Signatures of time

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Richard Corfield explains how stable isotopes are helping to pinpoint geologic 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, ¹⁶O, would be incorporated in shell material with a rise in temperature. This would be due purely to the fact that ¹⁶O has greater vibrational energy than the heavier isotope ¹⁸O, 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 ¹⁶O than the seawater left behind. Hence foraminifera growing in the oceans during glacial intervals will already have a heavier ¹⁸O signature than during interglacials. Conversely, during interglacials, both warmer temperatures and less ice equals more ¹⁶O 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).

Fluctuations

Fig 1. δ18O fluctuations for the past 2.5 million years*

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 ¹³C data from the same sample. The three most common species of CO₂ are ¹²C ¹⁶O ¹⁶O, ¹³C ¹⁶O ¹⁶O, and ¹²C ¹⁶O ¹⁸O. 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 ¹³C 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 ¹²C 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, ¹³C.

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 ¹²C to ¹³C 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 ¹²C is locked up in organic material and transported away from the sea surface, and the more ¹³C 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 ¹³C 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.

Vostok

Fig 2. Vostok CO₂ record of the Pleistocene

When it was found that the Pleistocene δ¹³C 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 δ¹³C and the changes in ice volume implied by the variations in δ¹⁸O?

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, δ¹³C 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.

1. Perfect Palaeocene timing

During the Palaeocene carbon isotope values rise steeply to a maximum at about 60 Ma (million years before present) then decline toward a minimum in the early Eocene (see Fig below). It seems that the meteorite collision that killed off the dinosaurs also destabilised the prevailing controls on carbon isotope change by killing off most of the oceanic plankton that control the partitioning of carbon isotopes between the atmospheric and marine reservoirs. As plankton evolution began again in the immediate aftermath of the catastrophe, CO₂ was pulled out of the atmosphere and sequestered - probably in the form of organic carbon - in the deep ocean.

Unlike the Pleistocene this process was unregulated by Milankovitch cyclicity and continued for more than four million years. The peak in δ13C values at about 60Ma represents the time of maximum atmospheric CO₂ depletion and it is no surprise therefore that at this time the seas near Antarctica began to cool, accelerating the formation of cold, deep waters there. What stopped this trend is unknown but after about 60 Ma δ13C values began their decline, signifying an increase in atmospheric CO₂.

From the stratigraphic perspective the Palaeocene carbon isotope maximum is an enormously useful marker that identifies rocks of Palaeocene age even in the absence of other dating clues. The figure (below) also shows a comparison of the δ13C record from four sites: two from a rock section outcropping on land in central Italy, one from a deep sea drill hole the southeast Atlantic and the other from a drill hole in the central Pacific. The striking thing about all four curves is their similarity which shows that the Palaeocene carbon isotope event was a truly global phenomenon.

A single example serves to illustrate the importance of this point. In early 1990 Leg 130 of the international Ocean Drilling Program retrieved carbonate sediments from very deep below the ocean floor of the Ontong Java Plateau in the western equatorial Pacific. This rock had been so compacted and heated that all the microfossils - which would ordinarily have served to identify age - had been cooked and compressed into unidentifiable husks within a rock fabric so hard that it rang like a bell when tapped with a hammer. Yet a dark layer lower in the core had been tentatively identified as the K/T boundary. In my group’s laboratory at Oxford we analysed the barren sediments immediately above this layer and immediately retrieved the characteristic carbon isotope signature of the Palaeocene period. In the absence of other clues we still knew exactly where we were in time.

Palaeocene

2. Strontium dating

Strontium-87 is a radiogenic isotope formed by the α-decay of ⁸⁷Rb. It is conventionally expressed with respect to the lighter isotope of strontium, ⁸⁶Sr. Normally this relationship takes the form of an absolute ratio (in the region of 0.708) and so is not expressed in the conventional ’δ’ notation used in oxygen and carbon isotope studies. The Sr isotopic composition of seawater - and the rock derived from it - is controlled by the isotopic ratios in two terrestrial reservoirs: continental crust and the Earth’s upper mantle.

Continental crust supplies dissolved Sr to the oceans via weathering while the mantle supplies Sr to the oceans via hydrothermal activities at deep sea vents or ’black smokers’. Due to early isotopic separation at the time of the Earth’s formation these two reservoirs have developed distinctly different geochemical signatures, with continental crust having significantly higher Sr-87/Sr-86 ratios.

It is the interplay of the relative importance of these two sources that gives rise to the changing Sr isotopic composition of seawater and derived rocks through time. Because of the slow rate of change and the long residence time of well-mixed strontium in the ocean a single Sr isotope curve can be generated that is globally applicable. Or, to put it another way, a single measurement compared with a reference curve (eg Fig below) will place you accurately in time.

One thing that is immediately apparent is that, despite the slow rate of change of the curve Sr values are not time-unique, ie the value of about 0.709 that characterises the present day has also occurred in the Silurian and in the latest Cambrian. The point is that these repeat values are so widely separated in time that there is no danger of mixing them up. Fossils from the Holocene, Silurian and late Cambrian are all quite different and easily recognisable as such. Once you have identified what ’ballpark’ you are in, you can use the Sr isotope curve to refine your position in time.

Today many more Sr measurements are being generated and the reference curve is becoming progressively more refined. We are already at the stage where Sr isotope stratigraphy can be performed routinely and looks set to outstrip even carbon isotopes as a technique for orientating oneself in time, at least on longer timescales.

3. Recent trends

As technology has advanced, the discipline of chemostratigraphy has spread beyond just isotopes. Today, Inductively Coupled Atomic Emission Spectrometry (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) are widely used to make rapid and simultaneous measurements of the abundance of up to 50 elements that can then be used to characterise a rock unit. This technique is increasingly applied in the petroleum exploration industry where the emphasis is on correlating local rocks (for example, tracing the lateral extent of hydrocarbon bearing sandstones). Future developments will see the use of portable laser spectrometers that can be used at the well site and which will remove the need for complex, laboratory based ICP-AES and ICP-MS analysis.

Sulphur, nitrogen and neodymium isotope chemostratigraphy is also advancing. Sulphur isotope variability is linked to the partitioning of sulphur between oxidised (sulphate) and reduced (sulphide) reservoirs. Bacterial reduction of sulphate preferentially incorporates the light isotope of sulphur, ³²S so that an increase in the rate of sulphide formation results in higher δ34S values.

Nitrogen isotopes in seawater and associated sediments reflect nitrate use and the rate of denitrification in seawater. Oxidation of dead plankton as it falls through the water column results in concentration of the heavy isotope, ¹⁵N over the light isotope, ¹⁴N, and an increase therefore in the δ15N ratio. Like elemental abundance chemostratigraphy, sulphur and nitrogen isotopes are susceptible to local effects and are therefore not likely to be useful on the global scale.

Neodymium isotopes in seawater and associated sediments, like strontium isotopes, reflect source rock composition and, as such, show promise as a potential tracer of global water mass movements.

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<div class="xs-separator"> Intro/teaser </div> Isotopic ratios in the feathers of birds hold evidence of their comings and goings, and may explain some of the mysteries of migration.<div class="xs-separator"> Access denied intro </div>  <div class="xs-separator"> Standfirst </div> Isotopic ratios in the feathers of birds hold evidence of their comings and goings, and may explain some of the mysteries of migration.<div class="xs-separator main"> Story text </div> Isotopic ratios in the feathers of birds hold evidence of their comings and goings, and may explain some of the mysteries of migration, Richard Corfield reports. At the end of Hitchcock’s classic suspense thriller  The Birds , a badly gashed Tippi Hedren is helped away from a white house on the shore of northern California’s Bodega Bay by a taciturn Rod Taylor. Every surface, every wall, every telegraph wire is covered by a menacing sea of birds, continually shifting but eerily silent. It is one of the most powerful images of 20th century cinema.  The central MacGuffin of the film (Hitchcock’s own term for the dramatic question that drives a story) is: Why are these birds acting in such a hostile fashion? Are they seeking revenge for our mistreatment of nature, are they a presentiment of some apocalyptic Doomsday, or are they sent by God as a punishment for the evil of humankind?  If Hitchcock were alive today he would perhaps be glad to know that if we still don’t know why the birds in the movie were attacking humankind we now at least have a method of determining where they came from. One of the greatest puzzles in ecology - the mystery of migration - is in the process of being solved.  Migration, the regular seasonal movement of individuals to and from breeding locations, is a fundamental part of the life cycle of many species and its most conspicuous practitioners are birds. For decades migratory biology has attracted the attention of ecologists but attempts to track migrating individuals - by banding or radio-telemetry, for example - have almost always resulted in failure.  Consider the problems: only a small subset of a very large population is ever tagged and the individual elements that are tagged typically weigh less than 50g. Is it any surprise that the recovery rate of tagged individuals is only between one-in-1000 and one-in-100,000? But in recent years all of that has changed through the use of isotopes - particularly the stable isotopes - as natural abundance tracers. The technique relies on the fact that the relative abundance of several species of stable isotopes in the tissues of animals is intimately linked to geographic location.  The use of natural abundance isotopes as migration tracers has been stimulated partly by fears over the decline in certain species. For example, in the US, southern breeding populations of the black-throated blue warbler ( Dendroica caerulescens ) have been declining and the worry is that this may be due to overwintering habitat loss.  In a recent issue of  <cite> Science </cite> a group based at Dartmouth College in New Hampshire, Stanford University and the Smithsonian Institution, have managed to track the movements of populations of the black-throated blue warbler using the abundances of isotopes of hydrogen and carbon in feathers. The two stable isotopes of hydrogen (H and D) are strongly latitude dependent because the heavier D isotope tends to condense out first and fall as precipitation.  Since water vapour enters the atmosphere in the low latitudes through evaporation, and the D-rich water condenses out first, the net result is a gradient of decreasing D abundance with increasing latitude. When this is expressed as a ratio of D to H we find that isotope ratios decline with increasing latitude ( Table 1 ). This gross pattern can be locally modified by seasonal changes, distance from the sea, and the altitude dependence of precipitation (rain tends to fall as warm, moist air is forced over mountain ranges).  Table 1. Isotropic species in migration and trophic studies <table border="0"> <tbody> <tr> <td> Isotope  </td> <td> Convention  </td> <td> Standard* </td> <td> Fractionation   </td> </tr> <tr> <td> Hydrogen/Deuterium </td> <td> δ </td> <td> SMOW  </td> <td> Precipitation/condensation </td> </tr> <tr> <td> Carbon-12/Carbon-13 </td> <td> δ 13 C </td> <td> PDB </td> <td> Proportion of C3/C4 plants </td> </tr> <tr> <td> Nitrogen-15/Nitrogen-14 </td> <td> δ 15 N </td> <td> Air </td> <td> Marine or terrestrial plants + degree of aridity </td> </tr> <tr> <td> Strontium-87/Strontium-86 </td> <td> δ 87 Sr </td> <td> NBS-987 </td> <td> Source rock type </td> </tr> </tbody> </table> *SMOW = Standard Mean Ocean Water; PDB = Pee Dee Belemnite In the continental US the overall effect of these interactions is that the hydrogen isotope ratios tend to decrease in a north-westerly direction. Since local vegetation reflects this unique isotopic signature, all higher elements in the food chain, including the migratory species under study do so too.  C3 plants are an ancient group of carbon-fixing green plants. They use rubisco (the most abundant enzyme on Earth) to make a three-carbon compound which is the initial, stable product of photosynthesis. However, such plants may subsequently lose up to half of this recently fixed carbon through photorespiration at night. But since the onset of the global decline in atmospheric CO 2 that started at the end of the Cretaceous period (65 million years ago) another type of photosynthetic pathway has evolved, the C4 pathway.  C4 plants possess biochemical and anatomical mechanisms that serve to concentrate CO 2 at the site of fixation, on the chloroplasts, which greatly reduces carbon loss through respiration. Their biochemistry is based on the enzyme PEP-carboxylase that makes an initial four-carbon compound. This is then transferred to specialised cells where the CO 2 is re-released and refixed using rubisco. These different biochemistries of C3 and C4 plants result in very different stable isotope signatures.  Stable isotope ratios are conventionally expressed in parts per thousand (per mil or  o/oo ) difference from an agreed standard. C3 plants tend to cluster around an average δ  13 C of -25 o/oo , while C4 plants have a much heavier isotopic composition, with an average δ  13 C of about -12 o/oo . Once again there are local complications but these are minor. Since C3 plants predominate at high latitudes and C4 plants are more common in the low latitudes there is a gradient of decreasing δ  13 C with increasing latitude.  The Dartmouth study used these isotopic gradients to identify in detail where birds from breeding populations of the black-throated blue warbler spent their breeding season and the winter. By analysing feather keratin (which preserves the isotopic ratios at the time of moult) they found that birds overwintering on the Caribbean islands from Cuba in the west to Puerto Rico in the east came from two latitudinally distinct breeding populations on the US mainland.  Those overwintering in the west of the Greater Antilles (Cuba and Jamaica) came predominantly from northern populations ranging from Lake Michigan to as far north as New Brunswick, Canada, while those overwintering to the east of the range (the islands of Hispaniola and Puerto Rico) came predominantly from the southern part of the warbler’s breeding range in Georgia and Virginia. Interestingly, although there is latitudinal segregation within these overwintering populations there was no longitudinal segregation in the northern populations overwintering on Cuba and Jamaica. (The southern population came from too restricted a longitudinal range to test for segregation.)  The study hinted at an explanation for why the southern breeding populations of the black-throated blue warbler are declining in abundance. These are the birds that overwinter in the eastern portion of the Greater Antilles and it may be significant that the island of Hispaniola has undergone massive deforestation in the past few years with consequent habitat destruction.  This approach is not restricted to migratory species that live in the US. A similar study has used nitrogen isotopes together with carbon isotopes to show that two subspecies of the Scandinavian Willow Warbler ( Phylloscopus trochilus trochilus and  Phylloscopus trochilus acredula ), which occupy distinct latitudinal ranges in Sweden, also occupy different overwintering grounds when they migrate to Africa.  Nitrogen isotopes, like carbon isotopes vary geographically, although here the control is more complicated. The largest differences are between marine and terrestrial plants. Marine plants have more positive δ  15 N values than terrestrial plants and these differences are reflected further up the food chain, for example in migrating song birds. But there also appears to be an effect that is rather enigmatically linked to aridity.  With increasing dryness δ  15 N ratios tend to increase (spanning a range from <10 o/oo in regions with >800mm of rainfall a year to >15 o/oo in areas withThe ratio of strontium-87 to Sr-86 can also be used as a migratory tracer (although note that Sr-87 can be formed radiogenically by the decay of Rb-87 so it is not a stable isotope in the sense of, for example, C-13). In an earlier study of the black-throated blue warbler the Dartmouth team found that breeding populations could be separated according to the δ  87 Sr ratio preserved in their bones. Birds breeding near the core of the Appalachian Mountains in Georgia and Virginia, had high isotopic ratios of around 9 o/oo , while those that lived further north near the Great Lakes region and southern Canada had lower values of around 4 o/oo .  This contrast is because of the higher proportion of geologically ancient, igneous, rocks in the Appalachians relative to the low δ  87 Sr sedimentary rocks of the Great Lakes and southern Canadian regions. It seems that even the basic geology of a region has an isotopic spoor that can reflected in the animals that live there.  So we can see then that to use stable isotopes as tracers of migratory animals various conditions must be satisfied. In the case of annually migrating animals the tissue to be analysed must be renewed seasonally otherwise the analysis will be an average of isotopic ratios incorporated into the animal throughout its lifetime. Feathers of course are ideal.  Secondly, the isotope ratios must be an accurate reflection of their local levels. In practice carbon, nitrogen and strontium tend to be true reflections because they do not exchange with the atmosphere after incorporation with feather keratin. Hydrogen isotopes tend to be more exchangeable. Thirdly, the geographic variability of the isotope system in question needs to have been carefully mapped if small scale differences in regional origin are to be discerned.  The use of Sr isotopes as migratory tracers, however, is more limited than the other isotope systems so far discussed for several reasons. First, geographic Sr variability has not been as well mapped as other isotope systems; secondly its areal distribution is not as straightforward as, for example, hydrogen or carbon; and thirdly, the technique demands the sacrifice of the animal in order that its bone tissue may be sampled, which rather defeats the underlying emphasis on conservation.  Birds are not the only animal group that is being studied using natural abundance isotopes. In some cases isotope tracking can help preserve even severely endangered species. Today isotopes are offering undreamt of insights into the ecology of several animal species. These tools will undoubtedly be refined in the future and offer the hope that we can at last understand the complex web of feeding and migratory relationships among animals and use this information to help in their conservation.  The final scene envisaged by Hitchcock for the end of  The Birds was cut for budgetary reasons. It was to be a composite matte of the Golden Gate Bridge covered in a seething mass of birds preparing to assault San Francisco. Their place of origin - the ’where’ of the movie might now be solved through the use of isotopes - but the master of suspense can rest easy; his ’why’ remains as enigmatic as ever.  Source: Chemistry in Britain Richard Corfield is an isotope geochemist and science writer in the department of earth sciences, Oxford University, where he directs the stable isotope laboratory. <div class="factfile"> 1. Tracking wildlife African elephants have long been at risk from ivory poachers and one way that this trade can be discouraged is by tracing where a piece of ivory originated. Studies using carbon, nitrogen, strontium and radiogenic lead isotopes in South African elephant populations have shown promise particularly when the data are combined using multivariate statistics to generate ’composite variables’.  Using this approach a team based in South Africa has shown that a combination of carbon and nitrogen data, and lead and strontium data, was sufficient to distinguish where elephant bone - and ivory - had originated. The technique is still in its infancy however and more recent studies hint that there may be much more geographic variability in isotope concentrations than originally thought. Once again though, use of a sufficiently large number of isotopic species - when combined statistically - may be the way forward.  Currently a project is under way at the University of Cape Town to map the distribution of isotopes of strontium, lead and neodymium throughout sub-Saharan Africa so that an isotopic database can be produced with which poached ivory can be compared. In this way regions prone to poaching can be identified and steps taken to eradicate the threat.  However, stable isotopes vary not only as a consequence of geographic region, they often vary as a result of food source as well. Scientists working for the US Geological Survey are using ratios of carbon, nitrogen and sulphur isotopes to investigate whether a change in the basic diet of the US Grizzly bear in the area around Yellowstone National Park will have any long term effects on bear populations.  In recent years the native cut-throat trout of the lakes in Yellowstone National Park in Wyoming have been under threat from an invasion of lake trout. By calibrating the stable isotope composition of Grizzly bear food sources and then tracking the changing composition in bear hair, scientists are monitoring whether the bears are able to change their diet without any damage to their population structure.  The diet of Arctic foxes has also been investigated using the ratio of carbon isotopes. Arctic foxes usually feed on lemmings which have the distinctive carbon isotope of a terrestrial herbivore that feeds predominantly on C3 plants (approximately -25o/oo). Research has shown that following winters where the lemming population has been low, Arctic fox hair has the significantly more positive carbon isotope signature indicative of a marine diet.  It seems that in low-lemming abundance winters Arctic foxes follow polar bears out onto the ice and feed on seal carrion or pups. A concern must be that as global warming results in a shorter sea-ice season Arctic fox populations may become more vulnerable to times of low lemming abundance.   </div> <div class="factfile"> 2. Measuring up Most stable isotope measurements (C, N, O, S, H) are routinely made on a magnetic sector mass spectrometer. There are two variants: ’dual-inlet’ and ’continuous-flow’ machines.  In dual-inlet MS, the sample gas (liberated by digestion in acid or by combustion) is frozen in a low-volume cold-trap where it is then sublimated back to gas. The pressure of this sample gas is then balanced against that of a reference gas of known isotopic composition before multiple measurements of each are made. This allows very accurate measurement of the differences in the isotopic ratios of the sample and reference gases.  In continuous-flow MS, the gas is carried in a helium carrier-flow to the analyser, where the isotopic abundance is measured directly from the height of the peak in the mass spectrum. (Until recently, this technique was considerably less accurate than the dual inlet approach.)  Subsequently, in both cases, the isotopic ratios (of D to H, or C-13 to C-12, for example), are then corrected for machine error and normalised to an internationally agreed standard (eg PDB or SMOW). The final δ values are not absolute isotope abundances, therefore, but differences between samples and one or other of several natural abundance standards. For example, if a sample is found to have a C-13/C-12 ratio greater than the standard PDB’s by 10 parts per thousand, this value is reported as δ C = +10o/oo.  Strontium isotope measurements are made on a thermal ionisation magnetic sector mass spectrometer. Samples are not introduced into the machine from an on-line device but rather are laboriously evaporated onto rhenium filaments before measurement in the machine. Although attempts have been made to express strontium isotope ratios as delta values (δ 87Sr) it is still more common to see them expressed as an absolute ratio of Sr-87 to Sr-86, normally, for terrestrial rocks, in the region of 0.47.  Before using stable isotopes as tracers of migratory animals, three conditions must be satisfied.  1. The tissue to be analysed must be renewed seasonally otherwise the analysis will be an average of isotopic ratios incorporated into the animal throughout its lifetime. Feathers are ideal because the primary (most complete) moult for most temperate song birds occurs at the end of the breeding season and before migration.  2. The isotope ratios must be an accurate reflection of their local levels. In practice carbon, nitrogen and strontium tend to be true reflections because they do not exchange with the atmosphere after incorporation with the feather keratin. Hydrogen isotopes tend to be more exchangeable but this does not appear to affect their value as a geographic tool.  3. The geographic variability of the isotopes in question needs to be mapped if small-scale differences in regional origin are to be discerned in migrating populations.   </div><div class="xs-separator after_main"> References </div>  <div class="xs-separator"> Additional information </div>