But it soon became clear that Urey’s idea was an oversimplification - oxygen isotope values do reflect temperature, but there is a contaminating influence: the growth and decay of the great ice sheets through glacial-interglacial cycles. When ice sheets grow they incorporate water that has been evaporated in the low latitudes and carried to the poles in the form of water vapour which is then precipitated. Evaporation favours the light isotope of oxygen and so polar ice has proportionally more ¹⁶ O than the seawater left behind. Hence foraminifera growing in the oceans during glacial intervals will already have a heavier ¹⁸ O signature than during interglacials. Conversely, during interglacials, both warmer temperatures and less ice equals more ¹⁶ O in fossil forams. 

Investigations of foraminifera living on the sea bed (benthics), where temperatures are supposedly constant, revealed that they too showed large variations between glacial-interglacial intervals. This in turn suggested that changes in ice volume were far more important than temperature. However, for the purposes of chemostratigraphy this debate is academic; what is important is that both planktonic and benthic fossil foraminifera show systematic variations in time that reflect the growth and decay of ice-caps. A way had been found of unambiguously identifying times of glaciation in the fossil record. 

Today, oxygen isotope stratigraphy has burgeoned into one of the most widely used analytical techniques in the palaeontological arsenal. To date researchers have identified more than 100 oxygen isotope ’stages’ and there seems no reason why this technique should not be extended to the beginning of the Cenozoic (Fig 1). 

The discovery that these oxygen isotope variations occur with a regular cyclicity was one of the most important breakthroughs of the 1970s. In a seminal paper published in Science in 1976, Jim Hays, John Imbrie and Nicholas Shackleton proved beyond doubt that the cycles of glacial-interglacial ages were controlled by variations in the Earth’s orbit around the sun. The theory had originally been proposed by an obscure Yugoslavian astronomer named Milutin Milankovitch in the early years of the 20th century. 

When the Earth’s orbit is in such a configuration as to minimise the amount of sunlight hitting the land masses of the northern hemisphere, greater amounts of ice and snow persist in the high latitudes during the summer months. Together with the increased reflection from the snow-bound Earth, eventually this plunges the world into an ice age. The reciprocal of this orbital configuration promotes interglacial ages. 

The importance of this link between a metronome in the heavens and oxygen isotope variations in deep sea sediments cannot be overestimated for it has allowed, by turning the idea on its head, the ultra-accurate calibration of time. Each glacial-interglacial cycle has a invariant duration that must be reflected in the length of the oxygen isotope cycles in deep sea cores. Knowing this relationship has allowed geologists to recalibrate the timing of the crucial magnetic polarity reversals in the cores that, in turn, are the pegs on which hang the dating of fossil extinction or origination events. So now, when a palaeontologist identifies one of these datums in a deep sea - or indeed land section - the age of that rock layer is known with a previously undreamed of accuracy. This new discipline - astrochronology - is probably the supreme achievement of chemostratigraphy. 

Bottom drawer data 

Another type of isotope data began to be collected very soon after the oxygen isotope revolution of the 1950s started. Since CO₂ contains both oxygen and carbon atoms it was a fairly trivial matter to acquire ¹³ C data from the same sample. The three most common species of CO₂ are ¹² C ¹⁶ O ¹⁶ O, ¹³ C ¹⁶ O ¹⁶ O, and ¹² C ¹⁶ O ¹⁸ O. These have atomic masses of 44, 45 and 46 respectively and are easily separated in the flight tube of a mass spectrometer. 

In the early days of oxygen isotope thermometry the most common MS instrument was the so-called ’dual-collector’ machine which had two Faraday cups for collecting the ionised CO₂ molecules as they came screaming down the flight tube from the source. It was a simple matter to retune the accelerating voltage of the machine so that the ¹³ C beam was captured by the second collector. In this way carbon isotope data were collected along with the oxygen isotope data although at that time there was no clear appreciation of their usefulness - at least in a palaeoceanographic context. All that was known was that more of the light ¹² C isotope was incorporated into the organic material formed by photosynthesis and that this left the surrounding seawater progressively more enriched in the heavy isotope of carbon, ¹³ C. 

It was this relationship with photosynthesis that precipitated another revolution in chemostratigraphy in the 1970s, on the back of emerging concerns about the possible role of greenhouse gases like CO₂ in controlling climate change. The intensity of photosynthesis occurring in the surface waters of the oceans controls the partial pressure of CO₂ in the atmosphere above. Scientists soon realised therefore that the ¹² C to ¹³ C ratio indicated variations in the partial pressure of atmospheric CO₂ in the geological past. 

The more green plants and photosynthesising bacteria there are, the more ¹² C is locked up in organic material and transported away from the sea surface, and the more ¹³ C is incorporated into the shells of planktonic foraminifera precipitating CaCO₃ from seawater. Deep water benthic foraminifera do not register these fluctuations in productivity, so that the difference between these forams calibrated against the recent past gives a proxy for the partial pressure of CO₂ over geological time. This ’difference’ approach removes the effect of changes to the gross δ ¹³ C composition of the ocean which takes place on longer timescales. 

Proof of the accuracy of the technique was its near perfect match with the record of CO₂ change during the Pleistocene retrieved from ice cores at Vostok station in Antarctica (Fig 2). The crucial advantage of the carbon isotope difference technique was that it was not limited to the depth that an ice core could penetrate but, like the oxygen isotope technique, only by the recoverable depth of deep sea cores. 

When it was found that the Pleistocene δ¹³ C changes occurred with a cyclicity as regular as those of the oxygen isotope record the search was on for a link between the two. Was there any relationship between the changes in atmospheric CO₂ concentration implied by the regular cycles in δ¹³ C and the changes in ice volume implied by the variations in δ¹⁸ O? 

The breakthrough came in the early 1980s when Nicholas Shackleton of Cambridge University showed, using spectral analysis, that the variations in CO₂ concentration led ice-volume change but lagged the Milankovitch variations. The conclusion was inescapable: CO₂ change was the mechanism by which changes in the Earth’s orbit triggered and terminated ice ages. Carbon dioxide was nothing less than the amplifier that turned subtle changes in the Earth’s orbital geometry into ice ages or warm periods. Any lingering doubts about the relationship between greenhouse gases and climate change were blown away in one fell swoop. 

Time dependence

Since those heady days of the 1980s the relationship between oceanic productivity change and carbon isotope variations has been explored in more detail and is known to be heavily timescale dependent. 

At one of end of the scale, δ¹³ C variations can be linked to high-frequency mechanisms such as changes in the Earth’s orbit around the sun, while at the other end (> a million years) long-term changes in productivity and the partitioning of organic carbon between the atmospheric, oceanic and terrestrial carbon reservoirs exert a profound effect. Nowhere is this better seen than in the period of Earth history known as the Palaeocene, a 10 million year interval that started immediately after the mass extinction that killed off the dinosaurs (the so-called K/T boundary) and which ended with the oceanic methane ’burp’ known as the Palaeocene-Eocene Thermal Maximum or PETM. 

In general carbon isotopes are more useful than oxygen isotopes for stratigraphic purposes in the remote past (broadly, >10 million years before present). This is because they are more resistant to diagenesis: the tendency of the original isotopic ratio to be altered by post-depositional processes such as through-flow of meteoric water. Despite this, there are still limitations to the use of carbon isotopes as stratigraphic tools, partly because they too ultimately succumb to diagenesis in rocks that have been deeply buried or cooked and partly because the residence time of carbon in the ocean is not significantly longer than the mixing time of the ocean. 

This means that the possibility always exists that different parts of the world’s oceans - and therefore the sediments and ultimately rocks derived from them - may have different carbon isotope ratios. By the mid-1980s therefore interest was focusing on yet another isotopic system - strontium - that does not suffer from this disadvantage because its residence time in the ocean is very long indeed compared to the mixing time of the oceans and also because the isotopic ratios are solely controlled by very long term processes. 

For all the utility of chemostratigraphy, the best results are obtained when the various proxies are combined with other types of stratigraphic information. The most important of these is biostratigraphy - the succession of fossils in the rock record. Biostratigraphy is qualitatively different to chemostratigraphy because it is an ordinal process that never repeats itself. The evolution and extinction of life forms is, after all, unique; the dinosaurs, for example, will never evolve again. 

Chemostratigraphy, in contrast, is iterative - there is no unique signature to an oxygen, carbon or strontium isotope ratio. Yet chemostratigraphy, the signatures of time, have transformed the way that we recognise the passage of eternity in the geological record. 

Source: Chemistry in Britain

Acknowledgements

Richard Corfield is an isotope geochemist and science writer in the department of earth sciences, Oxford University, where he directs the stable isotope laboratory.